Function NewFromFloatWithExponent

decimal.go:377–455

NewFromFloatWithExponent converts a float64 to Decimal, with an arbitrary number of fractional digits. Example: NewFromFloatWithExponent(123.456, -2).String() // output: "123.46"

(value float64, exp int32)

//
//	NewFromFloatWithExponent(123.456, -2).String() // output: "123.46"
func NewFromFloatWithExponent(value float64, exp int32) Decimal {
	if math.IsNaN(value) || math.IsInf(value, 0) {
		panic(fmt.Sprintf("Cannot create a Decimal from %v", value))
	}

	bits := math.Float64bits(value)
	mant := bits & (1<<52 - 1)
	exp2 := int32((bits >> 52) & (1<<11 - 1))
	sign := bits >> 63

	if exp2 == 0 {
		// specials
		if mant == 0 {
			return Decimal{}
		}
		// subnormal
		exp2++
	} else {
		// normal
		mant |= 1 << 52
	}

	exp2 -= 1023 + 52

	// normalizing base-2 values
	for mant&1 == 0 {
		mant = mant >> 1
		exp2++
	}

	// maximum number of fractional base-10 digits to represent 2^N exactly cannot be more than -N if N<0
	if exp < 0 && exp < exp2 {
		if exp2 < 0 {
			exp = exp2
		} else {
			exp = 0
		}
	}

	// representing 10^M * 2^N as 5^M * 2^(M+N)
	exp2 -= exp

	temp := big.NewInt(1)
	dMant := big.NewInt(int64(mant))

	// applying 5^M
	if exp > 0 {
		temp = temp.SetInt64(int64(exp))
		temp = temp.Exp(fiveInt, temp, nil)
	} else if exp < 0 {
		temp = temp.SetInt64(-int64(exp))
		temp = temp.Exp(fiveInt, temp, nil)
		dMant = dMant.Mul(dMant, temp)
		temp = temp.SetUint64(1)
	}

	// applying 2^(M+N)
	if exp2 > 0 {