// Copyright 2015 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 (
	"errors"
	"log"
	"sync"

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

// ErrCompacted is returned by Storage.Entries/Compact when a requested
// index is unavailable because it predates the last snapshot.
var ErrCompacted = errors.New("requested index is unavailable due to compaction")

var ErrUnavailable = errors.New("requested entry at index is unavailable")

// Storage is an interface that may be implemented by the application
// to retrieve log entries from storage.
//
// If any Storage method returns an error, the raft instance will
// become inoperable and refuse to participate in elections; the
// application is responsible for cleanup and recovery in this case.
type Storage interface {
	// InitialState returns the saved HardState and ConfState information.
	InitialState() (pb.HardState, pb.ConfState, error)
	// Entries returns a slice of log entries in the range [lo,hi).
	Entries(lo, hi uint64) ([]pb.Entry, error)
	// Term returns the term of entry i, which must be in the range
	// [FirstIndex()-1, LastIndex()]. The term of the entry before
	// FirstIndex is retained for matching purposes even though the
	// rest of that entry may not be available.
	Term(i uint64) (uint64, error)
	// LastIndex returns the index of the last entry in the log.
	LastIndex() (uint64, error)
	// FirstIndex returns the index of the first log entry that is
	// possibly available via Entries (older entries have been incorporated
	// into the latest Snapshot; if storage only contains the dummy entry the
	// first log entry is not available).
	FirstIndex() (uint64, error)
	// Snapshot returns the most recent snapshot.
	Snapshot() (pb.Snapshot, error)
}

// MemoryStorage implements the Storage interface backed by an
// in-memory array.
type MemoryStorage struct {
	// Protects access to all fields. Most methods of MemoryStorage are
	// run on the raft goroutine, but Append() is run on an application
	// goroutine.
	sync.Mutex

	hardState pb.HardState
	snapshot  pb.Snapshot
	// ents[i] has raft log position i+snapshot.Metadata.Index
	ents []pb.Entry
}

// NewMemoryStorage creates an empty MemoryStorage.
func NewMemoryStorage() *MemoryStorage {
	return &MemoryStorage{
		// When starting from scratch populate the list with a dummy entry at term zero.
		ents: make([]pb.Entry, 1),
	}
}

// InitialState implements the Storage interface.
func (ms *MemoryStorage) InitialState() (pb.HardState, pb.ConfState, error) {
	return ms.hardState, ms.snapshot.Metadata.ConfState, nil
}

// SetHardState saves the current HardState.
func (ms *MemoryStorage) SetHardState(st pb.HardState) error {
	ms.hardState = st
	return nil
}

// Entries implements the Storage interface.
func (ms *MemoryStorage) Entries(lo, hi uint64) ([]pb.Entry, error) {
	ms.Lock()
	defer ms.Unlock()
	offset := ms.snapshot.Metadata.Index
	if lo <= offset {
		return nil, ErrCompacted
	}
	// only contains dummy entries.
	if len(ms.ents) == 1 {
		return nil, ErrUnavailable
	}
	return ms.ents[lo-offset : hi-offset], nil
}

// Term implements the Storage interface.
func (ms *MemoryStorage) Term(i uint64) (uint64, error) {
	ms.Lock()
	defer ms.Unlock()
	offset := ms.snapshot.Metadata.Index
	if i < offset {
		return 0, ErrCompacted
	}
	return ms.ents[i-offset].Term, nil
}

// LastIndex implements the Storage interface.
func (ms *MemoryStorage) LastIndex() (uint64, error) {
	ms.Lock()
	defer ms.Unlock()
	return ms.snapshot.Metadata.Index + uint64(len(ms.ents)) - 1, nil
}

// FirstIndex implements the Storage interface.
func (ms *MemoryStorage) FirstIndex() (uint64, error) {
	ms.Lock()
	defer ms.Unlock()
	return ms.snapshot.Metadata.Index + 1, nil
}

// Snapshot implements the Storage interface.
func (ms *MemoryStorage) Snapshot() (pb.Snapshot, error) {
	ms.Lock()
	defer ms.Unlock()
	return ms.snapshot, nil
}

// ApplySnapshot overwrites the contents of this Storage object with
// those of the given snapshot.
func (ms *MemoryStorage) ApplySnapshot(snap pb.Snapshot) error {
	ms.Lock()
	defer ms.Unlock()

	ms.snapshot = snap
	ms.ents = []pb.Entry{{Term: snap.Metadata.Term, Index: snap.Metadata.Index}}
	return nil
}

// Compact discards all log entries prior to i. Creates a snapshot
// which can be retrieved with the Snapshot() method and can be used
// to reconstruct the state at that point.
// If any configuration changes have been made since the last compaction,
// the result of the last ApplyConfChange must be passed in.
// It is the application's responsibility to not attempt to compact an index
// greater than raftLog.applied.
func (ms *MemoryStorage) Compact(i uint64, cs *pb.ConfState, data []byte) error {
	ms.Lock()
	defer ms.Unlock()
	offset := ms.snapshot.Metadata.Index
	if i <= offset {
		return ErrCompacted
	}
	if i > offset+uint64(len(ms.ents))-1 {
		log.Panicf("compact %d is out of bound lastindex(%d)", i, offset+uint64(len(ms.ents))-1)
	}
	i -= offset
	ents := make([]pb.Entry, 1, 1+uint64(len(ms.ents))-i)
	ents[0].Term = ms.ents[i].Term
	ents = append(ents, ms.ents[i+1:]...)
	ms.ents = ents
	ms.snapshot.Metadata.Index += i
	ms.snapshot.Metadata.Term = ents[0].Term
	if cs != nil {
		ms.snapshot.Metadata.ConfState = *cs
	}
	ms.snapshot.Data = data
	return nil
}

// Append the new entries to storage.
func (ms *MemoryStorage) Append(entries []pb.Entry) error {
	ms.Lock()
	defer ms.Unlock()
	if len(entries) == 0 {
		return nil
	}
	first := ms.snapshot.Metadata.Index + 1
	last := entries[0].Index + uint64(len(entries)) - 1

	// shortcut if there is no new entry.
	if last < first {
		return nil
	}
	// truncate old entries
	if first > entries[0].Index {
		entries = entries[first-entries[0].Index:]
	}

	offset := entries[0].Index - ms.snapshot.Metadata.Index
	switch {
	case uint64(len(ms.ents)) > offset:
		ms.ents = append([]pb.Entry{}, ms.ents[:offset]...)
		ms.ents = append(ms.ents, entries...)
	case uint64(len(ms.ents)) == offset:
		ms.ents = append(ms.ents, entries...)
	default:
		log.Panicf("missing log entry [last: %d, append at: %d]",
			ms.snapshot.Metadata.Index+uint64(len(ms.ents)), entries[0].Index)
	}
	return nil
}