path: root/calendar/cal-util/cal-recur.c

/* -*- Mode: C; tab-width: 8; indent-tabs-mode: t; c-basic-offset: 8 -*- */
/*
 * Evolution calendar recurrence rule functions
 *
 * Copyright (C) 2000 Ximian, Inc.
 *
 * Author: Damon Chaplin <damon@ximian.com>
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
 */

#include <config.h>
#include <stdlib.h>
#include <string.h>
#include <cal-util/cal-recur.h>
#include <cal-util/timeutil.h>


/*
 * Introduction to The Recurrence Generation Functions:
 *
 * Note: This is pretty complicated. See the iCalendar spec (RFC 2445) for
 *       the specification of the recurrence rules and lots of examples
 *   (sections 4.3.10 & 4.8.5). We also want to support the older
 *   vCalendar spec, though this should be easy since it is basically a
 *   subset of iCalendar.
 *
 * o An iCalendar event can have any number of recurrence rules specifying
 *   occurrences of the event, as well as dates & times of specific
 *   occurrences. It can also have any number of recurrence rules and
 *   specific dates & times specifying exceptions to the occurrences.
 *   So we first merge all the occurrences generated, eliminating any
 *   duplicates, then we generate all the exceptions and remove these to
 *   form the final set of occurrences.
 *
 * o There are 7 frequencies of occurrences: YEARLY, MONTHLY, WEEKLY, DAILY,
 *   HOURLY, MINUTELY & SECONDLY. The 'interval' property specifies the
 *   multiples of the frequency which we step by. We generate a 'set' of
 *   occurrences for each period defined by the frequency & interval.
 *   So for a YEARLY frequency with an interval of 3, we generate a set of
 *   occurrences for every 3rd year. We use complete years here -  any
 *   generated occurrences that occur before the event's start (or after its
 *   end) are just discarded.
 *
 * o There are 8 frequency modifiers: BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY,
 *   BYDAY, BYHOUR, BYMINUTE & BYSECOND. These can either add extra occurrences
 *   or filter out occurrences. For example 'FREQ=YEARLY;BYMONTH=1,2' produces
 *   2 occurrences for each year rather than the default 1. And
 *   'FREQ=DAILY;BYMONTH=1' filters out all occurrences except those in Jan.
 *   If the modifier works on periods which are less than the recurrence
 *   frequency, then extra occurrences are added, otherwise occurrences are
 *   filtered. So we have 2 functions for each modifier - one to expand events
 *   and the other to filter. We use a table of functions for each frequency
 *   which points to the appropriate function to use for each modifier.
 *
 * o Any number of frequency modifiers can be used in a recurrence rule.
 *   (Though the iCalendar spec says that BYWEEKNO can only be used in a YEARLY
 *   rule, and some modifiers aren't appropriate for some frequencies - e.g.
 *   BYMONTHDAY is not really useful in a WEEKLY frequency, and BYYEARDAY is
 *   not useful in a MONTHLY or WEEKLY frequency).
 *   The frequency modifiers are applied in the order given above. The first 5
 *   modifier rules (BYMONTH, BYWEEKNO, BYYEARDAY, BYMONTHDAY & BYDAY) all
 *   produce the days on which the occurrences take place, and so we have to
 *   compute some of these in parallel rather than sequentially, or we may end
 *   up with too many days.
 *
 * o Note that some expansion functions may produce days which are invalid,
 *   e.g. 31st September, 30th Feb. These invalid days are removed before the
 *   BYHOUR, BYMINUTE & BYSECOND modifier functions are applied.
 *
 * o After the set of occurrences for the frequency interval are generated,
 *   the BYSETPOS property is used to select which of the occurrences are
 *   finally output. If BYSETPOS is not specified then all the occurrences are
 *   output.
 *
 *
 * FIXME: I think there are a few errors in this code:
 *
 *  1) I'm not sure it should be generating events in parallel like it says
 *     above. That needs to be checked.
 *
 *  2) I didn't think about timezone changes when implementing this. I just
 *     assumed all the occurrences of the event would be in local time.
 *     But when clocks go back or forwards due to daylight-saving time, some
 *     special handling may be needed, especially for the shorter frequencies.
 *     e.g. for a MINUTELY frequency it should probably iterate over all the
 *     minutes before and after clocks go back (i.e. some may be the same local
 *     time but have different UTC offsets). For longer frequencies, if an
 *     occurrence lands on the overlapping or non-existant time when clocks
 *     go back/forward, then it may need to choose which of the times to use
 *     or move the time forward or something. I'm not sure this is clear in the
 *     spec.
 */

/* Define this for some debugging output. */
#if 0
#define CAL_OBJ_DEBUG   1
#endif

/* We will use icalrecurrencetype instead of this eventually. */
typedef struct {
    icalrecurrencetype_frequency freq;

    int            interval;

    /* Specifies the end of the recurrence. No occurrences are generated
       after this date. If it is 0, the event recurs forever. */
    time_t         enddate;

    /* WKST property - the week start day: 0 = Monday to 6 = Sunday. */
    gint           week_start_day;


    /* NOTE: I've used GList's here, but it doesn't matter if we use
       other data structures like arrays. The code should be easy to
       change. So long as it is easy to see if the modifier is set. */

    /* For BYMONTH modifier. A list of GINT_TO_POINTERs, 0-11. */
    GList         *bymonth;

    /* For BYWEEKNO modifier. A list of GINT_TO_POINTERs, [+-]1-53. */
    GList         *byweekno;

    /* For BYYEARDAY modifier. A list of GINT_TO_POINTERs, [+-]1-366. */
    GList         *byyearday;

    /* For BYMONTHDAY modifier. A list of GINT_TO_POINTERs, [+-]1-31. */
    GList         *bymonthday;

    /* For BYDAY modifier. A list of GINT_TO_POINTERs, in pairs.
       The first of each pair is the weekday, 0 = Monday to 6 = Sunday.
       The second of each pair is the week number [+-]0-53. */
    GList         *byday;

    /* For BYHOUR modifier. A list of GINT_TO_POINTERs, 0-23. */
    GList         *byhour;

    /* For BYMINUTE modifier. A list of GINT_TO_POINTERs, 0-59. */
    GList         *byminute;

    /* For BYSECOND modifier. A list of GINT_TO_POINTERs, 0-60. */
    GList         *bysecond;

    /* For BYSETPOS modifier. A list of GINT_TO_POINTERs, +ve or -ve. */
    GList         *bysetpos;
} CalRecurrence;

/* This is what we use to pass to all the filter functions. */
typedef struct _RecurData RecurData;
struct _RecurData {
    CalRecurrence *recur;

    /* This is used for the WEEKLY frequency. It is the offset from the
       week_start_day. */
    gint weekday_offset;

    /* This is used for fast lookup in BYMONTH filtering. */
    guint8 months[12];

    /* This is used for fast lookup in BYYEARDAY filtering. */
    guint8 yeardays[367], neg_yeardays[367]; /* Days are 1 - 366. */

    /* This is used for fast lookup in BYMONTHDAY filtering. */
    guint8 monthdays[32], neg_monthdays[32]; /* Days are 1 to 31. */

    /* This is used for fast lookup in BYDAY filtering. */
    guint8 weekdays[7];

    /* This is used for fast lookup in BYHOUR filtering. */
    guint8 hours[24];

    /* This is used for fast lookup in BYMINUTE filtering. */
    guint8 minutes[60];

    /* This is used for fast lookup in BYSECOND filtering. */
    guint8 seconds[62];
};

/* This is what we use to represent a date & time. */
typedef struct _CalObjTime CalObjTime;
struct _CalObjTime {
    guint16 year;
    guint8 month;       /* 0 - 11 */
    guint8 day;     /* 1 - 31 */
    guint8 hour;        /* 0 - 23 */
    guint8 minute;      /* 0 - 59 */
    guint8 second;      /* 0 - 59 (maybe up to 61 for leap seconds) */
    guint8 is_rdate;    /* TRUE if this is an RDATE, which may have an
                   end or duration set. */
};

/* This is what we use to represent specific recurrence dates.
   Note that we assume it starts with a CalObjTime when sorting. */
typedef struct _CalObjRecurrenceDate CalObjRecurrenceDate;
struct _CalObjRecurrenceDate {
    CalObjTime start;
    CalComponentPeriod *period;
};

/* The paramter we use to store the enddate in RRULE and EXRULE properties. */
#define EVOLUTION_END_DATE_PARAMETER    "X-EVOLUTION-ENDDATE"

typedef gboolean (*CalObjFindStartFn) (CalObjTime *event_start,
                       CalObjTime *event_end,
                       RecurData  *recur_data,
                       CalObjTime *interval_start,
                       CalObjTime *interval_end,
                       CalObjTime *cotime);
typedef gboolean (*CalObjFindNextFn)  (CalObjTime *cotime,
                       CalObjTime *event_end,
                       RecurData  *recur_data,
                       CalObjTime *interval_end);
typedef GArray*  (*CalObjFilterFn)    (RecurData  *recur_data,
                       GArray     *occs);

typedef struct _CalRecurVTable CalRecurVTable;
struct _CalRecurVTable {
    CalObjFindStartFn find_start_position;
    CalObjFindNextFn find_next_position;

    CalObjFilterFn bymonth_filter;
    CalObjFilterFn byweekno_filter;
    CalObjFilterFn byyearday_filter;
    CalObjFilterFn bymonthday_filter;
    CalObjFilterFn byday_filter;
    CalObjFilterFn byhour_filter;
    CalObjFilterFn byminute_filter;
    CalObjFilterFn bysecond_filter;
};


/* This is used to specify which parts of the CalObjTime to compare in
   cal_obj_time_compare(). */
typedef enum {
    CALOBJ_YEAR,
    CALOBJ_MONTH,
    CALOBJ_DAY,
    CALOBJ_HOUR,
    CALOBJ_MINUTE,
    CALOBJ_SECOND
} CalObjTimeComparison;

static void cal_recur_generate_instances_of_rule (CalComponent  *comp,
                          icalproperty  *prop,
                          time_t     start,
                          time_t     end,
                          CalRecurInstanceFn cb,
                          gpointer       cb_data,
                          CalRecurResolveTimezoneFn  tz_cb,
                          gpointer   tz_cb_data);

static CalRecurrence * cal_recur_from_icalproperty (icalproperty *prop,
                            gboolean exception);
static gint cal_recur_ical_weekday_to_weekday   (enum icalrecurrencetype_weekday day);
static void cal_recur_free          (CalRecurrence  *r);


static gboolean cal_object_get_rdate_end    (CalObjTime *occ,
                         GArray     *rdate_periods);
static void cal_object_compute_duration (CalObjTime *start,
                         CalObjTime *end,
                         gint       *days,
                         gint       *seconds);

static gboolean generate_instances_for_chunk    (CalComponent       *comp,
                         time_t          comp_dtstart,
                         GSList         *rrules,
                         GSList         *rdates,
                         GSList         *exrules,
                         GSList         *exdates,
                         gboolean        single_rule,
                         CalObjTime     *event_start,
                         time_t          interval_start,
                         CalObjTime     *chunk_start,
                         CalObjTime     *chunk_end,
                         gint            duration_days,
                         gint            duration_seconds,
                         CalRecurInstanceFn  cb,
                         gpointer        cb_data);

static GArray* cal_obj_expand_recurrence    (CalObjTime   *event_start,
                         CalRecurrence    *recur,
                         CalObjTime   *interval_start,
                         CalObjTime   *interval_end,
                         gboolean     *finished);

static GArray*  cal_obj_generate_set_yearly (RecurData  *recur_data,
                         CalRecurVTable *vtable,
                         CalObjTime *occ);
static GArray*  cal_obj_generate_set_monthly    (RecurData  *recur_data,
                         CalRecurVTable *vtable,
                         CalObjTime *occ);
static GArray*  cal_obj_generate_set_default    (RecurData  *recur_data,
                         CalRecurVTable *vtable,
                         CalObjTime *occ);


static CalRecurVTable* cal_obj_get_vtable   (icalrecurrencetype_frequency recur_type);
static void cal_obj_initialize_recur_data   (RecurData  *recur_data,
                         CalRecurrence  *recur,
                         CalObjTime *event_start);
static void cal_obj_sort_occurrences    (GArray     *occs);
static gint cal_obj_time_compare_func   (const void *arg1,
                         const void *arg2);
static void cal_obj_remove_duplicates_and_invalid_dates (GArray *occs);
static void cal_obj_remove_exceptions   (GArray     *occs,
                         GArray     *ex_occs);
static GArray*  cal_obj_bysetpos_filter     (CalRecurrence  *recur,
                         GArray     *occs);


static gboolean cal_obj_yearly_find_start_position (CalObjTime *event_start,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_start,
                            CalObjTime *interval_end,
                            CalObjTime *cotime);
static gboolean cal_obj_yearly_find_next_position  (CalObjTime *cotime,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_end);

static gboolean cal_obj_monthly_find_start_position (CalObjTime *event_start,
                             CalObjTime *event_end,
                             RecurData  *recur_data,
                             CalObjTime *interval_start,
                             CalObjTime *interval_end,
                             CalObjTime *cotime);
static gboolean cal_obj_monthly_find_next_position  (CalObjTime *cotime,
                             CalObjTime *event_end,
                             RecurData  *recur_data,
                             CalObjTime *interval_end);

static gboolean cal_obj_weekly_find_start_position (CalObjTime *event_start,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_start,
                            CalObjTime *interval_end,
                            CalObjTime *cotime);
static gboolean cal_obj_weekly_find_next_position  (CalObjTime *cotime,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_end);

