1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95

use super::super::{RadixKey, Radixable};
use super::comparative_sort::insertion_sort;
use super::utils::{get_histogram, prefix_sums, Params};

fn serial_swap<T, K>(
    arr: &mut [T],
    heads: &mut Vec<usize>,
    tails: &[usize],
    p: &Params,
    mask: <<T as Radixable<K>>::Key as RadixKey>::Key,
    shift: usize,
) where
    T: Radixable<K>,
    K: RadixKey,
{
    for i in 0..(p.radix_range) - 1 {
        while heads[i] < tails[i] {
            unsafe {
                let mut bucket =
                    arr.get_unchecked(heads[i]).extract(mask, shift);
                while bucket != i {
                    arr.swap(heads[i], heads[bucket]);
                    heads[bucket] += 1;
                    bucket = arr.get_unchecked(heads[i]).extract(mask, shift);
                }
                heads[i] += 1;
            }
        }
    }
}

pub fn serial_radixsort_rec<T, K>(arr: &mut [T], p: Params)
where
    T: Radixable<K>,
    K: RadixKey,
{
    if arr.len() <= 64 {
        insertion_sort(arr);
        return;
    }

    let dummy = arr[0];

    let (mask, shift) = dummy.get_mask_and_shift_from_left(&p);
    let histogram = get_histogram(arr, &p, mask, shift);
    let (p_sums, mut heads, tails) = prefix_sums(&histogram);

    serial_swap(arr, &mut heads, &tails, &p, mask, shift);

    let mut rest = arr;
    if p.level < p.max_level - 1 {
        for i in 0..(p.radix_range) {
            let bucket_end = p_sums[i + 1] - p_sums[i];
            let (first_part, second_part) = rest.split_at_mut(bucket_end);
            rest = second_part;
            if histogram[i] > 1 {
                let new_params = p.new_level(p.level + 1);
                serial_radixsort_rec(first_part, new_params);
            }
        }
    }
}

/// # American flag sort
///
/// An implementation of the famous
/// [American flag sort](https://en.wikipedia.org/wiki/American_flag_sort)
/// algorithm.
///
/// This algorithm is used as a fallback in the Ska sort.
///
/// The American flag sort is an in place unstable radix sort.
pub fn american_flag_sort<T, K>(arr: &mut [T], radix: usize)
where
    T: Radixable<K>,
    K: RadixKey,
{
    if arr.len() <= 64 {
        insertion_sort(arr);
        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);

    serial_radixsort_rec(arr, params);
}