rust/src/libcore/tests/slice.rs

// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.

use core::result::Result::{Ok, Err};

#[test]
fn test_position() {
    let b = [1, 2, 3, 5, 5];
    assert!(b.iter().position(|&v| v == 9) == None);
    assert!(b.iter().position(|&v| v == 5) == Some(3));
    assert!(b.iter().position(|&v| v == 3) == Some(2));
    assert!(b.iter().position(|&v| v == 0) == None);
}

#[test]
fn test_rposition() {
    let b = [1, 2, 3, 5, 5];
    assert!(b.iter().rposition(|&v| v == 9) == None);
    assert!(b.iter().rposition(|&v| v == 5) == Some(4));
    assert!(b.iter().rposition(|&v| v == 3) == Some(2));
    assert!(b.iter().rposition(|&v| v == 0) == None);
}

#[test]
fn test_binary_search() {
    let b: [i32; 0] = [];
    assert_eq!(b.binary_search(&5), Err(0));

    let b = [4];
    assert_eq!(b.binary_search(&3), Err(0));
    assert_eq!(b.binary_search(&4), Ok(0));
    assert_eq!(b.binary_search(&5), Err(1));

    let b = [1, 2, 4, 6, 8, 9];
    assert_eq!(b.binary_search(&5), Err(3));
    assert_eq!(b.binary_search(&6), Ok(3));
    assert_eq!(b.binary_search(&7), Err(4));
    assert_eq!(b.binary_search(&8), Ok(4));

    let b = [1, 2, 4, 5, 6, 8];
    assert_eq!(b.binary_search(&9), Err(6));

    let b = [1, 2, 4, 6, 7, 8, 9];
    assert_eq!(b.binary_search(&6), Ok(3));
    assert_eq!(b.binary_search(&5), Err(3));
    assert_eq!(b.binary_search(&8), Ok(5));

    let b = [1, 2, 4, 5, 6, 8, 9];
    assert_eq!(b.binary_search(&7), Err(5));
    assert_eq!(b.binary_search(&0), Err(0));

    let b = [1, 3, 3, 3, 7];
    assert_eq!(b.binary_search(&0), Err(0));
    assert_eq!(b.binary_search(&1), Ok(0));
    assert_eq!(b.binary_search(&2), Err(1));
    assert!(match b.binary_search(&3) { Ok(1...3) => true, _ => false });
    assert!(match b.binary_search(&3) { Ok(1...3) => true, _ => false });
    assert_eq!(b.binary_search(&4), Err(4));
    assert_eq!(b.binary_search(&5), Err(4));
    assert_eq!(b.binary_search(&6), Err(4));
    assert_eq!(b.binary_search(&7), Ok(4));
    assert_eq!(b.binary_search(&8), Err(5));
}

#[test]
// Test implementation specific behavior when finding equivalent elements.
// It is ok to break this test but when you do a crater run is highly advisable.
fn test_binary_search_implementation_details() {
    let b = [1, 1, 2, 2, 3, 3, 3];
    assert_eq!(b.binary_search(&1), Ok(1));
    assert_eq!(b.binary_search(&2), Ok(3));
    assert_eq!(b.binary_search(&3), Ok(6));
    let b = [1, 1, 1, 1, 1, 3, 3, 3, 3];
    assert_eq!(b.binary_search(&1), Ok(4));
    assert_eq!(b.binary_search(&3), Ok(8));
    let b = [1, 1, 1, 1, 3, 3, 3, 3, 3];
    assert_eq!(b.binary_search(&1), Ok(3));
    assert_eq!(b.binary_search(&3), Ok(8));
}

#[test]
fn test_iterator_nth() {
    let v: &[_] = &[0, 1, 2, 3, 4];
    for i in 0..v.len() {
        assert_eq!(v.iter().nth(i).unwrap(), &v[i]);
    }
    assert_eq!(v.iter().nth(v.len()), None);

    let mut iter = v.iter();
    assert_eq!(iter.nth(2).unwrap(), &v[2]);
    assert_eq!(iter.nth(1).unwrap(), &v[4]);
}

