ac3dec: Move center&surround mix level tables to parser.

That way all mix levels as exported by the parser
will have the same meaning.

Previously the 3bit center mix level for eac3 was
used to index in a 4 entry table leading to out of array reads.
this change removes the table and offsets the ac3 variable by 4
so it matches the meanings for eac3 except the reserved case.
The reserved case is then explicitly handled.

Idea-by: Justin Ruggles <justin.ruggles@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/ac3_parser.c

@@ -34,6 +34,18 @@ static const uint8_t eac3_blocks[4] = {
     1, 2, 3, 6
 };
 
+/**
+ * Table for center mix levels
+ * reference: Section 5.4.2.4 cmixlev
+ */
+static const uint8_t center_levels[4] = { 4, 5, 6, 5 };
+
+/**
+ * Table for surround mix levels
+ * reference: Section 5.4.2.5 surmixlev
+ */
+static const uint8_t surround_levels[4] = { 4, 6, 7, 6 };
+
 
 int avpriv_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
 {
@@ -53,8 +65,8 @@ int avpriv_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
     hdr->num_blocks = 6;
 
     /* set default mix levels */
-    hdr->center_mix_level   = 1;  // -4.5dB
-    hdr->surround_mix_level = 1;  // -6.0dB
+    hdr->center_mix_level   = 5;  // -4.5dB
+    hdr->surround_mix_level = 6;  // -6.0dB
 
     if(hdr->bitstream_id <= 10) {
         /* Normal AC-3 */
@@ -76,9 +88,9 @@ int avpriv_ac3_parse_header(GetBitContext *gbc, AC3HeaderInfo *hdr)
             skip_bits(gbc, 2); // skip dsurmod
         } else {
             if((hdr->channel_mode & 1) && hdr->channel_mode != AC3_CHMODE_MONO)
-                hdr->center_mix_level = get_bits(gbc, 2);
+                hdr->  center_mix_level =   center_levels[get_bits(gbc, 2)];
             if(hdr->channel_mode & 4)
-                hdr->surround_mix_level = get_bits(gbc, 2);
+                hdr->surround_mix_level = surround_levels[get_bits(gbc, 2)];
         }
         hdr->lfe_on = get_bits1(gbc);
 

libavcodec/ac3dec.c

@@ -76,18 +76,6 @@ static const float gain_levels[9] = {
     LEVEL_MINUS_9DB
 };
 
-/**
- * Table for center mix levels
- * reference: Section 5.4.2.4 cmixlev
- */
-static const uint8_t center_levels[4] = { 4, 5, 6, 5 };
-
-/**
- * Table for surround mix levels
- * reference: Section 5.4.2.5 surmixlev
- */
-static const uint8_t surround_levels[4] = { 4, 6, 7, 6 };
-
 /**
  * Table for default stereo downmixing coefficients
  * reference: Section 7.8.2 Downmixing Into Two Channels
@@ -320,8 +308,8 @@ static int parse_frame_header(AC3DecodeContext *s)
 static void set_downmix_coeffs(AC3DecodeContext *s)
 {
     int i;
-    float cmix = gain_levels[center_levels[s->center_mix_level]];
-    float smix = gain_levels[surround_levels[s->surround_mix_level]];
+    float cmix = gain_levels[s->  center_mix_level];
+    float smix = gain_levels[s->surround_mix_level];
     float norm0, norm1;
 
     for (i = 0; i < s->fbw_channels; i++) {
@@ -1400,8 +1388,8 @@ static int ac3_decode_frame(AVCodecContext * avctx, void *data,
         avctx->channels       = s->out_channels;
         avctx->channel_layout = s->channel_layout;
 
-        s->loro_center_mix_level   = gain_levels[  center_levels[s->  center_mix_level]];
-        s->loro_surround_mix_level = gain_levels[surround_levels[s->surround_mix_level]];
+        s->loro_center_mix_level   = gain_levels[s->  center_mix_level];
+        s->loro_surround_mix_level = gain_levels[s->surround_mix_level];
         s->ltrt_center_mix_level   = LEVEL_MINUS_3DB;
         s->ltrt_surround_mix_level = LEVEL_MINUS_3DB;
         /* set downmixing coefficients if needed */