From c0e9a229b65f1c5ba8c8bd8789266af3d28f79ea Mon Sep 17 00:00:00 2001 From: Andreas Rheinhardt Date: Mon, 8 Apr 2024 02:49:03 +0200 Subject: [PATCH] avcodec/ac3enc: Avoid calculating the CRC multiple times Signed-off-by: Andreas Rheinhardt Signed-off-by: Paul B Mahol --- libavcodec/ac3enc.c | 21 +++++++++++---------- 1 file changed, 11 insertions(+), 10 deletions(-) diff --git a/libavcodec/ac3enc.c b/libavcodec/ac3enc.c index d66f4136ec..7166e04757 100644 --- a/libavcodec/ac3enc.c +++ b/libavcodec/ac3enc.c @@ -1941,7 +1941,7 @@ 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; @@ -1957,7 +1957,7 @@ 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... */ @@ -1967,16 +1967,17 @@ 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, crc2_partial, frame + s->frame_size - 3, 1); - /* ensure crc2 does not match sync word by flipping crcrsv bit if needed */ - if (crc2 == 0x770B) { - frame[s->frame_size - 3] ^= 0x1; - crc2 = av_crc(crc_ctx, crc2_partial, frame + s->frame_size - 3, 1); + crc2 = av_crc(crc_ctx, 0, frame + frame_size_58, + s->frame_size - frame_size_58 - 2); } crc2 = av_bswap16(crc2); + /* ensure crc2 does not match sync word by flipping crcrsv bit if needed */ + 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 ^= 0x8005; + } AV_WB16(frame + s->frame_size - 2, crc2); }