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fixed: compression ratio discrepancy

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depending on initialization,
the first byte of a new frame was invalidated or not.

As a consequence, one match opportunity was available or not,
resulting in slightly different compressed sizes
(on average, 1 or 2 bytes once every 20 frames).

It impacted ratio comparison between one-shot and streaming modes.

This fix makes the first byte of a new frame always a valid match.
Now compressed size is always the same.
It also improves compressed size by a negligible amount.
This commit is contained in:
Yann Collet
2018-12-19 10:11:06 -08:00
parent d0e15f8d32
commit 8e0e495ce8
3 changed files with 48 additions and 33 deletions
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lib/compress/zstd_compress_internal.h
+12 -9
View File
@@ -679,16 +679,19 @@ MEM_STATIC U32 ZSTD_window_correctOverflow(ZSTD_window_t* window, U32 cycleLog,
* dictMatchState mode, lowLimit and dictLimit are the same, and the dictionary
* is below them. forceWindow and dictMatchState are therefore incompatible.
*/
MEM_STATIC void ZSTD_window_enforceMaxDist(ZSTD_window_t* window,
void const* srcEnd, U32 maxDist,
U32* loadedDictEndPtr,
const ZSTD_matchState_t** dictMatchStatePtr)
MEM_STATIC void
ZSTD_window_enforceMaxDist(ZSTD_window_t* window,
void const* srcEnd,
U32 maxDist,
U32* loadedDictEndPtr,
const ZSTD_matchState_t** dictMatchStatePtr)
{
U32 const current = (U32)((BYTE const*)srcEnd - window->base);
U32 loadedDictEnd = loadedDictEndPtr != NULL ? *loadedDictEndPtr : 0;
DEBUGLOG(5, "ZSTD_window_enforceMaxDist: current=%u, maxDist=%u", current, maxDist);
if (current > maxDist + loadedDictEnd) {
U32 const newLowLimit = current - maxDist;
U32 const blockEndIdx = (U32)((BYTE const*)srcEnd - window->base);
U32 loadedDictEnd = (loadedDictEndPtr != NULL) ? *loadedDictEndPtr : 0;
DEBUGLOG(5, "ZSTD_window_enforceMaxDist: blockEndIdx=%u, maxDist=%u",
blockEndIdx, maxDist);
if (blockEndIdx > maxDist + loadedDictEnd) {
U32 const newLowLimit = blockEndIdx - maxDist;
if (window->lowLimit < newLowLimit) window->lowLimit = newLowLimit;
if (window->dictLimit < window->lowLimit) {
DEBUGLOG(5, "Update dictLimit to match lowLimit, from %u to %u",