path: root/Godeps/_workspace/src/github.com/syndtr/gosnappy/snappy/encode.go

// Copyright 2011 The Snappy-Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package snappy

import (
    "encoding/binary"
    "io"
)

// We limit how far copy back-references can go, the same as the C++ code.
const maxOffset = 1 << 15

// emitLiteral writes a literal chunk and returns the number of bytes written.
func emitLiteral(dst, lit []byte) int {
    i, n := 0, uint(len(lit)-1)
    switch {
    case n < 60:
        dst[0] = uint8(n)<<2 | tagLiteral
        i = 1
    case n < 1<<8:
        dst[0] = 60<<2 | tagLiteral
        dst[1] = uint8(n)
        i = 2
    case n < 1<<16:
        dst[0] = 61<<2 | tagLiteral
        dst[1] = uint8(n)
        dst[2] = uint8(n >> 8)
        i = 3
    case n < 1<<24:
        dst[0] = 62<<2 | tagLiteral
        dst[1] = uint8(n)
        dst[2] = uint8(n >> 8)
        dst[3] = uint8(n >> 16)
        i = 4
    case int64(n) < 1<<32:
        dst[0] = 63<<2 | tagLiteral
        dst[1] = uint8(n)
        dst[2] = uint8(n >> 8)
        dst[3] = uint8(n >> 16)
        dst[4] = uint8(n >> 24)
        i = 5
    default:
        panic("snappy: source buffer is too long")
    }
    if copy(dst[i:], lit) != len(lit) {
        panic("snappy: destination buffer is too short")
    }
    return i + len(lit)
}

// emitCopy writes a copy chunk and returns the number of bytes written.
func emitCopy(dst []byte, offset, length int) int {
    i := 0
    for length > 0 {
        x := length - 4
        if 0 <= x && x < 1<<3 && offset < 1<<11 {
            dst[i+0] = uint8(offset>>8)&0x07<<5 | uint8(x)<<2 | tagCopy1
            dst[i+1] = uint8(offset)
            i += 2
            break
        }

        x = length
        if x > 1<<6 {
            x = 1 << 6
        }
        dst[i+0] = uint8(x-1)<<2 | tagCopy2
        dst[i+1] = uint8(offset)
        dst[i+2] = uint8(offset >> 8)
        i += 3
        length -= x
    }
    return i
}

// Encode returns the encoded form of src. The returned slice may be a sub-
// slice of dst if dst was large enough to hold the entire encoded block.
// Otherwise, a newly allocated slice will be returned.
// It is valid to pass a nil dst.
func Encode(dst, src []byte) ([]byte, error) {
    if n := MaxEncodedLen(len(src)); len(dst) < n {
        dst = make([]byte, n)
    }

    // The block starts with the varint-encoded length of the decompressed bytes.
    d := binary.PutUvarint(dst, uint64(len(src)))

    // Return early if src is short.
    if len(src) <= 4 {
        if len(src) != 0 {
            d += emitLiteral(dst[d:], src)
        }
        return dst[:d], nil
    }

    // Initialize the hash table. Its size ranges from 1<<8 to 1<<14 inclusive.
    const maxTableSize = 1 << 14
    shift, tableSize := uint(32-8), 1<<8
    for tableSize < maxTableSize && tableSize < len(src) {
        shift--
        tableSize *= 2
    }
    var table [maxTableSize]int

    // Iterate over the source bytes.
    var (
        s   int // The iterator position.
        t   int // The last position with the same hash as s.
        lit int // The start position of any pending literal bytes.
    )
    for s+3 < len(src) {
        // Update the hash table.
        b0, b1, b2, b3 := src[s], src[s+1], src[s+2], src[s+3]
        h := uint32(b0) | uint32(b1)<<8 | uint32(b2)<<16 | uint32(b3)<<24
        p := &table[(h*0x1e35a7bd)>>shift]
        // We need to to store values in [-1, inf) in table. To save
        // some initialization time, (re)use the table's zero value
        // and shift the values against this zero: add 1 on writes,
        // subtract 1 on reads.
        t, *p = *p-1, s+1
        // If t is invalid or src[s:s+4] differs from src[t:t+4], accumulate a literal byte.
        if t < 0 || s-t >= maxOffset || b0 != src[t] || b1 != src[t+1] || b2 != src[t+2] || b3 != src[t+3] {
            s++
            continue
        }
        // Otherwise, we have a match. First, emit any pending literal bytes.
        if lit != s {
            d += emitLiteral(dst[d:], src[lit:s])
        }
        // Extend the match to be as long as possible.
        s0 := s
        s, t = s+4, t+4
        for s < len(src) && src[s] == src[t] {
            s++
            t++
        }
        // Emit the copied bytes.
        d += emitCopy(dst[d:], s-t, s-s0)
        lit = s
    }

