Fix integer overflow in combinations size-hint tests

src/adaptors/mod.rs

@@ -834,8 +834,9 @@ impl_tuple_combination!(Tuple10Combination Tuple9Combination; a b c d e f g h i)
 impl_tuple_combination!(Tuple11Combination Tuple10Combination; a b c d e f g h i j);
 impl_tuple_combination!(Tuple12Combination Tuple11Combination; a b c d e f g h i j k);
 
+/// Computes the binomial coefficient, returning `None` if it overflows `usize`.
 // https://en.wikipedia.org/wiki/Binomial_coefficient#In_programming_languages
-pub(crate) fn checked_binomial(mut n: usize, mut k: usize) -> Option<usize> {
+pub fn checked_binomial(mut n: usize, mut k: usize) -> Option<usize> {
     if n < k {
         return Some(0);
     }

src/lib.rs

@@ -158,6 +158,9 @@ pub mod traits {
     pub use crate::tuple_impl::HomogeneousTuple;
 }
 
+// Shared with integration tests.
+#[doc(hidden)]
+pub use crate::adaptors::checked_binomial;
 #[cfg(feature = "use_alloc")]
 use crate::combinations_with_replacement::ArrayCombinationsWithReplacement;
 pub use crate::concat_impl::concat;

tests/quick.rs

@@ -962,11 +962,7 @@ quickcheck! {
 }
 
 fn binomial(n: usize, k: usize) -> usize {
-    if k > n {
-        0
-    } else {
-        (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>()
-    }
+    itertools::checked_binomial(n, k).unwrap()
 }
 
 quickcheck! {

tests/test_std.rs

@@ -1138,11 +1138,19 @@ fn combinations_zero() {
 }
 
 fn binomial(n: usize, k: usize) -> usize {
-    if k > n {
-        0
-    } else {
-        (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>()
-    }
+    it::checked_binomial(n, k).unwrap()
+}
+
+#[test]
+fn binomial_avoids_intermediate_overflow() {
+    // The naive "product then divide" formula overflows `usize` even when the
+    // binomial coefficient itself fits. C(21, 19) == C(21, 2) == 210, but
+    // (3..=21).product::<usize>() overflows on 64-bit. The same class of bug
+    // panics `combinations_inexact_size_hints` on 32-bit (#995): that test
+    // evaluates C(27, 7) because `Filter`'s size-hint upper bound is 18 and
+    // the lazy buffer has already loaded 9 items.
+    assert_eq!(binomial(21, 19), 210);
+    assert_eq!(binomial(27, 7), 888030);
 }
 
 #[test]