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GH-38395: [Go] fix rounding errors in decimal256 string functions #38426

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merged 5 commits into from
Oct 24, 2023
Merged

GH-38395: [Go] fix rounding errors in decimal256 string functions #38426

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531d60d
GH-38395: [Go] fix rounding errors in decimal256 string functions
zeroshade Oct 23, 2023
87263cb
fix negative scale support
zeroshade Oct 24, 2023
ea06abe
negative scale ToString
zeroshade Oct 24, 2023
f9ea3ff
i hate fp precision
zeroshade Oct 24, 2023
0cf5021
updates from feedback
zeroshade Oct 24, 2023
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41 changes: 27 additions & 14 deletions go/arrow/decimal128/decimal128.go
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Original file line number Diff line number Diff line change
Expand Up @@ -266,23 +266,36 @@ func FromString(v string, prec, scale int32) (n Num, err error) {
return
}

// Since we're going to truncate this to get an integer, we need to round
// the value instead because of edge cases so that we match how other implementations
// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
// we're positive we'll add 0.5.
out.Mul(out, big.NewFloat(math.Pow10(int(scale)))).SetPrec(precInBits)
if out.Signbit() {
out.Sub(out, pt5)
if scale < 0 {
var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 127 {
return Num{}, errors.New("bitlen too large for decimal128")
}
n = FromBigInt(val)
n, _ = n.Div(scaleMultipliers[-scale])
} else {
out.Add(out, pt5)
}

var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 127 {
return Num{}, errors.New("bitlen too large for decimal128")
// Since we're going to truncate this to get an integer, we need to round
// the value instead because of edge cases so that we match how other implementations
// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
// we're positive we'll add 0.5.
p := (&big.Float{}).SetFloat64(float64PowersOfTen[scale+38])
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out.Mul(out, p).SetPrec(precInBits)
if out.Signbit() {
out.Sub(out, pt5)
} else {
out.Add(out, pt5)
}
Comment on lines +284 to +288
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This is done because the round is a "round up", right?

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more accurately it's a round away from zero

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So -7.0 will become -7.0-0.5=-7.5 and round to -8.0?

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Not quite, check the comment above the spot: we truncate the value to get an integer. So -7.0 with a scale of 0 becomes -7.5 and then we truncate it to -7. -7.5 becomes -8.0 and then we truncate it to -8.

A better example would be if we're given the value -0.75 with a scale of 1: The scale of 1 means that our final Decimal value is going to be the integer -8 because of rounding.

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Thank you for the explanation!


var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 127 {
return Num{}, errors.New("bitlen too large for decimal128")
}
n = FromBigInt(val)
}
n = FromBigInt(val)

if !n.FitsInPrecision(prec) {
err = fmt.Errorf("val %v doesn't fit in precision %d", n, prec)
}
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1 change: 1 addition & 0 deletions go/arrow/decimal128/decimal128_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -659,6 +659,7 @@ func TestFromString(t *testing.T) {
{"1e-37", 1, 37},
{"2112.33", 211233, 2},
{"-2112.33", -211233, 2},
{"12E2", 12, -2},
}

for _, tt := range tests {
Expand Down
39 changes: 25 additions & 14 deletions go/arrow/decimal256/decimal256.go
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Original file line number Diff line number Diff line change
Expand Up @@ -154,23 +154,34 @@ func FromString(v string, prec, scale int32) (n Num, err error) {
return
}

out.Mul(out, (&big.Float{}).SetInt(scaleMultipliers[scale].BigInt())).SetPrec(precInBits)
// Since we're going to truncate this to get an integer, we need to round
// the value instead because of edge cases so that we match how other implementations
// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
// we're positive we'll add 0.5.
if out.Signbit() {
out.Sub(out, pt5)
if scale < 0 {
var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 255 {
return Num{}, errors.New("bitlen too large for decimal256")
}
n = FromBigInt(val)

n, _ = n.Div(scaleMultipliers[-scale])
} else {
out.Add(out, pt5)
}
out.Mul(out, (&big.Float{}).SetFloat64(float64PowersOfTen[scale+76])).SetPrec(precInBits)
// Since we're going to truncate this to get an integer, we need to round
// the value instead because of edge cases so that we match how other implementations
// (e.g. C++) handles Decimal values. So if we're negative we'll subtract 0.5 and if
// we're positive we'll add 0.5.
if out.Signbit() {
out.Sub(out, pt5)
} else {
out.Add(out, pt5)
}

var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 255 {
return Num{}, errors.New("bitlen too large for decimal256")
var tmp big.Int
val, _ := out.Int(&tmp)
if val.BitLen() > 255 {
return Num{}, errors.New("bitlen too large for decimal256")
}
n = FromBigInt(val)
}
n = FromBigInt(val)
if !n.FitsInPrecision(prec) {
err = fmt.Errorf("value %v doesn't fit in precision %d", n, prec)
}
Expand Down
1 change: 1 addition & 0 deletions go/arrow/decimal256/decimal256_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -564,6 +564,7 @@ func TestFromString(t *testing.T) {
{"1e-37", 1, 37},
{"2112.33", 211233, 2},
{"-2112.33", -211233, 2},
{"12E2", 12, -2},
}

for _, tt := range tests {
Expand Down
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