path: root/src/buffer/selection.rs

//! Selection model — byte-indexed, following Helix's `helix-core/src/selection.rs`.
//!
//! This is what makes the editor selection-first rather than Vim-like: a motion
//! leaves a *range*, and an operator such as `d` acts on that range. There is no
//! operator-pending state anywhere in the editor.
//!
//! Conventions, preserved exactly from Helix:
//!
//! - A range is **half-open**: inclusive of `from()`, exclusive of `to()`,
//!   regardless of whether `head` precedes or follows `anchor`.
//! - The visible block cursor spans one grapheme *inward* from the head, so a
//!   forward range `0..1` shows its cursor on byte 0, not byte 1.
//!
//! Per MJB-LLR-009 a `Selection` holds exactly one range. It is a struct rather
//! than a bare `Range` so that multi-cursor support can be added later without
//! reworking call sites.

use ropey::RopeSlice;

use super::grapheme::{next_grapheme_boundary, prev_grapheme_boundary};

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Direction {
    Forward,
    Backward,
}

/// MJB-LLR-001: a range over the buffer, both offsets in **bytes**.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
pub struct Range {
    /// The side that stays put when extending.
    pub anchor: usize,
    /// The side that moves when extending.
    pub head: usize,
}

impl Range {
    pub fn new(anchor: usize, head: usize) -> Self {
        Self { anchor, head }
    }

    /// A zero-width range at `head`.
    pub fn point(head: usize) -> Self {
        Self { anchor: head, head }
    }

    /// MJB-LLR-011: clamp both offsets into the buffer and snap them to char
    /// boundaries. Byte indexing permits offsets that char indexing could not
    /// express, and ropey panics on them — see MJB-DR-002.
    pub fn clamped(self, text: RopeSlice) -> Self {
        let len = text.len();
        Self {
            anchor: text.floor_char_boundary(self.anchor.min(len)),
            head: text.floor_char_boundary(self.head.min(len)),
        }
    }

    /// MJB-LLR-002: lower bound, inclusive.
    pub fn from(&self) -> usize {
        self.anchor.min(self.head)
    }

    /// MJB-LLR-002: upper bound, exclusive.
    pub fn to(&self) -> usize {
        self.anchor.max(self.head)
    }

    /// MJB-LLR-003
    pub fn is_empty(&self) -> bool {
        self.anchor == self.head
    }

    /// Byte length of the span.
    pub fn len(&self) -> usize {
        self.to() - self.from()
    }

    /// MJB-LLR-004
    pub fn direction(&self) -> Direction {
        if self.head < self.anchor {
            Direction::Backward
        } else {
            Direction::Forward
        }
    }

    /// MJB-LLR-005: the byte offset the block cursor is drawn at.
    ///
    /// For a forward range the head sits *past* the last selected grapheme, so
    /// the cursor steps back one grapheme to land on it.
    pub fn cursor(&self, text: RopeSlice) -> usize {
        if self.head > self.anchor {
            prev_grapheme_boundary(text, self.head)
        } else {
            self.head
        }
    }

    /// MJB-LLR-006, MJB-LLR-007: move the cursor to `byte_idx`.
    ///
    /// Without `extend` this collapses to a point. With `extend` the anchor is
    /// nudged by one grapheme when the range flips direction across it, so the
    /// anchored grapheme stays selected — this is Helix's `put_cursor`.
    pub fn put_cursor(self, text: RopeSlice, byte_idx: usize, extend: bool) -> Self {
        if !extend {
            return Range::point(byte_idx).clamped(text);
        }

        let anchor = if self.head >= self.anchor && byte_idx < self.anchor {
            next_grapheme_boundary(text, self.anchor)
        } else if self.head < self.anchor && byte_idx >= self.anchor {
            prev_grapheme_boundary(text, self.anchor)
        } else {
            self.anchor
        };

        if anchor <= byte_idx {
            Range::new(anchor, next_grapheme_boundary(text, byte_idx)).clamped(text)
        } else {
            Range::new(anchor, byte_idx).clamped(text)
        }
    }

    /// The line the cursor lies on.
    pub fn cursor_line(&self, text: RopeSlice) -> usize {
        text.byte_to_line_idx(self.cursor(text), super::LINE_TYPE)
    }

    /// MJB-LLR-008: inclusive span of line indices the range covers.
    pub fn line_range(&self, text: RopeSlice) -> (usize, usize) {
        let lt = super::LINE_TYPE;
        let start = text.byte_to_line_idx(self.from(), lt);
        // An exclusive upper bound sitting exactly on a line start belongs to
        // the previous line, otherwise `x` on a full line would report two.
        let end_byte = if self.to() > self.from() {
            self.to() - 1
        } else {
            self.to()
        };
        let end = text.byte_to_line_idx(end_byte.min(text.len()), lt);
        (start, end)
    }

    /// Flip anchor and head, keeping the same span.
    pub fn flipped(self) -> Self {
        Range::new(self.head, self.anchor)
    }
}

/// MJB-LLR-009: exactly one range, with `primary_index` pinned at zero.
///
/// The vector and index exist so the multi-cursor shape is already in place;
/// the invariant is asserted, not assumed.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Selection {
    ranges: Vec<Range>,
    primary_index: usize,
}

impl Default for Selection {
    fn default() -> Self {
        Self::point(0)
    }
}

impl Selection {
    pub fn single(range: Range) -> Self {
        Self {
            ranges: vec![range],
            primary_index: 0,
        }
    }

    pub fn point(byte_idx: usize) -> Self {
        Self::single(Range::point(byte_idx))
    }

    /// MJB-LLR-010
    pub fn primary(&self) -> Range {
        self.ranges[self.primary_index]
    }

    pub fn set_primary(&mut self, range: Range) {
        self.ranges[self.primary_index] = range;
    }

