number.go RawHistoryFolder

1// SPDX-License-Identifier: MIT
2
3package template
4
5import (
6	"math"
7	"strconv"
8	"strings"
9)
10
11// JSNumberString formats x like JavaScript's Number.prototype.toString: the
12// shortest decimal that round-trips, positional for decimal exponents from -7
13// to 20, exponent notation beyond. The reference uses it for the text of a
14// number token.
15func JSNumberString(x float64) string {
16	switch {
17	case math.IsNaN(x):
18		return "NaN"
19	case x == 0:
20		return "0"
21	case math.IsInf(x, 1):
22		return "Infinity"
23	case math.IsInf(x, -1):
24		return "-Infinity"
25	}
26	sign := ""
27	if x < 0 {
28		sign = "-"
29		x = -x
30	}
31	s := strconv.FormatFloat(x, 'e', -1, 64)
32	mant, expStr, _ := strings.Cut(s, "e")
33	exp, _ := strconv.Atoi(expStr)
34	digits := strings.Replace(mant, ".", "", 1)
35	k := len(digits)
36	n := exp + 1
37	switch {
38	case k <= n && n <= 21:
39		return sign + digits + strings.Repeat("0", n-k)
40	case 0 < n && n <= 21:
41		return sign + digits[:n] + "." + digits[n:]
42	case -6 < n && n <= 0:
43		return sign + "0." + strings.Repeat("0", -n) + digits
44	}
45	e := n - 1
46	esign := "+"
47	if e < 0 {
48		esign = "-"
49		e = -e
50	}
51	if k == 1 {
52		return sign + digits + "e" + esign + strconv.Itoa(e)
53	}
54	return sign + digits[:1] + "." + digits[1:] + "e" + esign + strconv.Itoa(e)
55}