// Copyright 2023 PingCAP, 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 common

import (
	"github.com/pingcap/errors"
	"github.com/pingcap/tidb/pkg/util/codec"
)

// KeyAdapter is used to encode and decode keys so that duplicate key can be
// identified by rowID and avoid overwritten.
type KeyAdapter interface {
	// Encode encodes the key with its corresponding rowID. It appends the encoded
	// key to dst and returns the resulting slice. The encoded key is guaranteed to
	// be in ascending order for comparison.
	// rowID must be a coded mem-comparable value, one way to get it is to use
	// tidb/util/codec package.
	Encode(dst []byte, key []byte, rowID []byte) []byte

	// Decode decodes the original key to dst. It appends the encoded key to dst and returns the resulting slice.
	Decode(dst []byte, data []byte) ([]byte, error)

	// EncodedLen returns the encoded key length.
	EncodedLen(key []byte, rowID []byte) int
}

func reallocBytes(b []byte, n int) []byte {
	newSize := len(b) + n
	if cap(b) < newSize {
		bs := make([]byte, len(b), newSize)
		copy(bs, b)
		return bs
	}
	return b
}

// NoopKeyAdapter is a key adapter that does nothing.
type NoopKeyAdapter struct{}

// Encode implements KeyAdapter.
func (NoopKeyAdapter) Encode(dst []byte, key []byte, _ []byte) []byte {
	return append(dst, key...)
}

// Decode implements KeyAdapter.
func (NoopKeyAdapter) Decode(dst []byte, data []byte) ([]byte, error) {
	return append(dst, data...), nil
}

// EncodedLen implements KeyAdapter.
func (NoopKeyAdapter) EncodedLen(key []byte, _ []byte) int {
	return len(key)
}

var _ KeyAdapter = NoopKeyAdapter{}

// DupDetectKeyAdapter is a key adapter that appends rowID to the key to avoid
// overwritten.
type DupDetectKeyAdapter struct{}

// Encode implements KeyAdapter.
func (DupDetectKeyAdapter) Encode(dst []byte, key []byte, rowID []byte) []byte {
	dst = codec.EncodeBytes(dst, key)
	dst = reallocBytes(dst, len(rowID)+2)
	dst = append(dst, rowID...)
	rowIDLen := uint16(len(rowID))
	dst = append(dst, byte(rowIDLen>>8), byte(rowIDLen))
	return dst
}

// Decode implements KeyAdapter.
func (DupDetectKeyAdapter) Decode(dst []byte, data []byte) ([]byte, error) {
	if len(data) < 2 {
		return nil, errors.New("insufficient bytes to decode value")
	}
	rowIDLen := uint16(data[len(data)-2])<<8 | uint16(data[len(data)-1])
	tailLen := int(rowIDLen + 2)
	if len(data) < tailLen {
		return nil, errors.New("insufficient bytes to decode value")
	}
	_, key, err := codec.DecodeBytes(data[:len(data)-tailLen], dst[len(dst):cap(dst)])
	if err != nil {
		return nil, err
	}
	if len(dst) == 0 {
		return key, nil
	}
	if len(dst)+len(key) <= cap(dst) {
		dst = dst[:len(dst)+len(key)]
		return dst, nil
	}
	// New slice is allocated, append key to dst manually.
	return append(dst, key...), nil
}

// EncodedLen implements KeyAdapter.
func (DupDetectKeyAdapter) EncodedLen(key []byte, rowID []byte) int {
	return codec.EncodedBytesLength(len(key)) + len(rowID) + 2
}

var _ KeyAdapter = DupDetectKeyAdapter{}

var (
	// MinRowID is the minimum rowID value after DupDetectKeyAdapter.Encode().
	MinRowID = []byte{0, 0, 0, 0, 0, 0, 0, 0, 0}
)