package unpacker

import (
	"encoding/binary"

	unpackerpkg "git.noahlan.cn/noahlan/nnet/v2/pkg/unpacker"
)

// lengthFieldUnpacker 长度字段拆包器实现
type lengthFieldUnpacker struct {
	lengthFieldOffset   int
	lengthFieldLength   int
	lengthAdjustment    int
	initialBytesToStrip int
	buffer              []byte
	byteOrder           binary.ByteOrder
	maxBufferSize       int
}

// NewLengthFieldUnpacker 创建长度字段拆包器
func NewLengthFieldUnpacker(config unpackerpkg.LengthFieldUnpacker) unpackerpkg.Unpacker {
	lengthFieldOffset := config.LengthFieldOffset
	if lengthFieldOffset < 0 {
		lengthFieldOffset = 0
	}

	lengthFieldLength := config.LengthFieldLength
	if lengthFieldLength <= 0 {
		lengthFieldLength = 4 // 默认4字节
	}

	initialBytesToStrip := config.InitialBytesToStrip
	if initialBytesToStrip < 0 {
		initialBytesToStrip = 0
	}

	maxBufferSize := config.MaxBufferSize
	if maxBufferSize <= 0 {
		maxBufferSize = unpackerpkg.DefaultMaxBufferSize
	}

	return &lengthFieldUnpacker{
		lengthFieldOffset:   lengthFieldOffset,
		lengthFieldLength:   lengthFieldLength,
		lengthAdjustment:    config.LengthAdjustment,
		initialBytesToStrip: initialBytesToStrip,
		buffer:              nil, // 延迟分配，使用buffer pool
		byteOrder:           binary.BigEndian,
		maxBufferSize:       maxBufferSize,
	}
}

// Unpack 拆包
func (u *lengthFieldUnpacker) Unpack(data []byte) ([][]byte, []byte, int, error) {
	// 检查buffer大小限制
	newSize := len(u.buffer) + len(data)
	if newSize > u.maxBufferSize {
		return nil, nil, 0, unpackerpkg.NewErrorf("unpacker buffer size exceeded: %d > %d", newSize, u.maxBufferSize)
	}

	// 优化buffer管理：只在需要时分配和扩容
	if u.buffer == nil {
		// 首次使用，从buffer pool获取或分配
		if newSize <= 4096 {
			// 小数据，使用固定大小的buffer
			u.buffer = make([]byte, 0, 4096)
		} else {
			// 大数据，直接分配所需容量（但不超过maxBufferSize）
			cap := newSize
			if cap > u.maxBufferSize {
				cap = u.maxBufferSize
			}
			u.buffer = make([]byte, 0, cap)
		}
	} else if cap(u.buffer) < newSize {
		// 需要扩容：计算新容量（至少是当前大小的2倍，但不超过maxBufferSize）
		newCap := cap(u.buffer) * 2
		if newCap < newSize {
			newCap = newSize
		}
		if newCap > u.maxBufferSize {
			newCap = u.maxBufferSize
		}

		// 扩容：创建新buffer并复制数据
		newBuffer := make([]byte, len(u.buffer), newCap)
		copy(newBuffer, u.buffer)
		u.buffer = newBuffer
	}

	// 追加新数据
	u.buffer = append(u.buffer, data...)

	var messages [][]byte

	for {
		if len(u.buffer) < u.lengthFieldOffset+u.lengthFieldLength {
			// 数据不足，等待更多数据
			break
		}

		// 读取长度字段
		lengthBytes := u.buffer[u.lengthFieldOffset : u.lengthFieldOffset+u.lengthFieldLength]
		var length int
		switch u.lengthFieldLength {
		case 1:
			length = int(lengthBytes[0])
		case 2:
			length = int(u.byteOrder.Uint16(lengthBytes))
		case 4:
			length = int(u.byteOrder.Uint32(lengthBytes))
		case 8:
			length = int(u.byteOrder.Uint64(lengthBytes))
		default:
			return nil, u.buffer, 0, unpackerpkg.NewError("unsupported length field length")
		}

		// 验证长度字段的合理性（防止恶意数据）
		if length < 0 || length > u.maxBufferSize {
			return nil, nil, 0, unpackerpkg.NewErrorf("invalid length field: %d", length)
		}

