Go: precise float parse

parsefloat.go

package main

import (
  "fmt"
)

// ParseFloatPrecisely parses floating point from string as strconv.ParseFloat
// but avoids float point precision issue (ex: "0.21" parsed as 0.209999)
func ParseFloatPrecisely(str string, bitSize int) (num float64, err error) {
	if str == "" || str == "0" {
		return 0, nil
	}

	chunks := strings.SplitN(str, ".", 2)
	mantissaPart := chunks[0]
	isNegative := mantissaPart[0] == '-'
	mantissa, err := strconv.ParseInt(mantissaPart, 10, bitSize)
	if err != nil {
    		return 0, fmt.Errorf("ParseFloat: failed to parse mantissa part of number (%w)", err)
	}

	if len(chunks) < 2 {
		return float64(mantissa), nil
	}

	expoLen := len(chunks[1])
	expoPart, err := strconv.ParseUint(chunks[1], 10, bitSize)
	if err != nil {
    		return 0, fmt.Errorf("ParseFloat: failed to parse exponent part of number (%w)", err)
	}

	if expoPart == 0 {
		return float64(mantissa), nil
	}

	// concat exponent and mantissa to make f64 number
	exponent := float64(expoPart) / math.Pow10(expoLen)
	if isNegative {
		exponent *= -1
	}
	num = float64(mantissa) + exponent
	return num, nil
}