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slightly improved compression ratio at levels 3 & 4

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The compression ratio benefits are small but consistent, i.e. always positive.
On `silesia.tar` corpus, this modification saves ~75 KB at level 3.
The measured speed cost is negligible, i.e. below noise level, between 0 and -1%.
This commit is contained in:
Yann Collet
2024-10-17 09:37:23 -07:00
parent 18a42190c2
commit 6326775166
1 changed files with 13 additions and 9 deletions
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lib/compress/zstd_double_fast.c
+13 -9
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@@ -252,19 +252,23 @@ _cleanup:
_search_next_long:
/* check prefix long +1 match */
/* short match found: let's check for a longer one */
mLength = ZSTD_count(ip+4, matchs0+4, iend) + 4;
/* check long match at +1 position */
if (idxl1 > prefixLowestIndex) {
if (MEM_read64(matchl1) == MEM_read64(ip1)) {
ip = ip1;
mLength = ZSTD_count(ip+8, matchl1+8, iend) + 8;
offset = (U32)(ip-matchl1);
while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
goto _match_found;
}
size_t const llen = ZSTD_count(ip1+8, matchl1+8, iend) + 8;
if (llen > mLength) {
ip = ip1;
mLength = llen;
offset = (U32)(ip-matchl1);
while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
goto _match_found;
} }
}
/* if no long +1 match, explore the short match we found */
mLength = ZSTD_count(ip+4, matchs0+4, iend) + 4;
/* validate short match previously found */
offset = (U32)(ip - matchs0);
while (((ip>anchor) & (matchs0>prefixLowest)) && (ip[-1] == matchs0[-1])) { ip--; matchs0--; mLength++; } /* catch up */