use case for simd horizontal traits

variance.rs

use conv::{ValueFrom, ValueInto};
use core_simd::*;
use std::iter::Sum;
use std::ops::{Div, Mul, Sub};

/// Trait wrapper for Simd horizontal_sum()
pub trait HorizontalSum {
    type Output;
    fn hsum(&self) -> Self::Output;
}

macro_rules! hsum_impl {
    ($($t:ty)*) => ($(
        impl<const LANES: usize> HorizontalSum for Simd<$t, LANES>
        where
            LaneCount<LANES>: SupportedLaneCount,
        {
            type Output = $t;
            fn hsum(&self) -> Self::Output {
                self.horizontal_sum()
            }
        }
    )*)
}
hsum_impl! { i8 i16 i32 i64 isize u8 u16 u32 u64 usize f32 f64 }

/// Compute variance of a slice of Simd types
pub fn var<T, const LANES: usize>(xs: &[Simd<T, LANES>]) -> T
where
    T: SimdElement + Div<Output = T> + ValueFrom<usize>,
    LaneCount<LANES>: SupportedLaneCount,
    Simd<T, LANES>: Sum
        + Sub<Output = Simd<T, LANES>>
        + Mul<Output = Simd<T, LANES>>
        + HorizontalSum<Output = T>,
{
    let count: T = (LANES * xs.len()).value_into().unwrap();
    let mean = {
        let s: Simd<T, LANES> = xs.iter().cloned().sum();
        s.hsum() / count
    };
    let vmean = Simd::<T, LANES>::splat(mean);
    let vs: Simd<T, LANES> = xs
        .iter()
        .map(|x| {
            let d: Simd<T, LANES> = *x - vmean;
            d * d
        })
        .sum();
    vs.hsum() / count
}