factored the logic selecting lowest match index

as suggested by @terrelln

lib/compress/zstd_compress_internal.h

@@ -136,6 +136,8 @@ struct ZSTD_matchState_t {
     ZSTD_window_t window;   /* State for window round buffer management */
     U32 loadedDictEnd;      /* index of end of dictionary, within context's referential.
                              * When loadedDictEnd != 0, a dictionary is in use, and still valid.
+                             * This relies on a mechanism to set loadedDictEnd=0 when dictionary is no longer within distance.
+                             * Such mechanism is provided within ZSTD_window_enforceMaxDist() and ZSTD_checkDictValidity().
                              * When dict referential is copied into active context (i.e. not attached),
                              * loadedDictEnd == dictSize, since referential starts from zero.
                              */
@@ -354,7 +356,7 @@ MEM_STATIC void ZSTD_storeSeq(seqStore_t* seqStorePtr, size_t litLength, const v
     /* copy Literals */
     assert(seqStorePtr->maxNbLit <= 128 KB);
     assert(seqStorePtr->lit + litLength <= seqStorePtr->litStart + seqStorePtr->maxNbLit);
-    ZSTD_wildcopy(seqStorePtr->lit, literals, litLength, ZSTD_no_overlap);
+    ZSTD_wildcopy(seqStorePtr->lit, literals, (ptrdiff_t)litLength, ZSTD_no_overlap);
     seqStorePtr->lit += litLength;
 
     /* literal Length */
@@ -839,6 +841,17 @@ MEM_STATIC U32 ZSTD_window_update(ZSTD_window_t* window,
     return contiguous;
 }
 
+MEM_STATIC U32 ZSTD_getLowestMatchIndex(const ZSTD_matchState_t* ms, U32 current, unsigned windowLog)
+{
+    U32    const maxDistance = 1U << windowLog;
+    U32    const lowestValid = ms->window.lowLimit;
+    U32    const withinWindow = (current - lowestValid > maxDistance) ? current - maxDistance : lowestValid;
+    U32    const isDictionary = (ms->loadedDictEnd != 0);
+    U32    const matchLowest = isDictionary ? lowestValid : withinWindow;
+    return matchLowest;
+}
+
+
 
 /* debug functions */
 #if (DEBUGLEVEL>=2)

lib/compress/zstd_double_fast.c

@@ -65,6 +65,7 @@ size_t ZSTD_compressBlock_doubleFast_generic(
     const U32 endIndex = (U32)((size_t)(istart - base) + srcSize);
     const U32 lowestValid = ms->window.dictLimit;
     const U32 maxDistance = 1U << cParams->windowLog;
+    /* presumes that, if there is a dictionary, it must be using Attach mode */
     const U32 prefixLowestIndex = (endIndex - lowestValid > maxDistance) ? endIndex - maxDistance : lowestValid;
     const BYTE* const prefixLowest = base + prefixLowestIndex;
     const BYTE* const iend = istart + srcSize;
@@ -369,11 +370,7 @@ static size_t ZSTD_compressBlock_doubleFast_extDict_generic(
     const BYTE* const ilimit = iend - 8;
     const BYTE* const base = ms->window.base;
     const U32   endIndex = (U32)((size_t)(istart - base) + srcSize);
-    const U32   maxDistance = 1U << cParams->windowLog;
+    const U32   lowLimit = ZSTD_getLowestMatchIndex(ms, endIndex, cParams->windowLog);
-    const U32   validLowest = ms->window.lowLimit;
-    const int   isDictionary = (ms->loadedDictEnd != 0);
-    const U32   withinWindow = (endIndex - validLowest > maxDistance) ? endIndex - maxDistance : validLowest;
-    const U32   lowLimit = isDictionary ? validLowest : withinWindow;
     const U32   dictStartIndex = lowLimit;
     const U32   dictLimit = ms->window.dictLimit;
     const U32   prefixStartIndex = (dictLimit > lowLimit) ? dictLimit : lowLimit;

lib/compress/zstd_fast.c

@@ -381,11 +381,7 @@ static size_t ZSTD_compressBlock_fast_extDict_generic(
     const BYTE* ip = istart;
     const BYTE* anchor = istart;
     const U32   endIndex = (U32)((size_t)(istart - base) + srcSize);
-    const U32   maxDistance = 1U << cParams->windowLog;
+    const U32   lowLimit = ZSTD_getLowestMatchIndex(ms, endIndex, cParams->windowLog);
-    const U32   validLowest = ms->window.lowLimit;
-    const int   isDictionary = (ms->loadedDictEnd != 0);
-    const U32   withinWindow = (endIndex - validLowest > maxDistance) ? endIndex - maxDistance : validLowest;
-    const U32   lowLimit = isDictionary ? validLowest : withinWindow;
     const U32   dictStartIndex = lowLimit;
     const BYTE* const dictStart = dictBase + dictStartIndex;
     const U32   dictLimit = ms->window.dictLimit;

lib/compress/zstd_lazy.c

@@ -242,11 +242,7 @@ ZSTD_DUBT_findBestMatch(ZSTD_matchState_t* ms,
 
     const BYTE* const base = ms->window.base;
     U32    const current = (U32)(ip-base);
-    U32    const maxDistance = 1U << cParams->windowLog;
+    U32    const windowLow = ZSTD_getLowestMatchIndex(ms, current, cParams->windowLog);
-    U32    const lowestValid = ms->window.lowLimit;
-    U32    const withinWindow = (current - lowestValid > maxDistance) ? current - maxDistance : lowestValid;
-    U32    const isDictionary = (ms->loadedDictEnd != 0);
-    U32    const windowLow = isDictionary ? lowestValid : withinWindow;
 
     U32*   const bt = ms->chainTable;
     U32    const btLog  = cParams->chainLog - 1;

lib/compress/zstd_opt.c

@@ -552,7 +552,6 @@ 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;
@@ -569,10 +568,7 @@ U32 ZSTD_insertBtAndGetAllMatches (
     const BYTE* const dictEnd = dictBase + dictLimit;
     const BYTE* const prefixStart = base + dictLimit;
     U32 const btLow = (btMask >= current) ? 0 : current - btMask;
-    U32 const lowestValid = ms->window.lowLimit;
+    U32 const windowLow = ZSTD_getLowestMatchIndex(ms, current, cParams->windowLog);
-    U32 const withinWindow = (current - lowestValid > maxDistance) ? current - maxDistance : lowestValid;
-    U32 const isDictionary = (ms->loadedDictEnd != 0);
-    U32 const windowLow = isDictionary ? lowestValid : withinWindow;
     U32 const matchLow = windowLow ? windowLow : 1;
     U32* smallerPtr = bt + 2*(current&btMask);
     U32* largerPtr  = bt + 2*(current&btMask) + 1;