package audio

import (
	"encoding/binary"
	"fmt"
)

// pcmFormat describes raw PCM samples as oto needs them for playback.
type pcmFormat struct {
	SampleRate   int
	Channels     int
	BitsPerCoded int // 8 (unsigned), 16 (signed LE), 32 (IEEE float only)
	Float        bool
}

// decodeWAV splits RIFF/WAVE bytes into format and PCM data.
//
// Supported: PCM (AudioFormat 1) with 8/16 bit and IEEE float (AudioFormat
// 3) with 32 bit; mono and stereo. Everything else, compressed formats in
// particular, is a clear error; the OS TTS paths of osbuiltin deliver
// 16-bit PCM WAV exclusively.
func decodeWAV(data []byte) (pcmFormat, []byte, error) {
	var f pcmFormat
	if len(data) < 12 {
		return f, nil, fmt.Errorf("wav: too short for a RIFF header (%d bytes)", len(data))
	}
	if string(data[0:4]) != "RIFF" || string(data[8:12]) != "WAVE" {
		return f, nil, fmt.Errorf("wav: no RIFF/WAVE header")
	}

	var haveFmt bool
	var payload []byte
	pos := 12
	for pos+8 <= len(data) {
		id := string(data[pos : pos+4])
		size := int(binary.LittleEndian.Uint32(data[pos+4 : pos+8]))
		body := pos + 8
		if body+size > len(data) {
			return f, nil, fmt.Errorf("wav: chunk %q exceeds the data (size %d)", id, size)
		}
		switch id {
		case "fmt ":
			if size < 16 {
				return f, nil, fmt.Errorf("wav: fmt chunk too short (%d)", size)
			}
			audioFormat := binary.LittleEndian.Uint16(data[body : body+2])
			f.Channels = int(binary.LittleEndian.Uint16(data[body+2 : body+4]))
			f.SampleRate = int(binary.LittleEndian.Uint32(data[body+4 : body+8]))
			f.BitsPerCoded = int(binary.LittleEndian.Uint16(data[body+14 : body+16]))
			switch {
			case audioFormat == 1 && (f.BitsPerCoded == 8 || f.BitsPerCoded == 16):
			case audioFormat == 3 && f.BitsPerCoded == 32:
				f.Float = true
			default:
				return f, nil, fmt.Errorf("wav: unsupported format (AudioFormat %d, %d bit); allowed: PCM 8/16 bit, float 32 bit",
					audioFormat, f.BitsPerCoded)
			}
			if f.Channels < 1 || f.Channels > 2 {
				return f, nil, fmt.Errorf("wav: %d channels (allowed: 1, 2)", f.Channels)
			}
			if f.SampleRate <= 0 {
				return f, nil, fmt.Errorf("wav: sample rate %d", f.SampleRate)
			}
			haveFmt = true
		case "data":
			payload = data[body : body+size]
		}
		pos = body + size
		if size%2 == 1 { // chunks are word-aligned
			pos++
		}
	}
	if !haveFmt {
		return f, nil, fmt.Errorf("wav: no fmt chunk")
	}
	if payload == nil {
		return f, nil, fmt.Errorf("wav: no data chunk")
	}
	return f, payload, nil
}