static gboolean cal_obj_daily_find_start_position  (CalObjTime *event_start,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_start,
                            CalObjTime *interval_end,
                            CalObjTime *cotime);
static gboolean cal_obj_daily_find_next_position   (CalObjTime *cotime,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_end);

static gboolean cal_obj_hourly_find_start_position (CalObjTime *event_start,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_start,
                            CalObjTime *interval_end,
                            CalObjTime *cotime);
static gboolean cal_obj_hourly_find_next_position  (CalObjTime *cotime,
                            CalObjTime *event_end,
                            RecurData  *recur_data,
                            CalObjTime *interval_end);

static gboolean cal_obj_minutely_find_start_position (CalObjTime *event_start,
                              CalObjTime *event_end,
                              RecurData  *recur_data,
                              CalObjTime *interval_start,
                              CalObjTime *interval_end,
                              CalObjTime *cotime);
static gboolean cal_obj_minutely_find_next_position  (CalObjTime *cotime,
                              CalObjTime *event_end,
                              RecurData  *recur_data,
                              CalObjTime *interval_end);

static gboolean cal_obj_secondly_find_start_position (CalObjTime *event_start,
                              CalObjTime *event_end,
                              RecurData  *recur_data,
                              CalObjTime *interval_start,
                              CalObjTime *interval_end,
                              CalObjTime *cotime);
static gboolean cal_obj_secondly_find_next_position  (CalObjTime *cotime,
                              CalObjTime *event_end,
                              RecurData  *recur_data,
                              CalObjTime *interval_end);

static GArray* cal_obj_bymonth_expand       (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_bymonth_filter       (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byweekno_expand      (RecurData  *recur_data,
                         GArray     *occs);
#if 0
/* This isn't used at present. */
static GArray* cal_obj_byweekno_filter      (RecurData  *recur_data,
                         GArray     *occs);
#endif
static GArray* cal_obj_byyearday_expand     (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byyearday_filter     (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_bymonthday_expand    (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_bymonthday_filter    (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byday_expand_yearly  (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byday_expand_monthly (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byday_expand_weekly  (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byday_filter     (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byhour_expand        (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byhour_filter        (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byminute_expand      (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_byminute_filter      (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_bysecond_expand      (RecurData  *recur_data,
                         GArray     *occs);
static GArray* cal_obj_bysecond_filter      (RecurData  *recur_data,
                         GArray     *occs);

static void cal_obj_time_add_months     (CalObjTime *cotime,
                         gint        months);
static void cal_obj_time_add_days       (CalObjTime *cotime,
                         gint        days);
static void cal_obj_time_add_hours      (CalObjTime *cotime,
                         gint        hours);
static void cal_obj_time_add_minutes        (CalObjTime *cotime,
                         gint        minutes);
static void cal_obj_time_add_seconds        (CalObjTime *cotime,
                         gint        seconds);
static gint cal_obj_time_compare        (CalObjTime *cotime1,
                         CalObjTime *cotime2,
                         CalObjTimeComparison type);
static gint cal_obj_time_weekday        (CalObjTime *cotime);
static gint cal_obj_time_weekday_offset     (CalObjTime *cotime,
                         CalRecurrence *recur);
static gint cal_obj_time_day_of_year        (CalObjTime *cotime);
static void cal_obj_time_find_first_week    (CalObjTime *cotime,
                         RecurData  *recur_data);
static void cal_object_time_from_time       (CalObjTime *cotime,
                         time_t      t);
#if 0
static gint cal_obj_date_only_compare_func (const void *arg1,
                        const void *arg2);
#endif



static gboolean cal_recur_ensure_end_dates  (CalComponent   *comp,
                         gboolean    refresh,
                         CalRecurResolveTimezoneFn tz_cb,
                         gpointer    tz_cb_data);
static gboolean cal_recur_ensure_rule_end_date  (CalComponent   *comp,
                         icalproperty   *prop,
                         gboolean    exception,
                         gboolean    refresh,
                         CalRecurResolveTimezoneFn tz_cb,
                         gpointer    tz_cb_data);
static gboolean cal_recur_ensure_rule_end_date_cb   (CalComponent   *comp,
                             time_t      instance_start,
                             time_t      instance_end,
                             gpointer    data);
static time_t cal_recur_get_rule_end_date   (icalproperty   *prop);
static void cal_recur_set_rule_end_date     (icalproperty   *prop,
                         time_t      end_date);


#ifdef CAL_OBJ_DEBUG
static char* cal_obj_time_to_string     (CalObjTime *cotime);
#endif


CalRecurVTable cal_obj_yearly_vtable = {
    cal_obj_yearly_find_start_position,
    cal_obj_yearly_find_next_position,

    cal_obj_bymonth_expand,
    cal_obj_byweekno_expand,
    cal_obj_byyearday_expand,
    cal_obj_bymonthday_expand,
    cal_obj_byday_expand_yearly,
    cal_obj_byhour_expand,
    cal_obj_byminute_expand,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_monthly_vtable = {
    cal_obj_monthly_find_start_position,
    cal_obj_monthly_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    NULL, /* BYYEARDAY is not useful in a MONTHLY frequency. */
    cal_obj_bymonthday_expand,
    cal_obj_byday_expand_monthly,
    cal_obj_byhour_expand,
    cal_obj_byminute_expand,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_weekly_vtable = {
    cal_obj_weekly_find_start_position,
    cal_obj_weekly_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    NULL, /* BYYEARDAY is not useful in a WEEKLY frequency. */
    NULL, /* BYMONTHDAY is not useful in a WEEKLY frequency. */
    cal_obj_byday_expand_weekly,
    cal_obj_byhour_expand,
    cal_obj_byminute_expand,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_daily_vtable = {
    cal_obj_daily_find_start_position,
    cal_obj_daily_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    cal_obj_byyearday_filter,
    cal_obj_bymonthday_filter,
    cal_obj_byday_filter,
    cal_obj_byhour_expand,
    cal_obj_byminute_expand,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_hourly_vtable = {
    cal_obj_hourly_find_start_position,
    cal_obj_hourly_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    cal_obj_byyearday_filter,
    cal_obj_bymonthday_filter,
    cal_obj_byday_filter,
    cal_obj_byhour_filter,
    cal_obj_byminute_expand,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_minutely_vtable = {
    cal_obj_minutely_find_start_position,
    cal_obj_minutely_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    cal_obj_byyearday_filter,
    cal_obj_bymonthday_filter,
    cal_obj_byday_filter,
    cal_obj_byhour_filter,
    cal_obj_byminute_filter,
    cal_obj_bysecond_expand
};

CalRecurVTable cal_obj_secondly_vtable = {
    cal_obj_secondly_find_start_position,
    cal_obj_secondly_find_next_position,

    cal_obj_bymonth_filter,
    NULL, /* BYWEEKNO is only applicable to YEARLY frequency. */
    cal_obj_byyearday_filter,
    cal_obj_bymonthday_filter,
    cal_obj_byday_filter,
    cal_obj_byhour_filter,
    cal_obj_byminute_filter,
    cal_obj_bysecond_filter
};

/*
 * Calls the given callback function for each occurrence of the event that
 * intersects the range between the given start and end times (the end time is
 * not included). Note that the occurrences may start before the given start
 * time.
 *
 * If the callback routine returns FALSE the occurrence generation stops.
 *
 * Both start and end can be -1, in which case we start at the events first
 * instance and continue until it ends, or forever if it has no enddate.
 */
void
cal_recur_generate_instances (CalComponent      *comp,
                  time_t             start,
                  time_t             end,
                  CalRecurInstanceFn     cb,
                  gpointer                   cb_data,
                  CalRecurResolveTimezoneFn  tz_cb,
                  gpointer           tz_cb_data)
{
#if 0
    g_print ("In cal_recur_generate_instances comp: %p\n", comp);
    g_print ("  start: %li - %s", start, ctime (&start));
    g_print ("  end  : %li - %s", end, ctime (&end));
#endif
    cal_recur_generate_instances_of_rule (comp, NULL, start, end,
                          cb, cb_data, tz_cb, tz_cb_data);
}


/*
 * Calls the given callback function for each occurrence of the given
 * recurrence rule between the given start and end times. If the rule is NULL
 * it uses all the rules from the component.
 *
 * If the callback routine returns FALSE the occurrence generation stops.
 *
 * The use of the specific rule is for determining the end of a rule when
 * COUNT is set. The callback will count instances and store the enddate
 * when COUNT is reached.
 *
 * Both start and end can be -1, in which case we start at the events first
 * instance and continue until it ends, or forever if it has no enddate.
 */
static void
cal_recur_generate_instances_of_rule (CalComponent   *comp,
                      icalproperty   *prop,
                      time_t          start,
                      time_t          end,
                      CalRecurInstanceFn  cb,
                      gpointer            cb_data,
                      CalRecurResolveTimezoneFn  tz_cb,
                      gpointer           tz_cb_data)
{
    CalComponentDateTime dtstart, dtend;
    time_t dtstart_time, dtend_time;
    GSList *rrules = NULL, *rdates = NULL, elem;
    GSList *exrules = NULL, *exdates = NULL;
    CalObjTime interval_start, interval_end, event_start, event_end;
    CalObjTime chunk_start, chunk_end;
    gint days, seconds, year;
    gboolean single_rule;
    icaltimezone *start_zone = NULL, *end_zone = NULL;

    g_return_if_fail (comp != NULL);
    g_return_if_fail (cb != NULL);
    g_return_if_fail (start >= -1);
    g_return_if_fail (end >= -1);

    /* Get dtstart, dtend, recurrences, and exceptions */

    cal_component_get_dtstart (comp, &dtstart);
    cal_component_get_dtend (comp, &dtend);

    if (!dtstart.value) {
        g_message ("cal_recur_generate_instances_of_rule(): bogus "
               "component, does not have DTSTART.  Skipping...");
        goto out;
    }

    if (dtstart.tzid && tz_cb)
        start_zone = (*tz_cb) (dtstart.tzid, tz_cb_data);
    if (dtend.tzid && tz_cb)
        end_zone = (*tz_cb) (dtend.tzid, tz_cb_data);

    dtstart_time = icaltime_as_timet_with_zone (*dtstart.value,
                            start_zone);
    if (start == -1)
        start = dtstart_time;

    /* FIXME: DURATION could be used instead, couldn't it? - Damon */
    if (!dtend.value) {
        *dtend.value = *dtstart.value;
        dtend.value->hour = 0;
        dtend.value->minute = 0;
        dtend.value->second = 0;
        icaltime_adjust (dtend.value, 1, 0, 0, 0);
    }
    dtend_time = icaltime_as_timet_with_zone (*dtend.value, end_zone);

    /* If there is no recurrence, just call the callback if the event
       intersects the given interval. */
    if (!(cal_component_has_recurrences (comp)
          || cal_component_has_exceptions (comp))) {
        if ((end == -1 || dtstart_time < end) && dtend_time > start) {
            (* cb) (comp, dtstart_time, dtend_time, cb_data);
        }

        goto out;
    }

    /* If a specific recurrence rule is being used, set up a simple list,
       else get the recurrence rules from the component. */
    if (prop) {
        single_rule = TRUE;

        elem.data = prop;
        elem.next = NULL;
        rrules = &elem;
    } else {
        single_rule = FALSE;

        /* Make sure all the enddates for the rules are set. */
        cal_recur_ensure_end_dates (comp, FALSE, tz_cb, tz_cb_data);

        cal_component_get_rrule_property_list (comp, &rrules);
        cal_component_get_rdate_list (comp, &rdates);
        cal_component_get_exrule_property_list (comp, &exrules);
        cal_component_get_exdate_list (comp, &exdates);
    }

    /* Convert the interval start & end to CalObjTime. Note that if end
       is -1 interval_end won't be set, so don't use it!
       Also note that we use end - 1 since we want the interval to be
       inclusive as it makes the code simpler. */
    cal_object_time_from_time (&interval_start, start);
    if (end != -1)
        cal_object_time_from_time (&interval_end, end - 1);

    cal_object_time_from_time (&event_start, dtstart_time);
    cal_object_time_from_time (&event_end, dtend_time);
    
    /* Calculate the duration of the event, which we use for all
       occurrences. We can't just subtract start from end since that may
       be affected by daylight-saving time. So we want a value of days
       + seconds. */
    cal_object_compute_duration (&event_start, &event_end,
                     &days, &seconds);

    /* Take off the duration from interval_start, so we get occurrences
       that start just before the start time but overlap it. But only do
       that if the interval is after the event's start time. */
    if (start > dtstart_time) {
        cal_obj_time_add_days (&interval_start, -days);
        cal_obj_time_add_seconds (&interval_start, -seconds);
    }

