Limit the Rice parameter used for progressive decoding in ALS.

Originally committed as revision 21849 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/alsdec.c

@@ -192,6 +192,7 @@ typedef struct {
     unsigned int frame_id;          ///< the frame ID / number of the current frame
     unsigned int js_switch;         ///< if true, joint-stereo decoding is enforced
     unsigned int num_blocks;        ///< number of blocks used in the current frame
+    unsigned int s_max;             ///< maximum Rice parameter allowed in entropy coding
     uint8_t *bgmc_lut;              ///< pointer at lookup tables used for BGMC
     unsigned int *bgmc_lut_status;  ///< pointer at lookup table status flags used for BGMC
     int ltp_lag_length;             ///< number of bits used for ltp lag value
@@ -720,9 +721,9 @@ static int read_var_block_data(ALSDecContext *ctx, ALSBlockData *bd)
         if (opt_order)
             bd->raw_samples[0] = decode_rice(gb, avctx->bits_per_raw_sample - 4);
         if (opt_order > 1)
-            bd->raw_samples[1] = decode_rice(gb, s[0] + 3);
+            bd->raw_samples[1] = decode_rice(gb, FFMIN(s[0] + 3, ctx->s_max));
         if (opt_order > 2)
-            bd->raw_samples[2] = decode_rice(gb, s[0] + 1);
+            bd->raw_samples[2] = decode_rice(gb, FFMIN(s[0] + 1, ctx->s_max));
 
         start = FFMIN(opt_order, 3);
     }
@@ -1508,6 +1509,11 @@ static av_cold int decode_init(AVCodecContext *avctx)
         avctx->bits_per_raw_sample = (sconf->resolution + 1) * 8;
     }
 
+    // set maximum Rice parameter for progressive decoding based on resolution
+    // This is not specified in 14496-3 but actually done by the reference
+    // codec RM22 revision 2.
+    ctx->s_max = sconf->resolution > 1 ? 31 : 15;
+
     // set lag value for long-term prediction
     ctx->ltp_lag_length = 8 + (avctx->sample_rate >=  96000) +
                               (avctx->sample_rate >= 192000);