[PR] avcodec/ac3dec: fix dual-mono program metadata order (PR #24251)

PR #24251 opened by AYOUB NABIL BOUBAGRAT (ayoubnabil)
URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24251
Patch URL: https://code.ffmpeg.org/FFmpeg/FFmpeg/pulls/24251.patch

dual-mono dynamic range values were stored in reverse order to match a
reversed output lookup. that lookup also selects dialnorm and compression
data, which are already stored in program order.

store and apply all per-program metadata in bitstream order.



>From ed7f19d1a5b147ecf42e3b5b307ccfa42070872e Mon Sep 17 00:00:00 2001
From: Ayoub Nabil Boubagrat <[email protected]>
Date: Sun, 23 Aug 2026 21:50:47 +0200
Subject: [PATCH] avcodec/ac3dec: fix dual-mono program metadata order

dual-mono dynamic range values were stored in reverse order to match a
reversed output lookup. that lookup also selects dialnorm and compression
data, which are already stored in program order.

store and apply all per-program metadata in bitstream order.

Signed-off-by: Ayoub Nabil Boubagrat <[email protected]>
---
 libavcodec/ac3dec.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/libavcodec/ac3dec.c b/libavcodec/ac3dec.c
index 79a8751154..1091759991 100644
--- a/libavcodec/ac3dec.c
+++ b/libavcodec/ac3dec.c
@@ -1035,7 +1035,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk, int offset)
     }
 
     /* dynamic range */
-    i = !s->channel_mode;
+    i = 0;
     do {
         if (get_bits1(gbc)) {
             /* Allow asymmetric application of DRC when drc_scale > 1.
@@ -1049,7 +1049,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk, int offset)
         } else if (blk == 0) {
             s->dynamic_range[i] = AC3_DYNAMIC_RANGE1;
         }
-    } while (i--);
+    } while (++i < (s->channel_mode ? 1 : 2));
 
     /* spectral extension strategy */
     if (s->eac3 && (!blk || get_bits1(gbc))) {
@@ -1333,7 +1333,7 @@ static int decode_audio_block(AC3DecodeContext *s, int blk, int offset)
         int audio_channel = 0;
         INTFLOAT gain;
         if (s->channel_mode == AC3_CHMODE_DUALMONO && ch <= 2)
-            audio_channel = 2-ch;
+            audio_channel = ch-1;
         if (s->heavy_compression && s->compression_exists[audio_channel])
             gain = s->heavy_dynamic_range[audio_channel];
         else
-- 
2.52.0

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