lib/app/render-float.go

package app

/*

Author: https://github.com/gorhill
Source: https://gist.github.com/gorhill/5285193

A Go function to render a number to a string based on
the following user-specified criteria:

* thousands separator
* decimal separator
* decimal precision

Usage: s := RenderFloat(format, n)

The format parameter tells how to render the number n.

http://play.golang.org/p/LXc1Ddm1lJ

Examples of format strings, given n = 12345.6789:

"#,###.##" => "12,345.67"
"#,###." => "12,345"
"#,###" => "12345,678"
"#\u202F###,##" => "12 345,67"
"#.###,###### => 12.345,678900
"" (aka default format) => 12,345.67

The highest precision allowed is 9 digits after the decimal symbol.
There is also a version for integer number, RenderInteger(),
which is convenient for calls within template.

I didn't feel it was worth to publish a library just for this piece
of code, hence the snippet. Feel free to reuse as you wish.
const rPattern = "#,###.##"
*/

import (
	"math"
	"strconv"
)

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

var renderFloatPrecisionRounders = [10]float64{
	0.5,
	0.05,
	0.005,
	0.0005,
	0.00005,
	0.000005,
	0.0000005,
	0.00000005,
	0.000000005,
	0.0000000005,
}

// func renderInteger(format string, n int) string {
// 	return renderFloat(format, float64(n))
// }

// RenderFloat renders a number to a string
func RenderFloat(format string, n float64) string { //nolint:funlen,gocyclo // lots of comments
	// Special cases:
	//   NaN = "NaN"
	//   +Inf = "+Infinity"
	//   -Inf = "-Infinity"
	if math.IsNaN(n) {
		return "NaN"
	}
	if n > math.MaxFloat64 {
		return "Infinity"
	}
	if n < -math.MaxFloat64 {
		return "-Infinity"
	}

	// default format
	precision := 2
	decimalStr := "."
	thousandStr := ","
	positiveStr := ""
	negativeStr := "-"

	if len(format) > 0 {
		// If there is an explicit format directive,
		// then default values are these:
		precision = 9
		thousandStr = ""

		// collect indices of meaningful formatting directives
		formatDirectiveChars := []rune(format)
		formatDirectiveIndices := make([]int, 0)
		for i, char := range formatDirectiveChars {
			if char != '#' && char != '0' {
				formatDirectiveIndices = append(formatDirectiveIndices, i)
			}
		}

		if len(formatDirectiveIndices) > 0 {
			// Directive at index 0:
			//   Must be a '+'
			//   Raise an error if not the case
			// index: 0123456789
			//        +0.000,000
			//        +000,000.0
			//        +0000.00
			//        +0000
			if formatDirectiveIndices[0] == 0 {
				if formatDirectiveChars[formatDirectiveIndices[0]] != '+' {
					panic("RenderFloat(): invalid positive sign directive")
				}
				positiveStr = "+"
				formatDirectiveIndices = formatDirectiveIndices[1:]
			}

			// Two directives:
			//   First is thousands separator
			//   Raise an error if not followed by 3-digit
			// 0123456789
			// 0.000,000
			// 000,000.00
			if len(formatDirectiveIndices) == 2 { //nolint:gomnd // not my code
				if (formatDirectiveIndices[1] - formatDirectiveIndices[0]) != 4 { //nolint:gomnd // not my code
					panic("RenderFloat(): thousands separator directive must be followed by 3 digit-specifiers")
				}
				thousandStr = string(formatDirectiveChars[formatDirectiveIndices[0]])
				formatDirectiveIndices = formatDirectiveIndices[1:]
			}

			// One directive:
			//   Directive is decimal separator
			//   The number of digit-specifier following the separator indicates wanted precision
			// 0123456789
			// 0.00
			// 000,0000
			if len(formatDirectiveIndices) == 1 {
				decimalStr = string(formatDirectiveChars[formatDirectiveIndices[0]])
				precision = len(formatDirectiveChars) - formatDirectiveIndices[0] - 1
			}
		}
	}

	// generate sign part
	var signStr string
	if n >= 0.000000001 { //nolint:gomnd,gocritic // not my code!
		signStr = positiveStr
	} else if n <= -0.000000001 {
		signStr = negativeStr
		n = -n
	} else {
		signStr = ""
		n = 0.0
	}

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

	// generate integer part string
	intStr := strconv.Itoa(int(intf))

	// add thousand separator if required
	if len(thousandStr) > 0 {
		for i := len(intStr); i > 3; {
			i -= 3
			intStr = intStr[:i] + thousandStr + intStr[i:]
		}
	}

	// no fractional part, we can leave now
	if precision == 0 {
		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
}