combinations: count and size_hint

src/combinations.rs

@@ -120,9 +120,95 @@ 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;
+        // TODO: make `pool.it` private
+        let n = pool.len() + pool.it.count();
+        remaining_for(n, first, &indices).unwrap()
+    }
 }
 
 impl<I> FusedIterator for Combinations<I>
     where I: Iterator,
           I::Item: Clone
 {}
+
+// https://en.wikipedia.org/wiki/Binomial_coefficient#In_programming_languages
+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)
+}
+
+#[test]
+fn test_checked_binomial() {
+    // With the first row: [1, 0, 0, ...] and the first column full of 1s, we check
+    // row by row the recurrence relation of binomials (which is an equivalent definition).
+    // For n >= 1 and k >= 1 we have:
+    //   binomial(n, k) == binomial(n - 1, k - 1) + binomial(n - 1, k)
+    const LIMIT: usize = 500;
+    let mut row = vec![Some(0); LIMIT + 1];
+    row[0] = Some(1);
+    for n in 0..=LIMIT {
+        for k in 0..=LIMIT {
+            assert_eq!(row[k], checked_binomial(n, k));
+        }
+        row = std::iter::once(Some(1))
+            .chain((1..=LIMIT).map(|k| row[k - 1]?.checked_add(row[k]?)))
+            .collect();
+    }
+}
+
+/// 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 n < k {
+        Some(0)
+    } else if first {
+        checked_binomial(n, k)
+    } else {
+        // https://en.wikipedia.org/wiki/Combinatorial_number_system
+        // http://www.site.uottawa.ca/~lucia/courses/5165-09/GenCombObj.pdf
+
+        // The combinations generated after the current one can be counted by counting as follows:
+        // - The subsequent combinations that differ in indices[0]:
+        //   If subsequent combinations differ in indices[0], then their value for indices[0]
+        //   must be at least 1 greater than the current indices[0].
+        //   As indices is strictly monotonically sorted, this means we can effectively choose k values
+        //   from (n - 1 - indices[0]), leading to binomial(n - 1 - indices[0], k) possibilities.
+        // - The subsequent combinations with same indices[0], but differing indices[1]:
+        //   Here we can choose k - 1 values from (n - 1 - indices[1]) values,
+        //   leading to binomial(n - 1 - indices[1], k - 1) possibilities.
+        // - (...)
+        // - The subsequent combinations with same indices[0..=i], but differing indices[i]:
+        //   Here we can choose k - i values from (n - 1 - indices[i]) values: binomial(n - 1 - indices[i], k - i).
+        //   Since subsequent combinations can in any index, we must sum up the aforementioned binomial coefficients.
+
+        // Below, `n0` resembles indices[i].
+        indices
+            .iter()
+            .enumerate()
+            // TODO: Once the MSRV hits 1.37.0, we can sum options instead:
+            // .map(|(i, n0)| checked_binomial(n - 1 - *n0, k - i))
+            // .sum()
+            .fold(Some(0), |sum, (i, n0)| {
+                sum.and_then(|s| s.checked_add(checked_binomial(n - 1 - *n0, k - i)?))
+            })
+    }
+}

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 +909,28 @@ fn combinations_zero() {
     it::assert_equal((0..0).combinations(0), vec![vec![]]);
 }
 
+#[test]
+fn combinations_range_count() {
+    for n in 0..=10 {
+        for k in 0..=n {
+            let len = (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>();
+            let mut it = (0..n).combinations(k);
+            assert_eq!(len, it.clone().count());
+            assert_eq!(len, it.size_hint().0);
+            assert_eq!(Some(len), it.size_hint().1);
+            for count in (0..len).rev() {
+                let elem = it.next();
+                assert!(elem.is_some());
+                assert_eq!(count, it.clone().count());
+                assert_eq!(count, it.size_hint().0);
+                assert_eq!(Some(count), it.size_hint().1);
+            }
+            let should_be_none = it.next();
+            assert!(should_be_none.is_none());
+        }
+    }
+}
+
 #[test]
 fn permutations_zero() {
     it::assert_equal((1..3).permutations(0), vec![vec![]]);