Fix allocate panic

calculator.go

@@ -24,7 +24,11 @@ func (c *calculator) modulus(a Amount, d int64) Amount {
 	return a % d
 }
 
-func (c *calculator) allocate(a Amount, r, s int) Amount {
+func (c *calculator) allocate(a Amount, r, s uint) Amount {
+	if a == 0 || s == 0 {
+		return 0
+	}
+
 	return a * int64(r) / int64(s)
 }
 

money.go

@@ -265,23 +265,32 @@ func (m *Money) Allocate(rs ...int) ([]*Money, error) {
 	}
 
 	// Calculate sum of ratios.
-	var sum int
+	var sum uint
 	for _, r := range rs {
-		sum += r
+		if r < 0 {
+			return nil, errors.New("negative ratios not allowed")
+		}
+		sum += uint(r)
 	}
 
 	var total int64
 	ms := make([]*Money, 0, len(rs))
 	for _, r := range rs {
 		party := &Money{
-			amount:   mutate.calc.allocate(m.amount, r, sum),
+			amount:   mutate.calc.allocate(m.amount, uint(r), sum),
 			currency: m.currency,
 		}
 
 		ms = append(ms, party)
 		total += party.amount
 	}
 
+	// if the sum of all ratios is zero, then we just returns zeros and don't do anything
+	// with the leftover
+	if sum == 0 {
+		return ms, nil
+	}
+
 	// Calculate leftover value and divide to first parties.
 	lo := m.amount - total
 	sub := int64(1)

money_test.go

@@ -470,6 +470,10 @@ func TestMoney_Allocate(t *testing.T) {
 		{100, []int{30, 30, 30}, []int64{34, 33, 33}},
 		{200, []int{25, 25, 50}, []int64{50, 50, 100}},
 		{5, []int{50, 25, 25}, []int64{3, 1, 1}},
+		{0, []int{0, 0, 0, 0}, []int64{0, 0, 0, 0}},
+		{0, []int{50, 10}, []int64{0, 0}},
+		{10, []int{0, 100}, []int64{0, 10}},
+		{10, []int{0, 0}, []int64{0, 0}},
 	}
 
 	for _, tc := range tcs {