diff options
Diffstat (limited to 'decimal.go')
| -rw-r--r-- | decimal.go | 81 |
1 files changed, 74 insertions, 7 deletions
@@ -171,17 +171,84 @@ func RequireFromString(value string) Decimal { // NewFromFloat converts a float64 to Decimal. // -// Example: -// -// NewFromFloat(123.45678901234567).String() // output: "123.4567890123456" -// NewFromFloat(.00000000000000001).String() // output: "0.00000000000000001" +// The converted number will contain the number of significant digits that can be +// represented in a float with reliable roundtrip. +// This is typically 15 digits, but may be more in some cases. +// See https://www.exploringbinary.com/decimal-precision-of-binary-floating-point-numbers/ for more information. // -// NOTE: some float64 numbers can take up about 300 bytes of memory in decimal representation. -// Consider using NewFromFloatWithExponent if space is more important than precision. +// For slightly faster conversion, use NewFromFloatWithExponent where you can specify the precision in absolute terms. // // NOTE: this will panic on NaN, +/-inf func NewFromFloat(value float64) Decimal { - return NewFromFloatWithExponent(value, math.MinInt32) + if value == 0 { + return New(0, 0) + } + return newFromFloat(value, math.Float64bits(value), &float64info) +} + +// NewFromFloat converts a float32 to Decimal. +// +// The converted number will contain the number of significant digits that can be +// represented in a float with reliable roundtrip. +// This is typically 6-8 digits depending on the input. +// See https://www.exploringbinary.com/decimal-precision-of-binary-floating-point-numbers/ for more information. +// +// For slightly faster conversion, use NewFromFloatWithExponent where you can specify the precision in absolute terms. +// +// NOTE: this will panic on NaN, +/-inf +func NewFromFloat32(value float32) Decimal { + if value == 0 { + return New(0, 0) + } + // XOR is workaround for https://github.com/golang/go/issues/26285 + a := math.Float32bits(value) ^ 0x80808080 + return newFromFloat(float64(value), uint64(a)^0x80808080, &float32info) +} + +func newFromFloat(val float64, bits uint64, flt *floatInfo) Decimal { + if math.IsNaN(val) || math.IsInf(val, 0) { + panic(fmt.Sprintf("Cannot create a Decimal from %v", val)) + } + exp := int(bits>>flt.mantbits) & (1<<flt.expbits - 1) + mant := bits & (uint64(1)<<flt.mantbits - 1) + + switch exp { + case 0: + // denormalized + exp++ + + default: + // add implicit top bit + mant |= uint64(1) << flt.mantbits + } + exp += flt.bias + + var d decimal + d.Assign(mant) + d.Shift(exp - int(flt.mantbits)) + d.neg = bits>>(flt.expbits+flt.mantbits) != 0 + + roundShortest(&d, mant, exp, flt) + // If less than 19 digits, we can do calculation in an int64. + if d.nd < 19 { + tmp := int64(0) + m := int64(1) + for i := d.nd - 1; i >= 0; i-- { + tmp += m * int64(d.d[i]-'0') + m *= 10 + } + if d.neg { + tmp *= -1 + } + return Decimal{value: big.NewInt(tmp), exp: int32(d.dp) - int32(d.nd)} + } + dValue := new(big.Int) + dValue, ok := dValue.SetString(string(d.d[:d.nd]), 10) + if ok { + return Decimal{value: dValue, exp: int32(d.dp) - int32(d.nd)} + } + + return NewFromFloatWithExponent(val, int32(d.dp)-int32(d.nd)) } // NewFromFloatWithExponent converts a float64 to Decimal, with an arbitrary |