// Copyright 2018 The dexon-consensus-core Authors
// This file is part of the dexon-consensus-core library.
//
// The dexon-consensus-core library is free software: you can redistribute it
// and/or modify it under the terms of the GNU Lesser General Public License as
// published by the Free Software Foundation, either version 3 of the License,
// or (at your option) any later version.
//
// The dexon-consensus-core library 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 Lesser
// General Public License for more details.
//
// You should have received a copy of the GNU Lesser General Public License
// along with the dexon-consensus-core library. If not, see
// <http://www.gnu.org/licenses/>.

package core

import (
	"errors"
	"time"

	"github.com/dexon-foundation/dexon-consensus-core/core/types"
)

// consensusTimestamp is for Concensus Timestamp Algorithm.
type consensusTimestamp struct {
	chainTimestamps []time.Time

	// This part keeps configs for each round.
	numChainsForRounds []uint32
	numChainsRoundBase uint64

	// dMoment represents the genesis time.
	dMoment time.Time
}

var (
	// ErrTimestampNotIncrease would be reported if the timestamp is not strickly
	// increasing on the same chain.
	ErrTimestampNotIncrease = errors.New("timestamp is not increasing")
)

// newConsensusTimestamp creates timestamper object.
func newConsensusTimestamp(
	dMoment time.Time, numChains uint32) *consensusTimestamp {
	return &consensusTimestamp{
		numChainsForRounds: []uint32{numChains},
		numChainsRoundBase: uint64(0),
		dMoment:            dMoment,
	}
}

// appendConfig appends a configuration for upcoming round. When you append
// a config for round R, next time you can only append the config for round R+1.
func (ct *consensusTimestamp) appendConfig(
	round uint64, config *types.Config) error {

	if round != uint64(len(ct.numChainsForRounds))+ct.numChainsRoundBase {
		return ErrRoundNotIncreasing
	}
	ct.numChainsForRounds = append(ct.numChainsForRounds, config.NumChains)
	return nil
}

func (ct *consensusTimestamp) getNumChains(round uint64) uint32 {
	roundIndex := round - ct.numChainsRoundBase
	return ct.numChainsForRounds[roundIndex]
}

// ProcessBlocks is the entry function.
func (ct *consensusTimestamp) processBlocks(blocks []*types.Block) (err error) {
	for _, block := range blocks {
		numChains := ct.getNumChains(block.Position.Round)
		// Fulfill empty time slots with d-moment. This part also means
		// each time we increasing number of chains, we can't increase over
		// 49% of previous number of chains.
		for uint32(len(ct.chainTimestamps)) < numChains {
			ct.chainTimestamps = append(ct.chainTimestamps, ct.dMoment)
		}
		ts := ct.chainTimestamps[:numChains]
		if block.Finalization.Timestamp, err = getMedianTime(ts); err != nil {
			return
		}
		if !block.Timestamp.After(ct.chainTimestamps[block.Position.ChainID]) {
			return ErrTimestampNotIncrease
		}
		ct.chainTimestamps[block.Position.ChainID] = block.Timestamp
		// Purge configs for older rounds, rounds of blocks from total ordering
		// would increase.
		if block.Position.Round > ct.numChainsRoundBase {
			ct.numChainsRoundBase++
			ct.numChainsForRounds = ct.numChainsForRounds[1:]
		}
	}
	return
}