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lib/deflate_decompress: optimize codeword incrementing

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This commit is contained in:
Eric Biggers 2018-12-25 21:29:13 -06:00
parent a25f3b86d7
commit 1a3f34eab9
1 changed files with 17 additions and 13 deletions
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30
lib/deflate_decompress.c
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@ -729,22 +729,26 @@ build_decode_table(u32 decode_table[],
i += increment;
} while (i < end);
/* Advance to the next codeword by incrementing it. But since
* our codewords are bit-reversed, we must manipulate the bits
* ourselves rather than simply adding 1. */
bit = 1U << (codeword_len - 1);
while (codeword_reversed & bit)
bit >>= 1;
/* Advance to the next symbol and codeword */
if (++sym_idx == num_syms)
return true;
/*
* Increment the codeword, bit-reversed: find the last (highest
* order) 0 bit in the codeword, set it, and clear any later
* (higher order) bits.
*/
bit = 1U << bsr32(~codeword_reversed &
((1U << codeword_len) - 1));
codeword_reversed &= bit - 1;
codeword_reversed |= bit;
/* Advance to the next symbol. This will either increase the
* codeword length, or keep the same codeword length but
* increase the symbol value. Note: since we are using
* bit-reversed codewords, we don't need to explicitly append
* zeroes to the codeword when the codeword length increases. */
if (++sym_idx == num_syms)
return true;
/*
* If there are no more codewords of this length, proceed to the
* next lowest used length. Increasing the length logically
* appends 0's to the codeword, but this is a no-op due to the
* codeword being represented in bit-reversed form.
*/
len_counts[codeword_len]--;
while (len_counts[codeword_len] == 0)
codeword_len++;