/*
   Copyright 2014 CoreOS, Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package raft

import (
	"fmt"
	"log"

	pb "github.com/coreos/etcd/raft/raftpb"
)

type raftLog struct {
	ents      []pb.Entry
	unstable  uint64
	committed uint64
	applied   uint64
	offset    uint64
	snapshot  pb.Snapshot
}

func newLog() *raftLog {
	return &raftLog{
		ents:      make([]pb.Entry, 1),
		unstable:  0,
		committed: 0,
		applied:   0,
	}
}

func (l *raftLog) load(ents []pb.Entry) {
	if l.offset != ents[0].Index {
		panic("entries loaded don't match offset index")
	}
	l.ents = ents
	l.unstable = l.offset + uint64(len(ents))
}

func (l *raftLog) String() string {
	return fmt.Sprintf("offset=%d committed=%d applied=%d len(ents)=%d", l.offset, l.committed, l.applied, len(l.ents))
}

// maybeAppend returns (0, false) if the entries cannot be appended. Otherwise,
// it returns (last index of new entries, true).
func (l *raftLog) maybeAppend(index, logTerm, committed uint64, ents ...pb.Entry) (lastnewi uint64, ok bool) {
	lastnewi = index + uint64(len(ents))
	if l.matchTerm(index, logTerm) {
		from := index + 1
		ci := l.findConflict(from, ents)
		switch {
		case ci == 0:
		case ci <= l.committed:
			panic("conflict with committed entry")
		default:
			l.append(ci-1, ents[ci-from:]...)
		}
		tocommit := min(committed, lastnewi)
		// if toCommit > commitIndex, set commitIndex = toCommit
		if l.committed < tocommit {
			l.committed = tocommit
		}
		return lastnewi, true
	}
	return 0, false
}

func (l *raftLog) append(after uint64, ents ...pb.Entry) uint64 {
	l.ents = append(l.slice(l.offset, after+1), ents...)
	l.unstable = min(l.unstable, after+1)
	return l.lastIndex()
}

// findConflict finds the index of the conflict.
// It returns the first pair of conflicting entries between the existing
// entries and the given entries, if there are any.
// If there is no conflicting entries, and the existing entries contains
// all the given entries, zero will be returned.
// If there is no conflicting entries, but the given entries contains new
// entries, the index of the first new entry will be returned.
// An entry is considered to be conflicting if it has the same index but
// a different term.
// The first entry MUST have an index equal to the argument 'from'.
// The index of the given entries MUST be continuously increasing.
func (l *raftLog) findConflict(from uint64, ents []pb.Entry) uint64 {
	// TODO(xiangli): validate the index of ents
	for i, ne := range ents {
		if oe := l.at(from + uint64(i)); oe == nil || oe.Term != ne.Term {
			return from + uint64(i)
		}
	}
	return 0
}

func (l *raftLog) unstableEnts() []pb.Entry {
	ents := l.slice(l.unstable, l.lastIndex()+1)
	if ents == nil {
		return nil
	}
	cpy := make([]pb.Entry, len(ents))
	copy(cpy, ents)
	return cpy
}

// nextEnts returns all the available entries for execution.
// If applied is smaller than the index of snapshot, it returns all committed
// entries after the index of snapshot.
func (l *raftLog) nextEnts() (ents []pb.Entry) {
	off := max(l.applied, l.snapshot.Index)
	if l.committed > off {
		return l.slice(off+1, l.committed+1)
	}
	return nil
}

func (l *raftLog) appliedTo(i uint64) {
	if i == 0 {
		return
	}
	if l.committed < i || i < l.applied {
		log.Panicf("applied[%d] is out of range [prevApplied(%d), committed(%d)]", i, l.applied, l.committed)
	}
	l.applied = i
}

func (l *raftLog) stableTo(i uint64) {
	l.unstable = i + 1
}

func (l *raftLog) lastIndex() uint64 { return uint64(len(l.ents)) - 1 + l.offset }

func (l *raftLog) lastTerm() uint64 { return l.term(l.lastIndex()) }

func (l *raftLog) term(i uint64) uint64 {
	if e := l.at(i); e != nil {
		return e.Term
	}
	return 0
}

func (l *raftLog) entries(i uint64) []pb.Entry {
	// never send out the first entry
	// first entry is only used for matching
	// prevLogTerm
	if i == l.offset {
		panic("cannot return the first entry in log")
	}
	return l.slice(i, l.lastIndex()+1)
}

// isUpToDate determines if the given (lastIndex,term) log is more up-to-date
// by comparing the index and term of the last entries in the existing logs.
// If the logs have last entries with different terms, then the log with the
// later term is more up-to-date. If the logs end with the same term, then
// whichever log has the larger lastIndex is more up-to-date. If the logs are
// the same, the given log is up-to-date.
func (l *raftLog) isUpToDate(lasti, term uint64) bool {
	return term > l.lastTerm() || (term == l.lastTerm() && lasti >= l.lastIndex())
}

func (l *raftLog) matchTerm(i, term uint64) bool {
	if e := l.at(i); e != nil {
		return e.Term == term
	}
	return false
}

func (l *raftLog) maybeCommit(maxIndex, term uint64) bool {
	if maxIndex > l.committed && l.term(maxIndex) == term {
		l.committed = maxIndex
		return true
	}
	return false
}

// compact compacts all log entries until i.
// It removes the log entries before i, exclusive.
// i must be not smaller than the index of the first entry
// and not greater than the index of the last entry.
// the number of entries after compaction will be returned.
func (l *raftLog) compact(i uint64) uint64 {
	if l.isOutOfAppliedBounds(i) {
		panic(fmt.Sprintf("compact %d out of bounds [%d:%d]", i, l.offset, l.applied))
	}
	l.ents = l.slice(i, l.lastIndex()+1)
	l.unstable = max(i+1, l.unstable)
	l.offset = i
	return uint64(len(l.ents))
}

func (l *raftLog) snap(d []byte, index, term uint64, nodes []uint64) {
	l.snapshot = pb.Snapshot{
		Data:  d,
		Nodes: nodes,
		Index: index,
		Term:  term,
	}
}

func (l *raftLog) restore(s pb.Snapshot) {
	l.ents = []pb.Entry{{Term: s.Term}}
	l.unstable = s.Index + 1
	l.committed = s.Index
	l.offset = s.Index
	l.snapshot = s
}

func (l *raftLog) at(i uint64) *pb.Entry {
	if l.isOutOfBounds(i) {
		return nil
	}
	return &l.ents[i-l.offset]
}

// slice returns a slice of log entries from lo through hi-1, inclusive.
func (l *raftLog) slice(lo uint64, hi uint64) []pb.Entry {
	if lo >= hi {
		return nil
	}
	if l.isOutOfBounds(lo) || l.isOutOfBounds(hi-1) {
		return nil
	}
	return l.ents[lo-l.offset : hi-l.offset]
}

func (l *raftLog) isOutOfBounds(i uint64) bool {
	if i < l.offset || i > l.lastIndex() {
		return true
	}
	return false
}

func (l *raftLog) isOutOfAppliedBounds(i uint64) bool {
	if i < l.offset || i > l.applied {
		return true
	}
	return false
}

func min(a, b uint64) uint64 {
	if a > b {
		return b
	}
	return a
}

func max(a, b uint64) uint64 {
	if a > b {
		return a
	}
	return b
}