/*
NAME
  wav.go

DESCRIPTION
  wav.go contains functions for processing wav.

AUTHOR
  David Sutton <davidsutton@ausocean.org>

LICENSE
  wav.go is Copyright (C) 2023 the Australian Ocean Lab (AusOcean)

  It is free software: you can redistribute it and/or modify them
  under the terms of the GNU General Public License as published by the
  Free Software Foundation, either version 3 of the License, or (at your
  option) any later version.

  It is distributed in the hope that it will be useful, but WITHOUT
  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
  for more details.

  You should have received a copy of the GNU General Public License in gpl.txt.
  If not, see [GNU licenses](http://www.gnu.org/licenses).
*/

// Package wav provides functions for converting wav audio.
package wav

import (
	"encoding/binary"
	"fmt"
)

// ConvertFormat converts the common name for a format in a string type to the specific
// integer required by the wav encoder.
var ConvertFormat = map[string]int{"pcm": PCMFormat}

const PCMFormat = 1 // PCMFormat defines the value for pcm audio as defined by the wav std.

var (
	errInvalidFormat   = fmt.Errorf("invalid or no format defined")
	errInvalidRate     = fmt.Errorf("invalid or no sample rate defined")
	errInvalidChannels = fmt.Errorf("invalid or no number of channels defined")
	errInvalidBitDepth = fmt.Errorf("invalid or no bit depth defined")
)

// Metadata defines the format of the audio file for reading.
type Metadata struct {
	AudioFormat int
	Channels    int
	SampleRate  int
	BitDepth    int
}

type WAV struct {
	Metadata Metadata
	Audio    []byte
}

// Write writes the given audio byte slice to the WAV, encoding the appropriate headings.
func (w *WAV) Write(p []byte) (n int, err error) {
	// Create header slice.
	header := make([]byte, 44)

	// Write RIFF type.
	copy(header[0:4], []byte("RIFF"))

	// Write the size of overall file.
	buf := make([]byte, 4)
	binary.LittleEndian.PutUint32(buf, uint32(len(p)+44))
	copy(header[4:8], buf)

	// Write WAVE type.
	copy(header[8:12], []byte("WAVE"))

	// Write fmt chunk marker.
	copy(header[12:16], []byte("fmt "))

	// Write the subchunk1 Size.
	binary.LittleEndian.PutUint32(buf, 16)
	copy(header[16:20], buf)

	// Write the encoded audio format.
	if w.Metadata.AudioFormat != PCMFormat { // TODO: allow for more encoding formats.
		return 0, errInvalidFormat
	}
	binary.LittleEndian.PutUint16(buf[0:2], 1)
	copy(header[20:22], buf[0:2])

	// Write the number of channels.
	if w.Metadata.Channels == 0 {
		return 0, errInvalidChannels
	}
	binary.LittleEndian.PutUint16(buf[0:2], uint16(w.Metadata.Channels))
	copy(header[22:24], buf[0:2])

	// Write the sample rate.
	if w.Metadata.SampleRate == 0 {
		return 0, errInvalidRate
	}
	binary.LittleEndian.PutUint32(buf[0:4], uint32(w.Metadata.SampleRate))
	copy(header[24:28], buf[0:4])

	// Write bit depth values.
	if w.Metadata.BitDepth == 0 {
		return 0, errInvalidBitDepth
	}
	var val uint32 = uint32((w.Metadata.SampleRate * w.Metadata.BitDepth * w.Metadata.Channels) / 8)
	binary.LittleEndian.PutUint32(buf[0:4], val)
	copy(header[28:32], buf[0:4])

	val = uint32((w.Metadata.BitDepth * w.Metadata.Channels) / 8)
	binary.LittleEndian.PutUint32(buf[0:4], val)
	copy(header[32:34], buf[0:4])

	binary.LittleEndian.PutUint32(buf[0:4], uint32(w.Metadata.BitDepth))
	copy(header[34:36], buf[0:4])

	// Mark start of data.
	copy(header[36:40], []byte("data"))

	// Write size of data chunk.
	binary.LittleEndian.PutUint32(buf[0:4], uint32(len(p)))
	copy(header[40:44], buf[0:4])

	// Append audio data.
	w.Audio = header
	w.Audio = append(w.Audio, p...)

	// Return successful write.
	return len(p) + 44, nil

}