helix/helix-core/src/selection.rs

//! Selections are the primary editing construct. Even cursors are
//! defined as a selection range.
//!
//! All positioning is done via `char` offsets into the buffer.
use crate::{
    graphemes::{
        ensure_grapheme_boundary_next, ensure_grapheme_boundary_prev, next_grapheme_boundary,
        prev_grapheme_boundary,
    },
    Assoc, ChangeSet, RopeSlice,
};
use smallvec::{smallvec, SmallVec};
use std::borrow::Cow;

/// A single selection range.
///
/// A range consists of an "anchor" and "head" position in
/// the text.  The head is the part that the user moves when
/// directly extending a selection.  The head and anchor
/// can be in any order, or even share the same position.
///
/// The anchor and head positions use gap indexing, meaning
/// that their indices represent the the gaps *between* `char`s
/// rather than the `char`s themselves. For example, 1
/// represents the position between the first and second `char`.
///
/// Below are some example `Range` configurations to better
/// illustrate.  The anchor and head indices are show as
/// "(anchor, head)", followed by example text with "[" and "]"
/// inserted to represent the anchor and head positions:
///
/// - (0, 3): [Som]e text.
/// - (3, 0): ]Som[e text.
/// - (2, 7): So[me te]xt.
/// - (1, 1): S[]ome text.
///
/// Ranges are considered to be inclusive on the left and
/// exclusive on the right, regardless of anchor-head ordering.
/// This means, for example, that non-zero-width ranges that
/// are directly adjecent, sharing an edge, do not overlap.
/// However, a zero-width range will overlap with the shared
/// left-edge of another range.
///
/// By convention, user-facing ranges are considered to have
/// a block cursor on the head-side of the range that spans a
/// single grapheme inward from the range's edge.  There are a
/// variety of helper methods on `Range` for working in terms of
/// that block cursor, all of which have `cursor` in their name.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Range {
    /// The anchor of the range: the side that doesn't move when extending.
    pub anchor: usize,
    /// The head of the range, moved when extending.
    pub head: usize,
    pub horiz: Option<u32>,
}

impl Range {
    pub fn new(anchor: usize, head: usize) -> Self {
        Self {
            anchor,
            head,
            horiz: None,
        }
    }

    pub fn point(head: usize) -> Self {
        Self::new(head, head)
    }

    /// Start of the range.
    #[inline]
    #[must_use]
    pub fn from(&self) -> usize {
        std::cmp::min(self.anchor, self.head)
    }

    /// End of the range.
    #[inline]
    #[must_use]
    pub fn to(&self) -> usize {
        std::cmp::max(self.anchor, self.head)
    }

    /// The (inclusive) range of lines that the range overlaps.
    #[inline]
    #[must_use]
    pub fn line_range(&self, text: RopeSlice) -> (usize, usize) {
        let from = self.from();
        let to = if self.is_empty() {
            self.to()
        } else {
            prev_grapheme_boundary(text, self.to()).max(from)
        };

        (text.char_to_line(from), text.char_to_line(to))
    }

    /// `true` when head and anchor are at the same position.
    #[inline]
    pub fn is_empty(&self) -> bool {
        self.anchor == self.head
    }

    /// Check two ranges for overlap.
    #[must_use]
    pub fn overlaps(&self, other: &Self) -> bool {
        // To my eye, it's non-obvious why this works, but I arrived
        // at it after transforming the slower version that explicitly
        // enumerated more cases.  The unit tests are thorough.
        self.from() == other.from() || (self.to() > other.from() && other.to() > self.from())
    }

    pub fn contains(&self, pos: usize) -> bool {
        self.from() <= pos && pos < self.to()
    }

