package flac

import (
	"bytes"
	"errors"
	"io"

	"github.com/go-audio/audio"
	"github.com/go-audio/wav"
	"github.com/mewkiz/flac"
)

const wavAudioFormat = 1

type WriterSeeker struct {
	buf []byte
	pos int
}

func (ws *WriterSeeker) Bytes() []byte {
	return ws.buf
}

func (m *WriterSeeker) Write(p []byte) (n int, err error) {
	minCap := m.pos + len(p)
	if minCap > cap(m.buf) { // Make sure buf has enough capacity:
		buf2 := make([]byte, len(m.buf), minCap+len(p)) // add some extra
		copy(buf2, m.buf)
		m.buf = buf2
	}
	if minCap > len(m.buf) {
		m.buf = m.buf[:minCap]
	}
	copy(m.buf[m.pos:], p)
	m.pos += len(p)
	return len(p), nil
}

func (m *WriterSeeker) Seek(offset int64, whence int) (int64, error) {
	newPos, offs := 0, int(offset)
	switch whence {
	case io.SeekStart:
		newPos = offs
	case io.SeekCurrent:
		newPos = m.pos + offs
	case io.SeekEnd:
		newPos = len(m.buf) + offs
	}
	if newPos < 0 {
		return 0, errors.New("negative result pos")
	}
	m.pos = newPos
	return int64(newPos), nil
}

// Decode takes a slice of flac and decodes to wav
func Decode(buf []byte) ([]byte, error) {
	r := bytes.NewReader(buf)
	stream, err := flac.Parse(r)
	if err != nil {
		return nil, errors.New("Could not parse FLAC")
	}
	ws := &WriterSeeker{}
	enc := wav.NewEncoder(ws, int(stream.Info.SampleRate), int(stream.Info.BitsPerSample), int(stream.Info.NChannels), wavAudioFormat)
	defer enc.Close()
	var data []int
	for {
		// Decode FLAC audio samples.
		frame, err := stream.ParseNext()
		if err != nil {
			if err == io.EOF {
				break
			}
			return nil, err
		}

		// Encode WAV audio samples.
		data = data[:0]
		for i := 0; i < frame.Subframes[0].NSamples; i++ {
			for _, subframe := range frame.Subframes {
				data = append(data, int(subframe.Samples[i]))
			}
		}
		buf := &audio.IntBuffer{
			Format: &audio.Format{
				NumChannels: int(stream.Info.NChannels),
				SampleRate:  int(stream.Info.SampleRate),
			},
			Data:           data,
			SourceBitDepth: int(stream.Info.BitsPerSample),
		}
		if err := enc.Write(buf); err != nil {
			return nil, err
		}
	}

	return ws.Bytes(), nil
}