Merge 6e54f647f0 into 2b68c56fe0

* Adjust Ln method to prevent infinity iteration loops
* Add test case for infinity loop

decimal.go

@@ -51,6 +51,14 @@ var DivisionPrecision = 16
 // silently lose precision.
 var MarshalJSONWithoutQuotes = false
 
+// StringTrimTrailingZeros should be set to false if you want the decimal stringify without zeros trailing.
+// By default, when decimal is output as a string (for example, in JSON), zeros are truncated from it (2.00 -> 2, 3.11 -> 3.11, 13.000 -> 13).
+// But this logic can be changed by this variable.
+// For example, if you have numeric(10,2) values stored in your database,
+// and you want your API response to always be given 2 decimal places (even 2.00, 3.00, 17.00 [not 2,3,17]),
+// then this variable is a great way out.
+var StringTrimTrailingZeros = true
+
 // ExpMaxIterations specifies the maximum number of iterations needed to calculate
 // precise natural exponent value using ExpHullAbrham method.
 var ExpMaxIterations = 1000
@@ -129,11 +137,10 @@ 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 +927,10 @@ 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 +944,17 @@ 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
 		}
 	}
 
@@ -1162,7 +1180,7 @@ func (d Decimal) InexactFloat64() float64 {
 //
 //	-12.345
 func (d Decimal) String() string {
-	return d.string(true)
+	return d.string(StringTrimTrailingZeros)
 }
 
 // StringFixed returns a rounded fixed-point string with places digits after

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)
@@ -3452,3 +3453,48 @@ func ExampleNewFromFloat() {
 	//0.123123123123123
 	//-10000000000000
 }
+
+func TestDecimal_String(t *testing.T) {
+	type testData struct {
+		input string
+		expected string
+	}
+
+	tests := []testData{
+		{"1.22", "1.22"},
+		{"1.00", "1"},
+		{"153.192", "153.192"},
+		{"999.999", "999.999"},
+		{"0.0000000001", "0.0000000001"},
+		{"0.0000000000", "0"},
+	}
+
+	for _, test := range tests {
+		d, err := NewFromString(test.input);
+		if err != nil {
+			t.Fatal(err)
+		} else if d.String() != test.expected {
+			t.Errorf("expected %s, got %s", test.expected, d.String())
+		}
+	}
+
+	defer func() {
+		StringTrimTrailingZeros = true
+	}()
+
+	StringTrimTrailingZeros = false
+	tests = []testData{
+		{"1.00", "1.00"},
+		{"0.00", "0.00"},
+		{"129.123000", "129.123000"},
+	}
+
+	for _, test := range tests {
+		d, err := NewFromString(test.input);
+		if err != nil {
+			t.Fatal(err)
+		} else if d.String() != test.expected {
+			t.Errorf("expected %s, got %s", test.expected, d.String())
+		}
+	}
+}