/// Utilities.
use std::cmp::Ordering;
use std::ops::Range;

/// Additional methods for slices.
pub trait SliceExt<T> {
    /// Split a slice into consecutive groups with the same key.
    ///
    /// Returns an iterator of pairs of a key and the group with that key.
    fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F>
    where
        F: FnMut(&T) -> K,
        K: PartialEq;
}

impl<T> SliceExt<T> for [T] {
    fn group_by_key<K, F>(&self, f: F) -> GroupByKey<'_, T, F>
    where
        F: FnMut(&T) -> K,
        K: PartialEq,
    {
        GroupByKey { slice: self, f }
    }
}

/// This struct is produced by [`SliceExt::group_by_key`].
pub struct GroupByKey<'a, T, F> {
    slice: &'a [T],
    f: F,
}

impl<'a, T, K, F> Iterator for GroupByKey<'a, T, F>
where
    F: FnMut(&T) -> K,
    K: PartialEq,
{
    type Item = (K, &'a [T]);

    fn next(&mut self) -> Option<Self::Item> {
        let first = self.slice.first()?;
        let key = (self.f)(first);

        let mut i = 1;
        while self.slice.get(i).map_or(false, |t| (self.f)(t) == key) {
            i += 1;
        }

        let (head, tail) = self.slice.split_at(i);
        self.slice = tail;
        Some((key, head))
    }
}

/// Additional methods for [`Range<usize>`].
pub trait RangeExt {
    /// Locate a position relative to a range.
    ///
    /// This can be used for binary searching the range that contains the
    /// position as follows:
    /// ```
    /// # use typst::util::RangeExt;
    /// assert_eq!(
    ///     [1..2, 2..7, 7..10].binary_search_by(|r| r.locate(5)),
    ///     Ok(1),
    /// );
    /// ```
    fn locate(&self, pos: usize) -> Ordering;
}

impl RangeExt for Range<usize> {
    fn locate(&self, pos: usize) -> Ordering {
        if pos < self.start {
            Ordering::Greater
        } else if pos < self.end {
            Ordering::Equal
        } else {
            Ordering::Less
        }
    }
}