package frame

import (
	"encoding/binary"
	"fmt"
)

// ControlEnvelopeMagic is the 4-byte prefix marking a v2 control message multiplexed through the
// PacketConnection's send_packet path. An IPv4 packet's first byte is 0x4X and an IPv6 packet's
// first byte is 0x6X, so this magic (starting with 0xAA) never collides with a real IP packet.
var ControlEnvelopeMagic = [4]byte{0xAA, 0xAA, 0xC0, 0x01}

// ControlKind is the on-wire byte selector inside a control envelope.
type ControlKind byte

// Known control kinds (must match crates/aura-proto/src/frame.rs ControlKind).
const (
	ControlCrlPush       ControlKind = 0x01
	ControlCrlAck        ControlKind = 0x02
	ControlExtendBridge  ControlKind = 0x03
	ControlCircuitReady  ControlKind = 0x04
	ControlCircuitFailed ControlKind = 0x05
)

// EncodeControlEnvelope wraps (kind, payload) as
//
//	MAGIC(4) || kind(u8) || u32_be(payload_len) || payload
//
// suitable for shipping through PacketConnection.SendPacket.
func EncodeControlEnvelope(kind ControlKind, payload []byte) []byte {
	out := make([]byte, 0, len(ControlEnvelopeMagic)+1+4+len(payload))
	out = append(out, ControlEnvelopeMagic[:]...)
	out = append(out, byte(kind))
	var lb [4]byte
	binary.BigEndian.PutUint32(lb[:], uint32(len(payload)))
	out = append(out, lb[:]...)
	out = append(out, payload...)
	return out
}

// DecodeControlEnvelope returns (kind, payload, true, nil) if buf starts with the magic and
// parses cleanly. If buf does NOT start with the magic (i.e. it is a normal IP packet) the third
// return is false and the error is nil. A malformed envelope (truncated) returns an error.
func DecodeControlEnvelope(buf []byte) (ControlKind, []byte, bool, error) {
	if len(buf) < len(ControlEnvelopeMagic) {
		return 0, nil, false, nil
	}
	for i, b := range ControlEnvelopeMagic {
		if buf[i] != b {
			return 0, nil, false, nil
		}
	}
	rest := buf[len(ControlEnvelopeMagic):]
	if len(rest) < 1 {
		return 0, nil, true, fmt.Errorf("%w: control envelope: missing kind", ErrMalformedFrame)
	}
	kind := ControlKind(rest[0])
	if len(rest) < 5 {
		return 0, nil, true, fmt.Errorf("%w: control envelope: missing payload length", ErrMalformedFrame)
	}
	plen := int(binary.BigEndian.Uint32(rest[1:5]))
	if len(rest) < 5+plen {
		return 0, nil, true, fmt.Errorf("%w: control envelope: truncated payload", ErrMalformedFrame)
	}
	payload := make([]byte, plen)
	copy(payload, rest[5:5+plen])
	return kind, payload, true, nil
}