Optimize NumDigits

Dividing BitLen by math.Log2(10) is what math/big does underneath

Not including the Int64/Uint64 check makes this slightly slower than old method

Included 2 benchmarks, for 10 digit numbers & 100 digit numbers:

-- before
> go test -bench=NumDigit -run=NumDigit
goos: linux
goarch: amd64
pkg: github.com/shopspring/decimal
cpu: AMD Ryzen 7 7840U w/ Radeon  780M Graphics
BenchmarkDecimal_NumDigits10-16     	18317293	        63.87 ns/op
BenchmarkDecimal_NumDigits100-16    	 3645015	       329.6 ns/op

-- after
...
BenchmarkDecimal_NumDigits10-16     	143781325	         8.488 ns/op
BenchmarkDecimal_NumDigits100-16    	 5931247	       207.4 ns/op
This commit is contained in:
Philip Dubé 2024-03-28 19:15:46 +00:00
parent 547861c49b
commit afe2a66041
2 changed files with 55 additions and 10 deletions

View file

@ -1224,14 +1224,31 @@ func (d Decimal) Ln(precision int32) (Decimal, error) {
}
// NumDigits returns the number of digits of the decimal coefficient (d.Value)
// Note: Current implementation is extremely slow for large decimals and/or decimals with large fractional part
func (d Decimal) NumDigits() int {
d.ensureInitialized()
// Note(mwoss): It can be optimized, unnecessary cast of big.Int to string
if d.IsNegative() {
return len(d.value.String()) - 1
if d.value == nil {
return 1
}
return len(d.value.String())
if d.value.IsUint64() {
u64 := d.value.Uint64()
if u64 < (1 << 53) {
if u64 == 0 {
return 1
}
return int(math.Log10(float64(u64))) + 1
}
} else if d.value.IsInt64() {
i64 := d.value.Int64()
if i64 > -(1 << 53) {
return int(math.Log10(float64(-i64))) + 1
}
}
abs := new(big.Int).Abs(d.value)
// lg10 may be off by 1, need to verify
lg10 := int(float64(abs.BitLen()) / math.Log2(10))
check := big.NewInt(int64(lg10))
return lg10 + abs.Cmp(check.Exp(tenInt, check, nil))
}
// IsInteger returns true when decimal can be represented as an integer value, otherwise, it returns false.

View file

@ -121,6 +121,34 @@ func BenchmarkDecimal_RoundCash_Five(b *testing.B) {
}
}
func numDigits(b *testing.B, want int, val Decimal) {
b.Helper()
for i := 0; i < b.N; i++ {
if have := val.NumDigits(); have != want {
b.Fatalf("\nHave: %q\nWant: %q", have, want)
}
}
}
func BenchmarkDecimal_NumDigits10(b *testing.B) {
numDigits(b, 10, New(3478512345, -3))
}
func BenchmarkDecimal_NumDigits100(b *testing.B) {
s := make([]byte, 102)
for i := range s {
s[i] = byte('0' + i%10)
}
s[0] = '-'
s[100] = '.'
d, err := NewFromString(string(s))
if err != nil {
b.Log(d)
b.Error(err)
}
numDigits(b, 100, d)
}
func Benchmark_Cmp(b *testing.B) {
decimals := DecimalSlice([]Decimal{})
for i := 0; i < 1000000; i++ {
@ -132,7 +160,7 @@ func Benchmark_Cmp(b *testing.B) {
}
}
func Benchmark_decimal_Decimal_Add_different_precision(b *testing.B) {
func BenchmarkDecimal_Add_different_precision(b *testing.B) {
d1 := NewFromFloat(1000.123)
d2 := NewFromFloat(500).Mul(NewFromFloat(0.12))
@ -143,7 +171,7 @@ func Benchmark_decimal_Decimal_Add_different_precision(b *testing.B) {
}
}
func Benchmark_decimal_Decimal_Sub_different_precision(b *testing.B) {
func BenchmarkDecimal_Sub_different_precision(b *testing.B) {
d1 := NewFromFloat(1000.123)
d2 := NewFromFloat(500).Mul(NewFromFloat(0.12))
@ -154,7 +182,7 @@ func Benchmark_decimal_Decimal_Sub_different_precision(b *testing.B) {
}
}
func Benchmark_decimal_Decimal_Add_same_precision(b *testing.B) {
func BenchmarkDecimal_Add_same_precision(b *testing.B) {
d1 := NewFromFloat(1000.123)
d2 := NewFromFloat(500.123)
@ -165,7 +193,7 @@ func Benchmark_decimal_Decimal_Add_same_precision(b *testing.B) {
}
}
func Benchmark_decimal_Decimal_Sub_same_precision(b *testing.B) {
func BenchmarkDecimal_Sub_same_precision(b *testing.B) {
d1 := NewFromFloat(1000.123)
d2 := NewFromFloat(500.123)