Fix a.BigInt() for negative amounts (the result was always positive).

Fixes #27.

amount.go

@@ -122,15 +122,22 @@ func (a Amount) String() string {
 
 // BigInt returns a in minor units, as a big.Int.
 func (a Amount) BigInt() *big.Int {
-	r := a.Round()
-	return r.number.Coeff.MathBigInt()
+	a = a.Round()
+	n := a.number.Coeff.MathBigInt()
+	if a.IsNegative() {
+		// The coefficient is always positive, apd stores the sign separately.
+		n = n.Neg(n)
+	}
+
+	return n
 }
 
 // Int64 returns a in minor units, as an int64.
 // If a cannot be represented in an int64, an error is returned.
 func (a Amount) Int64() (int64, error) {
 	n := a.Round().number
 	n.Exponent = 0
+
 	return n.Int64()
 }
 

amount_test.go

@@ -90,6 +90,7 @@ func TestNewAmountFromBigInt(t *testing.T) {
 		wantNumber   string
 	}{
 		{big.NewInt(2099), "USD", "20.99"},
+		{big.NewInt(-2099), "USD", "-20.99"},
 		{big.NewInt(5000), "USD", "50.00"},
 		{big.NewInt(50), "JPY", "50"},
 		{hugeInt, "USD", "9223372036854775987.99"},
@@ -163,6 +164,9 @@ func TestAmount_BigInt(t *testing.T) {
 		// Number with no decimals.
 		{"50", "USD", big.NewInt(5000)},
 		{"50", "JPY", big.NewInt(50)},
+		// Negative number.
+		{"-12.3564", "USD", big.NewInt(-1236)},
+		{"-50", "JPY", big.NewInt(-50)},
 	}
 
 	for _, tt := range tests {