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| author | Andreas Rheinhardt <andreas.rheinhardt@outlook.com> | 2024-04-08 02:49:03 +0200 |
|---|---|---|
| committer | Andreas Rheinhardt <andreas.rheinhardt@outlook.com> | 2024-04-11 12:53:26 +0200 |
| commit | 0458fdbeeda4e82986a00febebc519aadbb75e3d (patch) | |
| tree | ca93a3deaa0727c7267523ec974ce94b70ffbecc /libavcodec/ac3enc.c | |
| parent | 28ffc39808f6e203291430515b22b7b6d1755329 (diff) | |
| download | ffmpeg-0458fdbeeda4e82986a00febebc519aadbb75e3d.tar.gz | |
avcodec/ac3enc: Avoid calculating the CRC multiple times
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Diffstat (limited to 'libavcodec/ac3enc.c')
| -rw-r--r-- | libavcodec/ac3enc.c | 17 |
1 files changed, 9 insertions, 8 deletions
diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c index b74ea6b2e7..1ba4ba549e 100644 --- a/libavcodec/ac3enc.c +++ b/libavcodec/ac3enc.c @@ -1942,7 +1942,7 @@ static unsigned int pow_poly(unsigned int a, unsigned int n, unsigned int poly) static void output_frame_end(AC3EncodeContext *s) { const AVCRC *crc_ctx = av_crc_get_table(AV_CRC_16_ANSI); - int frame_size_58, pad_bytes, crc1, crc2_partial, crc2, crc_inv; + int frame_size_58, pad_bytes, crc1, crc2, crc_inv; uint8_t *frame; frame_size_58 = ((s->frame_size >> 2) + (s->frame_size >> 4)) << 1; @@ -1958,7 +1958,7 @@ static void output_frame_end(AC3EncodeContext *s) if (s->eac3) { /* compute crc2 */ - crc2_partial = av_crc(crc_ctx, 0, frame + 2, s->frame_size - 5); + crc2 = av_crc(crc_ctx, 0, frame + 2, s->frame_size - 4); } else { /* compute crc1 */ /* this is not so easy because it is at the beginning of the data... */ @@ -1968,16 +1968,17 @@ static void output_frame_end(AC3EncodeContext *s) AV_WB16(frame + 2, crc1); /* compute crc2 */ - crc2_partial = av_crc(crc_ctx, 0, frame + frame_size_58, - s->frame_size - frame_size_58 - 3); + crc2 = av_crc(crc_ctx, 0, frame + frame_size_58, + s->frame_size - frame_size_58 - 2); } - crc2 = av_crc(crc_ctx, crc2_partial, frame + s->frame_size - 3, 1); + crc2 = av_bswap16(crc2); /* ensure crc2 does not match sync word by flipping crcrsv bit if needed */ - if (crc2 == 0x770B) { + if (crc2 == 0x0B77) { + /* The CRC generator polynomial is x^16 + x^15 + x^2 + 1, + * so xor'ing with 0x18005 does not affect the CRC. */ frame[s->frame_size - 3] ^= 0x1; - crc2 = av_crc(crc_ctx, crc2_partial, frame + s->frame_size - 3, 1); + crc2 ^= 0x8005; } - crc2 = av_bswap16(crc2); AV_WB16(frame + s->frame_size - 2, crc2); } |