From: W. Felix Handte <w@felixhandte.com>
Date: Thu, 20 Aug 2020 15:40:47 +0000 (-0400)
Subject: Simplify DDS Hash Table Construction
X-Git-Tag: v1.4.7~81^2~12
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=9e83c782f828f5a2f2ba6c48af74caf12be6c4f9;p=thirdparty%2Fzstd.git

Simplify DDS Hash Table Construction

No need to walk the chainTable; we can just keep shifting the entries in the
hashTable.
---

diff --git a/lib/compress/zstd_lazy.c b/lib/compress/zstd_lazy.c
index add107442..d8ed30a93 100644
--- a/lib/compress/zstd_lazy.c
+++ b/lib/compress/zstd_lazy.c
@@ -481,17 +481,22 @@ void ZSTD_dedicatedDictSearch_lazy_loadDictionary(ZSTD_matchState_t* ms, const B
     U32 const target = (U32)(ip - ms->window.base);
     U32* const chainTable = ms->chainTable;
     U32 const chainMask = (1 << ms->cParams.chainLog) - 1;
-    for (U32 idx = ms->nextToUpdate; idx < target; idx++) {
+    U32 idx = ms->nextToUpdate;
+    U32 bucketSize = 1 << ZSTD_LAZY_DDSS_BUCKET_LOG;
+    for ( ; idx < target; idx++) {
+        U32 i;
         U32 const h = ZSTD_hashPtr(
             ms->window.base + idx,
             ms->cParams.hashLog - ZSTD_LAZY_DDSS_BUCKET_LOG,
             ms->cParams.minMatch) << ZSTD_LAZY_DDSS_BUCKET_LOG;
+        /* Shift hash cache down 1. */
+        for (i = bucketSize - 1; i; i--)
+            ms->hashTable[h + i] = ms->hashTable[h + i - 1];
+        /* Insert new position. */
         chainTable[idx & chainMask] = ms->hashTable[h];
         ms->hashTable[h] = idx;
-        /* Same logic as before. But now, just copy the chain into the bucket */
-        for (U32 i = 0; i < (1 << ZSTD_LAZY_DDSS_BUCKET_LOG) - 1; i++)
-            ms->hashTable[h + i + 1] = chainTable[ms->hashTable[h + i] & chainMask];
     }
+
     ms->nextToUpdate = target;
 }