+/**
+ * Decode a macroblock.
+ * @return <0 if an error occurred
+ */
+static int h261_decode_block(H261Context *h, int16_t *block, int n, int coded)
+{
+ MpegEncContext *const s = &h->s;
+ int code, level, i, j, run;
+ RLTable *rl = &ff_h261_rl_tcoeff;
+ const uint8_t *scan_table;
+
+ /* For the variable length encoding there are two code tables, one being
+ * used for the first transmitted LEVEL in INTER, INTER + MC and
+ * INTER + MC + FIL blocks, the second for all other LEVELs except the
+ * first one in INTRA blocks which is fixed length coded with 8 bits.
+ * NOTE: The two code tables only differ in one VLC so we handle that
+ * manually. */
+ scan_table = s->intra_scantable.permutated;
+ if (s->mb_intra) {
+ /* DC coef */
+ level = bitstream_read(&s->bc, 8);
+ // 0 (00000000b) and -128 (10000000b) are FORBIDDEN
+ if ((level & 0x7F) == 0) {
+ av_log(s->avctx, AV_LOG_ERROR, "illegal dc %d at %d %d\n",
+ level, s->mb_x, s->mb_y);
+ return -1;
+ }
+ /* The code 1000 0000 is not used, the reconstruction level of 1024
+ * being coded as 1111 1111. */
+ if (level == 255)
+ level = 128;
+ block[0] = level;
+ i = 1;
+ } else if (coded) {
+ // Run Level Code
+ // EOB Not possible for first level when cbp is available (that's why the table is different)
+ // 0 1 1s
+ // * * 0*
+ int check = bitstream_peek(&s->bc, 2);
+ i = 0;
+ if (check & 0x2) {
+ bitstream_skip(&s->bc, 2);
+ block[0] = (check & 0x1) ? -1 : 1;
+ i = 1;
+ }
+ } else {
+ i = 0;
+ }
+ if (!coded) {
+ s->block_last_index[n] = i - 1;
+ return 0;
+ }
+ for (;;) {
+ code = bitstream_read_vlc(&s->bc, rl->vlc.table, TCOEFF_VLC_BITS, 2);
+ if (code < 0) {
+ av_log(s->avctx, AV_LOG_ERROR, "illegal ac vlc code at %dx%d\n",
+ s->mb_x, s->mb_y);
+ return -1;
+ }
+ if (code == rl->n) {
+ /* escape */
+ /* The remaining combinations of (run, level) are encoded with a
+ * 20-bit word consisting of 6 bits escape, 6 bits run and 8 bits
+ * level. */
+ run = bitstream_read(&s->bc, 6);
+ level = bitstream_read_signed(&s->bc, 8);
+ } else if (code == 0) {
+ break;
+ } else {
+ run = rl->table_run[code];
+ level = rl->table_level[code];
+ if (bitstream_read_bit(&s->bc))
+ level = -level;
+ }
+ i += run;
+ if (i >= 64) {
+ av_log(s->avctx, AV_LOG_ERROR, "run overflow at %dx%d\n",
+ s->mb_x, s->mb_y);
+ return -1;
+ }
+ j = scan_table[i];
+ block[j] = level;
+ i++;
+ }
+ s->block_last_index[n] = i - 1;
+ return 0;
+}
+
+static int h261_decode_mb(H261Context *h)
+{
+ MpegEncContext *const s = &h->s;