//! const fn combinations iter adapter
//!
//! # Examples
//!
//! ```
//! use const_combinations::IterExt;
//!
//! let mut combinations = (1..5).combinations();
//! assert_eq!(combinations.next(), Some([1, 2, 3]));
//! assert_eq!(combinations.next(), Some([1, 2, 4]));
//! assert_eq!(combinations.next(), Some([1, 3, 4]));
//! assert_eq!(combinations.next(), Some([2, 3, 4]));
//! assert_eq!(combinations.next(), None);
//! ```

#![no_std]
#![feature(maybe_uninit_uninit_array)]

extern crate alloc;

mod combinations;
mod permutations;

pub use combinations::{Combinations, SliceCombinations};
pub use permutations::{Permutations, SlicePermutations};

/// An extension trait adding `combinations` and `permutations` to `Iterator`.
pub trait IterExt: Iterator {
    /// Return an iterator adaptor that iterates over the k-length combinations of
    /// the elements from an iterator.
    ///
    /// The iterator produces a new array per iteration, and clones the iterator
    /// elements. If `K` is greater than the length of the input iterator the
    /// resulting iterator adaptor will yield no items.
    ///
    /// # Examples
    ///
    /// ```
    /// use const_combinations::IterExt;
    ///
    /// let mut combinations = (1..5).combinations();
    /// assert_eq!(combinations.next(), Some([1, 2, 3]));
    /// assert_eq!(combinations.next(), Some([1, 2, 4]));
    /// assert_eq!(combinations.next(), Some([1, 3, 4]));
    /// assert_eq!(combinations.next(), Some([2, 3, 4]));
    /// assert_eq!(combinations.next(), None);
    /// ```
    ///
    /// Note: Combinations does not take into account the equality of the iterated values.
    ///
    /// ```
    /// # use const_combinations::IterExt;
    /// let mut combinations = vec![1, 2, 2].into_iter().combinations();
    /// assert_eq!(combinations.next(), Some([1, 2])); // Note: these are the same
    /// assert_eq!(combinations.next(), Some([1, 2])); // Note: these are the same
    /// assert_eq!(combinations.next(), Some([2, 2]));
    /// assert_eq!(combinations.next(), None);
    /// ```
    fn combinations<const K: usize>(self) -> Combinations<Self, K>
    where
        Self: Sized,
        Self::Item: Clone,
    {
        Combinations::new(self)
    }

    /// Return an iterator adaptor that iterates over the k-length permutations of
    /// the elements from an iterator.
    ///
    /// The iterator produces a new array per iteration, and clones the iterator
    /// elements. If `K` is greater than the length of the input iterator the
    /// resulting iterator adaptor will yield no items.
    ///
    /// # Examples
    ///
    /// ```
    /// # use const_combinations::IterExt;
    /// let mut permutations = (0..3).permutations();
    /// assert_eq!(permutations.next(), Some([0, 1]));
    /// assert_eq!(permutations.next(), Some([1, 0]));
    /// assert_eq!(permutations.next(), Some([0, 2]));
    /// assert_eq!(permutations.next(), Some([2, 0]));
    /// assert_eq!(permutations.next(), Some([1, 2]));
    /// assert_eq!(permutations.next(), Some([2, 1]));
    /// assert_eq!(permutations.next(), None);
    /// ```
    ///
    /// Note: Permutations does not take into account the equality of the iterated values.
    ///
    /// ```
    /// # use const_combinations::IterExt;
    /// let mut permutations = vec![2, 2].into_iter().permutations();
    /// assert_eq!(permutations.next(), Some([2, 2])); // Note: these are the same
    /// assert_eq!(permutations.next(), Some([2, 2])); // Note: these are the same
    /// assert_eq!(permutations.next(), None);
    /// ```
    fn permutations<const K: usize>(self) -> Permutations<Self, K>
    where
        Self: Sized,
        Self::Item: Clone,
    {
        Permutations::new(self)
    }
}