#[test]
fn test_iterator_last() {
    let v: &[_] = &[0, 1, 2, 3, 4];
    assert_eq!(v.iter().last().unwrap(), &4);
    assert_eq!(v[..1].iter().last().unwrap(), &0);
}

#[test]
fn test_iterator_count() {
    let v: &[_] = &[0, 1, 2, 3, 4];
    assert_eq!(v.iter().count(), 5);

    let mut iter2 = v.iter();
    iter2.next();
    iter2.next();
    assert_eq!(iter2.count(), 3);
}

#[test]
fn test_chunks_count() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.chunks(3);
    assert_eq!(c.count(), 2);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.chunks(2);
    assert_eq!(c2.count(), 3);

    let v3: &[i32] = &[];
    let c3 = v3.chunks(2);
    assert_eq!(c3.count(), 0);
}

#[test]
fn test_chunks_nth() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let mut c = v.chunks(2);
    assert_eq!(c.nth(1).unwrap(), &[2, 3]);
    assert_eq!(c.next().unwrap(), &[4, 5]);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let mut c2 = v2.chunks(3);
    assert_eq!(c2.nth(1).unwrap(), &[3, 4]);
    assert_eq!(c2.next(), None);
}

#[test]
fn test_chunks_last() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.chunks(2);
    assert_eq!(c.last().unwrap()[1], 5);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.chunks(2);
    assert_eq!(c2.last().unwrap()[0], 4);
}

#[test]
fn test_chunks_zip() {
    let v1: &[i32] = &[0, 1, 2, 3, 4];
    let v2: &[i32] = &[6, 7, 8, 9, 10];

    let res = v1.chunks(2)
        .zip(v2.chunks(2))
        .map(|(a, b)| a.iter().sum::<i32>() + b.iter().sum::<i32>())
        .collect::<Vec<_>>();
    assert_eq!(res, vec![14, 22, 14]);
}

#[test]
fn test_chunks_mut_count() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let c = v.chunks_mut(3);
    assert_eq!(c.count(), 2);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let c2 = v2.chunks_mut(2);
    assert_eq!(c2.count(), 3);

    let v3: &mut [i32] = &mut [];
    let c3 = v3.chunks_mut(2);
    assert_eq!(c3.count(), 0);
}

#[test]
fn test_chunks_mut_nth() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let mut c = v.chunks_mut(2);
    assert_eq!(c.nth(1).unwrap(), &[2, 3]);
    assert_eq!(c.next().unwrap(), &[4, 5]);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let mut c2 = v2.chunks_mut(3);
    assert_eq!(c2.nth(1).unwrap(), &[3, 4]);
    assert_eq!(c2.next(), None);
}

#[test]
fn test_chunks_mut_last() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let c = v.chunks_mut(2);
    assert_eq!(c.last().unwrap(), &[4, 5]);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let c2 = v2.chunks_mut(2);
    assert_eq!(c2.last().unwrap(), &[4]);
}

#[test]
fn test_chunks_mut_zip() {
    let v1: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let v2: &[i32] = &[6, 7, 8, 9, 10];

    for (a, b) in v1.chunks_mut(2).zip(v2.chunks(2)) {
        let sum = b.iter().sum::<i32>();
        for v in a {
            *v += sum;
        }
    }
    assert_eq!(v1, [13, 14, 19, 20, 14]);
}

#[test]
fn test_exact_chunks_count() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.exact_chunks(3);
    assert_eq!(c.count(), 2);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.exact_chunks(2);
    assert_eq!(c2.count(), 2);

    let v3: &[i32] = &[];
    let c3 = v3.exact_chunks(2);
    assert_eq!(c3.count(), 0);
}

