conditional pattern analysis

Pattern analysis (currently limited to long ranges of identical bytes)
is actually detrimental to performance
when `nbSearches` is low.

Reason is : `nbSearches` provides a built-in protection for these cases.
The problem with patterns is that they dramatically increase the number of candidates to visit.
But with a low nbSearches, the match finder just aborts early.

In such cases, pattern analysis adds some complexity without reducing total nb of candidates.
It actually increases compression ratio a little bit, by filtering only "good" candidates,
but at a measurable speed cost, so it's not a good trade-off.

This patch makes pattern analysis optional.
It's enabled for levels 8+ only.

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/lz4opt.h b/lib/lz4opt.h
index 9917851..6c15598 100644
--- a/lib/lz4opt.h
+++ b/lib/lz4opt.h
@@ -85,7 +85,9 @@ LZ4HC_match_t LZ4HC_FindLongerMatch(LZ4HC_CCtx_internal* const ctx,
     /* note : LZ4HC_InsertAndGetWiderMatch() is able to modify the starting position of a match (*startpos),
      * but this won't be the case here, as we define iLowLimit==ip,
      * so LZ4HC_InsertAndGetWiderMatch() won't be allowed to search past ip */
-    int const matchLength = LZ4HC_InsertAndGetWiderMatch(ctx, ip, ip, iHighLimit, minLen, &matchPtr, &ip, nbSearches);
+    int const matchLength = LZ4HC_InsertAndGetWiderMatch(ctx,
+                                ip, ip, iHighLimit, minLen, &matchPtr, &ip,
+                                nbSearches, 1 /* patternAnalysis */);
     if (matchLength <= minLen) return match;
     match.len = matchLength;
     match.off = (int)(ip-matchPtr);