Draft: Const generic permutations

src/lib.rs

@@ -174,6 +174,8 @@ pub use crate::either_or_both::EitherOrBoth;
 pub mod free;
 #[doc(inline)]
 pub use crate::free::*;
+use crate::permutations_const::PermutationsConst;
+
 #[cfg(feature = "use_alloc")]
 mod combinations;
 #[cfg(feature = "use_alloc")]
@@ -233,6 +235,7 @@ mod with_position;
 mod zip_eq_impl;
 mod zip_longest;
 mod ziptuple;
+mod permutations_const;
 
 #[macro_export]
 /// Create an iterator over the “cartesian product” of iterators.
@@ -1786,6 +1789,15 @@ pub trait Itertools: Iterator {
         permutations::permutations(self, k)
     }
 
+    #[cfg(feature = "use_alloc")]
+    fn permutations_const<const K: usize>(self) -> PermutationsConst<Self, K>
+        where
+            Self: Sized,
+            Self::Item: Clone,
+    {
+        permutations_const::permutations_const(self)
+    }
+
     /// Return an iterator that iterates through the powerset of the elements from an
     /// iterator.
     ///

src/permutations_const.rs

@@ -0,0 +1,188 @@
+use alloc::boxed::Box;
+use alloc::vec::Vec;
+use std::fmt;
+use std::iter::once;
+use std::iter::FusedIterator;
+use std::convert::TryInto;
+
+use super::lazy_buffer::LazyBuffer;
+use crate::size_hint::{self, SizeHint};
+
+/// An iterator adaptor that iterates through all the `k`-permutations of the
+/// elements from an iterator.
+///
+/// See [`.permutations()`](crate::Itertools::permutations) for
+/// more information.
+#[must_use = "iterator adaptors are lazy and do nothing unless consumed"]
+pub struct PermutationsConst<I: Iterator, const K: usize> {
+    vals: LazyBuffer<I>,
+    state: PermutationState<K>,
+}
+
+impl<I, const K: usize> Clone for PermutationsConst<I, K>
+where
+    I: Clone + Iterator,
+    I::Item: Clone,
+{
+    clone_fields!(vals, state);
+}
+
+#[derive(Clone, Debug)]
+enum PermutationState<const K: usize> {
+    /// No permutation generated yet.
+    Start,
+    /// Values from the iterator are not fully loaded yet so `n` is still unknown.
+    Buffered { min_n: usize },
+    /// All values from the iterator are known so `n` is known.
+    Loaded {
+        indices: Box<[usize]>,
+        cycles: Box<[usize]>,
+    },
+    /// No permutation left to generate.
+    End,
+}
+
+impl<I, const K: usize> fmt::Debug for PermutationsConst<I, K>
+where
+    I: Iterator + fmt::Debug,
+    I::Item: fmt::Debug,
+{
+    debug_fmt_fields!(Permutations, vals, state);
+}
+
+pub fn permutations_const<I: Iterator, const K: usize>(iter: I) -> PermutationsConst<I, K> {
+    PermutationsConst {
+        vals: LazyBuffer::new(iter),
+        state: PermutationState::Start,
+    }
+}
+
+impl<I, const K: usize> Iterator for PermutationsConst<I, K>
+where
+    I: Iterator,
+    I::Item: Clone + Default,
+{
+    type Item = [I::Item; K];
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let Self { vals, state } = self;
+        match state {
+            PermutationState::Start => {
+                *state = PermutationState::End;
+                // TODO: Consider this case and handle it somehow, currently just using default
+                Some(std::array::from_fn(|_|I::Item::default()))
+            }
+            &mut PermutationState::Start => {
+                vals.prefill(K);
+                if vals.len() != K {
+                    *state = PermutationState::End;
+                    return None;
+                }
+                *state = PermutationState::Buffered { min_n: K };
+                let mut iter = vals[0..K].into_iter().cloned();
+                Some(std::array::from_fn(|_|iter.next().unwrap())) // TODO: Handle error case, maybe make this better
+            }
+            PermutationState::Buffered { min_n } => {
+                if vals.get_next() {
+                    let mut item = (0..K - 1)
