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use super::super::algo::k_way_merge::k_way_merge;
use super::super::algo::verge_sort_heuristic::verge_sort_preprocessing;
use super::super::{RadixKey, Radixable};
use super::counting_sort::counting_sort;
use super::msd_sort::copy_by_histogram;
use super::utils::{
    copy_nonoverlapping, only_one_bucket_filled, prefix_sums, Params,
};

pub fn lsd_radixsort_body<T, K>(arr: &mut [T], p: Params)
where
    T: Radixable<K> + Copy + PartialOrd,
    K: RadixKey,
{
    if arr.len() <= 128 {
        arr.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
        return;
    }

    let size = arr.len();
    let dummy = arr[0];
    let mut buffer: Vec<T> = vec![arr[0]; size];
    let mut index = 0;

    let histograms = dummy.get_full_histograms(arr, &p);

    let mut t1 = arr;
    let t2 = &mut buffer;
    let mut t2 = t2.as_mut_slice();

    for level in (p.level..p.max_level).rev() {
        if only_one_bucket_filled(&histograms[level]) {
            continue;
        }

        let (mut source, mut destination) =
            if index == 0 { (t1, t2) } else { (t2, t1) };
        let (mask, shift) = dummy.get_mask_and_shift(&p.new_level(level));
        let (_, mut heads, _) = prefix_sums(&histograms[level]);

        copy_by_histogram(
            source.len(),
            &mut source,
            &mut destination,
            &mut heads,
            mask,
            shift,
        );

        index = 1 - index;

        if index == 1 {
            t1 = source;
            t2 = destination;
        } else {
            t2 = source;
            t1 = destination;
        }
    }

    if index == 1 {
        copy_nonoverlapping(t2, t1, size);
    }
}

pub fn lsd_radixsort_aux<T, K>(
    arr: &mut [T],
    radix: usize,
    heuristic: bool,
    min_cs2: usize,
) where
    T: Radixable<K> + Copy + PartialOrd,
    K: RadixKey,
{
    if arr.len() <= 128 {
        arr.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
        return;
    }

    let dummy = arr[0];
    let (offset, _) = dummy.compute_offset(arr, radix);
    let max_level = dummy.compute_max_level(offset, radix);

    if max_level == 0 {
        return;
    }

    let params = Params::new(0, radix, offset, max_level);

    if heuristic {
        if max_level == 1 {
            counting_sort(arr, 8);
        } else if max_level == 2 && arr.len() >= min_cs2 {
            counting_sort(arr, 16);
        } else {
            lsd_radixsort_body(arr, params);
        }
    } else {
        lsd_radixsort_body(arr, params);
    }
}

/// # LSD sort
///
/// An implementation of the
/// [LSD sort](https://en.wikipedia.org/wiki/Radix_sort)
/// algorithm.
///
/// Implementation has been deeply optimized:
/// - Small preliminary check to skip prefix zero bits.
/// - Use ping pong copy.
/// - Use vectorization.
/// - Compute histograms in one pass.
/// - Check the number of non-empty buckets, if only one bucket is non-empty,
/// then skip the `copy_by_histogram`.
///
/// The Verge sort pre-processing heuristic is also added.
///
/// The LSD sort is an out of place unstable radix sort. The core algorithm is
/// stable, but fallback is unstable.
pub fn lsd_radixsort<T, K>(arr: &mut [T], radix: usize)
where
    T: Radixable<K> + Copy + PartialOrd,
    K: RadixKey,
{
    if arr.len() <= 128 {
        arr.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
        return;
    }

    let mut separators = verge_sort_preprocessing(arr, radix, &|arr, radix| {
        lsd_radixsort_aux(arr, radix, false, 0)
    });
    k_way_merge(arr, &mut separators);
}

pub fn lsd_radixsort_heu<T, K>(arr: &mut [T], radix: usize, min_cs2: usize)
where
    T: Radixable<K> + Copy + PartialOrd,
    K: RadixKey,
{
    if arr.len() <= 128 {
        arr.sort_unstable_by(|a, b| a.partial_cmp(b).unwrap());
        return;
    }

    let mut separators = verge_sort_preprocessing(arr, radix, &|arr, radix| {
        lsd_radixsort_aux(arr, radix, true, min_cs2)
    });
    k_way_merge(arr, &mut separators);
}