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avcodec/ac3enc: Avoid calculating the CRC multiple times

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Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>
Signed-off-by: Paul B Mahol <onemda@gmail.com>
This commit is contained in:
Andreas Rheinhardt 2024-04-08 02:49:03 +02:00 committed by Paul B Mahol
parent cdb00f9bc7
commit c0e9a229b6
1 changed files with 11 additions and 10 deletions
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21
libavcodec/ac3enc.c
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@ -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);
}