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Added fast path for bitreader

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This commit is contained in:
Dan Hirsch 2012-05-01 01:45:45 +01:00
parent bc51198354
commit ddcfa5b1b2
1 changed files with 46 additions and 33 deletions
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79
src/bitreader.c
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@ -12,41 +12,54 @@ long long read_bits(input_stream_t* state, int count, char signed_p) {
long long out = 0;
int offset = 0;
long long msb = (!!signed_p) << (count - 1); // 0 if unsigned, else 1 << (nbits - 1)
while (count) {
int segment, segment_len;
// Read a segment...
if (state->endianness & BIT_BIG_ENDIAN) {
if (count >= state->bit_offset) {
segment_len = state->bit_offset;
state->bit_offset = 8;
segment = state->input[state->index] & ((1 << segment_len) - 1);
state->index++;
} else {
segment_len = count;
state->bit_offset -= count;
segment = (state->input[state->index] >> state->bit_offset) & ((1 << segment_len) - 1);
}
} else { // BIT_LITTLE_ENDIAN
if (count + state->bit_offset >= 8) {
segment_len = 8 - state->bit_offset;
segment = (state->input[state->index] >> state->bit_offset);
state->index++;
state->bit_offset = 0;
} else {
segment_len = count;
segment = (state->input[state->index] >> state->bit_offset) & ((1 << segment_len) - 1);
state->bit_offset += segment_len;
}
}
// have a valid segment; time to assemble the byte
if ((state->bit_offset & 0x7) == 0 && (count & 0x7) == 0) {
// fast path
if (state->endianness & BYTE_BIG_ENDIAN) {
out = out << segment_len | segment;
} else { // BYTE_LITTLE_ENDIAN
out |= segment << offset;
offset += segment_len;
while (count > 0)
out = (out << 8) | state->input[state->index++];
} else {
while (count > 0) {
count -= 8;
out |= state->input[state->index++] << count;
}
}
} else {
while (count) {
int segment, segment_len;
// Read a segment...
if (state->endianness & BIT_BIG_ENDIAN) {
if (count >= state->bit_offset) {
segment_len = state->bit_offset;
state->bit_offset = 8;
segment = state->input[state->index] & ((1 << segment_len) - 1);
state->index++;
} else {
segment_len = count;
state->bit_offset -= count;
segment = (state->input[state->index] >> state->bit_offset) & ((1 << segment_len) - 1);
}
} else { // BIT_LITTLE_ENDIAN
if (count + state->bit_offset >= 8) {
segment_len = 8 - state->bit_offset;
segment = (state->input[state->index] >> state->bit_offset);
state->index++;
state->bit_offset = 0;
} else {
segment_len = count;
segment = (state->input[state->index] >> state->bit_offset) & ((1 << segment_len) - 1);
state->bit_offset += segment_len;
}
}
// have a valid segment; time to assemble the byte
if (state->endianness & BYTE_BIG_ENDIAN) {
out = out << segment_len | segment;
} else { // BYTE_LITTLE_ENDIAN
out |= segment << offset;
offset += segment_len;
}
count -= segment_len;
}
count -= segment_len;
}
return (out ^ msb) - msb; // perform sign extension
}