time: optimize the performance of the function appendFormatRFC3339

src/time/export_test.go

@@ -134,6 +134,7 @@ var StdChunkNames = map[int]string{
 var Quote = quote
 
 var AppendInt = appendInt
+var AppendIntWidth4 = appendIntWidth4
 var AppendFormatAny = Time.appendFormat
 var AppendFormatRFC3339 = Time.appendFormatRFC3339
 var ParseAny = parse

src/time/format.go

@@ -464,6 +464,18 @@ func appendInt(b []byte, x int, width int) []byte {
 	return b
 }
 
+const onesDigit = "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789"
+const tensDigit = "0000000000111111111122222222223333333333444444444455555555556666666666777777777788888888889999999999"
+
+// appendIntWidth4 is semantically identical to appendInt(b, x, 4)
+// but optimized for 0 ≤ x < 10000.
+func appendIntWidth4(b []byte, x int) []byte {
+	if x < 0 || x >= 1e4 {
+		return appendInt(b, x, 4)
+	}
+	return append(b, tensDigit[x/1e2], onesDigit[x/1e2], tensDigit[x%1e2], onesDigit[x%1e2])
+}
+
 // Never printed, just needs to be non-nil for return by atoi.
 var errAtoi = errors.New("time: invalid number")
 

src/time/format_rfc3339.go

@@ -18,23 +18,22 @@ import "errors"
 func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
 	_, offset, abs := t.locabs()
 
-	// Format date.
+	// Format date and time.
 	year, month, day := abs.days().date()
-	b = appendInt(b, year, 4)
-	b = append(b, '-')
-	b = appendInt(b, int(month), 2)
-	b = append(b, '-')
-	b = appendInt(b, day, 2)
-
-	b = append(b, 'T')
-
-	// Format time.
 	hour, min, sec := abs.clock()
-	b = appendInt(b, hour, 2)
-	b = append(b, ':')
-	b = appendInt(b, min, 2)
-	b = append(b, ':')
-	b = appendInt(b, sec, 2)
+
+	b = appendIntWidth4(b, year)
+	b = append(b, '-',
+		tensDigit[month], onesDigit[month],
+		'-',
+		tensDigit[day], onesDigit[day],
+		'T',
+		tensDigit[hour], onesDigit[hour],
+		':',
+		tensDigit[min], onesDigit[min],
+		':',
+		tensDigit[sec], onesDigit[sec],
+	)
 
 	if nanos {
 		std := stdFracSecond(stdFracSecond9, 9, '.')
@@ -53,9 +52,19 @@ func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
 	} else {
 		b = append(b, '+')
 	}
-	b = appendInt(b, zone/60, 2)
-	b = append(b, ':')
-	b = appendInt(b, zone%60, 2)
+
+	if zone > 3600 {
+		b = appendInt(b, zone/60, 2)
+		b = append(b, ':',
+			tensDigit[zone%60], onesDigit[zone%60])
+		return b
+	}
+
+	b = append(b,
+		tensDigit[zone/60], onesDigit[zone/60],
+		':',
+		tensDigit[zone%60], onesDigit[zone%60],
+	)
 	return b
 }
 

src/time/format_test.go

@@ -1091,3 +1091,40 @@ func FuzzParseRFC3339(f *testing.F) {
 		}
 	})
 }
+
+func TestAppendIntWidth(t *testing.T) {
+	values := []int{0, -1, 1, 10, -10, 99, -99, 9999, -9999, 10001}
+	for _, v := range values {
+		want := AppendInt(nil, v, 4)
+		got := AppendIntWidth4(nil, v)
+		if !bytes.Equal(got, want) {
+			t.Errorf("AppendIntWidth4(%d) = %s, want %s", v, got, want)
+		}
+	}
+}
+
+func BenchmarkAppendIntWidth4(b *testing.B) {
+	b.Run("name=AppendInt", func(b *testing.B) {
+		var buf = make([]byte, 0, 8)
+		b.ResetTimer()
+		for b.Loop() {
+			buf = AppendInt(buf[:0], 360, 4)
+		}
+	})
+	b.Run("name=AppendIntWidth4", func(b *testing.B) {
+		var buf = make([]byte, 0, 8)
+		b.ResetTimer()
+		for b.Loop() {
+			buf = AppendIntWidth4(buf[:0], 360)
+		}
+	})
+}
+
+func BenchmarkTimeFormatRFC3339(b *testing.B) {
+	tm := Unix(1661201140, 676836973)
+	buf := make([]byte, 0, 64)
+	b.ReportAllocs()
+	for b.Loop() {
+		buf = tm.AppendFormat(buf[:0], RFC3339)
+	}
+}