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https://github.com/cuberite/libdeflate.git
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3dca7de4bd
Further improve the way the near-optimal parser estimates symbol costs: - When setting a block's initial costs, weigh the default costs and previous block's costs differently, depending on how different the current block seems to be from the previous block. - When determining the "default" costs, take into account how many literals appear in the block and how frequent matches seem to be. - Increase BIT_COST from 8 to 16, to increase precision in calculations.
45 lines
1.2 KiB
Python
Executable File
45 lines
1.2 KiB
Python
Executable File
#!/usr/bin/env python3
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#
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# This script computes the default litlen symbol costs for the near-optimal
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# parser.
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from math import log2
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BIT_COST = 16 # Must match BIT_COST in deflate_compress.c
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NUM_LEN_SLOTS = 29
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print("""static const struct {
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u8 used_lits_to_lit_cost[257];
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u8 len_sym_cost;
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} default_litlen_costs[] = {""")
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MATCH_PROBS = [0.25, 0.50, 0.75]
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for i, match_prob in enumerate(MATCH_PROBS):
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len_prob = match_prob / NUM_LEN_SLOTS
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len_sym_cost = int(-log2(len_prob) * BIT_COST)
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if i == 0:
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print('\t{', end='')
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print(f' /* match_prob = {match_prob} */')
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print('\t\t.used_lits_to_lit_cost = {')
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j = 0
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for num_used_literals in range(0, 257):
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if num_used_literals == 0:
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num_used_literals = 1
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lit_prob = (1 - match_prob) / num_used_literals
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lit_cost = int(-log2(lit_prob) * BIT_COST)
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if j == 0:
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print('\t\t\t', end='')
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if j == 7 or num_used_literals == 256:
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print(f'{lit_cost},')
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j = 0
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else:
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print(f'{lit_cost}, ', end='')
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j += 1
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print('\t\t},')
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print(f'\t\t.len_sym_cost = {len_sym_cost},')
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if i < len(MATCH_PROBS) - 1:
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print('\t}, {', end='')
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else:
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print('\t},')
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print('};')
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