#[test]
fn test_exact_chunks_nth() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let mut c = v.exact_chunks(2);
    assert_eq!(c.nth(1).unwrap(), &[2, 3]);
    assert_eq!(c.next().unwrap(), &[4, 5]);

    let v2: &[i32] = &[0, 1, 2, 3, 4, 5, 6];
    let mut c2 = v2.exact_chunks(3);
    assert_eq!(c2.nth(1).unwrap(), &[3, 4, 5]);
    assert_eq!(c2.next(), None);
}

#[test]
fn test_exact_chunks_last() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.exact_chunks(2);
    assert_eq!(c.last().unwrap(), &[4, 5]);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.exact_chunks(2);
    assert_eq!(c2.last().unwrap(), &[2, 3]);
}

#[test]
fn test_exact_chunks_zip() {
    let v1: &[i32] = &[0, 1, 2, 3, 4];
    let v2: &[i32] = &[6, 7, 8, 9, 10];

    let res = v1.exact_chunks(2)
        .zip(v2.exact_chunks(2))
        .map(|(a, b)| a.iter().sum::<i32>() + b.iter().sum::<i32>())
        .collect::<Vec<_>>();
    assert_eq!(res, vec![14, 22]);
}

#[test]
fn test_exact_chunks_mut_count() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let c = v.exact_chunks_mut(3);
    assert_eq!(c.count(), 2);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let c2 = v2.exact_chunks_mut(2);
    assert_eq!(c2.count(), 2);

    let v3: &mut [i32] = &mut [];
    let c3 = v3.exact_chunks_mut(2);
    assert_eq!(c3.count(), 0);
}

#[test]
fn test_exact_chunks_mut_nth() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let mut c = v.exact_chunks_mut(2);
    assert_eq!(c.nth(1).unwrap(), &[2, 3]);
    assert_eq!(c.next().unwrap(), &[4, 5]);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4, 5, 6];
    let mut c2 = v2.exact_chunks_mut(3);
    assert_eq!(c2.nth(1).unwrap(), &[3, 4, 5]);
    assert_eq!(c2.next(), None);
}

#[test]
fn test_exact_chunks_mut_last() {
    let v: &mut [i32] = &mut [0, 1, 2, 3, 4, 5];
    let c = v.exact_chunks_mut(2);
    assert_eq!(c.last().unwrap(), &[4, 5]);

    let v2: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let c2 = v2.exact_chunks_mut(2);
    assert_eq!(c2.last().unwrap(), &[2, 3]);
}

#[test]
fn test_exact_chunks_mut_zip() {
    let v1: &mut [i32] = &mut [0, 1, 2, 3, 4];
    let v2: &[i32] = &[6, 7, 8, 9, 10];

    for (a, b) in v1.exact_chunks_mut(2).zip(v2.exact_chunks(2)) {
        let sum = b.iter().sum::<i32>();
        for v in a {
            *v += sum;
        }
    }
    assert_eq!(v1, [13, 14, 19, 20, 4]);
}

#[test]
fn test_windows_count() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.windows(3);
    assert_eq!(c.count(), 4);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.windows(6);
    assert_eq!(c2.count(), 0);

    let v3: &[i32] = &[];
    let c3 = v3.windows(2);
    assert_eq!(c3.count(), 0);
}

#[test]
fn test_windows_nth() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let mut c = v.windows(2);
    assert_eq!(c.nth(2).unwrap()[1], 3);
    assert_eq!(c.next().unwrap()[0], 3);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let mut c2 = v2.windows(4);
    assert_eq!(c2.nth(1).unwrap()[1], 2);
    assert_eq!(c2.next(), None);
}

#[test]
fn test_windows_last() {
    let v: &[i32] = &[0, 1, 2, 3, 4, 5];
    let c = v.windows(2);
    assert_eq!(c.last().unwrap()[1], 5);

    let v2: &[i32] = &[0, 1, 2, 3, 4];
    let c2 = v2.windows(2);
    assert_eq!(c2.last().unwrap()[0], 3);
}

