Powerset count

src/combinations.rs

@@ -77,6 +77,12 @@ impl<I: Iterator> Combinations<I> {
             self.pool.prefill(k);
         }
     }
+
+    pub(crate) fn n_and_count(self) -> (usize, usize) {
+        let Self { indices, pool, first } = self;
+        let n = pool.count();
+        (n, remaining_for(n, first, &indices).unwrap())
+    }
 }
 
 impl<I> Iterator for Combinations<I>
@@ -128,10 +134,9 @@ impl<I> Iterator for Combinations<I>
         (low, upp)
     }
 
+    #[inline]
     fn count(self) -> usize {
-        let Self { indices, pool, first } = self;
-        let n = pool.count();
-        remaining_for(n, first, &indices).unwrap()
+        self.n_and_count().1
     }
 }
 

src/powerset.rs

@@ -3,8 +3,7 @@ use std::iter::FusedIterator;
 use std::usize;
 use alloc::vec::Vec;
 
-use super::combinations::{Combinations, combinations};
-use super::size_hint;
+use super::combinations::{Combinations, checked_binomial, combinations};
 
 /// An iterator to iterate through the powerset of the elements from an iterator.
 ///
@@ -13,22 +12,20 @@ use super::size_hint;
 #[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
 pub struct Powerset<I: Iterator> {
     combs: Combinations<I>,
-    // Iterator `position` (equal to count of yielded elements).
-    pos: usize,
 }
 
 impl<I> Clone for Powerset<I>
     where I: Clone + Iterator,
           I::Item: Clone,
 {
-    clone_fields!(combs, pos);
+    clone_fields!(combs);
 }
 
 impl<I> fmt::Debug for Powerset<I>
     where I: Iterator + fmt::Debug,
           I::Item: fmt::Debug,
 {
-    debug_fmt_fields!(Powerset, combs, pos);
+    debug_fmt_fields!(Powerset, combs);
 }
 
 /// Create a new `Powerset` from a clonable iterator.
@@ -38,7 +35,6 @@ pub fn powerset<I>(src: I) -> Powerset<I>
 {
     Powerset {
         combs: combinations(src, 0),
-        pos: 0,
     }
 }
 
@@ -51,35 +47,32 @@ impl<I> Iterator for Powerset<I>
 
     fn next(&mut self) -> Option<Self::Item> {
         if let Some(elt) = self.combs.next() {
-            self.pos = self.pos.saturating_add(1);
             Some(elt)
         } else if self.combs.k() < self.combs.n()
             || self.combs.k() == 0
         {
             self.combs.reset(self.combs.k() + 1);
-            self.combs.next().map(|elt| {
-                self.pos = self.pos.saturating_add(1);
-                elt
-            })
+            self.combs.next()
         } else {
             None
         }
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
+        let k = self.combs.k();
         // Total bounds for source iterator.
-        let src_total = self.combs.src().size_hint();
-
-        // Total bounds for self ( length(powerset(set) == 2 ^ length(set) )
-        let self_total = size_hint::pow_scalar_base(2, src_total);
+        let (n_min, n_max) = self.combs.src().size_hint();
+        // Total bounds for the current combinations.
+        let (mut low, mut upp) = self.combs.size_hint();
+        low = remaining_for(low, n_min, k).unwrap_or(usize::MAX);
+        upp = upp.and_then(|upp| remaining_for(upp, n_max?, k));
+        (low, upp)
+    }
 
-        if self.pos < usize::MAX {
-            // Subtract count of elements already yielded from total.
-            size_hint::sub_scalar(self_total, self.pos)
-        } else {
-            // Fallback: self.pos is saturated and no longer reliable.
-            (0, self_total.1)
-        }
+    fn count(self) -> usize {
+        let k = self.combs.k();
+        let (n, combs_count) = self.combs.n_and_count();
+        remaining_for(combs_count, n, k).unwrap()
     }
 }
 
@@ -88,3 +81,9 @@ impl<I> FusedIterator for Powerset<I>
         I: Iterator,
         I::Item: Clone,
 {}
+
+fn remaining_for(init_count: usize, n: usize, k: usize) -> Option<usize> {
+    (k + 1..=n).fold(Some(init_count), |sum, i| {
+        sum.and_then(|s| s.checked_add(checked_binomial(n, i)?))
+    })
+}

src/size_hint.rs

@@ -77,6 +77,7 @@ pub fn mul_scalar(sh: SizeHint, x: usize) -> SizeHint {
 
 /// Raise `base` correctly by a `SizeHint` exponent.
 #[inline]
+#[allow(dead_code)]
 pub fn pow_scalar_base(base: usize, exp: SizeHint) -> SizeHint {
     let exp_low = cmp::min(exp.0, u32::MAX as usize) as u32;
     let low = base.saturating_pow(exp_low);

tests/test_std.rs

@@ -989,6 +989,23 @@ fn powerset() {
     assert_eq!((0..4).powerset().count(), 1 << 4);
     assert_eq!((0..8).powerset().count(), 1 << 8);
     assert_eq!((0..16).powerset().count(), 1 << 16);
+
+    for n in 0..=10 {
+        let mut it = (0..n).powerset();
+        let len = 1 << n;
+        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]