optimize the performance of the function appendFormatRFC3339

Author: fx408

src/time/format.go

@@ -464,34 +464,16 @@ func appendInt(b []byte, x int, width int) []byte {
 	return b
 }
 
-const unitsDigit = "01234567890123456789012345678901234567890123456789" +
-	"01234567890123456789012345678901234567890123456789"
-const tensDigit = "00000000001111111111222222222233333333334444444444" +
-	"55555555556666666666777777777788888888889999999999"
-
-// appendIntWidth2 special scenario for appendInt, with parameter width=2
-// Only applicable to integers with absolute value less than 100
-func appendIntWidth2(b []byte, x int) []byte {
-	if x < 0 {
-		b = append(b, '-')
-		x = -x
-	}
-	if x >= 1e2 {
-		x %= 1e2
-	}
-	return append(b, tensDigit[x], unitsDigit[x])
-}
+const onesDigit = "0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789"
+const tensDigit = "0000000000111111111122222222223333333333444444444455555555556666666666777777777788888888889999999999"
 
-// appendIntWidth4 special scenario for appendInt, with parameter width=4
+// 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 {
-		b = append(b, '-')
-		x = -x
-	}
-	if x >= 1e4 {
+	if x < 0 || x >= 1e4 {
 		return appendInt(b, x, 4)
 	}
-	return append(b, tensDigit[x/1e2], unitsDigit[x/1e2], tensDigit[x%1e2], unitsDigit[x%1e2])
+	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.

src/time/format_rfc3339.go

@@ -21,20 +21,18 @@ func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
 	// Format date.
 	year, month, day := abs.days().date()
 	b = appendIntWidth4(b, year)
-	b = append(b, '-')
-	b = appendIntWidth2(b, int(month))
-	b = append(b, '-')
-	b = appendIntWidth2(b, day)
+	b = append(b, '-',
+		tensDigit[month], onesDigit[month], '-',
+		tensDigit[day], onesDigit[day])
 
 	b = append(b, 'T')
 
 	// Format time.
 	hour, min, sec := abs.clock()
-	b = appendIntWidth2(b, hour)
-	b = append(b, ':')
-	b = appendIntWidth2(b, min)
-	b = append(b, ':')
-	b = appendIntWidth2(b, sec)
+	b = append(b,
+		tensDigit[hour], onesDigit[hour], ':',
+		tensDigit[min], onesDigit[min], ':',
+		tensDigit[sec], onesDigit[sec])
 
 	if nanos {
 		std := stdFracSecond(stdFracSecond9, 9, '.')
@@ -53,9 +51,11 @@ func (t Time) appendFormatRFC3339(b []byte, nanos bool) []byte {
 	} else {
 		b = append(b, '+')
 	}
-	b = appendIntWidth2(b, zone/60)
-	b = append(b, ':')
-	b = appendIntWidth2(b, zone%60)
+
+	zone %= 3600
+	b = append(b,
+		tensDigit[zone/60], onesDigit[zone/60], ':',
+		tensDigit[zone%60], onesDigit[zone%60])
 	return b
 }
 

src/time/format_test.go

@@ -1093,21 +1093,7 @@ func FuzzParseRFC3339(f *testing.F) {
 }
 
 func TestAppendIntWidth(t *testing.T) {
-	values := []int{0, -1, 1, 10, -10, 99, -99}
-	for _, v := range values {
-		exp := AppendInt(nil, v, 2)
-		got := AppendIntWidth2(nil, v)
-		if !bytes.Equal(got, exp) {
-			t.Errorf("AppendIntWidth2(%d) = %s, want %s", v, got, exp)
-		}
-	}
-
-	got := AppendIntWidth2(nil, 199)
-	if !bytes.Equal(got, []byte("99")) {
-		t.Errorf("AppendIntWidth2(199) = %s, want %s", got, []byte("99"))
-	}
-
-	values = append(values, 9999, -9999, 10001)
+	values := []int{0, -1, 1, 10, -10, 99, -99, 9999, -9999, 10001}
 	for _, v := range values {
 		exp := AppendInt(nil, v, 4)
 		got := AppendIntWidth4(nil, v)
@@ -1117,23 +1103,6 @@ func TestAppendIntWidth(t *testing.T) {
 	}
 }
 
-func BenchmarkAppendIntWidth2(b *testing.B) {
-	b.Run("name=AppendInt", func(b *testing.B) {
-		var buf = make([]byte, 0, 8)
-		b.ResetTimer()
-		for i := 0; i < b.N; i++ {
-			buf = AppendInt(buf[:0], 36, 2)
-		}
-	})
-	b.Run("name=AppendIntWidth2", func(b *testing.B) {
-		var buf = make([]byte, 0, 8)
-		b.ResetTimer()
-		for i := 0; i < b.N; i++ {
-			buf = AppendIntWidth2(buf[:0], 36)
-		}
-	})
-}
-
 func BenchmarkAppendIntWidth4(b *testing.B) {
 	b.Run("name=AppendInt", func(b *testing.B) {
 		var buf = make([]byte, 0, 8)