package mic

import (
	"encoding/binary"
	"errors"
)

const (
	SampleRate     = 16000
	Channels       = 1
	BitsPerSample  = 16
	BytesPerSample = BitsPerSample / 8 * Channels
)

var wavHeaderSize = 44

// EncodeWAV wraps 16-bit mono PCM in a complete WAV file.
func EncodeWAV(pcm []byte) []byte {
	if len(pcm) == 0 {
		return nil
	}
	if len(pcm)%BytesPerSample != 0 {
		pcm = pcm[:len(pcm)-len(pcm)%BytesPerSample]
	}
	out := make([]byte, wavHeaderSize+len(pcm))
	writeWAVHeader(out, len(pcm))
	copy(out[wavHeaderSize:], pcm)
	return out
}

func writeWAVHeader(buf []byte, dataLen int) {
	if len(buf) < wavHeaderSize {
		return
	}
	copy(buf[0:4], "RIFF")
	binary.LittleEndian.PutUint32(buf[4:8], uint32(36+dataLen))
	copy(buf[8:12], "WAVE")
	copy(buf[12:16], "fmt ")
	binary.LittleEndian.PutUint32(buf[16:20], 16)
	binary.LittleEndian.PutUint16(buf[20:22], 1) // PCM
	binary.LittleEndian.PutUint16(buf[22:24], Channels)
	binary.LittleEndian.PutUint32(buf[24:28], SampleRate)
	byteRate := SampleRate * Channels * BytesPerSample
	binary.LittleEndian.PutUint32(buf[28:32], uint32(byteRate))
	binary.LittleEndian.PutUint16(buf[32:34], uint16(Channels*BytesPerSample))
	binary.LittleEndian.PutUint16(buf[34:36], BitsPerSample)
	copy(buf[36:40], "data")
	binary.LittleEndian.PutUint32(buf[40:44], uint32(dataLen))
}

// PCMBytesFromWAV returns PCM payload length implied by a WAV header.
func PCMBytesFromWAV(hdr []byte) (int, error) {
	if len(hdr) < wavHeaderSize {
		return 0, errors.New("wav header too short")
	}
	if string(hdr[0:4]) != "RIFF" || string(hdr[8:12]) != "WAVE" {
		return 0, errors.New("not a wav file")
	}
	dataLen := int(binary.LittleEndian.Uint32(hdr[40:44]))
	return dataLen, nil
}