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avcodec/dca_core: probe extension headers directly

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Avoid using bitstream reader in a non-standard way by directly accessing
index. Use bit shifting/masking operations instead.

Signed-off-by: James Almer <jamrial@gmail.com>
pull/265/head
foo86 8 years ago committed by James Almer
parent 07a7f08b1e
commit 902cd30e9b
1 changed files with 16 additions and 15 deletions
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  1. 31
      libavcodec/dca_core.c

31
libavcodec/dca_core.c
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@ -1704,6 +1704,7 @@ static int parse_optional_info(DCACoreDecoder *s)
int sync_pos = FFMIN(s->frame_size / 4, s->gb.size_in_bits / 32) - 1;
int last_pos = get_bits_count(&s->gb) / 32;
int size, dist;
uint32_t w1, w2 = 0;
// Search for extension sync words aligned on 4-byte boundary. Search
// must be done backwards from the end of core frame to work around
@ -1718,15 +1719,15 @@ static int parse_optional_info(DCACoreDecoder *s)
// compatibility with legacy bitstreams. Minimum XCH frame size is
// 96 bytes. AMODE and PCHS are further checked to reduce
// probability of alias sync detection.
for (; sync_pos >= last_pos; sync_pos--) {
if (AV_RB32(s->gb.buffer + sync_pos * 4) == DCA_SYNCWORD_XCH) {
s->gb.index = (sync_pos + 1) * 32;
size = get_bits(&s->gb, 10) + 1;
for (; sync_pos >= last_pos; sync_pos--, w2 = w1) {
w1 = AV_RB32(s->gb.buffer + sync_pos * 4);
if (w1 == DCA_SYNCWORD_XCH) {
size = (w2 >> 22) + 1;
dist = s->frame_size - sync_pos * 4;
if (size >= 96
&& (size == dist || size - 1 == dist)
&& get_bits(&s->gb, 7) == 0x08) {
s->xch_pos = get_bits_count(&s->gb);
&& (w2 >> 15 & 0x7f) == 0x08) {
s->xch_pos = sync_pos * 32 + 49;
break;
}
}
@ -1743,13 +1744,13 @@ static int parse_optional_info(DCACoreDecoder *s)
// The distance between X96 sync word and end of the core frame
// must be equal to X96 frame size. Minimum X96 frame size is 96
// bytes.
for (; sync_pos >= last_pos; sync_pos--) {
if (AV_RB32(s->gb.buffer + sync_pos * 4) == DCA_SYNCWORD_X96) {
s->gb.index = (sync_pos + 1) * 32;
size = get_bits(&s->gb, 12) + 1;
for (; sync_pos >= last_pos; sync_pos--, w2 = w1) {
w1 = AV_RB32(s->gb.buffer + sync_pos * 4);
if (w1 == DCA_SYNCWORD_X96) {
size = (w2 >> 20) + 1;
dist = s->frame_size - sync_pos * 4;
if (size >= 96 && size == dist) {
s->x96_pos = get_bits_count(&s->gb);
s->x96_pos = sync_pos * 32 + 44;
break;
}
}
@ -1768,10 +1769,10 @@ static int parse_optional_info(DCACoreDecoder *s)
// XXCH frame header CRC must be valid. Minimum XXCH frame header
// size is 11 bytes.
for (; sync_pos >= last_pos; sync_pos--) {
if (AV_RB32(s->gb.buffer + sync_pos * 4) == DCA_SYNCWORD_XXCH) {
s->gb.index = (sync_pos + 1) * 32;
size = get_bits(&s->gb, 6) + 1;
for (; sync_pos >= last_pos; sync_pos--, w2 = w1) {
w1 = AV_RB32(s->gb.buffer + sync_pos * 4);
if (w1 == DCA_SYNCWORD_XXCH) {
size = (w2 >> 26) + 1;
dist = s->gb.size_in_bits / 8 - sync_pos * 4;
if (size >= 11 && size <= dist &&
!av_crc(dca->crctab, 0xffff, s->gb.buffer +

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