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Round FormatFloat from the exact value #173

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Round FormatFloat from the exact value #173

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54 changes: 14 additions & 40 deletions number.go
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Original file line number Diff line number Diff line change
Expand Up @@ -10,35 +10,9 @@ Source: https://gist.github.com/gorhill/5285193

import (
"math"
"math/big"
"strconv"
)

var (
renderFloatPrecisionMultipliers = [...]float64{
1,
10,
100,
1000,
10000,
100000,
1000000,
10000000,
100000000,
1000000000,
}

renderFloatPrecisionRounders = [...]float64{
0.5,
0.05,
0.005,
0.0005,
0.00005,
0.000005,
0.0000005,
0.00000005,
0.000000005,
0.0000000005,
}
"strings"
)

// FormatFloat produces a formatted number as string based on the following user-specified criteria:
Expand Down Expand Up @@ -157,11 +131,7 @@ func FormatFloat(format string, n float64) string {
n = 0.0
}

// split number into integer and fractional parts
intf, fracf := math.Modf(n + renderFloatPrecisionRounders[precision])

// generate integer part string
intStr := strconv.FormatFloat(intf, 'f', 0, 64)
intStr, fracStr, _ := strings.Cut(formatHalfUp(n, precision), ".")

// add thousand separator if required
if len(thousandStr) > 0 {
Expand All @@ -176,16 +146,20 @@ func FormatFloat(format string, n float64) string {
return signStr + intStr
}

// generate fractional part
fracStr := strconv.Itoa(int(fracf * renderFloatPrecisionMultipliers[precision]))
// may need padding
if len(fracStr) < precision {
fracStr = "000000000000000"[:precision-len(fracStr)] + fracStr
}

return signStr + intStr + decimalStr + fracStr
}

// formatHalfUp formats n with precision decimals, rounding halves up.
func formatHalfUp(n float64, precision int) string {
// strconv rounds the exact binary value correctly, but breaks ties to even.
if s := strconv.FormatFloat(n, 'f', precision+1, 64); s[len(s)-1] == '5' {
if r, _ := new(big.Rat).SetString(s); r.Cmp(new(big.Rat).SetFloat64(n)) == 0 {
return r.FloatString(precision)
}
}
return strconv.FormatFloat(n, 'f', precision, 64)
}

// FormatInteger produces a formatted number as string.
// See FormatFloat.
func FormatInteger(format string, n int) string {
Expand Down
7 changes: 7 additions & 0 deletions number_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,13 @@ func TestFormatFloat(t *testing.T) {
{"negative int64 boundary", "#,###.##", -9223372036854775808, "-9,223,372,036,854,775,808.00"},
{"large integer", "#.", 1e20, "100000000000000000000"},
{"large integer with sign", "+#.###,##", 1e20, "+100.000.000.000.000.000.000,00"},
{"carry into the last digit", "#,###.#", 8894.75, "8,894.8"},
{"carry into the last digit, negative", "#,###.#", -8894.75, "-8,894.8"},
{"just above a tie", "#,###.##", 71.575, "71.58"},
{"exact tie rounds up", "#,###.##", 945.125, "945.13"},
{"exact tie rounds up, small", "#,###.##", 0.125, "0.13"},
{"exact tie rounds up, large", "#,###.#", 731701778190334.75, "731,701,778,190,334.8"},
{"just below a tie", "#,###.##", 1.005, "1.00"},
{"#\u202f###,##", "#\u202f###,##", 12345.6789, "12 345,68"},

// special cases
Expand Down