    /// Map a range through a set of changes. Returns a new range representing the same position
    /// after the changes are applied.
    pub fn map(self, changes: &ChangeSet) -> Self {
        use std::cmp::Ordering;
        let (anchor, head) = match self.anchor.cmp(&self.head) {
            Ordering::Equal => (
                changes.map_pos(self.anchor, Assoc::After),
                changes.map_pos(self.head, Assoc::After),
            ),
            Ordering::Less => (
                changes.map_pos(self.anchor, Assoc::After),
                changes.map_pos(self.head, Assoc::Before),
            ),
            Ordering::Greater => (
                changes.map_pos(self.anchor, Assoc::Before),
                changes.map_pos(self.head, Assoc::After),
            ),
        };

        // We want to return a new `Range` with `horiz == None` every time,
        // even if the anchor and head haven't changed, because we don't
        // know if the *visual* position hasn't changed due to
        // character-width or grapheme changes earlier in the text.
        Self {
            anchor,
            head,
            horiz: None,
        }
    }

    /// Extend the range to cover at least `from` `to`.
    #[must_use]
    pub fn extend(&self, from: usize, to: usize) -> Self {
        debug_assert!(from <= to);

        if self.anchor <= self.head {
            Self {
                anchor: self.anchor.min(from),
                head: self.head.max(to),
                horiz: None,
            }
        } else {
            Self {
                anchor: self.anchor.max(to),
                head: self.head.min(from),
                horiz: None,
            }
        }
    }

    /// Returns a range that encompasses both input ranges.
    ///
    /// This is like `extend()`, but tries to negotiate the
    /// anchor/head ordering between the two input ranges.
    #[must_use]
    pub fn merge(&self, other: Self) -> Self {
        if self.anchor > self.head && other.anchor > other.head {
            Range {
                anchor: self.anchor.max(other.anchor),
                head: self.head.min(other.head),
                horiz: None,
            }
        } else {
            Range {
                anchor: self.from().min(other.from()),
                head: self.to().max(other.to()),
                horiz: None,
            }
        }
    }

    // groupAt

    #[inline]
    pub fn fragment<'a, 'b: 'a>(&'a self, text: RopeSlice<'b>) -> Cow<'b, str> {
        text.slice(self.from()..self.to()).into()
    }

    //--------------------------------
    // Alignment methods.

    /// Compute a possibly new range from this range, with its ends
    /// shifted as needed to align with grapheme boundaries.
    ///
    /// Zero-width ranges will always stay zero-width, and non-zero-width
    /// ranges will never collapse to zero-width.
    #[must_use]
    pub fn grapheme_aligned(&self, slice: RopeSlice) -> Self {
        use std::cmp::Ordering;
        let (new_anchor, new_head) = match self.anchor.cmp(&self.head) {
            Ordering::Equal => {
                let pos = ensure_grapheme_boundary_prev(slice, self.anchor);
                (pos, pos)
            }
            Ordering::Less => (
                ensure_grapheme_boundary_prev(slice, self.anchor),
                ensure_grapheme_boundary_next(slice, self.head),
            ),
            Ordering::Greater => (
                ensure_grapheme_boundary_next(slice, self.anchor),
                ensure_grapheme_boundary_prev(slice, self.head),
            ),
        };
        Range {
            anchor: new_anchor,
            head: new_head,
            horiz: if new_anchor == self.anchor {
                self.horiz
            } else {
                None
            },
        }
    }

    /// Compute a possibly new range from this range, attempting to ensure
    /// a minimum range width of 1 char by shifting the head in the forward
    /// direction as needed.
    ///
    /// This method will never shift the anchor, and will only shift the
    /// head in the forward direction.  Therefore, this method can fail
    /// at ensuring the minimum width if and only if the passed range is
    /// both zero-width and at the end of the `RopeSlice`.
    ///
    /// If the input range is grapheme-boundary aligned, the returned range
    /// will also be.  Specifically, if the head needs to shift to achieve
    /// the minimum width, it will shift to the next grapheme boundary.
    #[must_use]
    #[inline]
    pub fn min_width_1(&self, slice: RopeSlice) -> Self {
        if self.anchor == self.head {
            Range {
                anchor: self.anchor,
                head: next_grapheme_boundary(slice, self.head),
                horiz: self.horiz,
            }
        } else {
            *self
        }
    }

    //--------------------------------
    // Block-cursor methods.