    // Emit any final pending literal bytes and return.
    if lit != len(src) {
        d += emitLiteral(dst[d:], src[lit:])
    }
    return dst[:d], nil
}

// MaxEncodedLen returns the maximum length of a snappy block, given its
// uncompressed length.
func MaxEncodedLen(srcLen int) int {
    // Compressed data can be defined as:
    //    compressed := item* literal*
    //    item       := literal* copy
    //
    // The trailing literal sequence has a space blowup of at most 62/60
    // since a literal of length 60 needs one tag byte + one extra byte
    // for length information.
    //
    // Item blowup is trickier to measure. Suppose the "copy" op copies
    // 4 bytes of data. Because of a special check in the encoding code,
    // we produce a 4-byte copy only if the offset is < 65536. Therefore
    // the copy op takes 3 bytes to encode, and this type of item leads
    // to at most the 62/60 blowup for representing literals.
    //
    // Suppose the "copy" op copies 5 bytes of data. If the offset is big
    // enough, it will take 5 bytes to encode the copy op. Therefore the
    // worst case here is a one-byte literal followed by a five-byte copy.
    // That is, 6 bytes of input turn into 7 bytes of "compressed" data.
    //
    // This last factor dominates the blowup, so the final estimate is:
    return 32 + srcLen + srcLen/6
}

// NewWriter returns a new Writer that compresses to w, using the framing
// format described at
// https://code.google.com/p/snappy/source/browse/trunk/framing_format.txt
func NewWriter(w io.Writer) *Writer {
    return &Writer{
        w:   w,
        enc: make([]byte, MaxEncodedLen(maxUncompressedChunkLen)),
    }
}

// Writer is an io.Writer than can write Snappy-compressed bytes.
type Writer struct {
    w           io.Writer
    err         error
    enc         []byte
    buf         [checksumSize + chunkHeaderSize]byte
    wroteHeader bool
}

// Reset discards the writer's state and switches the Snappy writer to write to
// w. This permits reusing a Writer rather than allocating a new one.
func (w *Writer) Reset(writer io.Writer) {
    w.w = writer
    w.err = nil
    w.wroteHeader = false
}

// Write satisfies the io.Writer interface.
func (w *Writer) Write(p []byte) (n int, errRet error) {
    if w.err != nil {
        return 0, w.err
    }
    if !w.wroteHeader {
        copy(w.enc, magicChunk)
        if _, err := w.w.Write(w.enc[:len(magicChunk)]); err != nil {
            w.err = err
            return n, err
        }
        w.wroteHeader = true
    }
    for len(p) > 0 {
        var uncompressed []byte
        if len(p) > maxUncompressedChunkLen {
            uncompressed, p = p[:maxUncompressedChunkLen], p[maxUncompressedChunkLen:]
        } else {
            uncompressed, p = p, nil
        }
        checksum := crc(uncompressed)

        // Compress the buffer, discarding the result if the improvement
        // isn't at least 12.5%.
        chunkType := uint8(chunkTypeCompressedData)
        chunkBody, err := Encode(w.enc, uncompressed)
        if err != nil {
            w.err = err
            return n, err
        }
        if len(chunkBody) >= len(uncompressed)-len(uncompressed)/8 {
            chunkType, chunkBody = chunkTypeUncompressedData, uncompressed
        }

        chunkLen := 4 + len(chunkBody)
        w.buf[0] = chunkType
        w.buf[1] = uint8(chunkLen >> 0)
        w.buf[2] = uint8(chunkLen >> 8)
        w.buf[3] = uint8(chunkLen >> 16)
        w.buf[4] = uint8(checksum >> 0)
        w.buf[5] = uint8(checksum >> 8)
        w.buf[6] = uint8(checksum >> 16)
        w.buf[7] = uint8(checksum >> 24)
        if _, err = w.w.Write(w.buf[:]); err != nil {
            w.err = err
            return n, err
        }
        if _, err = w.w.Write(chunkBody); err != nil {
            w.err = err
            return n, err
        }
        n += len(uncompressed)
    }
    return n, nil
}