Merge #735

735: `Powerset` count r=phimuemue a=Philippe-Cholet

`@phimuemue` After counting combinations last week, I thought a simple PR would be nice to start the week.

The first three commits are about `Powerset::count` while the next four commits are about `Powerset::size_hint`.

Co-authored-by: Philippe-Cholet <phcholet@orange.fr>

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 +3,8 @@ 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};
+use crate::size_hint::{self, SizeHint};
 
 /// An iterator to iterate through the powerset of the elements from an iterator.
 ///
@@ -13,22 +13,20 @@ 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 +36,6 @@ pub fn powerset<I>(src: I) -> Powerset<I>
 {
     Powerset {
         combs: combinations(src, 0),
-        pos: 0,
     }
 }
 
@@ -51,35 +48,30 @@ 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>) {
+    fn size_hint(&self) -> SizeHint {
+        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();
+        let low = remaining_for(n_min, k).unwrap_or(usize::MAX);
+        let upp = n_max.and_then(|n| remaining_for(n, k));
+        size_hint::add(self.combs.size_hint(), (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();
+        combs_count + remaining_for(n, k).unwrap()
     }
 }
 
@@ -88,3 +80,9 @@ impl<I> FusedIterator for Powerset<I>
         I: Iterator,
         I::Item: Clone,
 {}
+
+fn remaining_for(n: usize, k: usize) -> Option<usize> {
+    (k + 1..=n).fold(Some(0), |sum, i| {
+        sum.and_then(|s| s.checked_add(checked_binomial(n, i)?))
+    })
+}

src/size_hint.rs

@@ -3,7 +3,6 @@
 
 use std::usize;
 use std::cmp;
-use std::u32;
 
 /// `SizeHint` is the return type of `Iterator::size_hint()`.
 pub type SizeHint = (usize, Option<usize>);
@@ -75,20 +74,6 @@ pub fn mul_scalar(sh: SizeHint, x: usize) -> SizeHint {
     (low, hi)
 }
 
-/// Raise `base` correctly by a `SizeHint` exponent.
-#[inline]
-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);
-
-    let hi = exp.1.and_then(|exp| {
-        let exp_hi = cmp::min(exp, u32::MAX as usize) as u32;
-        base.checked_pow(exp_hi)
-    });
-
-    (low, hi)
-}
-
 /// Return the maximum
 #[inline]
 pub fn max(a: SizeHint, b: SizeHint) -> SizeHint {

tests/test_std.rs

@@ -909,15 +909,19 @@ fn combinations_zero() {
     it::assert_equal((0..0).combinations(0), vec![vec![]]);
 }
 
+fn binomial(n: usize, k: usize) -> usize {
+    if k > n {
+        0
+    } else {
+        (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>()
+    }
+}
+
 #[test]
 fn combinations_range_count() {
     for n in 0..=10 {
         for k in 0..=10 {
-            let len = if k<=n {
-                (n - k + 1..=n).product::<usize>() / (1..=k).product::<usize>()
-            } else {
-                0
-            };
+            let len = binomial(n, k);
             let mut it = (0..n).combinations(k);
             assert_eq!(len, it.clone().count());
             assert_eq!(len, it.size_hint().0);
@@ -935,6 +939,47 @@ fn combinations_range_count() {
     }
 }
 
+#[test]
+fn combinations_inexact_size_hints() {
+    for k in 0..=10 {
+        let mut numbers = (0..18).filter(|i| i % 2 == 0); // 9 elements
+        let mut it = numbers.clone().combinations(k);
+        let real_n = numbers.clone().count();
+        let len = binomial(real_n, k);
+        assert_eq!(len, it.clone().count());
+
+        let mut nb_loaded = numbers.by_ref().take(k).count(); // because of `LazyBuffer::prefill(k)`
+        let sh = numbers.size_hint();
+        assert_eq!(binomial(sh.0 + nb_loaded, k), it.size_hint().0);
+        assert_eq!(sh.1.map(|n| binomial(n + nb_loaded, k)), it.size_hint().1);
+
+        for next_count in 1..=len {
+            let elem = it.next();
+            assert!(elem.is_some());
+            assert_eq!(len - next_count, it.clone().count());
+            // It does not load anything more the very first time (it's prefilled).
+            if next_count > 1 {
+                // Then it loads one item each time until exhausted.
+                let nb = numbers.next();
+                if nb.is_some() {
+                    nb_loaded += 1;
+                }
+            }
+            let sh = numbers.size_hint();
+            if next_count > real_n - k + 1 {
+                assert_eq!(0, sh.0);
+                assert_eq!(Some(0), sh.1);
+                assert_eq!(real_n, nb_loaded);
+                // Once it's fully loaded, size hints of `it` are exacts.
+            }
+            assert_eq!(binomial(sh.0 + nb_loaded, k) - next_count, it.size_hint().0);
+            assert_eq!(sh.1.map(|n| binomial(n + nb_loaded, k) - next_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![]]);
@@ -989,6 +1034,23 @@ 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 = 2_usize.pow(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]