Check deadline less frequently

Calling time.Now() is a bit expensive to do in a tight loop.
Check it only every 16th time.
This results in an average 0.16% time overrun when the deadline is hit,
which seems a small price to pay for up to a 25% speedup on the
actual work.

name                       old time/op    new time/op    delta
DiffHalfMatch-8               108µs ± 1%     108µs ± 0%     ~     (p=0.211 n=10+9)
DiffCleanupSemantic-8         973µs ± 0%     971µs ± 1%     ~     (p=0.673 n=8+9)
DiffMain-8                    1.01s ± 0%     1.01s ± 0%   +0.16%  (p=0.003 n=9+10)
DiffMainLarge-8               110ms ± 1%     102ms ± 3%   -7.44%  (p=0.000 n=8+10)
DiffMainRunesLargeLines-8     693µs ± 1%     515µs ± 1%  -25.70%  (p=0.000 n=8+9)

name                       old alloc/op   new alloc/op   delta
DiffHalfMatch-8               106kB ± 0%     106kB ± 0%     ~     (all equal)
DiffCleanupSemantic-8         177kB ± 0%     177kB ± 0%     ~     (all equal)
DiffMain-8                   16.4MB ± 0%    16.4MB ± 0%     ~     (all equal)
DiffMainLarge-8              4.81MB ± 0%    4.81MB ± 0%     ~     (p=0.764 n=10+9)
DiffMainRunesLargeLines-8     174kB ± 0%     174kB ± 0%   +0.01%  (p=0.014 n=10+10)

name                       old allocs/op  new allocs/op  delta
DiffHalfMatch-8                2.00 ± 0%      2.00 ± 0%     ~     (all equal)
DiffCleanupSemantic-8         3.12k ± 0%     3.12k ± 0%     ~     (all equal)
DiffMain-8                     83.0 ± 0%      83.0 ± 0%     ~     (all equal)
DiffMainLarge-8               46.3k ± 0%     46.3k ± 0%     ~     (p=1.000 n=10+10)
DiffMainRunesLargeLines-8     1.08k ± 0%     1.08k ± 0%     ~     (p=0.211 n=10+10)

Author: josharian

diffmatchpatch/diff.go

@@ -283,7 +283,7 @@ func (dmp *DiffMatchPatch) diffBisect(runes1, runes2 []rune, deadline time.Time)
 	k2end := 0
 	for d := 0; d < maxD; d++ {
 		// Bail out if deadline is reached.
-		if !deadline.IsZero() && time.Now().After(deadline) {
+		if !deadline.IsZero() && d%16 == 0 && time.Now().After(deadline) {
 			break
 		}