#[test]
fn test_windows_zip() {
    let v1: &[i32] = &[0, 1, 2, 3, 4];
    let v2: &[i32] = &[6, 7, 8, 9, 10];

    let res = v1.windows(2)
        .zip(v2.windows(2))
        .map(|(a, b)| a.iter().sum::<i32>() + b.iter().sum::<i32>())
        .collect::<Vec<_>>();

    assert_eq!(res, [14, 18, 22, 26]);
}

mod slice_index {
    // Test a slicing operation that should succeed,
    // testing it on all of the indexing methods.
    macro_rules! assert_range_eq {
        ($arr:expr, $range:expr, $expected:expr)
        => {
            let mut arr = $arr;
            let mut expected = $expected;
            {
                let s: &[_] = &arr;
                let expected: &[_] = &expected;

                assert_eq!(&s[$range], expected, "(in assertion for: index)");
                assert_eq!(s.get($range), Some(expected), "(in assertion for: get)");
                unsafe {
                    assert_eq!(
                        s.get_unchecked($range), expected,
                        "(in assertion for: get_unchecked)",
                    );
                }
            }
            {
                let s: &mut [_] = &mut arr;
                let expected: &mut [_] = &mut expected;

                assert_eq!(
                    &mut s[$range], expected,
                    "(in assertion for: index_mut)",
                );
                assert_eq!(
                    s.get_mut($range), Some(&mut expected[..]),
                    "(in assertion for: get_mut)",
                );
                unsafe {
                    assert_eq!(
                        s.get_unchecked_mut($range), expected,
                        "(in assertion for: get_unchecked_mut)",
                    );
                }
            }
        }
    }

    // Make sure the macro can actually detect bugs,
    // because if it can't, then what are we even doing here?
    //
    // (Be aware this only demonstrates the ability to detect bugs
    //  in the FIRST method it calls, as the macro is not designed
    //  to be used in `should_panic`)
    #[test]
    #[should_panic(expected = "out of range")]
    fn assert_range_eq_can_fail_by_panic() {
        assert_range_eq!([0, 1, 2], 0..5, [0, 1, 2]);
    }

    // (Be aware this only demonstrates the ability to detect bugs
    //  in the FIRST method it calls, as the macro is not designed
    //  to be used in `should_panic`)
    #[test]
    #[should_panic(expected = "==")]
    fn assert_range_eq_can_fail_by_inequality() {
        assert_range_eq!([0, 1, 2], 0..2, [0, 1, 2]);
    }

    // Test cases for bad index operations.
    //
    // This generates `should_panic` test cases for Index/IndexMut
    // and `None` test cases for get/get_mut.
    macro_rules! panic_cases {
        ($(
            mod $case_name:ident {
                let DATA: $Data: ty = $data:expr;

                // optional:
                //
                // a similar input for which DATA[input] succeeds, and the corresponding
                // output as an array.  This helps validate "critical points" where an
                // input range straddles the boundary between valid and invalid.
                // (such as the input `len..len`, which is just barely valid)
                $(
                    let GOOD_INPUT = $good:expr;
                    let GOOD_OUTPUT = $output:expr;
                )*

                let BAD_INPUT = $bad:expr;
                const EXPECT_MSG = $expect_msg:expr;

                !!generate_tests!!
            }
        )*) => {$(
            mod $case_name {
                #[test]
                fn pass() {
                    let mut v: $Data = $data;

                    $( assert_range_eq!($data, $good, $output); )*

                    {
                        let v: &[_] = &v;
                        assert_eq!(v.get($bad), None, "(in None assertion for get)");
                    }

                    {
                        let v: &mut [_] = &mut v;
                        assert_eq!(v.get_mut($bad), None, "(in None assertion for get_mut)");
                    }
                }

