ttadec: fix last frame handling when seeking

Using a frame count, as is done currently, does not work at all with
seeking. Instead, when the number of samples in the final frame has
been decoded, we check if only the 32-bit CRC is remaining. If so, we
assume that it is the final frame.

There is no longer a need to keep total_frames in TTAContext.

libavcodec/tta.c

@@ -64,7 +64,7 @@ typedef struct TTAContext {
 
     int format, channels, bps;
     unsigned data_length;
-    int frame_length, last_frame_length, total_frames;
+    int frame_length, last_frame_length;
 
     int32_t *decode_buffer;
 
@@ -198,6 +198,7 @@ static int tta_check_crc(TTAContext *s, const uint8_t *buf, int buf_size)
 static av_cold int tta_decode_init(AVCodecContext * avctx)
 {
     TTAContext *s = avctx->priv_data;
+    int total_frames;
 
     s->avctx = avctx;
 
@@ -262,24 +263,24 @@ static av_cold int tta_decode_init(AVCodecContext * avctx)
         s->frame_length = 256 * avctx->sample_rate / 245;
 
         s->last_frame_length = s->data_length % s->frame_length;
-        s->total_frames = s->data_length / s->frame_length +
-                        (s->last_frame_length ? 1 : 0);
+        total_frames = s->data_length / s->frame_length +
+                       (s->last_frame_length ? 1 : 0);
 
         av_log(s->avctx, AV_LOG_DEBUG, "format: %d chans: %d bps: %d rate: %d block: %d\n",
             s->format, avctx->channels, avctx->bits_per_coded_sample, avctx->sample_rate,
             avctx->block_align);
         av_log(s->avctx, AV_LOG_DEBUG, "data_length: %d frame_length: %d last: %d total: %d\n",
-            s->data_length, s->frame_length, s->last_frame_length, s->total_frames);
+            s->data_length, s->frame_length, s->last_frame_length, total_frames);
 
         // FIXME: seek table
-        if (avctx->extradata_size <= 26 || s->total_frames > INT_MAX / 4 ||
-            avctx->extradata_size - 26 < s->total_frames * 4)
+        if (avctx->extradata_size <= 26 || total_frames > INT_MAX / 4 ||
+            avctx->extradata_size - 26 < total_frames * 4)
             av_log(avctx, AV_LOG_WARNING, "Seek table missing or too small\n");
         else if (avctx->err_recognition & AV_EF_CRCCHECK) {
-            if (tta_check_crc(s, avctx->extradata + 22, s->total_frames * 4))
+            if (tta_check_crc(s, avctx->extradata + 22, total_frames * 4))
                 return AVERROR_INVALIDDATA;
         }
-        skip_bits_long(&s->gb, 32 * s->total_frames);
+        skip_bits_long(&s->gb, 32 * total_frames);
         skip_bits_long(&s->gb, 32); // CRC32 of seektable
 
         if(s->frame_length >= UINT_MAX / (s->channels * sizeof(int32_t))){
@@ -325,11 +326,6 @@ static int tta_decode_frame(AVCodecContext *avctx, void *data,
 
     init_get_bits(&s->gb, buf, buf_size*8);
 
-    // FIXME: seeking
-    s->total_frames--;
-    if (!s->total_frames && s->last_frame_length)
-        framelen = s->last_frame_length;
-
     /* get output buffer */
     s->frame.nb_samples = framelen;
     if ((ret = ff_get_buffer(avctx, &s->frame)) < 0) {
@@ -348,6 +344,7 @@ static int tta_decode_frame(AVCodecContext *avctx, void *data,
         rice_init(&s->ch_ctx[i].rice, 10, 10);
     }
 
+    i = 0;
     for (p = s->decode_buffer; p < s->decode_buffer + (framelen * s->channels); p++) {
         int32_t *predictor = &s->ch_ctx[cur_chan].predictor;
         TTAFilter *filter = &s->ch_ctx[cur_chan].filter;
@@ -424,9 +421,16 @@ static int tta_decode_frame(AVCodecContext *avctx, void *data,
                     *r = *(r + 1) - *r;
             }
             cur_chan = 0;
+            i++;
+            // check for last frame
+            if (i == s->last_frame_length && get_bits_left(&s->gb) / 8 == 4) {
+                s->frame.nb_samples = framelen = s->last_frame_length;
+                break;
+            }
         }
     }
 
+    align_get_bits(&s->gb);
     if (get_bits_left(&s->gb) < 32) {
         ret = AVERROR_INVALIDDATA;
         goto error;