// Package protocol implements the obs-teleport wire format.
//
// This file builds the audio ("WAVE") side of the protocol: Header +
// WaveHeader + raw interleaved PCM, byte-for-byte compatible with the
// reference implementation (GPL-2.0).
package protocol

// BuildWavePacket assembles a complete ""WAVE"" wire packet:
// Header + WaveHeader + interleaved PCM payload.
//
// pcm is already interleaved (L,R,L,R,… for stereo). format must be one of
// the AudioFormat* constants. frames is the PCM frame count (samples per
// channel). The returned slice is a freshly allocated buffer safe to hand to
// the network.
func BuildWavePacket(timestamp uint64, format int32, sampleRate, speakers, frames int32, pcm []byte) ([]byte, error) {
	expected := int64(speakers) * int64(frames) * int64(bytesPerSample(format))
	if int64(len(pcm)) != expected {
		return nil, errBadPCMLength{got: len(pcm), want: int(expected)}
	}

	h := Header{
		Type:      AudioType,
		Timestamp: timestamp,
		Size:      int32(len(pcm)),
	}
	wave := WaveHeader{
		Format:     format,
		SampleRate: sampleRate,
		Speakers:   speakers,
		Frames:     frames,
	}
	return WritePacket(h, nil, &wave, pcm)
}

// bytesPerSample maps an AudioFormat* value to its size in bytes.
func bytesPerSample(format int32) int {
	switch format {
	case AudioFormatU8:
		return 1
	case AudioFormatS16:
		return 2
	case AudioFormatS32, AudioFormatF32:
		return 4
	default:
		return 0
	}
}

// errBadPCMLength is returned when pcm length does not match the declared
// format/speakers/frames.
type errBadPCMLength struct{ got, want int }

func (e errBadPCMLength) Error() string {
	return "protocol: pcm length mismatch" +
		": got " + itoa(e.got) + " bytes, want " + itoa(e.want)
}

// itoa is a tiny local int→string helper to avoid importing strconv in the
// hot path callers.
func itoa(v int) string {
	if v == 0 {
		return "0"
	}
	neg := v < 0
	if neg {
		v = -v
	}
	var buf [20]byte
	i := len(buf)
	for v > 0 {
		i--
		buf[i] = byte('0' + v%10)
		v /= 10
	}
	if neg {
		i--
		buf[i] = '-'
	}
	return string(buf[i:])
}