                #[test]
                #[should_panic(expected = $expect_msg)]
                fn index_fail() {
                    let v: $Data = $data;
                    let v: &[_] = &v;
                    let _v = &v[$bad];
                }

                #[test]
                #[should_panic(expected = $expect_msg)]
                fn index_mut_fail() {
                    let mut v: $Data = $data;
                    let v: &mut [_] = &mut v;
                    let _v = &mut v[$bad];
                }
            }
        )*};
    }

    #[test]
    fn simple() {
        let v = [0, 1, 2, 3, 4, 5];

        assert_range_eq!(v, .., [0, 1, 2, 3, 4, 5]);
        assert_range_eq!(v, ..2, [0, 1]);
        assert_range_eq!(v, ..=1, [0, 1]);
        assert_range_eq!(v, 2.., [2, 3, 4, 5]);
        assert_range_eq!(v, 1..4, [1, 2, 3]);
        assert_range_eq!(v, 1..=3, [1, 2, 3]);
    }

    panic_cases! {
        mod rangefrom_len {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = 6..;
            let GOOD_OUTPUT = [];

            let BAD_INPUT = 7..;
            const EXPECT_MSG = "but ends at"; // perhaps not ideal

            !!generate_tests!!
        }

        mod rangeto_len {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = ..6;
            let GOOD_OUTPUT = [0, 1, 2, 3, 4, 5];

            let BAD_INPUT = ..7;
            const EXPECT_MSG = "out of range";

            !!generate_tests!!
        }

        mod rangetoinclusive_len {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = ..=5;
            let GOOD_OUTPUT = [0, 1, 2, 3, 4, 5];

            let BAD_INPUT = ..=6;
            const EXPECT_MSG = "out of range";

            !!generate_tests!!
        }

        mod range_len_len {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = 6..6;
            let GOOD_OUTPUT = [];

            let BAD_INPUT = 7..7;
            const EXPECT_MSG = "out of range";

            !!generate_tests!!
        }

        mod rangeinclusive_len_len{
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = 6..=5;
            let GOOD_OUTPUT = [];

            let BAD_INPUT = 7..=6;
            const EXPECT_MSG = "out of range";

            !!generate_tests!!
        }
    }

    panic_cases! {
        mod range_neg_width {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = 4..4;
            let GOOD_OUTPUT = [];

            let BAD_INPUT = 4..3;
            const EXPECT_MSG = "but ends at";

            !!generate_tests!!
        }

        mod rangeinclusive_neg_width {
            let DATA: [i32; 6] = [0, 1, 2, 3, 4, 5];

            let GOOD_INPUT = 4..=3;
            let GOOD_OUTPUT = [];

            let BAD_INPUT = 4..=2;
            const EXPECT_MSG = "but ends at";

            !!generate_tests!!
        }
    }

    panic_cases! {
        mod rangeinclusive_overflow {
            let DATA: [i32; 2] = [0, 1];

            // note: using 0 specifically ensures that the result of overflowing is 0..0,
            //       so that `get` doesn't simply return None for the wrong reason.
            let BAD_INPUT = 0 ..= ::std::usize::MAX;
            const EXPECT_MSG = "maximum usize";

            !!generate_tests!!
        }

        mod rangetoinclusive_overflow {
            let DATA: [i32; 2] = [0, 1];

            let BAD_INPUT = ..= ::std::usize::MAX;
            const EXPECT_MSG = "maximum usize";

            !!generate_tests!!
        }
    } // panic_cases!
}

#[test]
fn test_find_rfind() {
    let v = [0, 1, 2, 3, 4, 5];
    let mut iter = v.iter();
    let mut i = v.len();
    while let Some(&elt) = iter.rfind(|_| true) {
        i -= 1;
        assert_eq!(elt, v[i]);
    }
    assert_eq!(i, 0);
    assert_eq!(v.iter().rfind(|&&x| x <= 3), Some(&3));
}