    /// Gets the left-side position of the block cursor.
    #[must_use]
    #[inline]
    pub fn cursor(self, text: RopeSlice) -> usize {
        if self.head > self.anchor {
            prev_grapheme_boundary(text, self.head)
        } else {
            self.head
        }
    }

    /// Puts the left side of the block cursor at `char_idx`, optionally extending.
    ///
    /// This follows "1-width" semantics, and therefore does a combination of anchor
    /// and head moves to behave as if both the front and back of the range are 1-width
    /// blocks
    ///
    /// This method assumes that the range and `char_idx` are already properly
    /// grapheme-aligned.
    #[must_use]
    #[inline]
    pub fn put_cursor(self, text: RopeSlice, char_idx: usize, extend: bool) -> Range {
        if extend {
            let anchor = if self.head >= self.anchor && char_idx < self.anchor {
                next_grapheme_boundary(text, self.anchor)
            } else if self.head < self.anchor && char_idx >= self.anchor {
                prev_grapheme_boundary(text, self.anchor)
            } else {
                self.anchor
            };

            if anchor <= char_idx {
                Range::new(anchor, next_grapheme_boundary(text, char_idx))
            } else {
                Range::new(anchor, char_idx)
            }
        } else {
            Range::point(char_idx)
        }
    }

    /// The line number that the block-cursor is on.
    #[inline]
    #[must_use]
    pub fn cursor_line(&self, text: RopeSlice) -> usize {
        text.char_to_line(self.cursor(text))
    }
}

impl From<(usize, usize)> for Range {
    fn from(tuple: (usize, usize)) -> Self {
        Self {
            anchor: tuple.0,
            head: tuple.1,
            horiz: None,
        }
    }
}

/// A selection consists of one or more selection ranges.
/// invariant: A selection can never be empty (always contains at least primary range).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Selection {
    ranges: SmallVec<[Range; 1]>,
    primary_index: usize,
}

#[allow(clippy::len_without_is_empty)] // a Selection is never empty
impl Selection {
    // eq

    #[must_use]
    pub fn primary(&self) -> Range {
        self.ranges[self.primary_index]
    }

    /// Ensure selection containing only the primary selection.
    pub fn into_single(self) -> Self {
        if self.ranges.len() == 1 {
            self
        } else {
            Self {
                ranges: smallvec![self.ranges[self.primary_index]],
                primary_index: 0,
            }
        }
    }

    /// Adds a new range to the selection and makes it the primary range.
    pub fn push(mut self, range: Range) -> Self {
        self.ranges.push(range);
        self.set_primary_index(self.ranges().len() - 1);
        self.normalize()
    }

    /// Map selections over a set of changes. Useful for adjusting the selection position after
    /// applying changes to a document.
    pub fn map(self, changes: &ChangeSet) -> Self {
        if changes.is_empty() {
            return self;
        }

        Self::new(
            self.ranges
                .into_iter()
                .map(|range| range.map(changes))
                .collect(),
            self.primary_index,
        )
    }

    pub fn ranges(&self) -> &[Range] {
        &self.ranges
    }

    pub fn primary_index(&self) -> usize {
        self.primary_index
    }

    pub fn set_primary_index(&mut self, idx: usize) {
        assert!(idx < self.ranges.len());
        self.primary_index = idx;
    }

    #[must_use]
    /// Constructs a selection holding a single range.
    pub fn single(anchor: usize, head: usize) -> Self {
        Self {
            ranges: smallvec![Range {
                anchor,
                head,
                horiz: None
            }],
            primary_index: 0,
        }
    }

    /// Constructs a selection holding a single cursor.
    pub fn point(pos: usize) -> Self {
        Self::single(pos, pos)
    }

