/*
 * aacscan: FFmpeg's AAC decoder, asked one frame at a time how many bytes
 * each raw AAC frame takes. For johnmcm.com's Video Repair: a recording cut
 * off before its index holds its sound as raw AAC frames with no lengths
 * written down, so the page finds them by decoding, here, and copies them
 * into the repaired file untouched.
 *
 * The decoder's own decode callback returns the bytes it consumed; the
 * public API keeps that to itself, so it is called directly (the internal
 * header is part of the FFmpeg 5.1.6 tree this is built in).
 *
 * LGPL-2.1-or-later, as FFmpeg is.
 */
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include "libavcodec/avcodec.h"
#include "libavcodec/codec_internal.h"
#include "libavutil/frame.h"
#include "libavutil/mem.h"

static AVCodecContext *ctx;
static AVFrame *frame;
static AVPacket *pkt;
static const FFCodec *codec;

/* The last frame's sample count and channels, for the page to check. */
static int last_samples;
static int last_channels;

void *aacs_alloc(int n) { return malloc(n); }
void aacs_free(void *p) { free(p); }

void aacs_close(void) {
  if (ctx) avcodec_free_context(&ctx);
  if (frame) av_frame_free(&frame);
  if (pkt) av_packet_free(&pkt);
}

/* Opens the decoder with an AudioSpecificConfig; 0 when it took it. */
int aacs_open(const uint8_t *asc, int len) {
  aacs_close();
  const AVCodec *c = avcodec_find_decoder(AV_CODEC_ID_AAC);
  if (!c) return -1;
  codec = ffcodec(c);
  ctx = avcodec_alloc_context3(c);
  frame = av_frame_alloc();
  pkt = av_packet_alloc();
  if (!ctx || !frame || !pkt) return -2;
  ctx->extradata = av_mallocz(len + AV_INPUT_BUFFER_PADDING_SIZE);
  if (!ctx->extradata) return -2;
  memcpy(ctx->extradata, asc, len);
  ctx->extradata_size = len;
  if (avcodec_open2(ctx, c, NULL) < 0) return -3;
  return 0;
}

/*
 * Decodes one frame from the start of buf: the bytes it took (> 0), or
 * a negative number when the bytes there are not an AAC frame under this
 * configuration. buf must have AV_INPUT_BUFFER_PADDING_SIZE (64) spare
 * bytes after size, zeroed.
 */
int aacs_frame(uint8_t *buf, int size) {
  if (!ctx) return -100;
  int got = 0;
  pkt->data = buf;
  pkt->size = size;
  av_frame_unref(frame);
  int ret = codec->cb.decode(ctx, frame, &got, pkt);
  last_samples = got ? frame->nb_samples : 0;
  last_channels = got ? frame->ch_layout.nb_channels : 0;
  av_frame_unref(frame);
  if (ret < 0) return ret;
  if (!got) return -101;
  return ret;
}

int aacs_samples(void) { return last_samples; }
int aacs_channels(void) { return last_channels; }