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binomial: return 0 for k < 0 #84

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17 changes: 16 additions & 1 deletion src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1131,6 +1131,8 @@ fn multiply_and_divide<T: Integer + Clone>(r: T, a: T, b: T) -> T {

/// Calculate the binomial coefficient.
///
/// Returns 0 if `k > n` or `k < 0`.
///
/// Note that this might overflow, depending on `T`. For the primitive integer
/// types, the following n are the largest ones possible such that there will
/// be no overflow for any k:
Expand All @@ -1149,7 +1151,7 @@ fn multiply_and_divide<T: Integer + Clone>(r: T, a: T, b: T) -> T {
/// For larger n, consider using a bigint type for `T`.
pub fn binomial<T: Integer + Clone>(mut n: T, k: T) -> T {
// See http://blog.plover.com/math/choose.html for the idea.
if k > n {
if k > n || k < T::zero() {
return T::zero();
}
if k > n.clone() - k.clone() {
Expand All @@ -1169,6 +1171,8 @@ pub fn binomial<T: Integer + Clone>(mut n: T, k: T) -> T {
}

/// Calculate the multinomial coefficient.
///
/// Returns 0 if any entry of `k` is negative.
pub fn multinomial<T: Integer + Clone>(k: &[T]) -> T
where
for<'a> T: Add<&'a T, Output = T>,
Expand Down Expand Up @@ -1311,6 +1315,12 @@ fn test_binomial() {
check!(i64, 14, 4, 1001);
check!(i64, 0, 0, 1);
check!(i64, 2, 3, 0);

assert_eq!(binomial(5i64, -1), 0);
assert_eq!(binomial(5i64, -3), 0);
assert_eq!(binomial(0i64, -1), 0);
assert_eq!(binomial(-3i64, -5), 0);
assert_eq!(binomial(-3i64, 1), 0);
}

#[test]
Expand Down Expand Up @@ -1385,4 +1395,9 @@ fn test_multinomial() {
check_multinomial!(u64, &[], 1);
check_multinomial!(u64, &[0], 1);
check_multinomial!(u64, &[12345], 1);

assert_eq!(multinomial(&[-1i64, 0]), 0);
assert_eq!(multinomial(&[0i64, -1]), 0);
assert_eq!(multinomial(&[5i64, -3, 2]), 0);
assert_eq!(multinomial(&[-3i64, 5, 2]), 0);
}