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Adjust Ln method to prevent infinity iteration loops (#357)

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* Adjust Ln method to prevent infinity iteration loops
* Add test case for infinity loop
This commit is contained in:
Mateusz Woś 2024-04-01 22:02:25 +02:00 committed by GitHub
parent d00399e161
commit 2b68c56fe0
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2 changed files with 17 additions and 6 deletions
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22
decimal.go
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@ -129,11 +129,10 @@ func NewFromBigInt(value *big.Int, exp int32) Decimal {
//
// Example:
//
// d1 := NewFromBigRat(big.NewRat(0, 1), 0) // output: "0"
// d2 := NewFromBigRat(big.NewRat(4, 5), 1) // output: "0.8"
// d3 := NewFromBigRat(big.NewRat(1000, 3), 3) // output: "333.333"
// d4 := NewFromBigRat(big.NewRat(2, 7), 4) // output: "0.2857"
//
// d1 := NewFromBigRat(big.NewRat(0, 1), 0) // output: "0"
// d2 := NewFromBigRat(big.NewRat(4, 5), 1) // output: "0.8"
// d3 := NewFromBigRat(big.NewRat(1000, 3), 3) // output: "333.333"
// d4 := NewFromBigRat(big.NewRat(2, 7), 4) // output: "0.2857"
func NewFromBigRat(value *big.Rat, precision int32) Decimal {
return Decimal{
value: new(big.Int).Set(value.Num()),
@ -920,7 +919,10 @@ func (d Decimal) Ln(precision int32) (Decimal, error) {
// Halley's Iteration.
// Calculating n-th term of formula: a_(n+1) = a_n - 2 * (exp(a_n) - z) / (exp(a_n) + z),
// until the difference between current and next term is smaller than epsilon
for {
var prevStep Decimal
maxIters := calcPrecision*2 + 10
for i := int32(0); i < maxIters; i++ {
// exp(a_n)
comp3, _ = comp1.ExpTaylor(calcPrecision)
// exp(a_n) - z
@ -934,9 +936,17 @@ func (d Decimal) Ln(precision int32) (Decimal, error) {
// comp1 = a_(n+1) = a_n - 2 * (exp(a_n) - z) / (exp(a_n) + z)
comp1 = comp1.Sub(comp3)
if prevStep.Add(comp3).IsZero() {
// If iteration steps oscillate we should return early and prevent an infinity loop
// NOTE(mwoss): This should be quite a rare case, returning error is not necessary
break
}
if comp3.Abs().Cmp(epsilon) <= 0 {
break
}
prevStep = comp3
}
}

1
decimal_test.go
View file

@ -2822,6 +2822,7 @@ func TestDecimal_Ln(t *testing.T) {
{"839101.0351094726488848490572028502", 50, "13.64008640145229044389152437468283605382056561604272"},
{"5023583755703750094849.03519358513093500275017501750602739169823", 25, "49.9684305274348922267409953"},
{"5023583755703750094849.03519358513093500275017501750602739169823", -1, "50.0"},
{"66.12", 18, "4.191471272952823429"},
} {
d, _ := NewFromString(testCase.Dec)
expected, _ := NewFromString(testCase.Expected)