chore: add public method to expose Compare behavior (#120)

Co-authored-by: Emerson Costa Silva <emerson@Emersons-MacBook-Pro.local>

README.md

@@ -73,6 +73,7 @@ Comparison
 * GreaterThanOrEqual
 * LessThan
 * LessThanOrEqual
+* Compare
 
 Comparisons must be made between the same currency units.
 
@@ -84,6 +85,10 @@ twoEuros := money.New(200, money.EUR)
 pound.GreaterThan(twoPounds) // false, nil
 pound.LessThan(twoPounds) // true, nil
 twoPounds.Equals(twoEuros) // false, error: Currencies don't match
+twoPounds.Compare(pound) // 1, nil
+pound.Compare(twoPounds) // -1, nil
+pound.Compare(pound) // 0, nil
+pound.Compare(twoEuros) // pound.amount, ErrCurrencyMismatch
 ```
 Asserts
 -

money.go

@@ -318,3 +318,16 @@ func (m *Money) UnmarshalJSON(b []byte) error {
 func (m Money) MarshalJSON() ([]byte, error) {
 	return MarshalJSON(m)
 }
+
+// Compare function compares two money of the same type
+//  if m.amount > om.amount returns (1, nil)
+//  if m.amount == om.amount returns (0, nil
+//  if m.amount < om.amount returns (-1, nil)
+// If compare moneys from distinct currency, return (m.amount, ErrCurrencyMismatch)
+func (m *Money) Compare(om *Money) (int, error) {
+	if err := m.assertSameCurrency(om); err != nil {
+		return int(m.amount), err
+	}
+
+	return m.compare(om), nil
+}

money_test.go

@@ -611,6 +611,27 @@ func TestMoney_Comparison(t *testing.T) {
 	if r, err := pound.GreaterThanOrEqual(twoEuros); err == nil || r {
 		t.Error("Expected err")
 	}
+
+	if r, err := twoPounds.Compare(pound); r != 1 && err != nil {
+		t.Errorf("Expected %d Greater Than %d == %d got %d", pound.amount,
+			twoPounds.amount, 1, r)
+	}
+
+	if r, err := pound.Compare(twoPounds); r != -1 && err != nil {
+		t.Errorf("Expected %d Less Than %d == %d got %d", pound.amount,
+			twoPounds.amount, -1, r)
+	}
+
+	if _, err := pound.Compare(twoEuros); err != ErrCurrencyMismatch {
+		t.Error("Expected err")
+	}
+
+	anotherTwoEuros := New(200, EUR)
+	if r, err := twoEuros.Compare(anotherTwoEuros); r != 0 && err != nil {
+		t.Errorf("Expected %d Equals to %d == %d got %d", anotherTwoEuros.amount,
+			twoEuros.amount, 0, r)
+	}
+
 }
 
 func TestMoney_Currency(t *testing.T) {