    /* Expand the recurrence for each year between start & end, or until
       the callback returns 0 if end is 0. We do a year at a time to
       give the callback function a chance to break out of the loop, and
       so we don't get into problems with infinite recurrences. Since we
       have to work on complete sets of occurrences, if there is a yearly
       frequency it wouldn't make sense to break it into smaller chunks,
       since we would then be calculating the same sets several times.
       Though this does mean that we sometimes do a lot more work than
       is necessary, e.g. if COUNT is set to something quite low. */
    for (year = interval_start.year;
         end == -1 || year <= interval_end.year;
         year++) {
        chunk_start = interval_start;
        chunk_start.year = year;
        if (end != -1)
            chunk_end = interval_end;
        chunk_end.year = year;

        if (year != interval_start.year) {
            chunk_start.month  = 0;
            chunk_start.day    = 1;
            chunk_start.hour   = 0;
            chunk_start.minute = 0;
            chunk_start.second = 0;
        }
        if (end == -1 || year != interval_end.year) {
            chunk_end.month  = 11;
            chunk_end.day    = 31;
            chunk_end.hour   = 23;
            chunk_end.minute = 59;
            chunk_end.second = 61;
            chunk_end.is_rdate = FALSE;
        }

        if (!generate_instances_for_chunk (comp, dtstart_time,
                           rrules, rdates,
                           exrules, exdates,
                           single_rule,
                           &event_start,
                           start,
                           &chunk_start, &chunk_end,
                           days, seconds,
                           cb, cb_data))
            break;
    }

    if (!prop) {
        cal_component_free_period_list (rdates);
        cal_component_free_exdate_list (exdates);
    }

 out:
    cal_component_free_datetime (&dtstart);
    cal_component_free_datetime (&dtend);
}

/* Builds a list of GINT_TO_POINTER() elements out of a short array from a
 * struct icalrecurrencetype.
 */
static GList *
array_to_list (short *array, int max_elements)
{
    GList *l;
    int i;

    l = NULL;

    for (i = 0; i < max_elements && array[i] != ICAL_RECURRENCE_ARRAY_MAX; i++)
        l = g_list_prepend (l, GINT_TO_POINTER ((int) (array[i])));
    return g_list_reverse (l);
}

/**
 * cal_recur_from_icalproperty:
 * @ir: An RRULE or EXRULE #icalproperty.
 * 
 * Converts an #icalproperty to a #CalRecurrence.  This should be
 * freed using the cal_recur_free() function.
 * 
 * Return value: #CalRecurrence structure.
 **/
static CalRecurrence *
cal_recur_from_icalproperty (icalproperty *prop, gboolean exception)
{
    struct icalrecurrencetype ir;
    CalRecurrence *r;
    gint max_elements, i;

    g_return_val_if_fail (prop != NULL, NULL);

    r = g_new (CalRecurrence, 1);

    if (exception)
        ir = icalproperty_get_exrule (prop);
    else
        ir = icalproperty_get_rrule (prop);

    r->freq = ir.freq;
    r->interval = ir.interval;

    if (ir.count != 0) {
        r->enddate = cal_recur_get_rule_end_date (prop);
    } else {
        /* FIXME: icaltime_as_timet() seems to return -1 if UNTIL isn't
           set, but a simpler test would be better. */
        r->enddate = icaltime_as_timet (ir.until);
        if (r->enddate == -1)
            r->enddate = 0;
        else if (ir.until.is_date)
            /* FIXME: Decide what to do here. */
            r->enddate = time_add_day (r->enddate, 1) - 1;
    }

    r->week_start_day = cal_recur_ical_weekday_to_weekday (ir.week_start);

    r->bymonth = array_to_list (ir.by_month,
                    sizeof (ir.by_month) / sizeof (ir.by_month[0]));

    r->byweekno = array_to_list (ir.by_week_no,
                     sizeof (ir.by_week_no) / sizeof (ir.by_week_no[0]));

    r->byyearday = array_to_list (ir.by_year_day,
                      sizeof (ir.by_year_day) / sizeof (ir.by_year_day[0]));

    r->bymonthday = array_to_list (ir.by_month_day,
                       sizeof (ir.by_month_day) / sizeof (ir.by_month_day[0]));

    /* FIXME: libical only supports 8 values, out of possible 107 * 7. */
    r->byday = NULL;
    max_elements = sizeof (ir.by_day) / sizeof (ir.by_day[0]);
    for (i = 0; i < max_elements && ir.by_day[i] != ICAL_RECURRENCE_ARRAY_MAX; i++) {
        enum icalrecurrencetype_weekday day;
        gint weeknum, weekday;

        day = icalrecurrencetype_day_day_of_week (ir.by_day[i]);
        weeknum = icalrecurrencetype_day_position (ir.by_day[i]);

        weekday = cal_recur_ical_weekday_to_weekday (day);

        r->byday = g_list_prepend (r->byday,
                       GINT_TO_POINTER (weeknum));
        r->byday = g_list_prepend (r->byday,
                       GINT_TO_POINTER (weekday));
    }

    r->byhour = array_to_list (ir.by_hour,
                   sizeof (ir.by_hour) / sizeof (ir.by_hour[0]));

    r->byminute = array_to_list (ir.by_minute,
                     sizeof (ir.by_minute) / sizeof (ir.by_minute[0]));

    r->bysecond = array_to_list (ir.by_second,
                     sizeof (ir.by_second) / sizeof (ir.by_second[0]));

    r->bysetpos = array_to_list (ir.by_set_pos,
                     sizeof (ir.by_set_pos) / sizeof (ir.by_set_pos[0]));

    return r;
}


static gint
cal_recur_ical_weekday_to_weekday   (enum icalrecurrencetype_weekday day)
{
    gint weekday;

    switch (day) {
    case ICAL_NO_WEEKDAY:       /* Monday is the default in RFC2445. */
    case ICAL_MONDAY_WEEKDAY:
        weekday = 0;
        break;
    case ICAL_TUESDAY_WEEKDAY:
        weekday = 1;
        break;
    case ICAL_WEDNESDAY_WEEKDAY:
        weekday = 2;
        break;
    case ICAL_THURSDAY_WEEKDAY:
        weekday = 3;
        break;
    case ICAL_FRIDAY_WEEKDAY:
        weekday = 4;
        break;
    case ICAL_SATURDAY_WEEKDAY:
        weekday = 5;
        break;
    case ICAL_SUNDAY_WEEKDAY:
        weekday = 6;
        break;
    default:
        g_warning ("cal_recur_ical_weekday_to_weekday(): Unknown week day %d",
               day);
        weekday = 0;
    }

    return weekday;
}


/**
 * cal_recur_free:
 * @r: A #CalRecurrence structure.
 * 
 * Frees a #CalRecurrence structure.
 **/
static void
cal_recur_free (CalRecurrence *r)
{
    g_return_if_fail (r != NULL);

    g_list_free (r->bymonth);
    g_list_free (r->byweekno);
    g_list_free (r->byyearday);
    g_list_free (r->bymonthday);
    g_list_free (r->byday);
    g_list_free (r->byhour);
    g_list_free (r->byminute);
    g_list_free (r->bysecond);
    g_list_free (r->bysetpos);

    g_free (r);
}

/* Generates one year's worth of recurrence instances.  Returns TRUE if all the
 * callback invocations returned TRUE, or FALSE when any one of them returns
 * FALSE, i.e. meaning that the instance generation should be stopped.
 *
 * This should only output instances whose start time is between chunk_start
 * and chunk_end (inclusive), or we may generate duplicates when we do the next
 * chunk. (This applies mainly to weekly recurrences, since weeks can span 2
 * years.)
 *
 * It should also only output instances that are on or after the event's
 * DTSTART property and that intersect the required interval, between
 * interval_start and interval_end.
 */
static gboolean
generate_instances_for_chunk (CalComponent  *comp,
                  time_t             comp_dtstart,
                  GSList        *rrules,
                  GSList        *rdates,
                  GSList        *exrules,
                  GSList        *exdates,
                  gboolean       single_rule,
                  CalObjTime    *event_start,
                  time_t         interval_start,
                  CalObjTime    *chunk_start,
                  CalObjTime    *chunk_end,
                  gint       duration_days,
                  gint       duration_seconds,
                  CalRecurInstanceFn cb,
                  gpointer           cb_data)
{
    GArray *occs, *ex_occs, *tmp_occs, *rdate_periods;
    CalObjTime cotime, *occ;
    GSList *elem;
    gint i;
    time_t start_time, end_time;
    struct tm start_tm, end_tm;
    gboolean cb_status = TRUE, rule_finished, finished = TRUE;

#if 0
    g_print ("In generate_instances_for_chunk rrules: %p\n"
         "  %i/%i/%i %02i:%02i:%02i - %i/%i/%i %02i:%02i:%02i\n",
         rrules,
         chunk_start->day, chunk_start->month + 1,
         chunk_start->year, chunk_start->hour,
         chunk_start->minute, chunk_start->second,
         chunk_end->day, chunk_end->month + 1,
         chunk_end->year, chunk_end->hour,
         chunk_end->minute, chunk_end->second);
#endif

    occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
    ex_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
    rdate_periods = g_array_new (FALSE, FALSE,
                     sizeof (CalObjRecurrenceDate));

    /* The original DTSTART property is included in the occurrence set,
       but not if we are just generating occurrences for a single rule. */
    if (!single_rule) {
        /* We add it if it is in this chunk. If it is after this chunk
           we set finished to FALSE, since we know we aren't finished
           yet. */
        if (cal_obj_time_compare_func (event_start, chunk_end) >= 0)
            finished = FALSE;
        else if (cal_obj_time_compare_func (event_start, chunk_start) >= 0)
            g_array_append_vals (occs, event_start, 1);
    }
    
    /* Expand each of the recurrence rules. */
    for (elem = rrules; elem; elem = elem->next) {
        icalproperty *prop;
        CalRecurrence *r;

        prop = elem->data;
        r = cal_recur_from_icalproperty (prop, FALSE);

        tmp_occs = cal_obj_expand_recurrence (event_start, r,
                              chunk_start,
                              chunk_end,
                              &rule_finished);
        cal_recur_free (r);

        /* If any of the rules return FALSE for finished, we know we
           have to carry on so we set finished to FALSE. */
        if (!rule_finished)
            finished = FALSE;

        g_array_append_vals (occs, tmp_occs->data, tmp_occs->len);
        g_array_free (tmp_occs, TRUE);
    }

    /* Add on specific occurrence dates, flag them as RDATEs, and store
       a pointer to the period in the rdate_periods array. */
    for (elem = rdates; elem; elem = elem->next) {
        CalComponentPeriod *p;
        CalObjRecurrenceDate rdate;
        time_t t;

        p = elem->data;
        t = icaltime_as_timet (p->start);
        cal_object_time_from_time (&cotime, t);

        /* If the rdate is after the current chunk we set finished
           to FALSE, and we skip it. */
        if (cal_obj_time_compare_func (&cotime, chunk_end) >= 0) {
            finished = FALSE;
            continue;
        }

        /* Check if the end date or duration is set. If it is we need
           to store it so we can get it later. (libical seems to set
           second to -1 to denote an unset time. See icalvalue.c) */
        if (p->type != CAL_COMPONENT_PERIOD_DATETIME
            || p->u.end.second != -1) {
            cotime.is_rdate = TRUE;

            rdate.start = cotime;
            rdate.period = p;
            g_array_append_val (rdate_periods, rdate);
        }

        g_array_append_val (occs, cotime);
    }

    /* Expand each of the exception rules. */
    for (elem = exrules; elem; elem = elem->next) {
        icalproperty *prop;
        CalRecurrence *r;

        prop = elem->data;
        r = cal_recur_from_icalproperty (prop, FALSE);

        tmp_occs = cal_obj_expand_recurrence (event_start, r,
                              chunk_start,
                              chunk_end,
                              &rule_finished);
        cal_recur_free (r);

        g_array_append_vals (ex_occs, tmp_occs->data, tmp_occs->len);
        g_array_free (tmp_occs, TRUE);
    }

    /* Add on specific exception dates. */
    for (elem = exdates; elem; elem = elem->next) {
        CalComponentDateTime *cdt;
        time_t t;

        /* FIXME we should only be dealing with dates, not times too.

