From: Yann Collet <cyan@fb.com>
Date: Wed, 16 Oct 2024 19:13:57 +0000 (-0700)
Subject: slightly improved compression ratio at levels 3 & 4
X-Git-Tag: v1.5.7^2~73^2~4
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=632677516616434c312d2fff2d84dcf0c9b78012;p=thirdparty%2Fzstd.git

slightly improved compression ratio at levels 3 & 4

The compression ratio benefits are small but consistent, i.e. always positive.
On `silesia.tar` corpus, this modification saves ~75 KB at level 3.
The measured speed cost is negligible, i.e. below noise level, between 0 and -1%.
---

diff --git a/lib/compress/zstd_double_fast.c b/lib/compress/zstd_double_fast.c
index e2b3b4a14..72b541ea6 100644
--- a/lib/compress/zstd_double_fast.c
+++ b/lib/compress/zstd_double_fast.c
@@ -252,19 +252,23 @@ _cleanup:
 
 _search_next_long:
 
-        /* check prefix long +1 match */
+        /* short match found: let's check for a longer one */
+        mLength = ZSTD_count(ip+4, matchs0+4, iend) + 4;
+
+        /* check long match at +1 position */
         if (idxl1 > prefixLowestIndex) {
             if (MEM_read64(matchl1) == MEM_read64(ip1)) {
-                ip = ip1;
-                mLength = ZSTD_count(ip+8, matchl1+8, iend) + 8;
-                offset = (U32)(ip-matchl1);
-                while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
-                goto _match_found;
-            }
+                size_t const llen = ZSTD_count(ip1+8, matchl1+8, iend) + 8;
+                if (llen > mLength) {
+                    ip = ip1;
+                    mLength = llen;
+                    offset = (U32)(ip-matchl1);
+                    while (((ip>anchor) & (matchl1>prefixLowest)) && (ip[-1] == matchl1[-1])) { ip--; matchl1--; mLength++; } /* catch up */
+                    goto _match_found;
+            }   }
         }
 
-        /* if no long +1 match, explore the short match we found */
-        mLength = ZSTD_count(ip+4, matchs0+4, iend) + 4;
+        /* validate short match previously found */
         offset = (U32)(ip - matchs0);
         while (((ip>anchor) & (matchs0>prefixLowest)) && (ip[-1] == matchs0[-1])) { ip--; matchs0--; mLength++; } /* catch up */