From 2b68c56fe017397c40a776606c62df0a5f943c75 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Mateusz=20Wo=C5=9B?= <wos.mateusz16@gmail.com>
Date: Mon, 1 Apr 2024 22:02:25 +0200
Subject: [PATCH] Adjust Ln method to prevent infinity iteration loops (#357)

* Adjust Ln method to prevent infinity iteration loops
* Add test case for infinity loop
---
 decimal.go      | 22 ++++++++++++++++------
 decimal_test.go |  1 +
 2 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/decimal.go b/decimal.go
index eed845f..0224292 100644
--- a/decimal.go
+++ b/decimal.go
@@ -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
 		}
 	}
 
diff --git a/decimal_test.go b/decimal_test.go
index 0e7b641..841f205 100644
--- a/decimal_test.go
+++ b/decimal_test.go
@@ -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)