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

    /// MJB-LLR-009: the single-range invariant, checked rather than assumed.
    pub fn invariant_holds(&self) -> bool {
        self.ranges.len() == 1 && self.primary_index == 0
    }

    /// Clamp every range into `text`.
    pub fn clamped(mut self, text: RopeSlice) -> Self {
        for r in &mut self.ranges {
            *r = r.clamped(text);
        }
        self
    }
}

#[cfg(test)]
mod tests {
    use ropey::Rope;

    use super::*;

    /// MJB-LLR-001: offsets are bytes, not characters. A range over a
    /// multi-byte character must report its byte extent.
    #[test]
    fn mjb_llr_001_offsets_are_byte_indices() {
        let r = Rope::from_str("文字");
        let s = r.slice(..);
        assert_eq!(s.len(), 6, "two 3-byte characters");

        let whole = Range::new(0, 6).clamped(s);
        assert_eq!(whole.len(), 6, "length is in bytes, not characters");

        // A single character spans three byte offsets.
        let first = Range::new(0, 3).clamped(s);
        assert_eq!(first.len(), 3);
    }

    #[test]
    fn mjb_llr_002_from_and_to_ignore_direction() {
        assert_eq!(Range::new(2, 5).from(), 2);
        assert_eq!(Range::new(2, 5).to(), 5);
        assert_eq!(Range::new(5, 2).from(), 2, "backward range still orders");
        assert_eq!(Range::new(5, 2).to(), 5);
    }

    #[test]
    fn mjb_llr_003_is_empty() {
        assert!(Range::point(3).is_empty());
        assert!(!Range::new(3, 4).is_empty());
    }

    #[test]
    fn mjb_llr_004_direction() {
        assert_eq!(Range::new(1, 5).direction(), Direction::Forward);
        assert_eq!(Range::new(5, 1).direction(), Direction::Backward);
        assert_eq!(
            Range::point(2).direction(),
            Direction::Forward,
            "an empty range is forward by convention"
        );
    }

    #[test]
    fn mjb_llr_005_cursor_steps_back_on_forward_range() {
        let r = Rope::from_str("abcdef");
        let s = r.slice(..);
        // Forward 0..1 selects byte 0, so the cursor is drawn on byte 0.
        assert_eq!(Range::new(0, 1).cursor(s), 0);
        assert_eq!(Range::new(0, 3).cursor(s), 2);
        // A backward range's head already sits on the cursor.
        assert_eq!(Range::new(3, 0).cursor(s), 0);
        assert_eq!(Range::point(4).cursor(s), 4);
    }

    #[test]
    fn mjb_llr_005_cursor_respects_grapheme_clusters() {
        let r = Rope::from_str("文字");
        let s = r.slice(..);
        // Head past the first wide char: cursor lands on its start, not mid-char.
        assert_eq!(Range::new(0, 3).cursor(s), 0);
    }

    #[test]
    fn mjb_llr_006_put_cursor_without_extend_collapses() {
        let r = Rope::from_str("abcdef");
        let s = r.slice(..);
        let got = Range::new(0, 4).put_cursor(s, 2, false);
        assert_eq!(got, Range::point(2));
    }

    #[test]
    fn mjb_llr_007_put_cursor_with_extend_keeps_anchor() {
        let r = Rope::from_str("abcdef");
        let s = r.slice(..);
        let got = Range::new(1, 2).put_cursor(s, 4, true);
        assert_eq!(got.anchor, 1, "anchor stays put when extending forward");
        assert_eq!(got.head, 5, "head lands one grapheme past the target");
    }

    #[test]
    fn mjb_llr_007_put_cursor_extend_flips_direction() {
        let r = Rope::from_str("abcdef");
        let s = r.slice(..);
        // Forward range extended to before its anchor must flip and nudge the
        // anchor forward one grapheme so the anchored byte stays selected.
        let got = Range::new(2, 4).put_cursor(s, 0, true);
        assert_eq!(got.direction(), Direction::Backward);
        assert_eq!(got.anchor, 3);
        assert_eq!(got.head, 0);
    }

    #[test]
    fn mjb_llr_011_clamped_snaps_into_bounds_and_onto_char_boundary() {
        let r = Rope::from_str("文");
        let s = r.slice(..);
        assert_eq!(Range::new(0, 99).clamped(s).head, 3, "clamped to length");
        assert_eq!(
            Range::new(0, 1).clamped(s).head,
            0,
            "an offset inside a multi-byte char snaps back to its start"
        );
    }

    #[test]
    fn mjb_llr_008_line_range() {
        let r = Rope::from_str("aa\nbb\ncc\n");
        let s = r.slice(..);
        assert_eq!(Range::point(0).line_range(s), (0, 0));
        // Exactly one full line, terminator included, is still one line.
        assert_eq!(Range::new(0, 3).line_range(s), (0, 0));
        assert_eq!(Range::new(0, 6).line_range(s), (0, 1));
    }

    #[test]
    fn mjb_llr_009_selection_invariant() {
        let sel = Selection::point(0);
        assert!(sel.invariant_holds());
        assert_eq!(sel.ranges().len(), 1);
    }

    #[test]
    fn mjb_llr_010_primary_round_trips() {
        let mut sel = Selection::point(0);
        sel.set_primary(Range::new(1, 4));
        assert_eq!(sel.primary(), Range::new(1, 4));
    }

    #[test]
    fn flipped_preserves_span() {
        let r = Range::new(2, 7).flipped();
        assert_eq!((r.anchor, r.head), (7, 2));
        assert_eq!(r.from(), 2);
        assert_eq!(r.to(), 7);
    }
}