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/// A crate to add two more sorting methods
/// ---
///
/// Say you have a vector of tuples, which you would like to sort by one of the
/// elements (maybe you got it by using the enumerate or zip iterator methods,
/// then collected into a Vec). Now, if you write the following, you will get
/// lifetime errors:
/// ```compile_fail
/// let mut letters = vec![
///     (1, "C".to_string()), 
///     (2, "B".to_string()),
///     (3, "B".to_string()),
///     (4, "A".to_string())
/// ];
/// letters.sort_by_key(|x| &x.1);
/// println!("Letters: {:?}", letters);
/// ```
/// So you might try not borrowing, but then you realize String isn't Copy:
/// ```compile_fail
/// let mut letters = vec![
///     (1, "C".to_string()), 
///     (2, "B".to_string()),
///     (3, "B".to_string()),
///     (4, "A".to_string())
/// ];
/// letters.sort_by_key(|x| x.1);
/// println!("Letters: {:?}", letters);
/// ```
/// So this fails because you'd be moving out of the array! You don't want to 
/// have to expensively clone all of these strings just to compare them. That's
/// where this library comes in:
/// ```
/// use sort_by_borrowed_key::SortByBorrowedKey;
/// let mut letters = vec![
///     (1, "C".to_string()), 
///     (2, "B".to_string()),
///     (3, "B".to_string()),
///     (4, "A".to_string())
/// ];
/// letters.sort_by_borrowed_key(|x| &x.1);
/// let expected = vec![
///     (4, "A".to_string()), 
///     (2, "B".to_string()),
///     (3, "B".to_string()),
///     (1, "C".to_string())
/// ];
/// assert_eq!(letters, expected);
/// ```
/// That's it! The only other thing this crate adds is a unstable-sort version
/// of the same method, `sort_unstable_by_borrowed_key`, which is faster but may
/// change of the order of equal elements (see the docs for
/// `[T]::sort_unstable`).
///
pub trait SortByBorrowedKey<T> {
    fn sort_by_borrowed_key<K, F>(&mut self, f: F) where
        F: for<'a> Fn(&'a T) -> &'a K,
        K: Ord;
    fn sort_unstable_by_borrowed_key<K, F>(&mut self, f: F) where
        F: for<'a> Fn(&'a T) -> &'a K,
        K: Ord;
}

impl<T> SortByBorrowedKey<T> for [T] {
    fn sort_by_borrowed_key<K, F>(&mut self, f: F) where
        F: for<'a> Fn(&'a T) -> &'a K,
        K: Ord {
            self.sort_by(|a, b| {
                f(a).cmp(f(b))
            });
        }
    fn sort_unstable_by_borrowed_key<K, F>(&mut self, f: F) where
        F: for<'a> Fn(&'a T) -> &'a K,
        K: Ord {
            self.sort_unstable_by(|a, b| {
                f(a).cmp(f(b))
            });
        }
}

#[cfg(test)]
mod tests {
    use crate::*;
    #[test]
    fn test_stable() {
        let mut letters = vec![
            (1, "C".to_string()), 
            (2, "B".to_string()),
            (3, "B".to_string()),
            (4, "A".to_string())
        ];
        letters.sort_by_borrowed_key(|x| &x.1);
        let expected = vec![
            (4, "A".to_string()), 
            (2, "B".to_string()),
            (3, "B".to_string()),
            (1, "C".to_string())
        ];
        assert_eq!(letters, expected);
    }
}