combinations: count and size_hint

src/combinations.rs

@@ -120,9 +120,51 @@ impl<I> Iterator for Combinations<I>
         // Create result vector based on the indices
         Some(self.indices.iter().map(|i| self.pool[*i].clone()).collect())
     }
+
+    fn size_hint(&self) -> (usize, Option<usize>) {
+        let (mut low, mut upp) = self.pool.size_hint();
+        low = remaining_for(low, self.first, &self.indices).unwrap_or(usize::MAX);
+        upp = upp.and_then(|upp| remaining_for(upp, self.first, &self.indices));
+        (low, upp)
+    }
+
+    fn count(self) -> usize {
+        let Self { indices, pool, first } = self;
+        let n = pool.len() + pool.it.count();
+        remaining_for(n, first, &indices).expect("Iterator count greater than usize::MAX")
+    }
 }
 
 impl<I> FusedIterator for Combinations<I>
     where I: Iterator,
           I::Item: Clone
 {}
+
+pub(crate) fn checked_binomial(mut n: usize, mut k: usize) -> Option<usize> {
+    if n < k {
+        return Some(0);
+    }
+    // `factorial(n) / factorial(n - k) / factorial(k)` but trying to avoid it overflows:
+    k = (n - k).min(k); // symmetry
+    let mut c = 1;
+    for i in 1..=k {
+        c = (c / i).checked_mul(n)?.checked_add((c % i).checked_mul(n)? / i)?;
+        n -= 1;
+    }
+    Some(c)
+}
+
+/// For a given size `n`, return the count of remaining combinations or None if it would overflow.
+fn remaining_for(n: usize, first: bool, indices: &[usize]) -> Option<usize> {
+    let k = indices.len();
+    if first {
+        checked_binomial(n, k)
+    } else {
+        indices
+            .iter()
+            .enumerate()
+            .fold(Some(0), |sum, (k0, n0)| {
+                sum.and_then(|s| s.checked_add(checked_binomial(n - 1 - *n0, k - k0)?))
+            })
+    }
+}

src/lazy_buffer.rs

@@ -2,6 +2,8 @@ use std::iter::Fuse;
 use std::ops::Index;
 use alloc::vec::Vec;
 
+use crate::size_hint::{self, SizeHint};
+
 #[derive(Debug, Clone)]
 pub struct LazyBuffer<I: Iterator> {
     pub it: Fuse<I>,
@@ -23,6 +25,10 @@ where
         self.buffer.len()
     }
 
+    pub fn size_hint(&self) -> SizeHint {
+        size_hint::add_scalar(self.it.size_hint(), self.len())
+    }
+
     pub fn get_next(&mut self) -> bool {
         if let Some(x) = self.it.next() {
             self.buffer.push(x);

tests/test_std.rs

@@ -909,6 +909,21 @@ fn combinations_zero() {
     it::assert_equal((0..0).combinations(0), vec![vec![]]);
 }
 
+#[test]
+fn combinations_range_count() {
+    for n in 0..6 {
+        for k in 0..=n {
+            let len = (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>();
+            let mut it = (0..n).combinations(k);
+            for count in (0..=len).rev() {
+                assert_eq!(it.size_hint(), (count, Some(count)));
+                assert_eq!(it.clone().count(), count);
+                assert_eq!(it.next().is_none(), count == 0);
+            }
+        }
+    }
+}
+
 #[test]
 fn permutations_zero() {
     it::assert_equal((1..3).permutations(0), vec![vec![]]);