i_key_sort 0.10.3

Counting sort algorithm.
Documentation 
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use crate::sort::key::{KeyFn, SortKey};
use crate::sort::min_max::MinMax;

pub(crate) const BIN_SORT_MIN: usize = 64;
pub(crate) const MAX_BINS_POWER: u32 = 8;
pub(crate) const MAX_BINS_COUNT: usize = 1 << MAX_BINS_POWER;

#[derive(Debug, Clone)]
pub(crate) struct BinLayout<K> {
    pub(crate) min_key: K,
    pub(crate) max_key: K,
    pub(crate) power: usize,
    bin_width_is_one: bool,
}

#[derive(Clone, Copy)]
pub(crate) struct LayoutConstraints {
    pub(crate) max_split_count: usize,
}

impl Default for LayoutConstraints {
    #[inline]
    fn default() -> Self {
        Self {
            max_split_count: MAX_BINS_COUNT,
        }
    }
}

impl<K: SortKey> BinLayout<K> {
    #[inline(always)]
    pub(super) fn bin_width_is_one(&self) -> bool {
        self.bin_width_is_one
    }

    #[inline(always)]
    pub fn index(&self, value: K) -> usize {
        debug_assert!(value >= self.min_key, "value must be >= min_key");
        let offset = value.difference(self.min_key);
        offset >> self.power
    }

    #[inline(always)]
    pub fn count(&self) -> usize {
        self.index(self.max_key) + 1
    }

    pub(crate) fn with_constraints(min_key: K, max_key: K, constraints: LayoutConstraints) -> BinLayout<K> {
        let length = max_key.difference(min_key);
        if length < constraints.max_split_count {
            return Self {
                min_key,
                max_key,
                power: 0,
                bin_width_is_one: true,
            };
        }

        let scale = length.saturating_add(1).ilog2_ceil();
        let power = scale.saturating_sub(constraints.max_split_count.ilog2()) as usize;

        Self {
            min_key,
            max_key,
            power,
            bin_width_is_one: false,
        }
    }

    #[inline(always)]
    pub fn with_keys<T, F: KeyFn<T, K>>(array: &[T], key: F) -> Option<Self> {
        if array.is_empty() {
            return None;
        }

        let (min_key, max_key) = array.min_max(key);

        if min_key == max_key {
            return None;
        }

        Some(Self::with_constraints(min_key, max_key, Default::default()))
    }
}

trait Log2 {
    fn ilog2_ceil(&self) -> u32;
}

impl Log2 for usize {
    #[inline(always)]
    fn ilog2_ceil(&self) -> u32 {
        let floor = self.ilog2();
        if self.is_power_of_two() {
            floor
        } else {
            floor + 1
        }
    }
}

#[cfg(test)]
mod tests {
    use crate::sort::bin_layout::{BinLayout, Log2};

    #[test]
    fn test_0() {
        let layout = BinLayout::<i32>::with_constraints(0i32, 3i32, Default::default());
        assert_eq!(layout.power, 0);
    }

    #[test]
    fn test_1() {
        let layout = BinLayout::<i32>::with_constraints(0, 255, Default::default());

        assert_eq!(layout.power, 0);
    }

    #[test]
    fn test_2() {
        let layout = BinLayout::<i32>::with_constraints(0, 256, Default::default());

        assert_eq!(layout.power, 1);
    }

    #[test]
    fn test_log2_0() {
        assert_eq!(1usize.ilog2_ceil(), 0);
        assert_eq!(2usize.ilog2_ceil(), 1);
        assert_eq!(3usize.ilog2_ceil(), 2);
        assert_eq!(4usize.ilog2_ceil(), 2);
        assert_eq!(5usize.ilog2_ceil(), 3);
    }
}