           No, I think it is supposed to be dates & times - Damon.
           I'm not sure what the semantics of just a date would be,
           since the event could recur several times each day. */
        cdt = elem->data;
        t = icaltime_as_timet (*cdt->value);
        cal_object_time_from_time (&cotime, t);

        g_array_append_val (ex_occs, cotime);
    }


    /* Sort all the arrays. */
    cal_obj_sort_occurrences (occs);
    cal_obj_sort_occurrences (ex_occs);

    qsort (rdate_periods->data, rdate_periods->len,
           sizeof (CalObjRecurrenceDate), cal_obj_time_compare_func);

    /* Create the final array, by removing the exceptions from the
       occurrences, and removing any duplicates. */
    cal_obj_remove_exceptions (occs, ex_occs);

    /* Call the callback for each occurrence. If it returns 0 we break
       out of the loop. */
    for (i = 0; i < occs->len; i++) {
        /* Convert each CalObjTime into a start & end time_t, and
           check it is within the bounds of the event & interval. */
        occ = &g_array_index (occs, CalObjTime, i);
#if 0
        g_print ("Checking occurrence: %s\n",
             cal_obj_time_to_string (occ));
#endif
        start_tm.tm_year  = occ->year - 1900;
        start_tm.tm_mon   = occ->month;
        start_tm.tm_mday  = occ->day;
        start_tm.tm_hour  = occ->hour;
        start_tm.tm_min   = occ->minute;
        start_tm.tm_sec   = occ->second;
        start_tm.tm_isdst = -1;
        start_time = mktime (&start_tm);

        if (start_time == -1) {
            g_warning ("mktime failed - time_t out of range?");
            finished = TRUE;
            break;
        }

        /* Check to ensure that the start time is at or after the
           event's DTSTART time, and that it is inside the chunk that
           we are currently working on. (Note that the chunk_end time
           is never after the interval end time, so this also tests
           that we don't go past the end of the required interval). */
        if (start_time < comp_dtstart
            || cal_obj_time_compare_func (occ, chunk_start) < 0
            || cal_obj_time_compare_func (occ, chunk_end) > 0) {
#if 0
            g_print ("  start time invalid\n");
#endif
            continue;
        }

        if (occ->is_rdate) {
            if (!cal_object_get_rdate_end (occ, rdate_periods)) {
                cal_obj_time_add_days (occ, duration_days);
                cal_obj_time_add_seconds (occ,
                              duration_seconds);
            }
        } else {
            cal_obj_time_add_days (occ, duration_days);
            cal_obj_time_add_seconds (occ, duration_seconds);
        }

        end_tm.tm_year  = occ->year - 1900;
        end_tm.tm_mon   = occ->month;
        end_tm.tm_mday  = occ->day;
        end_tm.tm_hour  = occ->hour;
        end_tm.tm_min   = occ->minute;
        end_tm.tm_sec   = occ->second;
        end_tm.tm_isdst = -1;
        end_time = mktime (&end_tm);

        if (end_time == -1) {
            g_warning ("mktime failed - time_t out of range?");
            finished = TRUE;
            break;
        }

        /* Check that the end time is after the interval start, so we
           know that it intersects the required interval. */
        if (end_time <= interval_start) {
#if 0
            g_print ("  end time invalid\n");
#endif
            continue;
        }

        cb_status = (*cb) (comp, start_time, end_time, cb_data);
        if (!cb_status)
            break;
    }

    g_array_free (occs, TRUE);
    g_array_free (ex_occs, TRUE);
    g_array_free (rdate_periods, TRUE);

    /* We return TRUE (i.e. carry on) only if the callback has always
       returned TRUE and we know that we have more occurrences to generate
       (i.e. finished is FALSE). */
    return cb_status && !finished;
}


/* This looks up the occurrence time in the sorted rdate_periods array, and
   tries to compute the end time of the occurrence. If no end time or duration
   is set it returns FALSE and the default duration will be used. */
static gboolean
cal_object_get_rdate_end    (CalObjTime *occ,
                 GArray     *rdate_periods)
{
    CalObjRecurrenceDate *rdate = NULL;
    CalComponentPeriod *p;
    gint lower, upper, middle, cmp = 0;
    time_t t;

    lower = 0;
    upper = rdate_periods->len;

    while (lower < upper) {
        middle = (lower + upper) >> 1;
      
        rdate = &g_array_index (rdate_periods, CalObjRecurrenceDate,
                    middle);

        cmp = cal_obj_time_compare_func (occ, &rdate->start);
      
        if (cmp == 0)
            break;
        else if (cmp < 0)
            upper = middle;
        else
            lower = middle + 1;
    }

    /* This should never happen. */
    if (cmp == 0) {
        g_warning ("Recurrence date not found");
        return FALSE;
    }

    p = rdate->period;
    if (p->type == CAL_COMPONENT_PERIOD_DATETIME) {
        t = icaltime_as_timet (p->u.end);
        cal_object_time_from_time (occ, t);
    } else {
        cal_obj_time_add_days (occ, p->u.duration.weeks * 7
                       + p->u.duration.days);
        cal_obj_time_add_hours (occ, p->u.duration.hours);
        cal_obj_time_add_minutes (occ, p->u.duration.minutes);
        cal_obj_time_add_seconds (occ, p->u.duration.seconds);
    }

    return TRUE;
}


static void
cal_object_compute_duration (CalObjTime *start,
                 CalObjTime *end,
                 gint   *days,
                 gint   *seconds)
{
    GDate start_date, end_date;
    gint start_seconds, end_seconds;

    g_date_clear (&start_date, 1);
    g_date_clear (&end_date, 1);
    g_date_set_dmy (&start_date, start->day, start->month + 1,
            start->year);
    g_date_set_dmy (&end_date, end->day, end->month + 1,
            end->year);

    *days = g_date_julian (&end_date) - g_date_julian (&start_date);
    start_seconds = start->hour * 3600 + start->minute * 60
        + start->second;
    end_seconds = end->hour * 3600 + end->minute * 60 + end->second;

    *seconds = end_seconds - start_seconds;
    if (*seconds < 0) {
        *days = *days - 1;
        *seconds += 24 * 60 * 60;
    }
}


/* Returns an unsorted GArray of CalObjTime's resulting from expanding the
   given recurrence rule within the given interval. Note that it doesn't
   clip the generated occurrences to the interval, i.e. if the interval
   starts part way through the year this function still returns all the
   occurrences for the year. Clipping is done later.
   The finished flag is set to FALSE if there are more occurrences to generate
   after the given interval.*/
static GArray*
cal_obj_expand_recurrence       (CalObjTime   *event_start,
                     CalRecurrence    *recur,
                     CalObjTime   *interval_start,
                     CalObjTime   *interval_end,
                     gboolean     *finished)
{
    CalRecurVTable *vtable;
    CalObjTime *event_end = NULL, event_end_cotime;
    RecurData recur_data;
    CalObjTime occ, *cotime;
    GArray *all_occs, *occs;
    gint len;

    /* This is the resulting array of CalObjTime elements. */
    all_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    *finished = TRUE;

    vtable = cal_obj_get_vtable (recur->freq);
    if (!vtable)
        return all_occs;

    /* Calculate some useful data such as some fast lookup tables. */
    cal_obj_initialize_recur_data (&recur_data, recur, event_start);

    /* Compute the event_end, if the recur's enddate is set. */
    if (recur->enddate > 0) {
        cal_object_time_from_time (&event_end_cotime,
                       recur->enddate);
        event_end = &event_end_cotime;

        /* If the enddate is before the requested interval return. */
        if (cal_obj_time_compare_func (event_end, interval_start) < 0)
            return all_occs;
    }

    /* Set finished to FALSE if we know there will be more occurrences to
       do after this interval. */
    if (!interval_end || !event_end
        || cal_obj_time_compare_func (event_end, interval_end) > 0)
        *finished = FALSE;

    /* Get the first period based on the frequency and the interval that
       intersects the interval between start and end. */
    if ((*vtable->find_start_position) (event_start, event_end,
                        &recur_data,
                        interval_start, interval_end,
                        &occ))
        return all_occs;

    /* Loop until the event ends or we go past the end of the required
       interval. */
    for (;;) {
        /* Generate the set of occurrences for this period. */
        switch (recur->freq) {
        case ICAL_YEARLY_RECURRENCE:
            occs = cal_obj_generate_set_yearly (&recur_data,
                                vtable, &occ);
            break;
        case ICAL_MONTHLY_RECURRENCE:
            occs = cal_obj_generate_set_monthly (&recur_data,
                                 vtable, &occ);
            break;
        default:
            occs = cal_obj_generate_set_default (&recur_data,
                                 vtable, &occ);
            break;
        }

        /* Sort the occurrences and remove duplicates. */
        cal_obj_sort_occurrences (occs);
        cal_obj_remove_duplicates_and_invalid_dates (occs);

        /* Apply the BYSETPOS property. */
        occs = cal_obj_bysetpos_filter (recur, occs);

        /* Remove any occs after event_end. */
        len = occs->len - 1;
        if (event_end) {
            while (len >= 0) {
                cotime = &g_array_index (occs, CalObjTime,
                             len);
                if (cal_obj_time_compare_func (cotime,
                                   event_end) <= 0)
                    break;
                len--;
            }
        }

        /* Add the occurrences onto the main array. */
        if (len >= 0)
            g_array_append_vals (all_occs, occs->data, len + 1);

        g_array_free (occs, TRUE);

        /* Skip to the next period, or exit the loop if finished. */
        if ((*vtable->find_next_position) (&occ, event_end,
                           &recur_data, interval_end))
            break;
    }

    return all_occs;
}


static GArray*
cal_obj_generate_set_yearly (RecurData *recur_data,
                 CalRecurVTable *vtable,
                 CalObjTime *occ)
{
    CalRecurrence *recur = recur_data->recur;
    GArray *occs_arrays[4], *occs, *occs2;
    gint num_occs_arrays = 0, i;

    /* This is a bit complicated, since the iCalendar spec says that
       several BYxxx modifiers can be used simultaneously. So we have to
       be quite careful when determining the days of the occurrences.
       The BYHOUR, BYMINUTE & BYSECOND modifiers are no problem at all.

       The modifiers we have to worry about are: BYMONTH, BYWEEKNO,
       BYYEARDAY, BYMONTHDAY & BYDAY. We can't do these sequentially
       since each filter will mess up the results of the previous one.
       But they aren't all completely independant, e.g. BYMONTHDAY and
       BYDAY are related to BYMONTH, and BYDAY is related to BYWEEKNO.

       BYDAY & BYMONTHDAY can also be applied independently, which makes
       it worse. So we assume that if BYMONTH or BYWEEKNO is used, then
       the BYDAY modifier applies to those, else it is applied
       independantly.

       We expand the occurrences in parallel into the occs_arrays[] array,
       and then merge them all into one GArray before expanding BYHOUR,
       BYMINUTE & BYSECOND. */

    if (recur->bymonth) {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);

        occs = (*vtable->bymonth_filter) (recur_data, occs);

        /* If BYMONTHDAY & BYDAY are both set we need to expand them
           in parallel and add the results. */
        if (recur->bymonthday && recur->byday) {
            /* Copy the occs array. */
            occs2 = g_array_new (FALSE, FALSE,
                         sizeof (CalObjTime));
            g_array_append_vals (occs2, occs->data, occs->len);

            occs = (*vtable->bymonthday_filter) (recur_data, occs);
            /* Note that we explicitly call the monthly version
               of the BYDAY expansion filter. */
            occs2 = cal_obj_byday_expand_monthly (recur_data,
                                  occs2);

            /* Add the 2 resulting arrays together. */
            g_array_append_vals (occs, occs2->data, occs2->len);
            g_array_free (occs2, TRUE);
        } else {
            occs = (*vtable->bymonthday_filter) (recur_data, occs);
            /* Note that we explicitly call the monthly version
               of the BYDAY expansion filter. */
            occs = cal_obj_byday_expand_monthly (recur_data, occs);
        }

        occs_arrays[num_occs_arrays++] = occs;
    }

    if (recur->byweekno) {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);

        occs = (*vtable->byweekno_filter) (recur_data, occs);
        /* Note that we explicitly call the weekly version of the
           BYDAY expansion filter. */
        occs = cal_obj_byday_expand_weekly (recur_data, occs);

        occs_arrays[num_occs_arrays++] = occs;
    }

    if (recur->byyearday) {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);

        occs = (*vtable->byyearday_filter) (recur_data, occs);

        occs_arrays[num_occs_arrays++] = occs;
    }

    /* If BYMONTHDAY is set, and BYMONTH is not set, we need to
       expand BYMONTHDAY independantly. */
    if (recur->bymonthday && !recur->bymonth) {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);

        occs = (*vtable->bymonthday_filter) (recur_data, occs);

        occs_arrays[num_occs_arrays++] = occs;
    }

    /* If BYDAY is set, and BYMONTH and BYWEEKNO are not set, we need to
       expand BYDAY independantly. */
    if (recur->byday && !recur->bymonth && !recur->byweekno) {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);

        occs = (*vtable->byday_filter) (recur_data, occs);

        occs_arrays[num_occs_arrays++] = occs;
    }

    /* Add all the arrays together. If no filters were used we just
       create an array with one element. */
    if (num_occs_arrays > 0) {
        occs = occs_arrays[0];
        for (i = 1; i < num_occs_arrays; i++) {
            occs2 = occs_arrays[i];
            g_array_append_vals (occs, occs2->data, occs2->len);
            g_array_free (occs2, TRUE);
        }
    } else {
        occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs, occ, 1);
    }

    /* Now expand BYHOUR, BYMINUTE & BYSECOND. */
    occs = (*vtable->byhour_filter) (recur_data, occs);
    occs = (*vtable->byminute_filter) (recur_data, occs);
    occs = (*vtable->bysecond_filter) (recur_data, occs);

    return occs;
}


static GArray*
cal_obj_generate_set_monthly    (RecurData *recur_data,
                 CalRecurVTable *vtable,
                 CalObjTime *occ)
{
    GArray *occs, *occs2;

    /* We start with just the one time in each set. */
    occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
    g_array_append_vals (occs, occ, 1);

    occs = (*vtable->bymonth_filter) (recur_data, occs);

    /* We need to combine the output of BYMONTHDAY & BYDAY, by doing them
       in parallel rather than sequentially. If we did them sequentially
       then we would lose the occurrences generated by BYMONTHDAY, and
       instead have repetitions of the occurrences from BYDAY. */
    if (recur_data->recur->bymonthday && recur_data->recur->byday) {
        occs2 = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
        g_array_append_vals (occs2, occs->data, occs->len);

        occs = (*vtable->bymonthday_filter) (recur_data, occs);
        occs2 = (*vtable->byday_filter) (recur_data, occs2);

        g_array_append_vals (occs, occs2->data, occs2->len);
        g_array_free (occs2, TRUE);
    } else {
        occs = (*vtable->bymonthday_filter) (recur_data, occs);
        occs = (*vtable->byday_filter) (recur_data, occs);
    }

    occs = (*vtable->byhour_filter) (recur_data, occs);
    occs = (*vtable->byminute_filter) (recur_data, occs);
    occs = (*vtable->bysecond_filter) (recur_data, occs);

    return occs;
}


static GArray*
cal_obj_generate_set_default    (RecurData *recur_data,
                 CalRecurVTable *vtable,
                 CalObjTime *occ)
{
    GArray *occs;

