From 193dedc73fb1401be8835f4c37eff9603ea3052c Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers3@gmail.com>
Date: Fri, 31 Dec 2021 16:04:49 -0600
Subject: [PATCH] deflate_compress: improve match scoring in lazy compressor

In the lazy compressor, it's usually worthwhile to (quickly) consider
the match offset too, not just the match length.
---
 lib/deflate_compress.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/lib/deflate_compress.c b/lib/deflate_compress.c
index fe6b9b5..9b00fcd 100644
--- a/lib/deflate_compress.c
+++ b/lib/deflate_compress.c
@@ -2072,7 +2072,7 @@ deflate_compress_greedy(struct libdeflate_compressor * restrict c,
 
 /*
  * This is the "lazy" DEFLATE compressor.  Before choosing a match, it checks to
- * see if there's a longer match at the next position.  If yes, it outputs a
+ * see if there's a better match at the next position.  If yes, it outputs a
  * literal and continues to the next position.  If no, it outputs the match.
  */
 static size_t
@@ -2153,7 +2153,7 @@ deflate_compress_lazy(struct libdeflate_compressor * restrict c,
 			}
 
 			/*
-			 * Try to find a longer match at the next position.
+			 * Try to find a better match at the next position.
 			 *
 			 * Note: since we already have a match at the *current*
 			 * position, we use only half the 'max_search_depth'
@@ -2164,7 +2164,7 @@ deflate_compress_lazy(struct libdeflate_compressor * restrict c,
 			 * Note: it's possible to structure the code such that
 			 * there's only one call to longest_match(), which
 			 * handles both the "find the initial match" and "try to
-			 * find a longer match" cases.  However, it is faster to
+			 * find a better match" cases.  However, it is faster to
 			 * have two call sites, with longest_match() inlined at
 			 * each.
 			 */
@@ -2174,15 +2174,18 @@ deflate_compress_lazy(struct libdeflate_compressor * restrict c,
 						&c->p.g.hc_mf,
 						&in_cur_base,
 						in_next++,
-						cur_len,
+						cur_len - 1,
 						max_len,
 						nice_len,
 						c->max_search_depth >> 1,
 						next_hashes,
 						&next_offset);
-			if (next_len > cur_len) {
+			if (next_len >= cur_len &&
+			    4 * (int)(next_len - cur_len) +
+			    ((int)bsr32(cur_offset) -
+			     (int)bsr32(next_offset)) > 2) {
 				/*
-				 * Found a longer match at the next position.
+				 * Found a better match at the next position.
 				 * Output a literal.  Then the next match
 				 * becomes the current match.
 				 */
@@ -2193,7 +2196,7 @@ deflate_compress_lazy(struct libdeflate_compressor * restrict c,
 				goto have_cur_match;
 			}
 			/*
-			 * No longer match at the next position.  Output the
+			 * No better match at the next position.  Output the
 			 * current match.
 			 */
 			deflate_choose_match(c, cur_len, cur_offset,