Add Copy func (#123)

* Add Copy method
Co-authored-by: habuvo <Sin90is0>

decimal.go

@@ -1755,7 +1755,7 @@ func (d Decimal) Sin() Decimal {
 		sign = !sign
 		j -= 4
 	}
-	z := d.Sub(y.Mul(PI4A)).Sub(y.Mul(PI4B)).Sub(y.Mul(PI4C)) // Extended precision modular arithmetic
+  	z := d.Sub(y.Mul(PI4A)).Sub(y.Mul(PI4B)).Sub(y.Mul(PI4C)) // Extended precision modular arithmetic
 	zz := z.Mul(z)
 
 	if j == 1 || j == 2 {
@@ -1842,12 +1842,12 @@ var _tanQ = [...]Decimal{
 
 // Tan returns the tangent of the radian argument x.
 func (d Decimal) Tan() Decimal {
-
+	
 	PI4A := NewFromFloat(7.85398125648498535156e-1)                             // 0x3fe921fb40000000, Pi/4 split into three parts
 	PI4B := NewFromFloat(3.77489470793079817668e-8)                             // 0x3e64442d00000000,
 	PI4C := NewFromFloat(2.69515142907905952645e-15)                            // 0x3ce8469898cc5170,
 	M4PI := NewFromFloat(1.273239544735162542821171882678754627704620361328125) // 4/pi
-
+	
 	if d.Equal(NewFromFloat(0.0)) {
 		return d
 	}
@@ -1886,3 +1886,14 @@ func (d Decimal) Tan() Decimal {
 	}
 	return y
 }
+
+// Copy makes instance of d with same value and different pointer.
+func (d Decimal) Copy() Decimal {
+	d.ensureInitialized()
+	return Decimal{
+		value: &(*d.value),
+		exp:   d.exp,
+	}
+}
+
+

decimal_test.go

@@ -3288,3 +3288,21 @@ func ExampleNewFromFloat() {
 	//0.123123123123123
 	//-10000000000000
 }
+
+//For Copy
+func TestCopy(t *testing.T) {
+	origin := New(1, 0)
+	cpy := origin.Copy()
+
+	if cpy.Cmp(origin) != 0 {
+		t.Error("copy and origin not equal")
+	}
+
+	//change value
+	cpy = cpy.Add(New(1, 0))
+
+	if cpy.Cmp(origin) == 0 {
+		t.Error("copy and origin are equal but expected not")
+	}
+
+}