#if 0
    g_print ("Generating set for %i/%i/%i %02i:%02i:%02i\n",
         occ->day, occ->month + 1, occ->year, occ->hour, occ->minute,
         occ->second);
#endif

    /* We start with just the one time in the set. */
    occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));
    g_array_append_vals (occs, occ, 1);

    occs = (*vtable->bymonth_filter) (recur_data, occs);
    if (vtable->byweekno_filter)
        occs = (*vtable->byweekno_filter) (recur_data, occs);
    if (vtable->byyearday_filter)
        occs = (*vtable->byyearday_filter) (recur_data, occs);
    if (vtable->bymonthday_filter)
        occs = (*vtable->bymonthday_filter) (recur_data, occs);
    occs = (*vtable->byday_filter) (recur_data, occs);

    occs = (*vtable->byhour_filter) (recur_data, occs);
    occs = (*vtable->byminute_filter) (recur_data, occs);
    occs = (*vtable->bysecond_filter) (recur_data, occs);

    return occs;
}



/* Returns the function table corresponding to the recurrence frequency. */
static CalRecurVTable* cal_obj_get_vtable (icalrecurrencetype_frequency recur_type)
{
    CalRecurVTable* vtable;

    switch (recur_type) {
    case ICAL_YEARLY_RECURRENCE:
        vtable = &cal_obj_yearly_vtable;
        break;
    case ICAL_MONTHLY_RECURRENCE:
        vtable = &cal_obj_monthly_vtable;
        break;
    case ICAL_WEEKLY_RECURRENCE:
        vtable = &cal_obj_weekly_vtable;
        break;
    case ICAL_DAILY_RECURRENCE:
        vtable = &cal_obj_daily_vtable;
        break;
    case ICAL_HOURLY_RECURRENCE:
        vtable = &cal_obj_hourly_vtable;
        break;
    case ICAL_MINUTELY_RECURRENCE:
        vtable = &cal_obj_minutely_vtable;
        break;
    case ICAL_SECONDLY_RECURRENCE:
        vtable = &cal_obj_secondly_vtable;
        break;
    default:
        g_warning ("Unknown recurrence frequenct");
        vtable = NULL;
    }

    return vtable;
}


/* This creates a number of fast lookup tables used when filtering with the
   modifier properties BYMONTH, BYYEARDAY etc. */
static void
cal_obj_initialize_recur_data (RecurData  *recur_data,
                   CalRecurrence *recur,
                   CalObjTime *event_start)
{
    GList *elem;
    gint month, yearday, monthday, weekday, week_num, hour, minute, second;

    /* Clear the entire RecurData. */
    memset (recur_data, 0, sizeof (RecurData));

    recur_data->recur = recur;

    /* Set the weekday, used for the WEEKLY frequency and the BYWEEKNO
       modifier. */
    recur_data->weekday_offset = cal_obj_time_weekday_offset (event_start,
                                  recur);

    /* Create an array of months from bymonths for fast lookup. */
    elem = recur->bymonth;
    while (elem) {
        month = GPOINTER_TO_INT (elem->data);
        recur_data->months[month] = 1;
        elem = elem->next;
    }

    /* Create an array of yeardays from byyearday for fast lookup.
       We create a second array to handle the negative values. The first
       element there corresponds to the last day of the year. */
    elem = recur->byyearday;
    while (elem) {
        yearday = GPOINTER_TO_INT (elem->data);
        if (yearday >= 0)
            recur_data->yeardays[yearday] = 1;
        else
            recur_data->neg_yeardays[-yearday] = 1;
        elem = elem->next;
    }

    /* Create an array of monthdays from bymonthday for fast lookup.
       We create a second array to handle the negative values. The first
       element there corresponds to the last day of the month. */
    elem = recur->bymonthday;
    while (elem) {
        monthday = GPOINTER_TO_INT (elem->data);
        if (monthday >= 0)
            recur_data->monthdays[monthday] = 1;
        else
            recur_data->neg_monthdays[-monthday] = 1;
        elem = elem->next;
    }

    /* Create an array of weekdays from byday for fast lookup. */
    elem = recur->byday;
    while (elem) {
        weekday = GPOINTER_TO_INT (elem->data);
        elem = elem->next;
        /* The week number is not used when filtering. */
        week_num = GPOINTER_TO_INT (elem->data);
        elem = elem->next;

        recur_data->weekdays[weekday] = 1;
    }

    /* Create an array of hours from byhour for fast lookup. */
    elem = recur->byhour;
    while (elem) {
        hour = GPOINTER_TO_INT (elem->data);
        recur_data->hours[hour] = 1;
        elem = elem->next;
    }

    /* Create an array of minutes from byminutes for fast lookup. */
    elem = recur->byminute;
    while (elem) {
        minute = GPOINTER_TO_INT (elem->data);
        recur_data->minutes[minute] = 1;
        elem = elem->next;
    }

    /* Create an array of seconds from byseconds for fast lookup. */
    elem = recur->bysecond;
    while (elem) {
        second = GPOINTER_TO_INT (elem->data);
        recur_data->seconds[second] = 1;
        elem = elem->next;
    }
}


static void
cal_obj_sort_occurrences (GArray *occs)
{
    qsort (occs->data, occs->len, sizeof (CalObjTime),
           cal_obj_time_compare_func);
}


static void
cal_obj_remove_duplicates_and_invalid_dates (GArray *occs)
{
    static const int days_in_month[12] = {
        31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31
    };

    CalObjTime *occ, *prev_occ = NULL;
    gint len, i, j = 0, year, month, days;
    gboolean keep_occ;

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        keep_occ = TRUE;

        if (prev_occ && cal_obj_time_compare_func (occ,
                               prev_occ) == 0)
            keep_occ = FALSE;

        year = occ->year;
        month = occ->month;
        days = days_in_month[occ->month];
        /* If it is february and a leap year, add a day. */
        if (month == 1 && (year % 4 == 0
                   && (year % 100 != 0
                       || year % 400 == 0)))
            days++;
        if (occ->day > days)
            keep_occ = FALSE;

        if (keep_occ) {
            if (i != j)
                g_array_index (occs, CalObjTime, j)
                    = g_array_index (occs, CalObjTime, i);
            j++;
        }

        prev_occ = occ;
    }

    g_array_set_size (occs, j);
}


/* Removes the exceptions from the ex_occs array from the occurrences in the
   occs array, and removes any duplicates. Both arrays are sorted. */
static void
cal_obj_remove_exceptions (GArray *occs,
               GArray *ex_occs)
{
    CalObjTime *occ, *prev_occ = NULL, *ex_occ = NULL, *last_occ_kept;
    gint i, j = 0, cmp, ex_index, occs_len, ex_occs_len;
    gboolean keep_occ, current_time_is_exception = FALSE;

    if (occs->len == 0)
        return;

    ex_index = 0;
    occs_len = occs->len;
    ex_occs_len = ex_occs->len;

    if (ex_occs_len > 0)
        ex_occ = &g_array_index (ex_occs, CalObjTime, ex_index);

    for (i = 0; i < occs_len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        keep_occ = TRUE;

        /* If the occurrence is a duplicate of the previous one, skip
           it. */
        if (prev_occ
            && cal_obj_time_compare_func (occ, prev_occ) == 0) {
            keep_occ = FALSE;

            /* If this occurrence is an RDATE, and the previous
               occurrence in the array was kept, set the RDATE flag
               of the last one, so we still use the end date
               or duration. */
            if (occ->is_rdate && !current_time_is_exception) {
                last_occ_kept = &g_array_index (occs,
                                CalObjTime,
                                j - 1);
                last_occ_kept->is_rdate = TRUE;
            }
        } else {
            /* We've found a new occurrence time. Reset the flag
               to indicate that it hasn't been found in the
               exceptions array (yet). */
            current_time_is_exception = FALSE;

            if (ex_occ) {
                /* Step through the exceptions until we come
                   to one that matches or follows this
                   occurrence. */
                while (ex_occ) {
                    cmp = cal_obj_time_compare_func (ex_occ, occ);
                    /* I'm pretty sure this is wrong. */
                    /*cmp = cal_obj_date_only_compare_func (ex_occ, occ);*/
                    if (cmp > 0)
                        break;

                    /* Move to the next exception, or set
                       ex_occ to NULL when we reach the
                       end of array. */
                    ex_index++;
                    if (ex_index < ex_occs_len)
                        ex_occ = &g_array_index (ex_occs, CalObjTime, ex_index);
                    else
                        ex_occ = NULL;

                    /* If the exception did match this
                       occurrence we remove it, and set the
                       flag to indicate that the current
                       time is an exception. */
                    if (cmp == 0) {
                        current_time_is_exception = TRUE;
                        keep_occ = FALSE;
                        break;
                    }
                }
            }
        }

        if (keep_occ) {
            if (i != j)
                g_array_index (occs, CalObjTime, j)
                    = g_array_index (occs, CalObjTime, i);
            j++;
        }

        prev_occ = occ;
    }

    g_array_set_size (occs, j);
}



static GArray*
cal_obj_bysetpos_filter (CalRecurrence *recur,
             GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, pos;

    /* If BYSETPOS has not been specified, or the array is empty, just
       return the array. */
    elem = recur->bysetpos;
    if (!elem || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    /* Iterate over the indices given in bysetpos, adding the corresponding
       element from occs to new_occs. */
    len = occs->len;
    while (elem) {
        pos = GPOINTER_TO_INT (elem->data);

        /* Negative values count back from the end of the array. */
        if (pos < 0)
            pos += len;

        if (pos >= 0 && pos < len) {
            occ = &g_array_index (occs, CalObjTime, pos);
            g_array_append_vals (new_occs, occ, 1);
        }
        elem = elem->next;
    }

    g_array_free (occs, TRUE);

    return new_occs;
}




/* Finds the first year from the event_start, counting in multiples of the
   recurrence interval, that intersects the given interval. It returns TRUE
   if there is no intersection. */
static gboolean
cal_obj_yearly_find_start_position (CalObjTime *event_start,
                    CalObjTime *event_end,
                    RecurData  *recur_data,
                    CalObjTime *interval_start,
                    CalObjTime *interval_end,
                    CalObjTime *cotime)
{
    *cotime = *event_start;

    /* Move on to the next interval, if the event starts before the
       given interval. */
    if (cotime->year < interval_start->year) {
        gint years = interval_start->year - cotime->year
            + recur_data->recur->interval - 1;
        years -= years % recur_data->recur->interval;
        /* NOTE: The day may now be invalid, e.g. 29th Feb. */
        cotime->year += years;
    }

    if ((event_end && cotime->year > event_end->year)
        || (interval_end && cotime->year > interval_end->year))
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_yearly_find_next_position (CalObjTime *cotime,
                   CalObjTime *event_end,
                   RecurData  *recur_data,
                   CalObjTime *interval_end)
{
    /* NOTE: The day may now be invalid, e.g. 29th Feb. */
    cotime->year += recur_data->recur->interval;

    if ((event_end && cotime->year > event_end->year)
        || (interval_end && cotime->year > interval_end->year))
        return TRUE;

    return FALSE;
}



static gboolean
cal_obj_monthly_find_start_position (CalObjTime *event_start,
                     CalObjTime *event_end,
                     RecurData  *recur_data,
                     CalObjTime *interval_start,
                     CalObjTime *interval_end,
                     CalObjTime *cotime)
{
    *cotime = *event_start;

    /* Move on to the next interval, if the event starts before the
       given interval. */
    if (cal_obj_time_compare (cotime, interval_start, CALOBJ_MONTH) < 0) {
        gint months = (interval_start->year - cotime->year) * 12
            + interval_start->month - cotime->month
            + recur_data->recur->interval - 1;
        months -= months % recur_data->recur->interval;
        /* NOTE: The day may now be invalid, e.g. 31st Sep. */
        cal_obj_time_add_months (cotime, months);
    }

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_MONTH) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_MONTH) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_monthly_find_next_position (CalObjTime *cotime,
                    CalObjTime *event_end,
                    RecurData  *recur_data,
                    CalObjTime *interval_end)
{
    /* NOTE: The day may now be invalid, e.g. 31st Sep. */
    cal_obj_time_add_months (cotime, recur_data->recur->interval);

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_MONTH) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_MONTH) > 0)
        return TRUE;

    return FALSE;
}



static gboolean
cal_obj_weekly_find_start_position (CalObjTime *event_start,
                    CalObjTime *event_end,
                    RecurData  *recur_data,
                    CalObjTime *interval_start,
                    CalObjTime *interval_end,
                    CalObjTime *cotime)
{
    GDate event_start_date, interval_start_date;
    guint32 event_start_julian, interval_start_julian;
    gint interval_start_weekday_offset;
    CalObjTime week_start;

    if (event_end && cal_obj_time_compare (event_end, interval_start,
                           CALOBJ_DAY) < 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (event_start, interval_end,
                          CALOBJ_DAY) > 0)
        return TRUE;

    *cotime = *event_start;

    /* Convert the event start and interval start to GDates, so we can
       easily find the number of days between them. */
    g_date_clear (&event_start_date, 1);
    g_date_set_dmy (&event_start_date, event_start->day,
            event_start->month + 1, event_start->year);
    g_date_clear (&interval_start_date, 1);
    g_date_set_dmy (&interval_start_date, interval_start->day,
            interval_start->month + 1, interval_start->year);

