fixed NewFromFloat to calculate accurate values

[kotaroyamazaki]

Multiplying floats by primitive types is not precise enough, and there is a serious bug that the amount generated by NewFromFloat is different from the original value.
issue : #121, #124

Fixed to use "github.com/shopspring/decimal" for float multiplication.

This fix requires a dependency on an external module,, but I could find no other idea.

[azlancpool]

Hello there!
There is an approximate date when this change would be integrated into master?
Or some other approach where this bug would be fixed?

[totemcaf]

The "1.15" problem is related to the fact that the binary representation of 1.15 is closer to 114.99999999999999

This can solved by adding a small "epsilon" value to the parsed value.

const epsilon = 0.0001

func NewFromFloat(amount float64, currency string) *Money {
	currencyDecimals := math.Pow10(GetCurrency(currency).Fraction)
	return New(int64(amount*currencyDecimals + epsilon), currency)
}

This way it is not necessary to add a new dependency.

[totemcaf]

Another trick is to do:

func NewFromFloat(amount float64, currency string) *Money {

       var fixed float64

       if float64 >0 {
            fixed = math.Float64frombits(math.Float64bits(amount) + 1)
       } else {
            fixed = math.Float64frombits(math.Float64bits(amount) - 1)
       }

       currencyDecimals := math.Pow10(GetCurrency(currency).Fraction)

       return New(int64(fixed * currencyDecimals ), currency)
}

Here the +/- 1 is the minimum "epsilon".

Reference: https://pkg.go.dev/math#Nextafter

[ahummel25]

Hello @kotaroyamazaki, is there any update on this PR? Thank you