Implement IsInteger method (shopspring#179)

decimal.go

@@ -583,6 +583,33 @@ func (d Decimal) Pow(d2 Decimal) Decimal {
 	return temp.Mul(temp).Div(d)
 }
 
+// IsInteger returns true when decimal can be represented as an integer value, otherwise, it returns false.
+func (d Decimal) IsInteger() bool {
+	// The most typical case, all decimal with exponent higher or equal 0 can be represented as integer
+	if d.exp >= 0 {
+		return true
+	}
+	// When the exponent is negative we have to check every number after the decimal place
+	// If all of them are zeroes, we are sure that given decimal can be represented as an integer
+	var r big.Int
+	q := big.NewInt(0).Set(d.value)
+	for z := abs(d.exp); z > 0; z-- {
+		q.QuoRem(q, tenInt, &r)
+		if r.Cmp(zeroInt) != 0 {
+			return false
+		}
+	}
+	return true
+}
+
+// Abs calculates absolute value of any int32. Used for calculating absolute value of decimal's exponent.
+func abs(n int32) int32 {
+	if n < 0 {
+		return -n
+	}
+	return n
+}
+
 // Cmp compares the numbers represented by d and d2 and returns:
 //
 //     -1 if d <  d2

decimal_bench_test.go

@@ -173,3 +173,13 @@ func Benchmark_decimal_Decimal_Sub_same_precision(b *testing.B) {
 		d1.Add(d2)
 	}
 }
+
+func BenchmarkDecimal_IsInteger(b *testing.B) {
+	d := RequireFromString("12.000")
+
+	b.ReportAllocs()
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		d.IsInteger()
+	}
+}

decimal_test.go

@@ -2076,6 +2076,34 @@ func TestNegativePow(t *testing.T) {
 	}
 }
 
+func TestDecimal_IsInteger(t *testing.T) {
+	for _, testCase := range []struct {
+		Dec       string
+		IsInteger bool
+	}{
+		{"0", true},
+		{"0.0000", true},
+		{"0.01", false},
+		{"0.01010101010000", false},
+		{"12.0", true},
+		{"12.00000000000000", true},
+		{"12.10000", false},
+		{"9999.0000", true},
+		{"99999999.000000000", true},
+		{"-656323444.0000000000000", true},
+		{"-32768.01234", false},
+		{"-32768.0123423562623600000", false},
+	} {
+		d, err := NewFromString(testCase.Dec)
+		if err != nil {
+			t.Fatal(err)
+		}
+		if d.IsInteger() != testCase.IsInteger {
+			t.Errorf("expect %t, got %t, for %s", testCase.IsInteger, d.IsInteger(), testCase.Dec)
+		}
+	}
+}
+
 func TestDecimal_Sign(t *testing.T) {
 	if Zero.Sign() != 0 {
 		t.Errorf("%q should have sign 0", Zero)