comma.go

func comma(v int64) string {
	sign := ""
	// Min int64 can't be negated to a usable value, so it has to be special cased.
	if v == math.MinInt64 {
		return "-9,223,372,036,854,775,808"
	}
	if v < 0 {
		sign = "-"
		v = 0 - v
	}
	parts := []string{"", "", "", "", "", "", ""}
	j := len(parts) - 1
	for v > 999 {
		parts[j] = strconv.FormatInt(v%1000, 10)
		switch len(parts[j]) {
		case 2:
			parts[j] = "0" + parts[j]
		case 1:
			parts[j] = "00" + parts[j]
		}
		v = v / 1000
		j--
	}
	parts[j] = strconv.Itoa(int(v))
	return sign + strings.Join(parts[j:], ",")
}

// use string.builder
func comma2(v int64) string {
	sign := ""
	// Min int64 can't be negated to a usable value, so it has to be special cased.
	if v == math.MinInt64 {
		return "-9,223,372,036,854,775,808"
	}
	if v < 0 {
		sign = "-"
		v = 0 - v
	}
	parts := make([]int16, 7)
	j := len(parts) - 1
	for v > 999 {
		parts[j] = int16(v % 1000)
		v = v / 1000
		j--
	}
	parts[j] = int16(v)
	var buf strings.Builder
	buf.Grow(26)
	if len(sign) > 0 {
		buf.WriteString("-")
	}
	var start bool
	for _, v := range parts {
		if start {
			buf.WriteString(",")
		}
		if v == 0 && start {
			buf.WriteString("000")
		} else if v < 10 {
			if start {
				buf.WriteString("00")
			}
			buf.WriteString(strconv.Itoa(int(v)))
		} else if v < 100 {
			if start {
				buf.WriteString("0")
			}
			buf.WriteString(strconv.Itoa(int(v)))
		} else {
			buf.WriteString(strconv.Itoa(int(v)))
		}
		if v > 0 {
			start = true
		}
	}
	return buf.String() //sign + strings.Join(parts[j:], ",")
}

// use string.builder and sprintf
func comma3(v int64) string {
	sign := ""
	// Min int64 can't be negated to a usable value, so it has to be special cased.
	if v == math.MinInt64 {
		return "-9,223,372,036,854,775,808"
	}
	if v < 0 {
		sign = "-"
		v = 0 - v
	}
	parts := make([]int16, 7)
	j := len(parts) - 1
	for v > 999 {
		parts[j] = int16(v % 1000)
		v = v / 1000
		j--
	}
	parts[j] = int16(v)
	var buf strings.Builder
	buf.Grow(26)
	if len(sign) > 0 {
		buf.WriteString("-")
	}
	var start bool
	for _, v := range parts {
		if start {
			buf.WriteString(",")
		}
		if v > 0 && !start {
			buf.WriteString(strconv.Itoa(int(v)))
			start = true
		} else if v == 0 && start || v > 0 {
			buf.WriteString(fmt.Sprintf("%03d", v))
			start = true
		}
	}
	return buf.String()
}

// -------------------------
// bench
// -------------------------

func BenchmarkCommaStringJoin(b *testing.B) {
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		comma(math.MaxInt64)
	}
}

func BenchmarkCommaStringBuilder(b *testing.B) {
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		comma2(math.MaxInt64)
	}
}

func BenchmarkCommaSprintf(b *testing.B) {
	b.ResetTimer()
	for i := 0; i < b.N; i++ {
		comma3(math.MaxInt64)
	}
}

ベンチ結果

BenchmarkCommaStringJoin-8      	 3000000	       405 ns/op	      83 B/op	       8 allocs/op
BenchmarkCommaStringBuilder-8   	 5000000	       384 ns/op	      48 B/op	       6 allocs/op
BenchmarkCommaSprintf-8         	 1000000	      1061 ns/op	      70 B/op	      13 allocs/op
PASS```