exhaust 0.2.5

Trait and derive macro for working with all possible values of a type (exhaustive enumeration).
Documentation 
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use core::{fmt, iter};

use crate::iteration::{carry, peekable_exhaust};
use crate::Exhaust;

impl<T: Exhaust, const N: usize> Exhaust for [T; N] {
    type Iter = ExhaustArray<T, N>;
    type Factory = [T::Factory; N];
    fn exhaust_factories() -> Self::Iter {
        ExhaustArray {
            state: [(); N].map(|()| peekable_exhaust::<T>()),
            done_zero: false,
        }
    }
    fn from_factory(factory: Self::Factory) -> Self {
        factory.map(T::from_factory)
    }
}

/// Iterator implementation of `[T; N]::exhaust()`.
pub struct ExhaustArray<T: Exhaust, const N: usize> {
    state: [iter::Peekable<T::Iter>; N],
    done_zero: bool,
}

impl<T, const N: usize> fmt::Debug for ExhaustArray<T, N>
where
    T: Exhaust<Iter: fmt::Debug, Factory: fmt::Debug>,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("ExhaustArray")
            .field("state", &self.state)
            .field("done_zero", &self.done_zero)
            .finish()
    }
}

impl<T: Exhaust, const N: usize> Clone for ExhaustArray<T, N> {
    fn clone(&self) -> Self {
        Self {
            state: self.state.clone(),
            done_zero: self.done_zero,
        }
    }
}

impl<T: Exhaust, const N: usize> Iterator for ExhaustArray<T, N> {
    type Item = [T::Factory; N];

    fn next(&mut self) -> Option<Self::Item> {
        if N == 0 {
            return if self.done_zero {
                None
            } else {
                self.done_zero = true;
                // This is just `Some([])` in disguise
                Some([(); N].map(|()| unreachable!()))
            };
        }

        // Check if we have a next item
        let has_next = self
            .state
            .iter_mut()
            .all(|value_iter| value_iter.peek().is_some());

        if !has_next {
            return None;
        }

        // Gather that next item.
        // unwrap() cannot fail because we checked with peek().
        let mut i = 0;
        let item = [(); N].map(|()| {
            let element = if i == N - 1 {
                // Advance the "last digit".
                self.state[i].next().unwrap()
            } else {
                // Don't advance the others
                self.state[i].peek().unwrap().clone()
            };
            i += 1;
            element
        });

        // "Carry": if the rightmost iterator is exhausted, advance the one to the left,
        // and repeat for all but the leftmost. If the leftmost is exhausted, we'll stop
        // on the next iteration.
        for i in (1..N).rev() {
            let (high, low) = &mut self.state.split_at_mut(i);
            if !carry(high.last_mut().unwrap(), &mut low[0], peekable_exhaust::<T>) {
                break;
            }
        }

        Some(item)
    }
}

impl<T: Exhaust, const N: usize> iter::FusedIterator for ExhaustArray<T, N> {}