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Better decisions for partial matches

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- Ratio of recompressed ratio to decompressed ratio is a good indicator
This commit is contained in:
Christian Schneider 2017-08-24 07:09:58 +02:00
parent d4b0a8e58a
commit cfa602c1ce
1 changed files with 26 additions and 11 deletions
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37
precomp.cpp
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@ -6092,18 +6092,33 @@ void try_recompress(FILE* origfile, int comp_level, int mem_level, int windowbit
}
final_compression_found = (identical_bytes_decomp == decomp_bytes_total) && (identical_bytes == compressed_stream_size) && (penalty_bytes_len < PENALTY_BYTES_TOLERANCE);
}
best_identical_bytes_decomp = identical_bytes_decomp;
best_identical_bytes = identical_bytes;
best_compression = comp_level;
best_mem_level = mem_level;
best_windowbits = windowbits;
if (penalty_bytes_len > 0) {
memcpy(best_penalty_bytes, penalty_bytes, penalty_bytes_len);
best_penalty_bytes_len = penalty_bytes_len;
} else {
best_penalty_bytes_len = 0;
// Partial matches sometimes need all the decompressed bytes, but there are much less
// identical recompressed bytes - in these cases, all the decompressed bytes have to
// be stored together with the remaining recompressed bytes, so the result won't compress
// better than the original stream. What's important here is the ratio between recompressed ratio
// and decompressed ratio that shouldn't get too high.
// Example: A stream has 5 of 1000 identical recompressed bytes, but needs 1000 of 1000 decompressed bytes,
// so the ratio is (1000/1000)/(5/1000) = 200 which is too high. With 5 of 1000 decompressed bytes or
// 1000 of 1000 identical recompressed bytes, ratio would've been 1 and we'd accept it.
float partial_ratio = ((float)identical_bytes_decomp / decomp_bytes_total) / ((float)identical_bytes / compressed_stream_size);
if (partial_ratio < 3.0f) {
best_identical_bytes_decomp = identical_bytes_decomp;
best_identical_bytes = identical_bytes;
best_compression = comp_level;
best_mem_level = mem_level;
best_windowbits = windowbits;
if (penalty_bytes_len > 0) {
memcpy(best_penalty_bytes, penalty_bytes, penalty_bytes_len);
best_penalty_bytes_len = penalty_bytes_len;
} else {
best_penalty_bytes_len = 0;
}
} else {
if (DEBUG_MODE) {
printf("Not enough identical recompressed bytes\n");
}
}
}
}
}