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use std::cmp::Ordering;

/// An implementation of binary search on indices only
/// that does not require slices to be constructed. Mirrors
/// the semantics of binary_search_by in the standard library.
pub fn binary_search<F>(mut start: usize, mut end: usize, mut f: F) -> Result<usize, usize>
where F: FnMut(usize) -> Ordering
{
    loop {
        let mid = start + (end - start) / 2;
        if mid == end { return Err(start); }
        match f(mid) {
            Ordering::Less => start = mid + 1,
            Ordering::Greater => end = mid,
            Ordering::Equal => return Ok(mid),
        }
    }
}

#[test]
fn test_binary_search() {
    assert_eq!(binary_search(0, 5000, |x| x.cmp(&1337)), Ok(1337));
    assert_eq!(binary_search(0, 5000, |x| x.cmp(&9000)), Err(5000));
    assert_eq!(binary_search(30, 50,  |x| x.cmp(&42)), Ok(42));
    assert_eq!(binary_search(300, 500,|x| x.cmp(&42)), Err(300));
    assert_eq!(binary_search(0, 500,  |x| if x < 42 {Ordering::Less} else {Ordering::Greater}), Err(42));
}