Avoid hashing same memory location twice by truncating overlapping byte ranges,

it's speed optimization as the inner code also checks that previous hash value
is not same as new hash value. Essentially those two checks together makes the
compression a little more efficient as it can remember matches further apart.
As far as I remember from my tests, the secondary path was triggered only twice
in very long uncompressed file, but the gain in compression rate was still noticeable.

diff --git a/deflate_medium.c b/deflate_medium.c
index 82301d1c4472be6e855837745469265942f927cf..046449fb9b2159ae1221a75fc6570016a169573d 100644 (file)
--- a/deflate_medium.c
+++ b/deflate_medium.c
@@ -68,17 +68,18 @@ static void insert_match(deflate_state *s, struct match match) {
             }
         }
 #else
-        if (likely(match.match_length == 1)) {
-            match.strstart++;
-            match.match_length = 0;
-        }else{
-            match.strstart++;
-            match.match_length--;
+        match.strstart++;
+        match.match_length--;
+        if (match.match_length > 0) {
             if (match.strstart >= match.orgstart) {
-                insert_string(s, match.strstart, match.match_length);
+                if (match.strstart + match.match_length - 1 >= match.orgstart) {
+                    insert_string(s, match.strstart, match.match_length);
+                } else {
+                    insert_string(s, match.strstart, match.orgstart - match.strstart + 1);
+                }
+                match.strstart += match.match_length;
+                match.match_length = 0;
             }
-            match.strstart += match.match_length;
-            match.match_length = 0;
         }
 #endif
         return;
@@ -102,7 +103,11 @@ static void insert_match(deflate_state *s, struct match match) {
         } while (--match.match_length != 0);
 #else
         if (likely(match.strstart >= match.orgstart)) {
-            insert_string(s, match.strstart, match.match_length);
+            if (likely(match.strstart + match.match_length - 1 >= match.orgstart)) {
+                insert_string(s, match.strstart, match.match_length);
+            } else {
+                insert_string(s, match.strstart, match.orgstart - match.strstart + 1);
+            }
         }
         match.strstart += match.match_length;
         match.match_length = 0;
@@ -111,7 +116,7 @@ static void insert_match(deflate_state *s, struct match match) {
         match.strstart += match.match_length;
         match.match_length = 0;
         s->ins_h = s->window[match.strstart];
-        if (match.strstart >= 1)
+        if (match.strstart >= (MIN_MATCH - 2))
 #ifndef NOT_TWEAK_COMPILER
             insert_string(s, match.strstart + 2 - MIN_MATCH, MIN_MATCH - 2);
 #else