fmt: optimize string truncation

Count only the runes up to the requested precision
to decide where to truncate a string.

Change the loop within truncate to need fewer jumps.

name                     old time/op  new time/op  delta
SprintfTruncateString-2   188ns ± 3%   155ns ± 3%  -17.43%  (p=0.000 n=20+20)

Change-Id: I17ca9fc0bb8bf7648599df48e4785251bbc31e99
Reviewed-on: https://go-review.googlesource.com/20098Reviewed-by: Rob Pike <r@golang.org>

src/fmt/fmt_test.go

@@ -199,7 +199,9 @@ var fmtTests = []struct {
 	{"%08q", "abc", `000"abc"`},
 	{"%5s", "abcdefghijklmnopqrstuvwxyz", "abcdefghijklmnopqrstuvwxyz"},
 	{"%.5s", "abcdefghijklmnopqrstuvwxyz", "abcde"},
+	{"%.0s", "日本語日本語", ""},
 	{"%.5s", "日本語日本語", "日本語日本"},
+	{"%.10s", "日本語日本語", "日本語日本語"},
 	{"%.5s", []byte("日本語日本語"), "日本語日本"},
 	{"%.5q", "abcdefghijklmnopqrstuvwxyz", `"abcde"`},
 	{"%.5x", "abcdefghijklmnopqrstuvwxyz", `6162636465`},
@@ -928,6 +930,14 @@ func BenchmarkSprintfString(b *testing.B) {
 	})
 }
 
+func BenchmarkSprintfTruncateString(b *testing.B) {
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			Sprintf("%.3s", "日本語日本語日本語")
+		}
+	})
+}
+
 func BenchmarkSprintfInt(b *testing.B) {
 	b.RunParallel(func(pb *testing.PB) {
 		for pb.Next() {

src/fmt/format.go

@@ -282,14 +282,13 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 
 // truncate truncates the string to the specified precision, if present.
 func (f *fmt) truncate(s string) string {
-	if f.precPresent && f.prec < utf8.RuneCountInString(s) {
+	if f.precPresent {
 		n := f.prec
 		for i := range s {
-			if n == 0 {
-				s = s[:i]
-				break
-			}
 			n--
+			if n < 0 {
+				return s[:i]
+			}
 		}
 	}
 	return s