math: correct Pow(..., x, 0.5) (#147)

Fixes #146

Author: ericlagergren

math/math_test.go

@@ -3,11 +3,26 @@ package math_test
 import (
 	"testing"
 
+	"github.com/ericlagergren/decimal"
 	"github.com/ericlagergren/decimal/internal/test"
+	"github.com/ericlagergren/decimal/math"
 )
 
 func TestExp(t *testing.T)   { test.Exp.Test(t) }
 func TestLog(t *testing.T)   { test.Log.Test(t) }
 func TestLog10(t *testing.T) { test.Log10.Test(t) }
 func TestPow(t *testing.T)   { test.Pow.Test(t) }
 func TestSqrt(t *testing.T)  { test.Sqrt.Test(t) }
+
+func TestIssue146(t *testing.T) {
+	one := decimal.New(1, 0)
+	for i := int64(0); i < 10; i++ {
+		n := decimal.New(i, 1)
+		firstPow := math.Pow(new(decimal.Big), n, one)
+		if n.Cmp(firstPow) != 0 {
+			t.Errorf("%v^1 != %v", n, firstPow)
+		} else {
+			t.Logf("%v^1 == %v", n, firstPow)
+		}
+	}
+}

math/pow.go

@@ -54,7 +54,7 @@ func Pow(z, x, y *decimal.Big) *decimal.Big {
 		return z.SetUint64(1)
 	}
 
-	if x.Cmp(ptFive) == 0 {
+	if y.Cmp(ptFive) == 0 {
 		// x ** 0.5 = sqrt(x)
 		return Sqrt(z, x)
 	}