    /// Normalizes a `Selection`.
    fn normalize(mut self) -> Self {
        let primary = self.ranges[self.primary_index];
        self.ranges.sort_unstable_by_key(Range::from);
        self.primary_index = self
            .ranges
            .iter()
            .position(|&range| range == primary)
            .unwrap();

        let mut prev_i = 0;
        for i in 1..self.ranges.len() {
            if self.ranges[prev_i].overlaps(&self.ranges[i]) {
                self.ranges[prev_i] = self.ranges[prev_i].merge(self.ranges[i]);
            } else {
                prev_i += 1;
                self.ranges[prev_i] = self.ranges[i];
            }
            if i == self.primary_index {
                self.primary_index = prev_i;
            }
        }

        self.ranges.truncate(prev_i + 1);

        self
    }

    // TODO: consume an iterator or a vec to reduce allocations?
    #[must_use]
    pub fn new(ranges: SmallVec<[Range; 1]>, primary_index: usize) -> Self {
        assert!(!ranges.is_empty());
        debug_assert!(primary_index < ranges.len());

        let mut selection = Self {
            ranges,
            primary_index,
        };

        if selection.ranges.len() > 1 {
            // TODO: only normalize if needed (any ranges out of order)
            selection = selection.normalize();
        }

        selection
    }

    /// Takes a closure and maps each `Range` over the closure.
    pub fn transform<F>(mut self, f: F) -> Self
    where
        F: Fn(Range) -> Range,
    {
        for range in self.ranges.iter_mut() {
            *range = f(*range)
        }

        self.normalize()
    }

    /// A convenience short-cut for `transform(|r| r.min_width_1(text))`.
    pub fn min_width_1(self, text: RopeSlice) -> Self {
        self.transform(|r| r.min_width_1(text))
    }

    /// Transforms the selection into all of the left-side head positions,
    /// using block-cursor semantics.
    pub fn cursors(self, text: RopeSlice) -> Self {
        self.transform(|range| Range::point(range.cursor(text)))
    }

    pub fn fragments<'a>(&'a self, text: RopeSlice<'a>) -> impl Iterator<Item = Cow<str>> + 'a {
        self.ranges.iter().map(move |range| range.fragment(text))
    }

    #[inline(always)]
    pub fn iter(&self) -> std::slice::Iter<'_, Range> {
        self.ranges.iter()
    }

    #[inline(always)]
    pub fn len(&self) -> usize {
        self.ranges.len()
    }
}

impl<'a> IntoIterator for &'a Selection {
    type Item = &'a Range;
    type IntoIter = std::slice::Iter<'a, Range>;

    fn into_iter(self) -> std::slice::Iter<'a, Range> {
        self.ranges().iter()
    }
}

// TODO: checkSelection -> check if valid for doc length && sorted

pub fn keep_matches(
    text: RopeSlice,
    selection: &Selection,
    regex: &crate::regex::Regex,
) -> Option<Selection> {
    let result: SmallVec<_> = selection
        .iter()
        .filter(|range| regex.is_match(&range.fragment(text)))
        .copied()
        .collect();

    // TODO: figure out a new primary index
    if !result.is_empty() {
        return Some(Selection::new(result, 0));
    }
    None
}

pub fn select_on_matches(
    text: RopeSlice,
    selection: &Selection,
    regex: &crate::regex::Regex,
) -> Option<Selection> {
    let mut result = SmallVec::with_capacity(selection.len());

    for sel in selection {
        // TODO: can't avoid occasional allocations since Regex can't operate on chunks yet
        let fragment = sel.fragment(text);

        let sel_start = sel.from();
        let start_byte = text.char_to_byte(sel_start);

        for mat in regex.find_iter(&fragment) {
            // TODO: retain range direction

            let start = text.byte_to_char(start_byte + mat.start());
            let end = text.byte_to_char(start_byte + mat.end());
            result.push(Range::new(start, end));
        }
    }

    // TODO: figure out a new primary index
    if !result.is_empty() {
        return Some(Selection::new(result, 0));
    }

