bdiff: adjust criteria for getting optimal longest match in the A side middle

We prefer matches closer to the middle to balance recursion, as introduced in
f1ca249696ed.

For ranges with uneven length, matches starting exactly in the middle should
have preference. That will be optimal for matches of length 1. We will thus
accept equality in the half check.

For ranges with even length, half was ceil'ed when calculated but we got the
preference for low matches from the 'less than half' check. To get the same
result as before when we also accept equality, floor it. Without that,
test-annotate.t would show some different (still correct but less optimal)
results.

This will change the heuristics. Tests shows a slightly different output - and
sometimes slightly smaller bundles.

The bundle size for 4.0 (hg bundle --base null -r 4.0 x.hg) happens to go from
22804885 to 22803824 bytes - an 0.005% reduction.

mercurial/bdiff.c

@@ -151,7 +151,7 b' 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 b' 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;

tests/test-bdiff.py

@@ -88,7 +88,7 b' 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')

tests/test-bdiff.py.out

@@ -67,16 +67,17 b' 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' -> ''