    /* Calculate the start of the weeks corresponding to the event start
       and interval start. */
    event_start_julian = g_date_julian (&event_start_date);
    event_start_julian -= recur_data->weekday_offset;

    interval_start_julian = g_date_julian (&interval_start_date);
    interval_start_weekday_offset = cal_obj_time_weekday_offset (interval_start, recur_data->recur);
    interval_start_julian -= interval_start_weekday_offset;

    /* We want to find the first full week using the recurrence interval
       that intersects the given interval dates. */
    if (event_start_julian < interval_start_julian) {
        gint weeks = (interval_start_julian - event_start_julian) / 7;
        weeks += recur_data->recur->interval - 1;
        weeks -= weeks % recur_data->recur->interval;
        cal_obj_time_add_days (cotime, weeks * 7);
    }

    week_start = *cotime;
    cal_obj_time_add_days (&week_start, -recur_data->weekday_offset);

    if (event_end && cal_obj_time_compare (&week_start, event_end,
                           CALOBJ_DAY) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (&week_start, interval_end,
                          CALOBJ_DAY) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_weekly_find_next_position (CalObjTime *cotime,
                   CalObjTime *event_end,
                   RecurData  *recur_data,
                   CalObjTime *interval_end)
{
    CalObjTime week_start;

    cal_obj_time_add_days (cotime, recur_data->recur->interval * 7);

    /* Return TRUE if the start of this week is after the event finishes
       or is after the end of the required interval. */
    week_start = *cotime;
    cal_obj_time_add_days (&week_start, -recur_data->weekday_offset);

#ifdef CAL_OBJ_DEBUG
    g_print ("Next  day: %s\n", cal_obj_time_to_string (cotime));
    g_print ("Week Start: %s\n", cal_obj_time_to_string (&week_start));
#endif

    if (event_end && cal_obj_time_compare (&week_start, event_end,
                           CALOBJ_DAY) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (&week_start, interval_end,
                          CALOBJ_DAY) > 0) {
#ifdef CAL_OBJ_DEBUG
        g_print ("Interval end reached: %s\n",
             cal_obj_time_to_string (interval_end));
#endif
        return TRUE;
    }

    return FALSE;
}


static gboolean
cal_obj_daily_find_start_position  (CalObjTime *event_start,
                    CalObjTime *event_end,
                    RecurData  *recur_data,
                    CalObjTime *interval_start,
                    CalObjTime *interval_end,
                    CalObjTime *cotime)
{
    GDate event_start_date, interval_start_date;
    guint32 event_start_julian, interval_start_julian, days;

    if (interval_end && cal_obj_time_compare (event_start, interval_end,
                          CALOBJ_DAY) > 0)
        return TRUE;
    if (event_end && cal_obj_time_compare (event_end, interval_start,
                           CALOBJ_DAY) < 0)
        return TRUE;

    *cotime = *event_start;

    /* Convert the event start and interval start to GDates, so we can
       easily find the number of days between them. */
    g_date_clear (&event_start_date, 1);
    g_date_set_dmy (&event_start_date, event_start->day,
            event_start->month + 1, event_start->year);
    g_date_clear (&interval_start_date, 1);
    g_date_set_dmy (&interval_start_date, interval_start->day,
            interval_start->month + 1, interval_start->year);

    event_start_julian = g_date_julian (&event_start_date);
    interval_start_julian = g_date_julian (&interval_start_date);

    if (event_start_julian < interval_start_julian) {
        days = interval_start_julian - event_start_julian
            + recur_data->recur->interval - 1;
        days -= days % recur_data->recur->interval;
        cal_obj_time_add_days (cotime, days);
    }

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_DAY) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_DAY) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_daily_find_next_position  (CalObjTime *cotime,
                   CalObjTime *event_end,
                   RecurData  *recur_data,
                   CalObjTime *interval_end)
{
    cal_obj_time_add_days (cotime, recur_data->recur->interval);

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_DAY) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_DAY) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_hourly_find_start_position (CalObjTime *event_start,
                    CalObjTime *event_end,
                    RecurData  *recur_data,
                    CalObjTime *interval_start,
                    CalObjTime *interval_end,
                    CalObjTime *cotime)
{
    GDate event_start_date, interval_start_date;
    guint32 event_start_julian, interval_start_julian, hours;

    if (interval_end && cal_obj_time_compare (event_start, interval_end,
                          CALOBJ_HOUR) > 0)
        return TRUE;
    if (event_end && cal_obj_time_compare (event_end, interval_start,
                           CALOBJ_HOUR) < 0)
        return TRUE;

    *cotime = *event_start;

    if (cal_obj_time_compare (event_start, interval_start,
                  CALOBJ_HOUR) < 0) {
        /* Convert the event start and interval start to GDates, so we
           can easily find the number of days between them. */
        g_date_clear (&event_start_date, 1);
        g_date_set_dmy (&event_start_date, event_start->day,
                event_start->month + 1, event_start->year);
        g_date_clear (&interval_start_date, 1);
        g_date_set_dmy (&interval_start_date, interval_start->day,
                interval_start->month + 1,
                interval_start->year);

        event_start_julian = g_date_julian (&event_start_date);
        interval_start_julian = g_date_julian (&interval_start_date);

        hours = (interval_start_julian - event_start_julian) * 24;
        hours += interval_start->hour - event_start->hour;
        hours += recur_data->recur->interval - 1;
        hours -= hours % recur_data->recur->interval;
        cal_obj_time_add_hours (cotime, hours);
    }

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_HOUR) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_HOUR) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_hourly_find_next_position (CalObjTime *cotime,
                   CalObjTime *event_end,
                   RecurData  *recur_data,
                   CalObjTime *interval_end)
{
    cal_obj_time_add_hours (cotime, recur_data->recur->interval);

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_HOUR) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_HOUR) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_minutely_find_start_position (CalObjTime *event_start,
                      CalObjTime *event_end,
                      RecurData  *recur_data,
                      CalObjTime *interval_start,
                      CalObjTime *interval_end,
                      CalObjTime *cotime)
{
    GDate event_start_date, interval_start_date;
    guint32 event_start_julian, interval_start_julian, minutes;

    if (interval_end && cal_obj_time_compare (event_start, interval_end,
                          CALOBJ_MINUTE) > 0)
        return TRUE;
    if (event_end && cal_obj_time_compare (event_end, interval_start,
                           CALOBJ_MINUTE) < 0)
        return TRUE;

    *cotime = *event_start;

    if (cal_obj_time_compare (event_start, interval_start,
                  CALOBJ_MINUTE) < 0) {
        /* Convert the event start and interval start to GDates, so we
           can easily find the number of days between them. */
        g_date_clear (&event_start_date, 1);
        g_date_set_dmy (&event_start_date, event_start->day,
                event_start->month + 1, event_start->year);
        g_date_clear (&interval_start_date, 1);
        g_date_set_dmy (&interval_start_date, interval_start->day,
                interval_start->month + 1,
                interval_start->year);

        event_start_julian = g_date_julian (&event_start_date);
        interval_start_julian = g_date_julian (&interval_start_date);

        minutes = (interval_start_julian - event_start_julian)
            * 24 * 60;
        minutes += (interval_start->hour - event_start->hour) * 24;
        minutes += interval_start->minute - event_start->minute;
        minutes += recur_data->recur->interval - 1;
        minutes -= minutes % recur_data->recur->interval;
        cal_obj_time_add_minutes (cotime, minutes);
    }

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_MINUTE) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_MINUTE) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_minutely_find_next_position (CalObjTime *cotime,
                     CalObjTime *event_end,
                     RecurData  *recur_data,
                     CalObjTime *interval_end)
{
    cal_obj_time_add_minutes (cotime, recur_data->recur->interval);

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_MINUTE) > 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_MINUTE) > 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_secondly_find_start_position (CalObjTime *event_start,
                      CalObjTime *event_end,
                      RecurData  *recur_data,
                      CalObjTime *interval_start,
                      CalObjTime *interval_end,
                      CalObjTime *cotime)
{
    GDate event_start_date, interval_start_date;
    guint32 event_start_julian, interval_start_julian, seconds;

    if (interval_end && cal_obj_time_compare (event_start, interval_end,
                          CALOBJ_SECOND) > 0)
        return TRUE;
    if (event_end && cal_obj_time_compare (event_end, interval_start,
                           CALOBJ_SECOND) < 0)
        return TRUE;

    *cotime = *event_start;

    if (cal_obj_time_compare (event_start, interval_start,
                  CALOBJ_SECOND) < 0) {
        /* Convert the event start and interval start to GDates, so we
           can easily find the number of days between them. */
        g_date_clear (&event_start_date, 1);
        g_date_set_dmy (&event_start_date, event_start->day,
                event_start->month + 1, event_start->year);
        g_date_clear (&interval_start_date, 1);
        g_date_set_dmy (&interval_start_date, interval_start->day,
                interval_start->month + 1,
                interval_start->year);

        event_start_julian = g_date_julian (&event_start_date);
        interval_start_julian = g_date_julian (&interval_start_date);

        seconds = (interval_start_julian - event_start_julian)
            * 24 * 60 * 60;
        seconds += (interval_start->hour - event_start->hour)
            * 24 * 60;
        seconds += (interval_start->minute - event_start->minute) * 60;
        seconds += interval_start->second - event_start->second;
        seconds += recur_data->recur->interval - 1;
        seconds -= seconds % recur_data->recur->interval;
        cal_obj_time_add_seconds (cotime, seconds);
    }

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_SECOND) >= 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_SECOND) >= 0)
        return TRUE;

    return FALSE;
}


static gboolean
cal_obj_secondly_find_next_position (CalObjTime *cotime,
                     CalObjTime *event_end,
                     RecurData  *recur_data,
                     CalObjTime *interval_end)
{
    cal_obj_time_add_seconds (cotime, recur_data->recur->interval);

    if (event_end && cal_obj_time_compare (cotime, event_end,
                           CALOBJ_SECOND) >= 0)
        return TRUE;
    if (interval_end && cal_obj_time_compare (cotime, interval_end,
                          CALOBJ_SECOND) >= 0)
        return TRUE;

    return FALSE;
}





/* If the BYMONTH rule is specified it expands each occurrence in occs, by
   using each of the months in the bymonth list. */
static GArray*
cal_obj_bymonth_expand      (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i;

    /* If BYMONTH has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bymonth || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->bymonth;
        while (elem) {
            /* NOTE: The day may now be invalid, e.g. 31st Feb. */
            occ->month = GPOINTER_TO_INT (elem->data);
            g_array_append_vals (new_occs, occ, 1);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* If the BYMONTH rule is specified it filters out all occurrences in occs
   which do not match one of the months in the bymonth list. */
static GArray*
cal_obj_bymonth_filter      (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i;

    /* If BYMONTH has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bymonth || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        if (recur_data->months[occ->month])
            g_array_append_vals (new_occs, occ, 1);
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



static GArray*
cal_obj_byweekno_expand     (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ, year_start_cotime, year_end_cotime, cotime;
    GList *elem;
    gint len, i, weekno;

    /* If BYWEEKNO has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byweekno || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        /* Find the day that would correspond to week 1 (note that
           week 1 is the first week starting from the specified week
           start day that has 4 days in the new year). */
        year_start_cotime = *occ;
        cal_obj_time_find_first_week (&year_start_cotime,
                          recur_data);

        /* Find the day that would correspond to week 1 of the next
           year, which we use for -ve week numbers. */
        year_end_cotime = *occ;
        year_end_cotime.year++;
        cal_obj_time_find_first_week (&year_end_cotime,
                          recur_data);

        /* Now iterate over the week numbers in byweekno, generating a
           new occurrence for each one. */
        elem = recur_data->recur->byweekno;
        while (elem) {
            weekno = GPOINTER_TO_INT (elem->data);
            if (weekno > 0) {
                cotime = year_start_cotime;
                cal_obj_time_add_days (&cotime,
                               (weekno - 1) * 7);
            } else {
                cotime = year_end_cotime;
                cal_obj_time_add_days (&cotime, weekno * 7);
            }

            /* Skip occurrences if they fall outside the year. */
            if (cotime.year == occ->year)
                g_array_append_val (new_occs, cotime);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


#if 0
/* This isn't used at present. */
static GArray*
cal_obj_byweekno_filter     (RecurData  *recur_data,
                 GArray     *occs)
{

    return occs;
}
#endif


static GArray*
cal_obj_byyearday_expand    (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ, year_start_cotime, year_end_cotime, cotime;
    GList *elem;
    gint len, i, dayno;

    /* If BYYEARDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byyearday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        /* Find the day that would correspond to day 1. */
        year_start_cotime = *occ;
        year_start_cotime.month = 0;
        year_start_cotime.day = 1;

        /* Find the day that would correspond to day 1 of the next
           year, which we use for -ve day numbers. */
        year_end_cotime = *occ;
        year_end_cotime.year++;
        year_end_cotime.month = 0;
        year_end_cotime.day = 1;

        /* Now iterate over the day numbers in byyearday, generating a
           new occurrence for each one. */
        elem = recur_data->recur->byyearday;
        while (elem) {
            dayno = GPOINTER_TO_INT (elem->data);
            if (dayno > 0) {
                cotime = year_start_cotime;
                cal_obj_time_add_days (&cotime, dayno - 1);
            } else {
                cotime = year_end_cotime;
                cal_obj_time_add_days (&cotime, dayno);
            }

            /* Skip occurrences if they fall outside the year. */
            if (cotime.year == occ->year)
                g_array_append_val (new_occs, cotime);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* Note: occs must not contain invalid dates, e.g. 31st September. */
static GArray*
cal_obj_byyearday_filter    (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint yearday, len, i, days_in_year;