#[test]
fn test_iter_folds() {
    let a = [1, 2, 3, 4, 5]; // len>4 so the unroll is used
    assert_eq!(a.iter().fold(0, |acc, &x| 2*acc + x), 57);
    assert_eq!(a.iter().rfold(0, |acc, &x| 2*acc + x), 129);
    let fold = |acc: i32, &x| acc.checked_mul(2)?.checked_add(x);
    assert_eq!(a.iter().try_fold(0, &fold), Some(57));
    assert_eq!(a.iter().try_rfold(0, &fold), Some(129));

    // short-circuiting try_fold, through other methods
    let a = [0, 1, 2, 3, 5, 5, 5, 7, 8, 9];
    let mut iter = a.iter();
    assert_eq!(iter.position(|&x| x == 3), Some(3));
    assert_eq!(iter.rfind(|&&x| x == 5), Some(&5));
    assert_eq!(iter.len(), 2);
}

#[test]
fn test_rotate_left() {
    const N: usize = 600;
    let a: &mut [_] = &mut [0; N];
    for i in 0..N {
        a[i] = i;
    }

    a.rotate_left(42);
    let k = N - 42;

    for i in 0..N {
        assert_eq!(a[(i + k) % N], i);
    }
}

#[test]
fn test_rotate_right() {
    const N: usize = 600;
    let a: &mut [_] = &mut [0; N];
    for i in 0..N {
        a[i] = i;
    }

    a.rotate_right(42);

    for i in 0..N {
        assert_eq!(a[(i + 42) % N], i);
    }
}

#[test]
#[cfg(not(target_arch = "wasm32"))]
fn sort_unstable() {
    use core::cmp::Ordering::{Equal, Greater, Less};
    use core::slice::heapsort;
    use rand::{Rng, XorShiftRng};

    let mut v = [0; 600];
    let mut tmp = [0; 600];
    let mut rng = XorShiftRng::new_unseeded();

    for len in (2..25).chain(500..510) {
        let v = &mut v[0..len];
        let tmp = &mut tmp[0..len];

        for &modulus in &[5, 10, 100, 1000] {
            for _ in 0..100 {
                for i in 0..len {
                    v[i] = rng.gen::<i32>() % modulus;
                }

                // Sort in default order.
                tmp.copy_from_slice(v);
                tmp.sort_unstable();
                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));

                // Sort in ascending order.
                tmp.copy_from_slice(v);
                tmp.sort_unstable_by(|a, b| a.cmp(b));
                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));

                // Sort in descending order.
                tmp.copy_from_slice(v);
                tmp.sort_unstable_by(|a, b| b.cmp(a));
                assert!(tmp.windows(2).all(|w| w[0] >= w[1]));

                // Test heapsort using `<` operator.
                tmp.copy_from_slice(v);
                heapsort(tmp, |a, b| a < b);
                assert!(tmp.windows(2).all(|w| w[0] <= w[1]));

                // Test heapsort using `>` operator.
                tmp.copy_from_slice(v);
                heapsort(tmp, |a, b| a > b);
                assert!(tmp.windows(2).all(|w| w[0] >= w[1]));
            }
        }
    }

    // Sort using a completely random comparison function.
    // This will reorder the elements *somehow*, but won't panic.
    for i in 0..v.len() {
        v[i] = i as i32;
    }
    v.sort_unstable_by(|_, _| *rng.choose(&[Less, Equal, Greater]).unwrap());
    v.sort_unstable();
    for i in 0..v.len() {
        assert_eq!(v[i], i as i32);
    }

    // Should not panic.
    [0i32; 0].sort_unstable();
    [(); 10].sort_unstable();
    [(); 100].sort_unstable();

    let mut v = [0xDEADBEEFu64];
    v.sort_unstable();
    assert!(v == [0xDEADBEEF]);
}