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use crate::SIMD_LEN;
use std::simd::cmp::SimdPartialEq;
use std::simd::Mask;
use std::simd::Simd;
use std::simd::SimdElement;
use std::slice;

pub trait AllEqualSimd<'a, T>
where
    T: SimdElement + std::cmp::PartialEq,
    Simd<T, SIMD_LEN>: SimdPartialEq<Mask = Mask<T::Mask, SIMD_LEN>>,
{
    fn all_equal_simd(&self) -> bool;
}

impl<'a, T> AllEqualSimd<'a, T> for slice::Iter<'a, T>
where
    T: SimdElement + std::cmp::PartialEq,
    Simd<T, SIMD_LEN>: SimdPartialEq<Mask = Mask<T::Mask, SIMD_LEN>>,
{
    fn all_equal_simd(&self) -> bool {
        let arr = self.as_slice();
        if arr.is_empty() {
            return true;
        }
        let first = arr[0];
        let (prefix, simd_data, suffix) = arr.as_simd::<SIMD_LEN>();
        // Prefix
        if !prefix.iter().all(|x| *x == first) {
            return false;
        }
        // SIMD
        let simd_needle = Simd::splat(first);
        for rest_slice in simd_data {
            let mask = rest_slice.simd_ne(simd_needle).to_bitmask();
            if mask != 0 {
                return false;
            }
        }
        // Suffix
        suffix.iter().all(|x| *x == first)
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use itertools::Itertools;
    use rand::distributions::Standard;
    use rand::prelude::Distribution;
    use rand::Rng;
    use std::fmt::Debug;

    fn test_simd_for_type<T>()
    where
        T: rand::distributions::uniform::SampleUniform
            + PartialEq
            + Debug
            + Copy
            + Default
            + SimdElement
            + std::cmp::PartialEq,
        Simd<T, SIMD_LEN>: SimdPartialEq<Mask = Mask<T::Mask, SIMD_LEN>>,
        Standard: Distribution<T>,
    {
        for len in 0..1000 {
            for _ in 0..5 {
                let mut v: Vec<T> = vec![T::default(); len];
                let mut rng = rand::thread_rng();
                for x in v.iter_mut() {
                    *x = rng.gen()
                }
                let ans = v.iter().all_equal_simd();
                let correct = v.iter().all_equal();
                assert_eq!(
                    ans,
                    correct,
                    "Failed for length {} and type {:?}",
                    len,
                    std::any::type_name::<T>()
                );
            }
        }
    }

    #[test]
    fn test_all_equal() {
        test_simd_for_type::<i8>();
        test_simd_for_type::<i16>();
        test_simd_for_type::<i32>();
        test_simd_for_type::<i64>();
        test_simd_for_type::<u8>();
        test_simd_for_type::<u16>();
        test_simd_for_type::<u32>();
        test_simd_for_type::<u64>();
        test_simd_for_type::<usize>();
        test_simd_for_type::<isize>();
        test_simd_for_type::<f32>();
        test_simd_for_type::<f64>();
    }
}