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#![feature(unchecked_math)]
#![feature(inline_const)]

use std::mem;

#[derive(PartialEq)]
struct Foo(u8, u8);

pub fn cmp(a: &[Foo], b: &[Foo]) -> bool {
    slice_equal(a, b)
}

fn slice_equal<A, B>(this: &[A], other: &[B]) -> bool
    where A: PartialEq<B> {
        if this.len() != other.len() {
            return false;
        }

if this.len() == 0 {
            return true;
        }

// ZSTs have no identity and slices don't guarantee which addresses-to-ZSTs they produce
        // so we only need to compare them once to determine the behavior of the PartialEq impl
        if const { mem::size_of::<A>() == 0 && mem::size_of::<B>() == 0 } {
            // zero-length slices are always equal
            // SAFETY: A and B are ZSTs so it's ok to conjure them out of thin air
            return unsafe { mem::zeroed::<A>() == mem::zeroed::<B>() };
        }

const UNROLL: usize = 4;
        let mut i = 0;
        let mut is_eq = true;

let a = this.as_ptr();
        let b = other.as_ptr();
        let len = this.len();

// compare items 1 by 1 in case comparisons are expensive. at least one item, then
        // until the remainder is a multiple of UNROLL
        loop {
            // SAFETY: slices are of the same length and loop conditions ensure indexes are in bounds
            unsafe {
                is_eq = is_eq & PartialEq::eq(&*a.add(i), &*b.add(i));
                i = i.unchecked_add(1);
            }

if !is_eq || i == len || (len - i) % UNROLL == 0 {
                break;
            }
        }
        while is_eq && i + UNROLL <= len {
            // SAFETY: slices are of the same length and loop conditions ensure indexes are in bounds
            unsafe {
                is_eq = is_eq & PartialEq::eq(&*a.add(i), &*b.add(i));
                is_eq = is_eq & PartialEq::eq(&*a.add(i+1), &*b.add(i+1));
                is_eq = is_eq & PartialEq::eq(&*a.add(i+2), &*b.add(i+2));
                is_eq = is_eq & PartialEq::eq(&*a.add(i+3), &*b.add(i+3));
                i = i.unchecked_add(UNROLL);
            }
        }

is_eq
    }