Protocol/app_packet.go

package eq2net

import (
	"encoding/binary"
	"fmt"
)

// AppPacket handles application-level game packets with opcode abstraction
// This layer sits above the protocol layer and handles game-specific opcodes
type AppPacket struct {
	EQPacket

	EmuOpcode  uint16 // Emulator opcode (internal representation)
	OpcodeSize uint8  // Size of opcode in bytes (1 or 2)
}

// Default opcode size for application packets
var DefaultAppOpcodeSize uint8 = 2

// NewAppPacket creates a new application packet with emulator opcode
func NewAppPacket(emuOpcode uint16, data []byte) *AppPacket {
	p := &AppPacket{
		EQPacket:   *NewEQPacket(0, data), // Network opcode will be set during conversion
		EmuOpcode:  emuOpcode,
		OpcodeSize: DefaultAppOpcodeSize,
	}
	return p
}

// NewAppPacketWithSize creates an application packet with specified opcode size
func NewAppPacketWithSize(emuOpcode uint16, data []byte, opcodeSize uint8) *AppPacket {
	p := &AppPacket{
		EQPacket:   *NewEQPacket(0, data),
		EmuOpcode:  emuOpcode,
		OpcodeSize: opcodeSize,
	}
	return p
}

// ParseAppPacket creates an application packet from raw data
// The data should NOT include the protocol header, just the app opcode + payload
func ParseAppPacket(data []byte, opcodeSize uint8) (*AppPacket, error) {
	if len(data) < int(opcodeSize) {
		return nil, fmt.Errorf("packet too small for opcode: need %d bytes, got %d", opcodeSize, len(data))
	}

	// Extract opcode
	var opcode uint16
	if opcodeSize == 1 {
		opcode = uint16(data[0])
	} else {
		// Handle special encoding for 2-byte opcodes
		if data[0] == 0x00 && len(data) > 2 {
			// Special case: extra 0x00 prefix for opcodes with low byte = 0x00
			opcode = binary.BigEndian.Uint16(data[1:3])
			data = data[3:] // Skip the extra byte
		} else {
			opcode = binary.BigEndian.Uint16(data[0:2])
			data = data[2:]
		}
	}

	p := &AppPacket{
		EQPacket:   *NewEQPacket(opcode, data[opcodeSize:]),
		EmuOpcode:  opcode, // Initially same as network opcode
		OpcodeSize: opcodeSize,
	}

	return p, nil
}

// SetEmuOpcode sets the emulator opcode
// This is used when converting between emulator and network opcodes
func (p *AppPacket) SetEmuOpcode(opcode uint16) {
	p.EmuOpcode = opcode
}

// GetEmuOpcode returns the emulator opcode
func (p *AppPacket) GetEmuOpcode() uint16 {
	return p.EmuOpcode
}

// SerializeApp serializes the application packet with proper opcode encoding
func (p *AppPacket) SerializeApp() []byte {
	opcodeBytes := p.OpcodeSize
	extraBytes := 0

	// Handle special encoding rules for 2-byte opcodes
	if p.OpcodeSize == 2 && (p.Opcode&0x00FF) == 0 {
		// Opcodes with low byte = 0x00 need an extra 0x00 prefix
		extraBytes = 1
	}

	// Create buffer
	buf := make([]byte, int(opcodeBytes)+extraBytes+len(p.Buffer))
	offset := 0

	// Write opcode
	if p.OpcodeSize == 1 {
		buf[offset] = byte(p.Opcode)
		offset++
	} else {
		if extraBytes > 0 {
			// Special encoding: add 0x00 prefix
			buf[offset] = 0x00
			offset++
			binary.BigEndian.PutUint16(buf[offset:], p.Opcode)
			offset += 2
		} else {
			binary.BigEndian.PutUint16(buf[offset:], p.Opcode)
			offset += 2
		}
	}

	// Copy packet data
	if len(p.Buffer) > 0 {
		copy(buf[offset:], p.Buffer)
	}

	return buf
}

// Combine combines multiple application packets into a single packet
// Returns true if successful, false if size limit exceeded
func (p *AppPacket) Combine(other *AppPacket) bool {
	// Create a new buffer with both packets
	myData := p.SerializeApp()
	otherData := other.SerializeApp()