		// 计算实际消息长度
		actualLength := length + u.lengthAdjustment
		if actualLength < 0 {
			return nil, nil, 0, unpackerpkg.NewErrorf("invalid actual length: %d", actualLength)
		}
		totalLength := u.lengthFieldOffset + u.lengthFieldLength + actualLength

		if totalLength > u.maxBufferSize {
			return nil, nil, 0, unpackerpkg.NewErrorf("message too large: %d > %d", totalLength, u.maxBufferSize)
		}

		if len(u.buffer) < totalLength {
			// 数据不足，等待更多数据
			break
		}

		// 提取消息（必须复制数据，因为buffer可能会被修改）
		// 优化：预分配messages slice的容量以减少重新分配
		start := u.initialBytesToStrip
		end := totalLength
		if start > end {
			return nil, nil, 0, unpackerpkg.NewErrorf("initial bytes to strip exceeds frame length: %d > %d", start, end)
		}
		messageLen := end - start
		if messages == nil {
			// 预分配messages slice（假设至少有一个消息）
			messages = make([][]byte, 0, 1)
		}
		message := make([]byte, messageLen)
		copy(message, u.buffer[start:end])
		messages = append(messages, message)

		// 移除已处理的数据（使用切片操作，避免复制）
		u.buffer = u.buffer[totalLength:]

		// 优化：如果buffer使用率太低，压缩buffer以减少内存占用
		// 使用率阈值：25%（即剩余数据 < 容量的25%）
		// 注意：压缩不会改变buffer的长度，只改变容量
		if len(u.buffer) < cap(u.buffer)/4 && cap(u.buffer) > 4096 {
			// 压缩：新容量为当前容量的50%，但至少能容纳当前数据
			newCap := cap(u.buffer) / 2
			if newCap < len(u.buffer) {
				newCap = len(u.buffer)
			}
			// 如果新容量仍然合理，执行压缩
			if newCap >= 1024 {
				compressed := make([]byte, len(u.buffer), newCap)
				copy(compressed, u.buffer)
				u.buffer = compressed
			}
		}
	}

	// 输入字节已经被复制进连接级buffer；调用方应从底层读缓冲中丢弃本次输入，避免重复处理。
	return messages, u.buffer, len(data), nil
}

// Pack 打包
func (u *lengthFieldUnpacker) Pack(data []byte) ([]byte, error) {
	dataLength := len(data)
	totalLength := u.lengthFieldOffset + u.lengthFieldLength + dataLength - u.lengthAdjustment
	if totalLength < u.lengthFieldOffset+u.lengthFieldLength {
		return nil, unpackerpkg.NewErrorf("invalid packed length: %d", totalLength)
	}
	if u.initialBytesToStrip > len(data) {
		return nil, unpackerpkg.NewErrorf("initial bytes to strip exceeds data length: %d > %d", u.initialBytesToStrip, len(data))
	}

	result := make([]byte, totalLength)

	// 填充长度字段之前的字节（如果有）
	if u.lengthFieldOffset > 0 {
		// 通常为0，如果需要可以扩展
	}

	// 写入长度字段
	lengthValue := dataLength - u.lengthAdjustment
	if lengthValue < 0 {
		return nil, unpackerpkg.NewErrorf("invalid length value: %d", lengthValue)
	}
	lengthBytes := make([]byte, u.lengthFieldLength)
	switch u.lengthFieldLength {
	case 1:
		if lengthValue > 0xff {
			return nil, unpackerpkg.NewErrorf("length value too large for 1-byte field: %d", lengthValue)
		}
		lengthBytes[0] = byte(lengthValue)
	case 2:
		if lengthValue > 0xffff {
			return nil, unpackerpkg.NewErrorf("length value too large for 2-byte field: %d", lengthValue)
		}
		u.byteOrder.PutUint16(lengthBytes, uint16(lengthValue))
	case 4:
		u.byteOrder.PutUint32(lengthBytes, uint32(lengthValue))
	case 8:
		u.byteOrder.PutUint64(lengthBytes, uint64(lengthValue))
	default:
		return nil, unpackerpkg.NewError("unsupported length field length")
	}
	copy(result[u.lengthFieldOffset:], lengthBytes)

	// 写入数据
	copy(result[u.lengthFieldOffset+u.lengthFieldLength:], data[u.initialBytesToStrip:])

	return result, nil
}