    None
}

// TODO: support to split on capture #N instead of whole match
pub fn split_on_matches(
    text: RopeSlice,
    selection: &Selection,
    regex: &crate::regex::Regex,
) -> Selection {
    let mut result = SmallVec::with_capacity(selection.len());

    for sel in selection {
        // Special case: zero-width selection.
        if sel.from() == sel.to() {
            result.push(*sel);
            continue;
        }

        // TODO: can't avoid occasional allocations since Regex can't operate on chunks yet
        let fragment = sel.fragment(text);

        let sel_start = sel.from();
        let sel_end = sel.to();

        let start_byte = text.char_to_byte(sel_start);

        let mut start = sel_start;

        for mat in regex.find_iter(&fragment) {
            // TODO: retain range direction
            let end = text.byte_to_char(start_byte + mat.start());
            result.push(Range::new(start, end));
            start = text.byte_to_char(start_byte + mat.end());
        }

        if start < sel_end {
            result.push(Range::new(start, sel_end));
        }
    }

    // TODO: figure out a new primary index
    Selection::new(result, 0)
}

#[cfg(test)]
mod test {
    use super::*;
    use crate::Rope;

    #[test]
    #[should_panic]
    fn test_new_empty() {
        let _ = Selection::new(smallvec![], 0);
    }

    #[test]
    fn test_create_normalizes_and_merges() {
        let sel = Selection::new(
            smallvec![
                Range::new(10, 12),
                Range::new(6, 7),
                Range::new(4, 5),
                Range::new(3, 4),
                Range::new(0, 6),
                Range::new(7, 8),
                Range::new(9, 13),
                Range::new(13, 14),
            ],
            0,
        );

        let res = sel
            .ranges
            .into_iter()
            .map(|range| format!("{}/{}", range.anchor, range.head))
            .collect::<Vec<String>>()
            .join(",");

        assert_eq!(res, "0/6,6/7,7/8,9/13,13/14");

        // it correctly calculates a new primary index
        let sel = Selection::new(
            smallvec![Range::new(0, 2), Range::new(1, 5), Range::new(4, 7)],
            2,
        );

        let res = sel
            .ranges
            .into_iter()
            .map(|range| format!("{}/{}", range.anchor, range.head))
            .collect::<Vec<String>>()
            .join(",");

        assert_eq!(res, "0/7");
        assert_eq!(sel.primary_index, 0);
    }

    #[test]
    fn test_create_merges_adjacent_points() {
        let sel = Selection::new(
            smallvec![
                Range::new(10, 12),
                Range::new(12, 12),
                Range::new(12, 12),
                Range::new(10, 10),
                Range::new(8, 10),
            ],
            0,
        );

        let res = sel
            .ranges
            .into_iter()
            .map(|range| format!("{}/{}", range.anchor, range.head))
            .collect::<Vec<String>>()
            .join(",");

        assert_eq!(res, "8/10,10/12,12/12");
    }

    #[test]
    fn test_contains() {
        let range = Range::new(10, 12);

        assert_eq!(range.contains(9), false);
        assert_eq!(range.contains(10), true);
        assert_eq!(range.contains(11), true);
        assert_eq!(range.contains(12), false);
        assert_eq!(range.contains(13), false);

        let range = Range::new(9, 6);
        assert_eq!(range.contains(9), false);
        assert_eq!(range.contains(7), true);
        assert_eq!(range.contains(6), true);
    }

    #[test]
    fn test_overlaps() {
        fn overlaps(a: (usize, usize), b: (usize, usize)) -> bool {
            Range::new(a.0, a.1).overlaps(&Range::new(b.0, b.1))
        }

        // Two non-zero-width ranges, no overlap.
        assert!(!overlaps((0, 3), (3, 6)));
        assert!(!overlaps((0, 3), (6, 3)));
        assert!(!overlaps((3, 0), (3, 6)));
        assert!(!overlaps((3, 0), (6, 3)));
        assert!(!overlaps((3, 6), (0, 3)));
        assert!(!overlaps((3, 6), (3, 0)));
        assert!(!overlaps((6, 3), (0, 3)));
        assert!(!overlaps((6, 3), (3, 0)));

