Added comparisons to Amount struct

money.go

@@ -29,8 +29,10 @@ import (
 	"strings"
 )
 
+// Errors returned by the Amount functions.
 var (
-	errInvalidJSONString = errors.New("Invalid JSON, field must be a string \"xxx.cc\"")
+	ErrInvalidJSONString = errors.New("Invalid JSON, field must be a string \"xxx.cc\"")
+	ErrMixedCurrency     = errors.New("amounts have different currencies")
 )
 
 // Amount describes a monetary amount.
@@ -44,8 +46,83 @@ type Amount struct {
 // Cents represents an integer number of cents (1/100th unit of currency).
 type Cents int
 
+// NewAmount creates a new amount of integer cents.
+func NewAmount(currency string, cents int) Amount {
+	return Amount{
+		Currency: currency,
+		Value:    Cents(cents),
+	}
+}
+
+// Add returns the sum. Currencies must match.
+func (amt Amount) Add(other Amount) (sum Amount, err error) {
+	if amt.Currency != other.Currency {
+		err = ErrMixedCurrency
+		return
+	}
+
+	sum = Amount{
+		Currency: amt.Currency,
+		Value:    amt.Value.Add(other.Value),
+	}
+	return
+}
+
+// Sub returns the difference. Currencies must match.
+func (amt Amount) Sub(other Amount) (sum Amount, err error) {
+	if amt.Currency != other.Currency {
+		err = ErrMixedCurrency
+		return
+	}
+
+	sum = Amount{
+		Currency: amt.Currency,
+		Value:    amt.Value.Sub(other.Value),
+	}
+	return
+}
+
+func (amt Amount) cmp(other Amount) (int, error) {
+	if amt.Currency != other.Currency {
+		return 0, ErrMixedCurrency
+	}
+
+	diff := amt.Value.Sub(other.Value).Cents()
+	switch {
+	case diff < 0:
+		return -1, nil
+	case diff > 0:
+		return 1, nil
+	default:
+		return 0, nil
+	}
+}
+
+// LessThan returns true iff amt < other
+func (amt Amount) LessThan(other Amount) (bool, error) {
+	cmp, err := amt.cmp(other)
+	if err != nil {
+		return false, err
+	}
+	return cmp < 0, nil
+}
+
+// GreaterThan returns true iff amt > other
+func (amt Amount) GreaterThan(other Amount) (bool, error) {
+	cmp, err := amt.cmp(other)
+	if err != nil {
+		return false, err
+	}
+	return cmp > 0, nil
+}
+
+// Equals returns true if both amounts are equal in currency and value.
+func (amt Amount) Equals(other Amount) bool {
+	return amt.Currency == other.Currency && amt.Value == other.Value
+}
+
 func (amt Amount) String() string {
-	return fmt.Sprintf("%s %s", amt.Value.String(), amt.Currency)
+	return fmt.Sprintf("%s %s", amt.Currency, amt.Value.String())
 }
 
 // Cents returns the number of cents as int.
@@ -53,6 +130,16 @@ func (c Cents) Cents() int {
 	return int(c)
 }
 
+// Add returns the sum
+func (c Cents) Add(other Cents) Cents {
+	return Cents(int(c) + int(other))
+}
+
+// Sub returns the difference
+func (c Cents) Sub(other Cents) Cents {
+	return Cents(int(c) - int(other))
+}
+
 func (c Cents) String() string {
 	cents := int(c)
 	return fmt.Sprintf("%d.%02d", cents/100, cents%100)
@@ -62,13 +149,13 @@ func (c Cents) String() string {
 func (c *Cents) UnmarshalJSON(input []byte) error {
 	inLen := len(input)
 	if inLen < 2 || input[0] != '"' || input[inLen-1] != '"' {
-		return errInvalidJSONString
+		return ErrInvalidJSONString
 	}
 
 	asString := string(input[1 : inLen-1])
 	parts := strings.Split(asString, ".")
 	if len(parts) != 2 {
-		return errInvalidJSONString
+		return ErrInvalidJSONString
 	}
 
 	wholeUnits, err := strconv.Atoi(parts[0])
@@ -80,7 +167,7 @@ func (c *Cents) UnmarshalJSON(input []byte) error {
 		return err
 	}
 	if cents >= 100 {
-		return errInvalidJSONString
+		return ErrInvalidJSONString
 	}
 
 	*c = Cents(wholeUnits*100 + cents)

money_test.go

@@ -54,3 +54,41 @@ func TestCents_MarshalJSON_Happy(t *testing.T) {
 	assert.Nil(t, err)
 	assert.Equal(t, `{"value":"123.45","currency":"EUR"}`, string(asJSON))
 }
+
+func TestAmount_Comparisons(t *testing.T) {
+	a := NewAmount("EUR", 47)
+	b := NewAmount("EUR", 327)
+
+	sum, err := a.Add(b)
+	assert.Nil(t, err)
+	assert.Equal(t, a.Currency, sum.Currency)
+	assert.Equal(t, 47+327, sum.Value.Cents())
+	assert.Equal(t, 47, a.Value.Cents())
+	assert.Equal(t, 327, b.Value.Cents())
+
+	diff, err := a.Sub(b)
+	assert.Nil(t, err)
+	assert.Equal(t, a.Currency, diff.Currency)
+	assert.Equal(t, 47-327, diff.Value.Cents())
+	assert.Equal(t, 47, a.Value.Cents())
+	assert.Equal(t, 327, b.Value.Cents())
+
+	isLess, err := a.LessThan(b)
+	assert.Nil(t, err)
+	assert.True(t, isLess)
+	isLess, err = b.LessThan(a)
+	assert.Nil(t, err)
+	assert.False(t, isLess)
+
+	isGreater, err := a.GreaterThan(b)
+	assert.Nil(t, err)
+	assert.False(t, isGreater)
+	isGreater, err = b.GreaterThan(a)
+	assert.Nil(t, err)
+	assert.True(t, isGreater)
+
+	assert.True(t, a.Equals(a))
+	assert.True(t, a.Equals(NewAmount("EUR", 47)))
+	assert.False(t, a.Equals(b))
+	assert.False(t, a.Equals(NewAmount("HRK", a.Value.Cents())))
+}