// Package opus encodes and decodes the reflector's 16 kHz mono frames with libopus.
package opus

/*
#cgo pkg-config: opus
#include <opus.h>

static int configure_encoder(OpusEncoder *enc) {
	int err;
	err = opus_encoder_ctl(enc, OPUS_SET_BITRATE(20000));
	if (err < 0) return err;
	err = opus_encoder_ctl(enc, OPUS_SET_VBR(1));
	if (err < 0) return err;
	err = opus_encoder_ctl(enc, OPUS_SET_SIGNAL(OPUS_SIGNAL_VOICE));
	if (err < 0) return err;
	err = opus_encoder_ctl(enc, OPUS_SET_MAX_BANDWIDTH(OPUS_BANDWIDTH_MEDIUMBAND));
	if (err < 0) return err;
	err = opus_encoder_ctl(enc, OPUS_SET_DTX(0));
	return err;
}
*/
import "C"

import (
	"fmt"
	"unsafe"
)

// FrameSamples is the 20 ms frame at 16 kHz, which is what the reflector carries.
const FrameSamples = 320

// Encoder produces one raw Opus packet per 20 ms frame.
type Encoder struct {
	enc *C.OpusEncoder
}

// Decoder turns one raw Opus packet into 320 mono samples.
type Decoder struct {
	dec *C.OpusDecoder
}

// NewEncoder returns a mono 16 kHz voice encoder at 20 kbit/s.
func NewEncoder() (*Encoder, error) {
	var err C.int
	enc := C.opus_encoder_create(16000, 1, C.OPUS_APPLICATION_AUDIO, &err)
	if err < 0 || enc == nil {
		return nil, fmt.Errorf("opus encoder %d", int(err))
	}
	if c := C.configure_encoder(enc); c < 0 {
		C.opus_encoder_destroy(enc)
		return nil, fmt.Errorf("opus encoder setup %d", int(c))
	}
	return &Encoder{enc: enc}, nil
}

// Encode encodes exactly 320 samples.
func (e *Encoder) Encode(pcm []int16) ([]byte, error) {
	if len(pcm) != FrameSamples {
		return nil, fmt.Errorf("opus frame is %d samples", len(pcm))
	}
	out := make([]byte, 4000)
	n := C.opus_encode(
		e.enc,
		(*C.opus_int16)(unsafe.Pointer(&pcm[0])),
		FrameSamples,
		(*C.uchar)(unsafe.Pointer(&out[0])),
		C.opus_int32(len(out)),
	)
	if n < 0 {
		return nil, fmt.Errorf("opus encode %d", int(n))
	}
	return append([]byte(nil), out[:n]...), nil
}

// Close releases the encoder.
func (e *Encoder) Close() {
	if e == nil || e.enc == nil {
		return
	}
	C.opus_encoder_destroy(e.enc)
	e.enc = nil
}

// NewDecoder returns a mono 16 kHz decoder.
func NewDecoder() (*Decoder, error) {
	var err C.int
	dec := C.opus_decoder_create(16000, 1, &err)
	if err < 0 || dec == nil {
		return nil, fmt.Errorf("opus decoder %d", int(err))
	}
	return &Decoder{dec: dec}, nil
}

// Decode decodes one packet into 320 samples.
func (d *Decoder) Decode(packet []byte) ([]int16, error) {
	if len(packet) == 0 {
		return nil, fmt.Errorf("empty opus packet")
	}
	pcm := make([]int16, FrameSamples)
	n := C.opus_decode(
		d.dec,
		(*C.uchar)(unsafe.Pointer(&packet[0])),
		C.opus_int32(len(packet)),
		(*C.opus_int16)(unsafe.Pointer(&pcm[0])),
		FrameSamples,
		0,
	)
	if n < 0 {
		return nil, fmt.Errorf("opus decode %d", int(n))
	}
	if int(n) != FrameSamples {
		return nil, fmt.Errorf("opus decode returned %d samples", int(n))
	}
	return pcm, nil
}

// Close releases the decoder.
func (d *Decoder) Close() {
	if d == nil || d.dec == nil {
		return
	}
	C.opus_decoder_destroy(d.dec)
	d.dec = nil
}