Add support for RoundHalfEven aka "Banker's Rounding" (#22)

amount.go

@@ -26,6 +26,9 @@ const (
 	RoundUp
 	// RoundDown rounds towards 0, truncating extra digits.
 	RoundDown
+	// RoundHalfEven rounds up if the next digit is > 5. If the next digit is equal
+	// to 5, it rounds to the nearest even decimal. Also called bankers' rounding.
+	RoundHalfEven
 )
 
 // InvalidNumberError is returned when a numeric string can't be converted to a decimal.
@@ -401,6 +404,7 @@ func roundingContext(decimal *apd.Decimal, mode RoundingMode) *apd.Context {
 		RoundHalfDown: apd.RoundHalfDown,
 		RoundUp:       apd.RoundUp,
 		RoundDown:     apd.RoundDown,
+		RoundHalfEven: apd.RoundHalfEven,
 	}
 	ctx := *decimalContext(decimal)
 	ctx.Rounding = extModes[mode]

amount_test.go

@@ -529,11 +529,21 @@ func TestAmount_RoundTo(t *testing.T) {
 		{"12.345", 2, currency.RoundDown, "12.34"},
 		{"12.347", 2, currency.RoundDown, "12.34"},
 
+		{"12.344", 2, currency.RoundHalfEven, "12.34"},
+		{"12.345", 2, currency.RoundHalfEven, "12.34"},
+		{"12.346", 2, currency.RoundHalfEven, "12.35"},
+
+		{"12.334", 2, currency.RoundHalfEven, "12.33"},
+		{"12.335", 2, currency.RoundHalfEven, "12.34"},
+		{"12.336", 2, currency.RoundHalfEven, "12.34"},
+
 		// Negative amounts.
 		{"-12.345", 2, currency.RoundHalfUp, "-12.35"},
 		{"-12.345", 2, currency.RoundHalfDown, "-12.34"},
 		{"-12.345", 2, currency.RoundUp, "-12.35"},
 		{"-12.345", 2, currency.RoundDown, "-12.34"},
+		{"-12.345", 2, currency.RoundHalfEven, "-12.34"},
+		{"-12.335", 2, currency.RoundHalfEven, "-12.34"},
 
 		// More digits that the amount has.
 		{"12.345", 4, currency.RoundHalfUp, "12.3450"},