	// Check size limit (application packets can be larger than protocol)
	if len(myData)+len(otherData) > 8192 { // Reasonable max size
		return false
	}

	// Combine the data
	newBuffer := make([]byte, len(myData)+len(otherData))
	copy(newBuffer, myData)
	copy(newBuffer[len(myData):], otherData)

	// Update packet
	p.Buffer = newBuffer
	return true
}

// Copy creates a deep copy of the application packet
func (p *AppPacket) Copy() *AppPacket {
	newPacket := &AppPacket{
		EQPacket:   *p.Clone(),
		EmuOpcode:  p.EmuOpcode,
		OpcodeSize: p.OpcodeSize,
	}
	return newPacket
}

// SetVersion sets the protocol version for opcode conversion
func (p *AppPacket) SetVersion(version int16) {
	p.Version = version
}

// ConvertToProtocolPacket wraps this application packet in a protocol packet
// This is used when sending application data over the network
func (p *AppPacket) ConvertToProtocolPacket() *ProtocolPacket {
	// Serialize the application packet
	appData := p.SerializeApp()

	// Create protocol packet with OP_Packet opcode
	proto := &ProtocolPacket{
		EQPacket: *NewEQPacket(OP_Packet, appData),
	}

	// Copy network info
	proto.SrcIP = p.SrcIP
	proto.SrcPort = p.SrcPort
	proto.DstIP = p.DstIP
	proto.DstPort = p.DstPort
	proto.Timestamp = p.Timestamp
	proto.Version = p.Version

	return proto
}

// AppCombine performs application-level packet combining (EQ2-specific)
// This is different from protocol-level combining
func AppCombine(packets []*AppPacket) *AppPacket {
	if len(packets) == 0 {
		return nil
	}
	if len(packets) == 1 {
		return packets[0]
	}

	// Calculate total size needed
	var totalSize int
	for _, p := range packets {
		data := p.SerializeApp()
		if len(data) >= 255 {
			totalSize += 3 + len(data) // 0xFF marker + 2-byte size + data
		} else {
			totalSize += 1 + len(data) // 1-byte size + data
		}
	}

	// Build combined buffer
	buffer := make([]byte, totalSize)
	offset := 0

	for _, p := range packets {
		data := p.SerializeApp()
		size := len(data)

		if size >= 255 {
			// Oversized packet: use 0xFF marker followed by 2-byte size
			buffer[offset] = 0xFF
			offset++
			binary.BigEndian.PutUint16(buffer[offset:], uint16(size))
			offset += 2
		} else {
			// Normal packet: 1-byte size
			buffer[offset] = byte(size)
			offset++
		}

		// Copy packet data
		copy(buffer[offset:], data)
		offset += size
	}

	// Create combined packet with OP_AppCombined
	combined := &AppPacket{
		EQPacket:   *NewEQPacket(OP_AppCombined, buffer),
		OpcodeSize: DefaultAppOpcodeSize,
	}

	return combined
}

// ExtractAppPackets extracts individual packets from an app-combined packet
func ExtractAppPackets(combined *AppPacket) ([]*AppPacket, error) {
	if combined.Opcode != OP_AppCombined {
		return nil, fmt.Errorf("not an app-combined packet")
	}

	var packets []*AppPacket
	data := combined.Buffer
	offset := 0

	for offset < len(data) {
		if offset >= len(data) {
			break
		}

		var size int
		// Read size
		if data[offset] == 0xFF {
			// Oversized packet
			if offset+3 > len(data) {
				return nil, fmt.Errorf("invalid oversized packet header")
			}
			offset++
			size = int(binary.BigEndian.Uint16(data[offset:]))
			offset += 2
		} else {
			// Normal packet
			size = int(data[offset])
			offset++
		}

		// Extract packet data
		if offset+size > len(data) {
			return nil, fmt.Errorf("packet size exceeds buffer")
		}

		packetData := data[offset : offset+size]
		offset += size

		// Parse the packet
		app, err := ParseAppPacket(packetData, combined.OpcodeSize)
		if err != nil {
			return nil, fmt.Errorf("failed to parse app packet: %w", err)
		}

		packets = append(packets, app)
	}

	return packets, nil
}