Added array_permutations (attempt 2)

src/combinations.rs

@@ -1,12 +1,12 @@
 use core::array;
-use core::borrow::BorrowMut;
 use std::fmt;
 use std::iter::FusedIterator;
 
-use super::lazy_buffer::LazyBuffer;
+use super::lazy_buffer::{LazyBuffer, ArrayOrVecHelper};
 use alloc::vec::Vec;
 
 use crate::adaptors::checked_binomial;
+use crate::lazy_buffer::MaybeConstUsize as _;
 
 /// Iterator for `Vec` valued combinations returned by [`.combinations()`](crate::Itertools::combinations)
 pub type Combinations<I> = CombinationsGeneric<I, Vec<usize>>;
@@ -39,42 +39,6 @@ pub struct CombinationsGeneric<I: Iterator, Idx> {
     first: bool,
 }
 
-/// A type holding indices of elements in a pool or buffer of items from an inner iterator
-/// and used to pick out different combinations in a generic way.
-pub trait PoolIndex<T>: BorrowMut<[usize]> {
-    type Item;
-
-    fn extract_item<I: Iterator<Item = T>>(&self, pool: &LazyBuffer<I>) -> Self::Item
-    where
-        T: Clone;
-
-    fn len(&self) -> usize {
-        self.borrow().len()
-    }
-}
-
-impl<T> PoolIndex<T> for Vec<usize> {
-    type Item = Vec<T>;
-
-    fn extract_item<I: Iterator<Item = T>>(&self, pool: &LazyBuffer<I>) -> Vec<T>
-    where
-        T: Clone,
-    {
-        pool.get_at(self)
-    }
-}
-
-impl<T, const K: usize> PoolIndex<T> for [usize; K] {
-    type Item = [T; K];
-
-    fn extract_item<I: Iterator<Item = T>>(&self, pool: &LazyBuffer<I>) -> [T; K]
-    where
-        T: Clone,
-    {
-        pool.get_array(*self)
-    }
-}
-
 impl<I, Idx> Clone for CombinationsGeneric<I, Idx>
 where
     I: Iterator + Clone,
@@ -93,7 +57,7 @@ where
     debug_fmt_fields!(Combinations, indices, pool, first);
 }
 
-impl<I: Iterator, Idx: PoolIndex<I::Item>> CombinationsGeneric<I, Idx> {
+impl<I: Iterator, Idx: ArrayOrVecHelper> CombinationsGeneric<I, Idx> {
     /// Constructor with arguments the inner iterator and the initial state for the indices.
     fn new(iter: I, indices: Idx) -> Self {
         Self {
@@ -105,7 +69,7 @@ impl<I: Iterator, Idx: PoolIndex<I::Item>> CombinationsGeneric<I, Idx> {
 
     /// Returns the length of a combination produced by this iterator.
     #[inline]
-    pub fn k(&self) -> usize {
+    pub fn k(&self) -> Idx::Length {
         self.indices.len()
     }
 
@@ -136,8 +100,8 @@ impl<I: Iterator, Idx: PoolIndex<I::Item>> CombinationsGeneric<I, Idx> {
     /// Initialises the iterator by filling a buffer with elements from the
     /// iterator. Returns true if there are no combinations, false otherwise.
     fn init(&mut self) -> bool {
-        self.pool.prefill(self.k());
-        let done = self.k() > self.n();
+        self.pool.prefill(self.k().value());
+        let done = self.k().value() > self.n();
         if !done {
             self.first = false;
         }
@@ -210,9 +174,9 @@ impl<I, Idx> Iterator for CombinationsGeneric<I, Idx>
 where
     I: Iterator,
     I::Item: Clone,
-    Idx: PoolIndex<I::Item>,
+    Idx: ArrayOrVecHelper,
 {
-    type Item = Idx::Item;
+    type Item = Idx::Item<I::Item>;
     fn next(&mut self) -> Option<Self::Item> {
         let done = if self.first {
             self.init()
@@ -248,7 +212,7 @@ impl<I, Idx> FusedIterator for CombinationsGeneric<I, Idx>
 where
     I: Iterator,
     I::Item: Clone,
-    Idx: PoolIndex<I::Item>,
+    Idx: ArrayOrVecHelper,
 {
 }
 

src/lazy_buffer.rs

@@ -1,4 +1,5 @@
 use alloc::vec::Vec;
+use core::borrow::BorrowMut;
 use std::iter::Fuse;
 use std::ops::Index;
 