    /* If BYYEARDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byyearday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        yearday = cal_obj_time_day_of_year (occ);
        if (recur_data->yeardays[yearday]) {
            g_array_append_vals (new_occs, occ, 1);
        } else {
            days_in_year = g_date_is_leap_year (occ->year)
                ? 366 : 365;
            if (recur_data->neg_yeardays[days_in_year + 1
                            - yearday])
                g_array_append_vals (new_occs, occ, 1);
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



static GArray*
cal_obj_bymonthday_expand   (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ, month_start_cotime, month_end_cotime, cotime;
    GList *elem;
    gint len, i, dayno;

    /* If BYMONTHDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bymonthday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        /* Find the day that would correspond to day 1. */
        month_start_cotime = *occ;
        month_start_cotime.day = 1;

        /* Find the day that would correspond to day 1 of the next
           month, which we use for -ve day numbers. */
        month_end_cotime = *occ;
        month_end_cotime.month++;
        month_end_cotime.day = 1;

        /* Now iterate over the day numbers in bymonthday, generating a
           new occurrence for each one. */
        elem = recur_data->recur->bymonthday;
        while (elem) {
            dayno = GPOINTER_TO_INT (elem->data);
            if (dayno > 0) {
                cotime = month_start_cotime;
                cal_obj_time_add_days (&cotime, dayno - 1);
            } else {
                cotime = month_end_cotime;
                cal_obj_time_add_days (&cotime, dayno);
            }

            /* Skip occurrences if they fall outside the month. */
            if (cotime.month == occ->month)
                g_array_append_val (new_occs, cotime);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


static GArray*
cal_obj_bymonthday_filter   (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i, days_in_month;

    /* If BYMONTHDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bymonthday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        if (recur_data->monthdays[occ->day]) {
            g_array_append_vals (new_occs, occ, 1);
        } else {
            days_in_month = time_days_in_month (occ->year,
                                occ->month);
            if (recur_data->neg_monthdays[days_in_month + 1
                             - occ->day])
                g_array_append_vals (new_occs, occ, 1);
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



static GArray*
cal_obj_byday_expand_yearly (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i, weekday, week_num;
    gint first_weekday, last_weekday, offset;
    guint16 year;

    /* If BYDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->byday;
        while (elem) {
            weekday = GPOINTER_TO_INT (elem->data);
            elem = elem->next;
            week_num = GPOINTER_TO_INT (elem->data);
            elem = elem->next;

            year = occ->year;
            if (week_num == 0) {
                /* Expand to every Mon/Tue/etc. in the year. */
                occ->month = 0;
                occ->day = 1;
                first_weekday = cal_obj_time_weekday (occ);
                offset = (weekday + 7 - first_weekday) % 7;
                cal_obj_time_add_days (occ, offset);

                while (occ->year == year) {
                    g_array_append_vals (new_occs, occ, 1);
                    cal_obj_time_add_days (occ, 7);
                }

            } else if (week_num > 0) {
                /* Add the nth Mon/Tue/etc. in the year. */
                occ->month = 0;
                occ->day = 1;
                first_weekday = cal_obj_time_weekday (occ);
                offset = (weekday + 7 - first_weekday) % 7;
                offset += (week_num - 1) * 7;
                cal_obj_time_add_days (occ, offset);
                if (occ->year == year)
                    g_array_append_vals (new_occs, occ, 1);

            } else {
                /* Add the -nth Mon/Tue/etc. in the year. */
                occ->month = 11;
                occ->day = 31;
                last_weekday = cal_obj_time_weekday (occ);
                offset = (last_weekday + 7 - weekday) % 7;
                offset += (week_num - 1) * 7;
                cal_obj_time_add_days (occ, -offset);
                if (occ->year == year)
                    g_array_append_vals (new_occs, occ, 1);
            }

            /* Reset the year, as we may have gone past the end. */
            occ->year = year;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


static GArray*
cal_obj_byday_expand_monthly    (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i, weekday, week_num;
    gint first_weekday, last_weekday, offset;
    guint16 year;
    guint8 month;

    /* If BYDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->byday;
        while (elem) {
            weekday = GPOINTER_TO_INT (elem->data);
            elem = elem->next;
            week_num = GPOINTER_TO_INT (elem->data);
            elem = elem->next;

            year = occ->year;
            month = occ->month;
            if (week_num == 0) {
                /* Expand to every Mon/Tue/etc. in the month.*/
                occ->day = 1;
                first_weekday = cal_obj_time_weekday (occ);
                offset = (weekday + 7 - first_weekday) % 7;
                cal_obj_time_add_days (occ, offset);

                while (occ->year == year
                       && occ->month == month) {
                    g_array_append_vals (new_occs, occ, 1);
                    cal_obj_time_add_days (occ, 7);
                }

            } else if (week_num > 0) {
                /* Add the nth Mon/Tue/etc. in the month. */
                occ->day = 1;
                first_weekday = cal_obj_time_weekday (occ);
                offset = (weekday + 7 - first_weekday) % 7;
                offset += (week_num - 1) * 7;
                cal_obj_time_add_days (occ, offset);
                if (occ->year == year && occ->month == month)
                    g_array_append_vals (new_occs, occ, 1);

            } else {
                /* Add the -nth Mon/Tue/etc. in the month. */
                occ->day = time_days_in_month (occ->year,
                                   occ->month);
                last_weekday = cal_obj_time_weekday (occ);
                offset = (last_weekday + 7 - weekday) % 7;
                offset += (week_num - 1) * 7;
                cal_obj_time_add_days (occ, -offset);
                if (occ->year == year && occ->month == month)
                    g_array_append_vals (new_occs, occ, 1);
            }

            /* Reset the year & month, as we may have gone past
               the end. */
            occ->year = year;
            occ->month = month;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* Note: occs must not contain invalid dates, e.g. 31st September. */
static GArray*
cal_obj_byday_expand_weekly (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i, weekday, week_num;
    gint weekday_offset, new_weekday_offset;

    /* If BYDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->byday;
        while (elem) {
            weekday = GPOINTER_TO_INT (elem->data);
            elem = elem->next;

            /* FIXME: Currently we just ignore this, but maybe we
               should skip all elements where week_num != 0.
               The spec isn't clear about this. */
            week_num = GPOINTER_TO_INT (elem->data);
            elem = elem->next;

            weekday_offset = cal_obj_time_weekday_offset (occ, recur_data->recur);
            new_weekday_offset = (weekday + 7 - recur_data->recur->week_start_day) % 7;
            cal_obj_time_add_days (occ, new_weekday_offset - weekday_offset);
            g_array_append_vals (new_occs, occ, 1);
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* Note: occs must not contain invalid dates, e.g. 31st September. */
static GArray*
cal_obj_byday_filter        (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i, weekday;

    /* If BYDAY has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byday || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        weekday = cal_obj_time_weekday (occ);

        /* See if the weekday on its own is set. */
        if (recur_data->weekdays[weekday])
            g_array_append_vals (new_occs, occ, 1);
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



/* If the BYHOUR rule is specified it expands each occurrence in occs, by
   using each of the hours in the byhour list. */
static GArray*
cal_obj_byhour_expand       (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i;

    /* If BYHOUR has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byhour || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->byhour;
        while (elem) {
            occ->hour = GPOINTER_TO_INT (elem->data);
            g_array_append_vals (new_occs, occ, 1);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* If the BYHOUR rule is specified it filters out all occurrences in occs
   which do not match one of the hours in the byhour list. */
static GArray*
cal_obj_byhour_filter       (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i;

    /* If BYHOUR has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byhour || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        if (recur_data->hours[occ->hour])
            g_array_append_vals (new_occs, occ, 1);
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



/* If the BYMINUTE rule is specified it expands each occurrence in occs, by
   using each of the minutes in the byminute list. */
static GArray*
cal_obj_byminute_expand     (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i;

    /* If BYMINUTE has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byminute || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->byminute;
        while (elem) {
            occ->minute = GPOINTER_TO_INT (elem->data);
            g_array_append_vals (new_occs, occ, 1);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* If the BYMINUTE rule is specified it filters out all occurrences in occs
   which do not match one of the minutes in the byminute list. */
static GArray*
cal_obj_byminute_filter     (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i;

    /* If BYMINUTE has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->byminute || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        if (recur_data->minutes[occ->minute])
            g_array_append_vals (new_occs, occ, 1);
    }

    g_array_free (occs, TRUE);

    return new_occs;
}



/* If the BYSECOND rule is specified it expands each occurrence in occs, by
   using each of the seconds in the bysecond list. */
static GArray*
cal_obj_bysecond_expand     (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    GList *elem;
    gint len, i;

    /* If BYSECOND has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bysecond || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);

        elem = recur_data->recur->bysecond;
        while (elem) {
            occ->second = GPOINTER_TO_INT (elem->data);
            g_array_append_vals (new_occs, occ, 1);
            elem = elem->next;
        }
    }

    g_array_free (occs, TRUE);

    return new_occs;
}


/* If the BYSECOND rule is specified it filters out all occurrences in occs
   which do not match one of the seconds in the bysecond list. */
static GArray*
cal_obj_bysecond_filter     (RecurData  *recur_data,
                 GArray     *occs)
{
    GArray *new_occs;
    CalObjTime *occ;
    gint len, i;

    /* If BYSECOND has not been specified, or the array is empty, just
       return the array. */
    if (!recur_data->recur->bysecond || occs->len == 0)
        return occs;

    new_occs = g_array_new (FALSE, FALSE, sizeof (CalObjTime));

    len = occs->len;
    for (i = 0; i < len; i++) {
        occ = &g_array_index (occs, CalObjTime, i);
        if (recur_data->seconds[occ->second])
            g_array_append_vals (new_occs, occ, 1);
    }

    g_array_free (occs, TRUE);

    return new_occs;
}





/* Adds a positive or negative number of months to the given CalObjTime,
   updating the year appropriately so we end up with a valid month.
   Note that the day may be invalid, e.g. 30th Feb. */
static void
cal_obj_time_add_months     (CalObjTime *cotime,
                 gint        months)
{
    guint month, years;

    /* We use a guint to avoid overflow on the guint8. */
    month = cotime->month + months;
    cotime->month = month % 12;
    if (month > 0) {
        cotime->year += month / 12;
    } else {
        years = month / 12;
        if (cotime->month != 0) {
            cotime->month += 12;
            years -= 1;
        }
        cotime->year += years;
    }
}


/* Adds a positive or negative number of days to the given CalObjTime,
   updating the month and year appropriately so we end up with a valid day. */
static void
cal_obj_time_add_days       (CalObjTime *cotime,
                 gint        days)
{
    gint day, days_in_month;

    /* We use a guint to avoid overflow on the guint8. */
    day = cotime->day;
    day += days;

    if (days >= 0) {
        for (;;) {
            days_in_month = time_days_in_month (cotime->year,
                                cotime->month);
            if (day <= days_in_month)
                break;

            cotime->month++;
            if (cotime->month >= 12) {
                cotime->year++;
                cotime->month = 0;
            }

            day -= days_in_month;
        }

        cotime->day = (guint8) day;
    } else {
        while (day <= 0) {
            if (cotime->month == 0) {
                cotime->year--;
                cotime->month = 11;
            } else {
                cotime->month--;
            }

            days_in_month = time_days_in_month (cotime->year,
                                cotime->month);
            day += days_in_month;
        }

        cotime->day = (guint8) day;
    }
}


/* Adds a positive or negative number of hours to the given CalObjTime,
   updating the day, month & year appropriately so we end up with a valid
   time. */
static void
cal_obj_time_add_hours      (CalObjTime *cotime,
                 gint        hours)
{
    gint hour, days;

    /* We use a gint to avoid overflow on the guint8. */
    hour = cotime->hour + hours;
    cotime->hour = hour % 24;
    if (hour >= 0) {
        if (hour >= 24)
            cal_obj_time_add_days (cotime, hour / 24);
    } else {
        days = hour / 24;
        if (cotime->hour != 0) {
            cotime->hour += 24;
            days -= 1;
        }
        cal_obj_time_add_days (cotime, days);
    }
}


/* Adds a positive or negative number of minutes to the given CalObjTime,
   updating the rest of the CalObjTime appropriately. */
static void
cal_obj_time_add_minutes    (CalObjTime *cotime,
                 gint        minutes)
{
    gint minute, hours;

    /* We use a gint to avoid overflow on the guint8. */
    minute = cotime->minute + minutes;
    cotime->minute = minute % 60;
    if (minute >= 0) {
        if (minute >= 60)
            cal_obj_time_add_hours (cotime, minute / 60);
    } else {
        hours = minute / 60;
        if (cotime->minute != 0) {
            cotime->minute += 60;
            hours -= 1;
        }
        cal_obj_time_add_hours (cotime, hours);
    }
}


/* Adds a positive or negative number of seconds to the given CalObjTime,
   updating the rest of the CalObjTime appropriately. */
static void
cal_obj_time_add_seconds    (CalObjTime *cotime,
                 gint        seconds)
{
    gint second, minutes;

    /* We use a gint to avoid overflow on the guint8. */
    second = cotime->second + seconds;
    cotime->second = second % 60;
    if (second >= 0) {
        if (second >= 60)
            cal_obj_time_add_minutes (cotime, second / 60);
    } else {
        minutes = second / 60;
        if (cotime->second != 0) {
            cotime->second += 60;
            minutes -= 1;
        }
        cal_obj_time_add_minutes (cotime, minutes);
    }
}


