ddidderr/zstd-rs
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first implementation of small window size for btopt

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noticeably improves compression ratio
when window size is small (< 18).

enwik7	level 19

windowLog	`dev`	`smallwlog`	improvement
23	3.577	3.577	0.02%
22	3.536	3.538	0.06%
21	3.462	3.467	0.14%
20	3.364	3.377	0.39%
19	3.244	3.272	0.86%
18	3.110	3.166	1.80%
17	2.843	3.057	7.53%
16	2.724	2.943	8.04%
15	2.594	2.822	8.79%
14	2.456	2.686	9.36%
13	2.312	2.523	9.13%
12	2.162	2.361	9.20%
11	2.003	2.182	8.94%
This commit is contained in:
Yann Collet
2019-05-31 15:55:12 -07:00
parent b13a9207f9
commit bc601bdc6d
3 changed files with 42 additions and 8 deletions
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lib/compress/zstd_opt.c
+4 -2
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@@ -545,6 +545,7 @@ U32 ZSTD_insertBtAndGetAllMatches (
{
const ZSTD_compressionParameters* const cParams = &ms->cParams;
U32 const sufficient_len = MIN(cParams->targetLength, ZSTD_OPT_NUM -1);
U32 const maxDistance = 1U << cParams->windowLog;
const BYTE* const base = ms->window.base;
U32 const current = (U32)(ip-base);
U32 const hashLog = cParams->hashLog;
@@ -560,8 +561,9 @@ U32 ZSTD_insertBtAndGetAllMatches (
U32 const dictLimit = ms->window.dictLimit;
const BYTE* const dictEnd = dictBase + dictLimit;
const BYTE* const prefixStart = base + dictLimit;
U32 const btLow = btMask >= current ? 0 : current - btMask;
U32 const windowLow = ms->window.lowLimit;
U32 const btLow = (btMask >= current) ? 0 : current - btMask;
U32 const windowValid = ms->window.lowLimit;
U32 const windowLow = ((current - windowValid) > maxDistance) ? current - maxDistance : windowValid;
U32 const matchLow = windowLow ? windowLow : 1;
U32* smallerPtr = bt + 2*(current&btMask);
U32* largerPtr = bt + 2*(current&btMask) + 1;