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[decode.c] use binary search in Log2Floor

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Log2Floor implementation uses lineal search to find the floor of the log2, doing a binary search is slightly faster.

Because Log2Floor is only called from ReadHuffmanCode and the parameter passed is always less than 1024, l16_Log2Floor does a binary search in the lower 16 bits of the number, thus, in only 3 loops it gets the result
This commit is contained in:
Nicolas De Carli 2015-10-12 22:56:50 -03:00 committed by Nicoshev11
parent 1886f77ac7
commit 5e59adfc30
1 changed files with 25 additions and 1 deletions
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26
dec/decode.c
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@ -331,6 +331,30 @@ static BROTLI_INLINE uint32_t Log2Floor(uint32_t x) {
return result;
}
/* Calculates the Floor of Log2(x) using a binary search in the lower 16 bits,
Precondition: x<2**16
*/
static BROTLI_INLINE uint32_t l16_Log2Floor(uint32_t x) {
uint32_t result = 8;
uint32_t mask = 0x0000FF00;
uint32_t i = 4;
if (x == 0)
return 0;
while (i){
if (mask & x){
result += i;
mask <<= i;
}else{
result -= i;
mask >>= i;
}
i /= 2;
}
if (x & mask)
result++;
return result;
}
/* Decodes the Huffman tables.
There are 2 scenarios:
A) Huffman code contains only few symbols (1..4). Those symbols are read
@ -364,7 +388,7 @@ static BrotliResult ReadHuffmanCode(uint32_t alphabet_size,
BROTLI_LOG_UINT(i);
if (i == 1) {
/* Read symbols, codes & code lengths directly. */
uint32_t max_bits = Log2Floor(alphabet_size - 1);
uint32_t max_bits = l16_Log2Floor(alphabet_size - 1);
uint32_t num_symbols = BrotliReadBits(br, 2);
for (i = 0; i < 4; ++i) {
s->symbols_lists_array[i] = 0;