@@ -77,3 +78,96 @@ where
         self.buffer.index(index)
     }
 }
+
+pub trait MaybeConstUsize: Clone + Copy + std::fmt::Debug {
+    /*TODO const*/
+    fn value(self) -> usize;
+}
+
+#[derive(Clone, Copy, Debug)]
+pub struct ConstUsize<const N: usize>;
+impl<const N: usize> MaybeConstUsize for ConstUsize<N> {
+    fn value(self) -> usize {
+        N
+    }
+}
+
+impl MaybeConstUsize for usize {
+    fn value(self) -> usize {
+        self
+    }
+}
+
+/// A type holding indices of elements in a pool or buffer of items from an inner iterator
+/// and used to pick out different combinations in a generic way.
+pub trait ArrayOrVecHelper: BorrowMut<[usize]> {
+    type Item<T>;
+    type Length: MaybeConstUsize;
+
+    fn extract_item<I: Iterator>(&self, pool: &LazyBuffer<I>) -> Self::Item<I::Item>
+    where
+        I::Item: Clone;
+
+    // TODO if only ever used to index into LazyBuffer, specialize to
+    // extract_from_fn(Self::Length, Fn(usize) -> usize, &LazyBuffer) -> Item<T>?
+    fn item_from_fn<T, F: Fn(usize) -> T>(len: Self::Length, f: F) -> Self::Item<T>;
+
+    fn len(&self) -> Self::Length;
+
+    fn from_fn<F: Fn(usize) -> usize>(k: Self::Length, f: F) -> Self;
+
+    fn start(len: Self::Length) -> Self
+    where
+        Self: Sized,
+    {
+        Self::from_fn(len, |i| i)
+    }
+}
+
+impl ArrayOrVecHelper for Vec<usize> {
+    type Item<T> = Vec<T>;
+    type Length = usize;
+
+    fn extract_item<I: Iterator>(&self, pool: &LazyBuffer<I>) -> Self::Item<I::Item>
+    where
+        I::Item: Clone,
+    {
+        pool.get_at(self)
+    }
+
+    fn item_from_fn<T, F: Fn(usize) -> T>(len: Self::Length, f: F) -> Self::Item<T> {
+        (0..len).map(f).collect()
+    }
+
+    fn len(&self) -> Self::Length {
+        self.len()
+    }
+
+    fn from_fn<F: Fn(usize) -> usize>(k: Self::Length, f: F) -> Self {
+        (0..k).map(f).collect()
+    }
+}
+
+impl<const K: usize> ArrayOrVecHelper for [usize; K] {
+    type Item<T> = [T; K];
+    type Length = ConstUsize<K>;
+
+    fn extract_item<I: Iterator>(&self, pool: &LazyBuffer<I>) -> Self::Item<I::Item>
+    where
+        I::Item: Clone,
+    {
+        pool.get_array(*self)
+    }
+
+    fn item_from_fn<T, F: Fn(usize) -> T>(_len: Self::Length, f: F) -> Self::Item<T> {
+        std::array::from_fn(f)
+    }
+
+    fn len(&self) -> Self::Length {
+        ConstUsize::<K>
+    }
+
+    fn from_fn<F: Fn(usize) -> usize>(_len: Self::Length, f: F) -> Self {
+        std::array::from_fn(f)
+    }
+}