        // Two non-zero-width ranges, overlap.
        assert!(overlaps((0, 4), (3, 6)));
        assert!(overlaps((0, 4), (6, 3)));
        assert!(overlaps((4, 0), (3, 6)));
        assert!(overlaps((4, 0), (6, 3)));
        assert!(overlaps((3, 6), (0, 4)));
        assert!(overlaps((3, 6), (4, 0)));
        assert!(overlaps((6, 3), (0, 4)));
        assert!(overlaps((6, 3), (4, 0)));

        // Zero-width and non-zero-width range, no overlap.
        assert!(!overlaps((0, 3), (3, 3)));
        assert!(!overlaps((3, 0), (3, 3)));
        assert!(!overlaps((3, 3), (0, 3)));
        assert!(!overlaps((3, 3), (3, 0)));

        // Zero-width and non-zero-width range, overlap.
        assert!(overlaps((1, 4), (1, 1)));
        assert!(overlaps((4, 1), (1, 1)));
        assert!(overlaps((1, 1), (1, 4)));
        assert!(overlaps((1, 1), (4, 1)));

        assert!(overlaps((1, 4), (3, 3)));
        assert!(overlaps((4, 1), (3, 3)));
        assert!(overlaps((3, 3), (1, 4)));
        assert!(overlaps((3, 3), (4, 1)));

        // Two zero-width ranges, no overlap.
        assert!(!overlaps((0, 0), (1, 1)));
        assert!(!overlaps((1, 1), (0, 0)));

        // Two zero-width ranges, overlap.
        assert!(overlaps((1, 1), (1, 1)));
    }

    #[test]
    fn test_graphem_aligned() {
        let r = Rope::from_str("\r\nHi\r\n");
        let s = r.slice(..);

        // Zero-width.
        assert_eq!(Range::new(0, 0).grapheme_aligned(s), Range::new(0, 0));
        assert_eq!(Range::new(1, 1).grapheme_aligned(s), Range::new(0, 0));
        assert_eq!(Range::new(2, 2).grapheme_aligned(s), Range::new(2, 2));
        assert_eq!(Range::new(3, 3).grapheme_aligned(s), Range::new(3, 3));
        assert_eq!(Range::new(4, 4).grapheme_aligned(s), Range::new(4, 4));
        assert_eq!(Range::new(5, 5).grapheme_aligned(s), Range::new(4, 4));
        assert_eq!(Range::new(6, 6).grapheme_aligned(s), Range::new(6, 6));

        // Forward.
        assert_eq!(Range::new(0, 1).grapheme_aligned(s), Range::new(0, 2));
        assert_eq!(Range::new(1, 2).grapheme_aligned(s), Range::new(0, 2));
        assert_eq!(Range::new(2, 3).grapheme_aligned(s), Range::new(2, 3));
        assert_eq!(Range::new(3, 4).grapheme_aligned(s), Range::new(3, 4));
        assert_eq!(Range::new(4, 5).grapheme_aligned(s), Range::new(4, 6));
        assert_eq!(Range::new(5, 6).grapheme_aligned(s), Range::new(4, 6));

        assert_eq!(Range::new(0, 2).grapheme_aligned(s), Range::new(0, 2));
        assert_eq!(Range::new(1, 3).grapheme_aligned(s), Range::new(0, 3));
        assert_eq!(Range::new(2, 4).grapheme_aligned(s), Range::new(2, 4));
        assert_eq!(Range::new(3, 5).grapheme_aligned(s), Range::new(3, 6));
        assert_eq!(Range::new(4, 6).grapheme_aligned(s), Range::new(4, 6));

