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//
// Copyright 2021 Signal Messenger, LLC.
// SPDX-License-Identifier: AGPL-3.0-only
//

use std::ops::Index;

use partial_default::PartialDefault;
use serde::{Deserialize, Serialize};

/// Abstracts over fixed-length arrays (and similar types) with an element type `T`.
///
/// Provides `iter` and `Index` rather than `Deref` or `AsRef<[T]>` to allow for alternate forms of
/// indexing, for which exposing a slice could be confusing. See [`OneBased`].
pub trait ArrayLike<T>: Index<usize, Output = T> {
    const LEN: usize;
    fn create(create_element: impl FnMut() -> T) -> Self;
    fn iter(&self) -> std::slice::Iter<T>;
}

impl<T, const LEN: usize> ArrayLike<T> for [T; LEN] {
    const LEN: usize = LEN;
    fn create(mut create_element: impl FnMut() -> T) -> Self {
        [0; LEN].map(|_| create_element())
    }
    fn iter(&self) -> std::slice::Iter<T> {
        self[..].iter()
    }
}

/// A wrapper around an array or slice to use one-based indexing.
#[derive(Clone, Copy, PartialEq, Eq, Debug, Default, Serialize, Deserialize)]
pub struct OneBased<T>(pub T);

impl<T> Index<usize> for OneBased<T>
where
    T: Index<usize>,
{
    type Output = T::Output;
    fn index(&self, index: usize) -> &Self::Output {
        assert!(index > 0, "one-based index cannot be zero");
        &self.0[index - 1]
    }
}

impl<T, Ts> ArrayLike<T> for OneBased<Ts>
where
    Ts: ArrayLike<T>,
{
    const LEN: usize = Ts::LEN;

    fn create(create_element: impl FnMut() -> T) -> Self {
        OneBased(Ts::create(create_element))
    }

    fn iter(&self) -> std::slice::Iter<T> {
        self.0.iter()
    }
}

pub(crate) fn collect_permutation<T: PartialDefault + Clone>(
    iter: impl ExactSizeIterator<Item = (T, usize)>,
) -> Vec<T> {
    let mut result = vec![T::partial_default(); iter.len()];

    for (value, position) in iter {
        result[position] = value
    }

    result
}

#[cfg(test)]
mod tests {
    use rand::Rng as _;

    use super::*;

    #[test]
    fn test_one_based_indexing() {
        let array = OneBased([10, 20, 30]);
        assert_eq!(10, array[1]);
        assert_eq!(20, array[2]);
        assert_eq!(30, array[3]);
    }

    #[test]
    #[should_panic]
    fn test_one_based_indexing_with_zero() {
        let array = OneBased([10, 20, 30]);
        let _ = array[0];
    }

    #[test]
    #[should_panic]
    fn test_one_based_indexing_past_end() {
        let array = OneBased([10, 20, 30]);
        let _ = array[4];
    }

    #[test]
    fn test_one_based_iter() {
        let array = OneBased([10, 20, 30]);
        assert_eq!(vec![10, 20, 30], array.iter().copied().collect::<Vec<_>>());
    }

    #[test]
    fn test_permute_simple() {
        let elements = [5, 6, 7, 8];
        let permutation = [3, 2, 1, 0];
        let result = collect_permutation(elements.into_iter().zip(permutation));
        assert_eq!([8, 7, 6, 5].as_slice(), result.as_slice());
    }

    #[test]
    fn test_permute_scramble_and_unscramble() {
        for _ in 0..100 {
            let mut elements = [0u32; 512];
            rand::thread_rng().fill(&mut elements);

            let mut elements_with_indexes: Vec<_> = elements.into_iter().zip(0..).collect();
            elements_with_indexes.sort_unstable();

            let result = collect_permutation(elements_with_indexes.into_iter());
            assert_eq!(elements.as_slice(), result.as_slice());
        }
    }
}