/* Compares 2 CalObjTimes. Returns -1 if the cotime1 is before cotime2, 0 if
   they are the same, or 1 if cotime1 is after cotime2. The comparison type
   specifies which parts of the times we are interested in, e.g. if CALOBJ_DAY
   is used we only want to know if the days are different. */
static gint
cal_obj_time_compare        (CalObjTime *cotime1,
                 CalObjTime *cotime2,
                 CalObjTimeComparison type)
{
    if (cotime1->year < cotime2->year)
        return -1;
    if (cotime1->year > cotime2->year)
        return 1;

    if (type == CALOBJ_YEAR)
        return 0;

    if (cotime1->month < cotime2->month)
        return -1;
    if (cotime1->month > cotime2->month)
        return 1;

    if (type == CALOBJ_MONTH)
        return 0;

    if (cotime1->day < cotime2->day)
        return -1;
    if (cotime1->day > cotime2->day)
        return 1;

    if (type == CALOBJ_DAY)
        return 0;

    if (cotime1->hour < cotime2->hour)
        return -1;
    if (cotime1->hour > cotime2->hour)
        return 1;

    if (type == CALOBJ_HOUR)
        return 0;

    if (cotime1->minute < cotime2->minute)
        return -1;
    if (cotime1->minute > cotime2->minute)
        return 1;

    if (type == CALOBJ_MINUTE)
        return 0;

    if (cotime1->second < cotime2->second)
        return -1;
    if (cotime1->second > cotime2->second)
        return 1;

    return 0;
}


/* This is the same as the above function, but without the comparison type.
   It is used for qsort(). */
static gint
cal_obj_time_compare_func (const void *arg1,
               const void *arg2)
{
    CalObjTime *cotime1, *cotime2;
    gint retval;

    cotime1 = (CalObjTime*) arg1;
    cotime2 = (CalObjTime*) arg2;

    if (cotime1->year < cotime2->year)
        retval = -1;
    else if (cotime1->year > cotime2->year)
        retval = 1;

    else if (cotime1->month < cotime2->month)
        retval = -1;
    else if (cotime1->month > cotime2->month)
        retval = 1;

    else if (cotime1->day < cotime2->day)
        retval = -1;
    else if (cotime1->day > cotime2->day)
        retval = 1;

    else if (cotime1->hour < cotime2->hour)
        retval = -1;
    else if (cotime1->hour > cotime2->hour)
        retval = 1;

    else if (cotime1->minute < cotime2->minute)
        retval = -1;
    else if (cotime1->minute > cotime2->minute)
        retval = 1;

    else if (cotime1->second < cotime2->second)
        retval = -1;
    else if (cotime1->second > cotime2->second)
        retval = 1;

    else
        retval = 0;

#if 0
    g_print ("%s - ", cal_obj_time_to_string (cotime1));
    g_print ("%s : %i\n", cal_obj_time_to_string (cotime2), retval);
#endif

    return retval;
}

#if 0
static gint
cal_obj_date_only_compare_func (const void *arg1,
                const void *arg2)
{
    CalObjTime *cotime1, *cotime2;

    cotime1 = (CalObjTime*) arg1;
    cotime2 = (CalObjTime*) arg2;

    if (cotime1->year < cotime2->year)
        return -1;
    if (cotime1->year > cotime2->year)
        return 1;

    if (cotime1->month < cotime2->month)
        return -1;
    if (cotime1->month > cotime2->month)
        return 1;

    if (cotime1->day < cotime2->day)
        return -1;
    if (cotime1->day > cotime2->day)
        return 1;

    return 0;
}
#endif

/* Returns the weekday of the given CalObjTime, from 0 (Mon) - 6 (Sun). */
static gint
cal_obj_time_weekday        (CalObjTime *cotime)
{
    GDate date;
    gint weekday;

    g_date_clear (&date, 1);
    g_date_set_dmy (&date, cotime->day, cotime->month + 1, cotime->year);

    /* This results in a value of 0 (Monday) - 6 (Sunday). */
    weekday = g_date_weekday (&date) - 1;

    return weekday;
}


/* Returns the weekday of the given CalObjTime, from 0 - 6. The week start
   day is Monday by default, but can be set in the recurrence rule. */
static gint
cal_obj_time_weekday_offset (CalObjTime *cotime,
                 CalRecurrence *recur)
{
    GDate date;
    gint weekday, offset;

    g_date_clear (&date, 1);
    g_date_set_dmy (&date, cotime->day, cotime->month + 1, cotime->year);

    /* This results in a value of 0 (Monday) - 6 (Sunday). */
    weekday = g_date_weekday (&date) - 1;

    /* This calculates the offset of our day from the start of the week.
       We just add on a week (to avoid any possible negative values) and
       then subtract the specified week start day, then convert it into a
       value from 0-6. */
    offset = (weekday + 7 - recur->week_start_day) % 7;

    return offset;
}


/* Returns the day of the year of the given CalObjTime, from 1 - 366. */
static gint
cal_obj_time_day_of_year        (CalObjTime *cotime)
{
    GDate date;

    g_date_clear (&date, 1);
    g_date_set_dmy (&date, cotime->day, cotime->month + 1, cotime->year);

    return g_date_day_of_year (&date);
}


/* Finds the first week in the given CalObjTime's year, using the same weekday
   as the event start day (i.e. from the RecurData).
   The first week of the year is the first week starting from the specified
   week start day that has 4 days in the new year. It may be in the previous
   year. */
static void
cal_obj_time_find_first_week    (CalObjTime *cotime,
                 RecurData  *recur_data)
{
    GDate date;
    gint weekday, week_start_day, first_full_week_start_offset, offset;

    /* Find out the weekday of the 1st of the year, 0 (Mon) - 6 (Sun). */
    g_date_clear (&date, 1);
    g_date_set_dmy (&date, 1, 1, cotime->year);
    weekday = g_date_weekday (&date) - 1;

    /* Calculate the first day of the year that starts a new week, i.e. the
       first week_start_day after weekday, using 0 = 1st Jan.
       e.g. if the 1st Jan is a Tuesday (1) and week_start_day is a
       Monday (0), the result will be (0 + 7 - 1) % 7 = 6 (7th Jan). */
    week_start_day = recur_data->recur->week_start_day;
    first_full_week_start_offset = (week_start_day + 7 - weekday) % 7;

    /* Now see if we have to move backwards 1 week, i.e. if the week
       starts on or after Jan 5th (since the previous week has 4 days in
       this year and so will be the first week of the year). */
    if (first_full_week_start_offset >= 4)
        first_full_week_start_offset -= 7;

    /* Now add the days to get to the event's weekday. */
    offset = first_full_week_start_offset + recur_data->weekday_offset;

    /* Now move the cotime to the appropriate day. */
    cotime->month = 0;
    cotime->day = 1;
    cal_obj_time_add_days (cotime, offset);
}


static void
cal_object_time_from_time (CalObjTime *cotime,
               time_t      t)
{
    struct tm *tmp_tm;
    time_t tmp_time_t;

    tmp_time_t = t;
    /* FIXME */
    tmp_tm = localtime (&tmp_time_t);

    cotime->year     = tmp_tm->tm_year + 1900;
    cotime->month    = tmp_tm->tm_mon;
    cotime->day      = tmp_tm->tm_mday;
    cotime->hour     = tmp_tm->tm_hour;
    cotime->minute   = tmp_tm->tm_min;
    cotime->second   = tmp_tm->tm_sec;
    cotime->is_rdate = FALSE;
}


/* Debugging function to convert a CalObjTime to a string. It uses a static
   buffer so beware. */
#ifdef CAL_OBJ_DEBUG
static char*
cal_obj_time_to_string      (CalObjTime *cotime)
{
    static char buffer[20];
    char *weekdays[] = { "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun",
                 "   " };
    gint weekday;

    weekday = cal_obj_time_weekday (cotime);

    sprintf (buffer, "%s %02i/%02i/%04i %02i:%02i:%02i",
         weekdays[weekday],
         cotime->day, cotime->month + 1, cotime->year,
         cotime->hour, cotime->minute, cotime->second);
    return buffer;
}
#endif


/* This recalculates the end dates for recurrence & exception rules which use
   the COUNT property. If refresh is TRUE it will recalculate all enddates
   for rules which use COUNT. If refresh is FALSE, it will only calculate
   the enddate if it hasn't already been set. It returns TRUE if the component
   was changed, i.e. if the component should be saved at some point.
   We store the enddate in the "X-EVOLUTION-ENDDATE" parameter of the RRULE
   or EXRULE. */
static gboolean
cal_recur_ensure_end_dates (CalComponent    *comp,
                gboolean         refresh,
                CalRecurResolveTimezoneFn  tz_cb,
                gpointer         tz_cb_data)
{
    GSList *rrules, *exrules, *elem;
    gboolean changed = FALSE;

    /* Do the RRULEs. */
    cal_component_get_rrule_property_list (comp, &rrules);
    for (elem = rrules; elem; elem = elem->next) {
        changed |= cal_recur_ensure_rule_end_date (comp, elem->data,
                               FALSE, refresh,
                               tz_cb, tz_cb_data);
    }

    /* Do the EXRULEs. */
    cal_component_get_exrule_property_list (comp, &exrules);
    for (elem = exrules; elem; elem = elem->next) {
        changed |= cal_recur_ensure_rule_end_date (comp, elem->data,
                               TRUE, refresh,
                               tz_cb, tz_cb_data);
    }

    return changed;
}


typedef struct _CalRecurEnsureEndDateData CalRecurEnsureEndDateData;
struct _CalRecurEnsureEndDateData {
    gint count;
    gint instances;
    time_t end_date;
};


static gboolean
cal_recur_ensure_rule_end_date (CalComponent            *comp,
                icalproperty            *prop,
                gboolean             exception,
                gboolean             refresh,
                CalRecurResolveTimezoneFn    tz_cb,
                gpointer             tz_cb_data)
{
    struct icalrecurrencetype rule;
    CalRecurEnsureEndDateData cb_data;

    if (exception)
        rule = icalproperty_get_exrule (prop);
    else
        rule = icalproperty_get_rrule (prop);

    /* If the rule doesn't use COUNT just return. */
    if (rule.count == 0)
        return FALSE;

    /* If refresh is FALSE, we check if the enddate is already set, and
       if it is we just return. */
    if (!refresh) {
        if (cal_recur_get_rule_end_date (prop) != -1)
            return FALSE;
    }

    /* Calculate the end date. */
    cb_data.count = rule.count;
    cb_data.instances = 0;
    cal_recur_generate_instances_of_rule (comp, prop, -1, -1,
                          cal_recur_ensure_rule_end_date_cb,
                          &cb_data, tz_cb, tz_cb_data);

    /* Store the end date in the "X-EVOLUTION-ENDDATE" parameter of the
       rule. */
    cal_recur_set_rule_end_date (prop, cb_data.end_date);
        
    return TRUE;
}


static gboolean
cal_recur_ensure_rule_end_date_cb   (CalComponent   *comp,
                     time_t      instance_start,
                     time_t      instance_end,
                     gpointer    data)
{
    CalRecurEnsureEndDateData *cb_data;

    cb_data = (CalRecurEnsureEndDateData*) data;

    cb_data->instances++;

    if (cb_data->instances == cb_data->count) {
        cb_data->end_date = instance_start;
        return FALSE;
    }

    return TRUE;
}


static time_t
cal_recur_get_rule_end_date (icalproperty   *prop)
{
    icalparameter *param;
    const char *xname, *xvalue;
    icalvalue *value;
    struct icaltimetype icaltime;

    param = icalproperty_get_first_parameter (prop, ICAL_X_PARAMETER);
    while (param) {
        xname = icalparameter_get_xname (param);
        if (xname && !strcmp (xname, EVOLUTION_END_DATE_PARAMETER)) {
            xvalue = icalparameter_get_x (param);
            value = icalvalue_new_from_string (ICAL_DATETIME_VALUE,
                               xvalue);
            if (value) {
                icaltime = icalvalue_get_datetime (value);
                icalvalue_free (value);

                return icaltime_as_timet (icaltime);
            }
        }

        param = icalproperty_get_next_parameter (prop,
                             ICAL_X_PARAMETER);
    }

    return -1;
}


static void
cal_recur_set_rule_end_date (icalproperty   *prop,
                 time_t      end_date)
{
    icalparameter *param;
    icalvalue *value;
    struct icaltimetype icaltime;
    const char *end_date_string, *xname;

    icaltime = icaltime_from_timet (end_date, FALSE);
    value = icalvalue_new_datetime (icaltime);
    end_date_string = icalvalue_as_ical_string (value);
    icalvalue_free (value);

    /* If we already have an X-EVOLUTION-ENDDATE parameter, set the value
       to the new date-time. */
    param = icalproperty_get_first_parameter (prop, ICAL_X_PARAMETER);
    while (param) {
        xname = icalparameter_get_xname (param);
        if (xname && !strcmp (xname, EVOLUTION_END_DATE_PARAMETER)) {
            icalparameter_set_x (param, end_date_string);
            return;
        }
        param = icalproperty_get_next_parameter (prop, ICAL_X_PARAMETER);
    }

    /* Create a new X-EVOLUTION-ENDDATE and add it to the property. */
    param = icalparameter_new_x (end_date_string);
    icalparameter_set_xname (param, EVOLUTION_END_DATE_PARAMETER);
    icalproperty_add_parameter (prop, param);
}