        // Reverse.
        assert_eq!(Range::new(1, 0).grapheme_aligned(s), Range::new(2, 0));
        assert_eq!(Range::new(2, 1).grapheme_aligned(s), Range::new(2, 0));
        assert_eq!(Range::new(3, 2).grapheme_aligned(s), Range::new(3, 2));
        assert_eq!(Range::new(4, 3).grapheme_aligned(s), Range::new(4, 3));
        assert_eq!(Range::new(5, 4).grapheme_aligned(s), Range::new(6, 4));
        assert_eq!(Range::new(6, 5).grapheme_aligned(s), Range::new(6, 4));

        assert_eq!(Range::new(2, 0).grapheme_aligned(s), Range::new(2, 0));
        assert_eq!(Range::new(3, 1).grapheme_aligned(s), Range::new(3, 0));
        assert_eq!(Range::new(4, 2).grapheme_aligned(s), Range::new(4, 2));
        assert_eq!(Range::new(5, 3).grapheme_aligned(s), Range::new(6, 3));
        assert_eq!(Range::new(6, 4).grapheme_aligned(s), Range::new(6, 4));
    }

    #[test]
    fn test_min_width_1() {
        let r = Rope::from_str("\r\nHi\r\n");
        let s = r.slice(..);

        // Zero-width.
        assert_eq!(Range::new(0, 0).min_width_1(s), Range::new(0, 2));
        assert_eq!(Range::new(1, 1).min_width_1(s), Range::new(1, 2));
        assert_eq!(Range::new(2, 2).min_width_1(s), Range::new(2, 3));
        assert_eq!(Range::new(3, 3).min_width_1(s), Range::new(3, 4));
        assert_eq!(Range::new(4, 4).min_width_1(s), Range::new(4, 6));
        assert_eq!(Range::new(5, 5).min_width_1(s), Range::new(5, 6));
        assert_eq!(Range::new(6, 6).min_width_1(s), Range::new(6, 6));

        // Forward.
        assert_eq!(Range::new(0, 1).min_width_1(s), Range::new(0, 1));
        assert_eq!(Range::new(1, 2).min_width_1(s), Range::new(1, 2));
        assert_eq!(Range::new(2, 3).min_width_1(s), Range::new(2, 3));
        assert_eq!(Range::new(3, 4).min_width_1(s), Range::new(3, 4));
        assert_eq!(Range::new(4, 5).min_width_1(s), Range::new(4, 5));
        assert_eq!(Range::new(5, 6).min_width_1(s), Range::new(5, 6));

        // Reverse.
        assert_eq!(Range::new(1, 0).min_width_1(s), Range::new(1, 0));
        assert_eq!(Range::new(2, 1).min_width_1(s), Range::new(2, 1));
        assert_eq!(Range::new(3, 2).min_width_1(s), Range::new(3, 2));
        assert_eq!(Range::new(4, 3).min_width_1(s), Range::new(4, 3));
        assert_eq!(Range::new(5, 4).min_width_1(s), Range::new(5, 4));
        assert_eq!(Range::new(6, 5).min_width_1(s), Range::new(6, 5));
    }

    #[test]
    fn test_split_on_matches() {
        use crate::regex::Regex;

        let text = Rope::from(" abcd efg wrs   xyz 123 456");

        let selection = Selection::new(smallvec![Range::new(0, 9), Range::new(11, 20),], 0);

        let result = split_on_matches(text.slice(..), &selection, &Regex::new(r"\s+").unwrap());

        assert_eq!(
            result.ranges(),
            &[
                // TODO: rather than this behavior, maybe we want it
                // to be based on which side is the anchor?
                //
                // We get a leading zero-width range when there's
                // a leading match because ranges are inclusive on
                // the left.  Imagine, for example, if the entire
                // selection range were matched: you'd still want
                // at least one range to remain after the split.
                Range::new(0, 0),
                Range::new(1, 5),
                Range::new(6, 9),
                Range::new(11, 13),
                Range::new(16, 19),
                // In contrast to the comment above, there is no
                // _trailing_ zero-width range despite the trailing
                // match, because ranges are exclusive on the right.
            ]
        );

        assert_eq!(
            result.fragments(text.slice(..)).collect::<Vec<_>>(),
            &["", "abcd", "efg", "rs", "xyz"]
        );
    }
}