+                        .chain(once(*min_n))
+                        .map(|i| vals[i].clone());
+                    *min_n += 1;
+                    Some(std::array::from_fn(|_|item.next().unwrap()))
+                } else {
+                    let n = *min_n;
+                    let prev_iteration_count = n - K + 1;
+                    let mut indices: Box<[_]> = (0..n).collect();
+                    let mut cycles: Box<[_]> = (n - K..n).rev().collect();
+                    // Advance the state to the correct point.
+                    for _ in 0..prev_iteration_count {
+                        if advance(&mut indices, &mut cycles) {
+                            *state = PermutationState::End;
+                            return None;
+                        }
+                    }
+                    let item = vals.get_at(&indices[0..K]); // TODO: Impl const sized variant otherwise this is pointless
+                    *state = PermutationState::Loaded { indices, cycles };
+                    Some(item.try_into().ok()?) // TODO: Handle error case
+                }
+            }
+            PermutationState::Loaded { indices, cycles } => {
+                if advance(indices, cycles) {
+                    *state = PermutationState::End;
+                    return None;
+                }
+                let k = cycles.len();
+                Some(vals.get_at(&indices[0..k]).try_into().ok()?) // TODO: Handle error case and const size indexing
+            }
+            PermutationState::End => None,
+        }
+    }
+
+    fn count(self) -> usize {
+        let Self { vals, state } = self;
+        let n = vals.count();
+        state.size_hint_for(n).1.unwrap()
+    }
+
+    fn size_hint(&self) -> SizeHint {
+        let (mut low, mut upp) = self.vals.size_hint();
+        low = self.state.size_hint_for(low).0;
+        upp = upp.and_then(|n| self.state.size_hint_for(n).1);
+        (low, upp)
+    }
+}
+
+impl<I, const K: usize> FusedIterator for PermutationsConst<I, K>
+where
+    I: Iterator,
+    I::Item: Clone + Default,
+{
+}
+
+fn advance(indices: &mut [usize], cycles: &mut [usize]) -> bool {
+    let n = indices.len();
+    let k = cycles.len();
+    // NOTE: if `cycles` are only zeros, then we reached the last permutation.
+    for i in (0..k).rev() {
+        if cycles[i] == 0 {
+            cycles[i] = n - i - 1;
+            indices[i..].rotate_left(1);
+        } else {
+            let swap_index = n - cycles[i];
+            indices.swap(i, swap_index);
+            cycles[i] -= 1;
+            return false;
+        }
+    }
+    true
+}
+
+impl <const K: usize>PermutationState<K> {
+    fn size_hint_for(&self, n: usize) -> SizeHint {
+        // At the beginning, there are `n!/(n-k)!` items to come.
+        let at_start = |n, k| {
+            debug_assert!(n >= k);
+            let total = (n - k + 1..=n).try_fold(1usize, |acc, i| acc.checked_mul(i));
+            (total.unwrap_or(usize::MAX), total)
+        };
+        match *self {
+            Self::Start if n < K => (0, Some(0)),
+            Self::Start => at_start(n, K),
+            Self::Buffered { min_n } => {
+                // Same as `Start` minus the previously generated items.
+                size_hint::sub_scalar(at_start(n, K), min_n - K + 1)
+            }
+            Self::Loaded {
+                ref indices,
+                ref cycles,
+            } => {
+                let count = cycles.iter().enumerate().try_fold(0usize, |acc, (i, &c)| {
+                    acc.checked_mul(indices.len() - i)
+                        .and_then(|count| count.checked_add(c))
+                });
+                (count.unwrap_or(usize::MAX), count)
+            }
+            Self::End => (0, Some(0)),
+        }
+    }
+}