try_max_by_key

try_max_by_key.rs

/// short-circuiting version of iter.max_by_key
///
/// partially taken from https://doc.rust-lang.org/src/core/iter/traits/iterator.rs.html#2591
///
/// Example:
///
/// ```
/// let list = [[1, 2, 3], [5, 4, 3], [1, 1, 4]];
/// assert_eq!(try_max_by_key(list.into_iter(), |vec| vec.last().ok_or(())), Some(Ok(&[1, 1, 4])));
///
/// let lengths = [vec![], vec![1], vec![1, 2]];
/// assert_eq!(try_max_by_key(lengths.into_iter(), |vec| vec.last().ok_or(())), Some(Err(())));
/// ```
pub fn try_max_by_key<I, T, C, R, F>(mut iter: I, mut f: F) -> Option<Result<T, R>>
where
    I: Iterator<Item = T>,
    C: std::cmp::Ord,
    F: FnMut(&T) -> Result<C, R>,
{
    iter.next().map(|initial| {
        // if this gives an error, return it immediately
        // avoids f not being called if there's only one element
        f(&initial)?;
        iter.try_fold(initial, |current, next| {
            if f(&current)? >= f(&next)? {
                Ok(current)
            } else {
                Ok(next)
            }
        })
    })
}