Precompute fixed huffman tables #39
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Incidentally, zlib has analogous decoding tables hard-coded in its source code as well: https://github.com/madler/zlib/blob/ef24c4c7502169f016dcd2a26923dbaf3216748c/inffixed.h |
This was referenced Dec 1, 2024
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This PR switches to using precomputed decoding tables for fixed huffman blocks. Because fixed blocks have all literal/length symbols between 7-9 bits, a 12-bit decoding table is actually just a 9-bit decoding table repeated 8 times. Similarly the distance codes are all 5-bits so 16 repetitions of a 5-bit table are used.
Also switched the decoding tables to use Box instead of raw arrays.
I tried other options to shrink the size of decoding tables for fixed blocks, but having two code paths depending on table size was slower. Making the decoding tables
Vecs rather than boxed slices (so they didn't have to be zero-initialized for the fixed-block case) also didn't help performance.