time: unify formatting of decimals for timestamps

Change function appendUint to appendInt with variable-width 0-padding.

This allows the decimal for the year to be generated without extra code
to handle the wider padding and directly handles negative numbers.

Removes the special casing for numbers with one and two digits.
The special case for 0 was unreachable.

The new version is slightly slower.

benchmark              old ns/op     new ns/op     delta
BenchmarkFormat        444           454           +2.25%
BenchmarkFormatNow     398           415           +4.27%

Change-Id: I4ddef96bf07ad35dca76053321d510441ec6d4f5
Reviewed-on: https://go-review.googlesource.com/2751Reviewed-by: Robert Griesemer <gri@golang.org>

src/time/format.go

@@ -315,36 +315,34 @@ func lookup(tab []string, val string) (int, string, error) {
 	return -1, val, errBad
 }
 
-// appendUint appends the decimal form of x to b and returns the result.
-// If x is a single-digit number and pad != 0, appendUint inserts the pad byte
-// before the digit.
+// appendInt appends the decimal form of x to b and returns the result.
+// If the decimal form (excluding sign) is shorter than width, the result is padded with leading 0's.
 // Duplicates functionality in strconv, but avoids dependency.
-func appendUint(b []byte, x uint, pad byte) []byte {
-	if x < 10 {
-		if pad != 0 {
-			b = append(b, pad)
-		}
-		return append(b, byte('0'+x))
-	}
-	if x < 100 {
-		b = append(b, byte('0'+x/10))
-		b = append(b, byte('0'+x%10))
-		return b
+func appendInt(b []byte, x int, width int) []byte {
+	u := uint(x)
+	if x < 0 {
+		b = append(b, '-')
+		u = uint(-x)
 	}
 
-	var buf [32]byte
-	n := len(buf)
-	if x == 0 {
-		return append(b, '0')
+	// Assemble decimal in reverse order.
+	var buf [20]byte
+	i := len(buf)
+	for u >= 10 {
+		i--
+		q := u / 10
+		buf[i] = byte('0' + u - q*10)
+		u = q
 	}
-	for x >= 10 {
-		n--
-		buf[n] = byte(x%10 + '0')
-		x /= 10
+	i--
+	buf[i] = byte('0' + u)
+
+	// Add 0-padding.
+	for w := len(buf) - i; w < width; w++ {
+		b = append(b, '0')
 	}
-	n--
-	buf[n] = byte(x + '0')
-	return append(b, buf[n:]...)
+
+	return append(b, buf[i:]...)
 }
 
 // Never printed, just needs to be non-nil for return by atoi.
@@ -458,75 +456,56 @@ func (t Time) Format(layout string) string {
 			if y < 0 {
 				y = -y
 			}
-			b = appendUint(b, uint(y%100), '0')
+			b = appendInt(b, y%100, 2)
 		case stdLongYear:
-			// Pad year to at least 4 digits.
-			y := year
-			switch {
-			case year <= -1000:
-				b = append(b, '-')
-				y = -y
-			case year <= -100:
-				b = append(b, "-0"...)
-				y = -y
-			case year <= -10:
-				b = append(b, "-00"...)
-				y = -y
-			case year < 0:
-				b = append(b, "-000"...)
-				y = -y
-			case year < 10:
-				b = append(b, "000"...)
-			case year < 100:
-				b = append(b, "00"...)
-			case year < 1000:
-				b = append(b, '0')
-			}
-			b = appendUint(b, uint(y), 0)
+			b = appendInt(b, year, 4)
 		case stdMonth:
 			b = append(b, month.String()[:3]...)
 		case stdLongMonth:
 			m := month.String()
 			b = append(b, m...)
 		case stdNumMonth:
-			b = appendUint(b, uint(month), 0)
+			b = appendInt(b, int(month), 0)
 		case stdZeroMonth:
-			b = appendUint(b, uint(month), '0')
+			b = appendInt(b, int(month), 2)
 		case stdWeekDay:
 			b = append(b, absWeekday(abs).String()[:3]...)
 		case stdLongWeekDay:
 			s := absWeekday(abs).String()
 			b = append(b, s...)
 		case stdDay:
-			b = appendUint(b, uint(day), 0)
+			b = appendInt(b, day, 0)
 		case stdUnderDay:
-			b = appendUint(b, uint(day), ' ')
+			if day < 10 {
+				b = append(b, ' ')
+			}
+			b = appendInt(b, day, 0)
 		case stdZeroDay:
-			b = appendUint(b, uint(day), '0')
+			b = appendInt(b, day, 2)
 		case stdHour:
-			b = appendUint(b, uint(hour), '0')
+			b = appendInt(b, hour, 2)
 		case stdHour12:
 			// Noon is 12PM, midnight is 12AM.
 			hr := hour % 12
 			if hr == 0 {
 				hr = 12
 			}
-			b = appendUint(b, uint(hr), 0)
+			b = appendInt(b, hr, 0)
 		case stdZeroHour12:
 			// Noon is 12PM, midnight is 12AM.
 			hr := hour % 12
 			if hr == 0 {
 				hr = 12
 			}
-			b = appendUint(b, uint(hr), '0')
+			b = appendInt(b, hr, 2)
 		case stdMinute:
-			b = appendUint(b, uint(min), 0)
+			b = appendInt(b, min, 0)
 		case stdZeroMinute:
-			b = appendUint(b, uint(min), '0')
+			b = appendInt(b, min, 2)
 		case stdSecond:
-			b = appendUint(b, uint(sec), 0)
+			b = appendInt(b, sec, 2)
 		case stdZeroSecond:
-			b = appendUint(b, uint(sec), '0')
+			b = appendInt(b, sec, 2)
 		case stdPM:
 			if hour >= 12 {
 				b = append(b, "PM"...)
@@ -555,18 +534,18 @@ func (t Time) Format(layout string) string {
 			} else {
 				b = append(b, '+')
 			}
-			b = appendUint(b, uint(zone/60), '0')
+			b = appendInt(b, zone/60, 2)
 			if std == stdISO8601ColonTZ || std == stdNumColonTZ || std == stdISO8601ColonSecondsTZ || std == stdNumColonSecondsTZ {
 				b = append(b, ':')
 			}
-			b = appendUint(b, uint(zone%60), '0')
+			b = appendInt(b, zone%60, 2)
 
 			// append seconds if appropriate
 			if std == stdISO8601SecondsTZ || std == stdNumSecondsTz || std == stdNumColonSecondsTZ || std == stdISO8601ColonSecondsTZ {
 				if std == stdNumColonSecondsTZ || std == stdISO8601ColonSecondsTZ {
 					b = append(b, ':')
 				}
-				b = appendUint(b, uint(absoffset%60), '0')
+				b = appendInt(b, absoffset%60, 2)
 			}
 
 		case stdTZ:
@@ -583,8 +562,8 @@ func (t Time) Format(layout string) string {
 			} else {
 				b = append(b, '+')
 			}
-			b = appendUint(b, uint(zone/60), '0')
-			b = appendUint(b, uint(zone%60), '0')
+			b = appendInt(b, zone/60, 2)
+			b = appendInt(b, zone%60, 2)
 		case stdFracSecond0, stdFracSecond9:
 			b = formatNano(b, uint(t.Nanosecond()), std>>stdArgShift, std&stdMask == stdFracSecond9)
 		}