diff --git a/mercurial/bdiff.c b/mercurial/bdiff.c
--- a/mercurial/bdiff.c
+++ b/mercurial/bdiff.c
@@ -151,7 +151,7 @@ static int longest_match(struct bdiff_li
 	if (a2 - a1 > 30000)
 		a1 = a2 - 30000;
 
-	half = (a1 + a2) / 2;
+	half = (a1 + a2 - 1) / 2;
 
 	for (i = a1; i < a2; i++) {
 		/* skip all lines in b after the current block */
@@ -177,7 +177,7 @@ static int longest_match(struct bdiff_li
 
 			/* best match so far? we prefer matches closer
 			   to the middle to balance recursion */
-			if (k > mk || (k == mk && (i <= mi || i < half))) {
+			if (k > mk || (k == mk && (i <= mi || i <= half))) {
 				mi = i;
 				mj = j;
 				mk = k;
diff --git a/tests/test-bdiff.py b/tests/test-bdiff.py
--- a/tests/test-bdiff.py
+++ b/tests/test-bdiff.py
@@ -88,7 +88,7 @@ print("Diff 1 to 3 lines - preference fo
 showdiff('a\n', 'a\n' * 3)
 print("Diff 1 to 5 lines - preference for adding / removing at the end of sequences:")
 showdiff('a\n', 'a\n' * 5)
-print("Diff 3 to 1 lines - preference for adding / removing at the end of sequences:")
+print("Diff 3 to 1 lines - preference for balanced recursion:")
 showdiff('a\n' * 3, 'a\n')
-print("Diff 5 to 1 lines - this diff seems weird:")
+print("Diff 5 to 1 lines - preference for balanced recursion:")
 showdiff('a\n' * 5, 'a\n')
diff --git a/tests/test-bdiff.py.out b/tests/test-bdiff.py.out
--- a/tests/test-bdiff.py.out
+++ b/tests/test-bdiff.py.out
@@ -67,16 +67,17 @@ showdiff(
   'a\na\na\na\na\n'):
  'a\n'
  2 2 '' -> 'a\na\na\na\n'
-Diff 3 to 1 lines - preference for adding / removing at the end of sequences:
+Diff 3 to 1 lines - preference for balanced recursion:
 showdiff(
   'a\na\na\n',
   'a\n'):
+ 0 2 'a\n' -> ''
  'a\n'
- 2 6 'a\na\n' -> ''
-Diff 5 to 1 lines - this diff seems weird:
+ 4 6 'a\n' -> ''
+Diff 5 to 1 lines - preference for balanced recursion:
 showdiff(
   'a\na\na\na\na\n',
   'a\n'):
- 0 2 'a\n' -> ''
+ 0 4 'a\na\n' -> ''
  'a\n'
- 4 10 'a\na\na\n' -> ''
+ 6 10 'a\na\n' -> ''