// Copyright (C) 2018 The Syncthing Authors.
//
// This Source Code Form is subject to the terms of the Mozilla Public
// License, v. 2.0. If a copy of the MPL was not distributed with this file,
// You can obtain one at https://mozilla.org/MPL/2.0/.

package db

import (
	"encoding/binary"

	"github.com/syncthing/syncthing/lib/sync"
	"github.com/syndtr/goleveldb/leveldb"
	"github.com/syndtr/goleveldb/leveldb/util"
)

// A smallIndex is an in memory bidirectional []byte to uint32 map. It gives
// fast lookups in both directions and persists to the database. Don't use for
// storing more items than fit comfortably in RAM.
type smallIndex struct {
	db     *leveldb.DB
	prefix []byte
	id2val map[uint32]string
	val2id map[string]uint32
	nextID uint32
	mut    sync.Mutex
}

func newSmallIndex(db *leveldb.DB, prefix []byte) *smallIndex {
	idx := &smallIndex{
		db:     db,
		prefix: prefix,
		id2val: make(map[uint32]string),
		val2id: make(map[string]uint32),
		mut:    sync.NewMutex(),
	}
	idx.load()
	return idx
}

// load iterates over the prefix space in the database and populates the in
// memory maps.
func (i *smallIndex) load() {
	it := i.db.NewIterator(util.BytesPrefix(i.prefix), nil)
	defer it.Release()
	for it.Next() {
		val := string(it.Value())
		id := binary.BigEndian.Uint32(it.Key()[len(i.prefix):])
		i.id2val[id] = val
		i.val2id[val] = id
		if id >= i.nextID {
			i.nextID = id + 1
		}
	}
}

// ID returns the index number for the given byte slice, allocating a new one
// and persisting this to the database if necessary.
func (i *smallIndex) ID(val []byte) uint32 {
	i.mut.Lock()
	// intentionally avoiding defer here as we want this call to be as fast as
	// possible in the general case (folder ID already exists). The map lookup
	// with the conversion of []byte to string is compiler optimized to not
	// copy the []byte, which is why we don't assign it to a temp variable
	// here.
	if id, ok := i.val2id[string(val)]; ok {
		i.mut.Unlock()
		return id
	}

	id := i.nextID
	i.nextID++

	valStr := string(val)
	i.val2id[valStr] = id
	i.id2val[id] = valStr

	key := make([]byte, len(i.prefix)+8) // prefix plus uint32 id
	copy(key, i.prefix)
	binary.BigEndian.PutUint32(key[len(i.prefix):], id)
	i.db.Put(key, val, nil)

	i.mut.Unlock()
	return id
}

// Val returns the value for the given index number, or (nil, false) if there
// is no such index number.
func (i *smallIndex) Val(id uint32) ([]byte, bool) {
	i.mut.Lock()
	val, ok := i.id2val[id]
	i.mut.Unlock()
	if !ok {
		return nil, false
	}

	return []byte(val), true
}