impl<I> IterExt for I where I: Iterator {}

/// An extension trait adding `combinations` and `permutations` to `Slice`.
pub trait SliceExt<T> {
    /// Return an iterator that iterates over the k-length combinations of
    /// the elements from a slice.
    ///
    /// The iterator produces a new array per iteration, and returns references to the
    /// elements of the slice. If `K` is greater than the length of the input slice the
    /// resulting iterator will yield no items.
    ///
    /// # Examples
    ///
    /// ```
    /// use const_combinations::SliceExt;
    ///
    /// let mut combinations = [1, 2, 3, 4].combinations();
    /// assert_eq!(combinations.next(), Some([&1, &2, &3]));
    /// assert_eq!(combinations.next(), Some([&1, &2, &4]));
    /// assert_eq!(combinations.next(), Some([&1, &3, &4]));
    /// assert_eq!(combinations.next(), Some([&2, &3, &4]));
    /// assert_eq!(combinations.next(), None);
    /// ```
    ///
    /// Note: Combinations does not take into account the equality of the slice elements.
    ///
    /// ```
    /// # use const_combinations::SliceExt;
    /// let mut combinations = [1, 2, 2].combinations();
    /// assert_eq!(combinations.next(), Some([&1, &2])); // Note: these are the same
    /// assert_eq!(combinations.next(), Some([&1, &2])); // Note: these are the same
    /// assert_eq!(combinations.next(), Some([&2, &2]));
    /// assert_eq!(combinations.next(), None);
    /// ```
    fn combinations<const K: usize>(&self) -> SliceCombinations<T, K>;

    /// Return an iterator that iterates over the k-length permutations of
    /// the elements from a slice.
    ///
    /// The iterator produces a new array per iteration, and clones the iterator
    /// elements. If `K` is greater than the length of the input slice the
    /// resulting iterator adaptor will yield no items.
    ///
    /// # Examples
    ///
    /// ```
    /// # use const_combinations::SliceExt;
    /// let mut permutations = [0, 1, 2].permutations();
    /// assert_eq!(permutations.next(), Some([&0, &1]));
    /// assert_eq!(permutations.next(), Some([&1, &0]));
    /// assert_eq!(permutations.next(), Some([&0, &2]));
    /// assert_eq!(permutations.next(), Some([&2, &0]));
    /// assert_eq!(permutations.next(), Some([&1, &2]));
    /// assert_eq!(permutations.next(), Some([&2, &1]));
    /// assert_eq!(permutations.next(), None);
    /// ```
    ///
    /// Note: Permutations does not take into account the equality of the slice elements.
    ///
    /// ```
    /// # use const_combinations::SliceExt;
    /// let mut permutations = [2, 2].permutations();
    /// assert_eq!(permutations.next(), Some([&2, &2])); // Note: these are the same
    /// assert_eq!(permutations.next(), Some([&2, &2])); // Note: these are the same
    /// assert_eq!(permutations.next(), None);
    /// ```
    fn permutations<const K: usize>(&self) -> SlicePermutations<T, K>;
}

impl<T> SliceExt<T> for [T] {
    fn combinations<const K: usize>(&self) -> SliceCombinations<T, K> {
        SliceCombinations::new(self)
    }
    fn permutations<const K: usize>(&self) -> SlicePermutations<T, K> {
        SlicePermutations::new(self)
    }
}

fn make_array<T, F, const N: usize>(f: F) -> [T; N]
where
    F: Fn(usize) -> T,
{
    use core::mem::MaybeUninit;

    // Create the result array based on the indices
    let mut out: [MaybeUninit<T>; N] = MaybeUninit::uninit_array();

    // NOTE: this clippy attribute can be removed once we can `collect` into `[usize; K]`.
    #[allow(clippy::clippy::needless_range_loop)]
    for i in 0..N {
        out[i] = MaybeUninit::new(f(i));
    }
    unsafe { out.as_ptr().cast::<[T; N]>().read() }
}