mathtex-editor-core 0.1.0

//! Editor owns the `Tree`, cursor, selection, host callbacks, render output, and undo snapshots.

use crate::command::{Command, CommandResult, Dir, ExitDir};
use crate::doc::{Document, DocFragment};
use crate::host::{Host, RenderOutput};
use crate::menu::{Menu, RowEffect};
use crate::model::{Cursor, NodeId, Selection, Symbol};
use crate::{nav, selection};

/// An opaque undo entry containing the previous document snapshot.
#[derive(Debug, Clone)]
pub struct UndoEntry(pub(crate) Document);

/// The headless editor. Generic over the host's callback implementation.
pub struct Editor<H: Host> {
    tree: crate::model::Tree,
    cursor: Cursor,
    /// Selection anchor whose seq can differ from `cursor.seq` during structural drag detours.
    sel_anchor: Option<Cursor>,
    /// The Backspace swap or delete dropdown while it is open.
    pending_menu: Option<Menu>,
    host: H,
}

impl<H: Host> Editor<H> {
    /// Create an editor, optionally seeded with a document.
    pub fn new(host: H, initial: Option<Document>) -> Self {
        let tree = match initial {
            Some(d) => crate::model::Tree::from_doc(&d),
            None => crate::model::Tree::new(),
        };
        let cursor = Cursor {
            seq: tree.root(),
            index: 0,
        };
        Self {
            tree,
            cursor,
            sel_anchor: None,
            pending_menu: None,
            host,
        }
    }

    /// Return the current cursor.
    pub fn cursor(&self) -> Cursor {
        self.cursor
    }

    /// Return matrix shape at the cursor so the host can show row and column controls.
    pub fn matrix_shape(&self) -> Option<(usize, usize)> {
        self.tree.matrix_shape_at(self.cursor)
    }

    /// Return the active selection when anchor and cursor share a seq and distinct indexes.
    pub fn current_selection(&self) -> Option<Selection> {
        self.sel_anchor
            .filter(|a| a.seq == self.cursor.seq && a.index != self.cursor.index)
            .map(|a| Selection {
                seq: self.cursor.seq,
                anchor: a.index,
                focus: self.cursor.index,
            })
    }

    /// The single mutation command entry point. The host maps keys and pointer input to `Command`.
    pub fn exec(&mut self, cmd: Command) -> CommandResult {
        if self.pending_menu.is_some() {
            if let Some(result) = self.exec_menu(&cmd) {
                return result;
            }
            // Unrecognized menu commands close it and then execute normally.
            self.pending_menu = None;
        }
        match cmd {
            Command::Move(Dir::Left) => match nav::move_left(&self.tree, self.cursor) {
                Some(c) => self.moved(c),
                None => self.boundary(ExitDir::Left),
            },
            Command::Move(Dir::Right) => match nav::move_right(&self.tree, self.cursor) {
                Some(c) => self.moved(c),
                None => self.boundary(ExitDir::Right),
            },
            Command::Move(Dir::Up) => match nav::vertical(&self.tree, self.cursor, true) {
                Some(c) => self.moved(c),
                None => self.vertical_boundary(true),
            },
            Command::Move(Dir::Down) => match nav::vertical(&self.tree, self.cursor, false) {
                Some(c) => self.moved(c),
                None => self.vertical_boundary(false),
            },
            Command::MoveLineStart => self.moved(Cursor {
                seq: self.cursor.seq,
                index: 0,
            }),
            Command::MoveLineEnd => self.moved(Cursor {
                seq: self.cursor.seq,
                index: self.tree.len(self.cursor.seq),
            }),
            Command::Tab => match self.next_fill_target(true) {
                Some(c) => self.moved(c),
                None => self.exit_result(ExitDir::Right),
            },
            Command::ShiftTab => match self.next_fill_target(false) {
                Some(c) => self.moved(c),
                None => self.exit_result(ExitDir::Left),
            },
            Command::MoveTo(p) => {
                let (latex, spans) = self.tree.export_latex();
                let ir = self.host.typeset(&latex);
                let c = crate::matcher::hit_test(&ir, &spans, &self.tree, p);
                self.moved(crate::nav::normalize(&self.tree, c))
            }
            Command::ExtendTo(p) => {
                let (latex, spans) = self.tree.export_latex();
                let ir = self.host.typeset(&latex);
                let target = crate::nav::normalize(
                    &self.tree,
                    crate::matcher::hit_test(&ir, &spans, &self.tree, p),
                );
                self.extend_to(target)
            }

            Command::Extend(Dir::Left) => self.extend(false),
            Command::Extend(Dir::Right) => self.extend(true),
            Command::Extend(Dir::Up) | Command::Extend(Dir::Down) => self.no_op(),
            Command::SelectAll => {
                let s = selection::select_all(&self.tree);
                self.cursor = Cursor {
                    seq: s.seq,
                    index: s.focus,
                };
                self.sel_anchor = Some(Cursor { seq: s.seq, index: s.anchor });
                self.no_op()
            }
            Command::Collapse => {
                self.sel_anchor = None;
                self.no_op()
            }
            // With no pending menu, `Confirm` behaves like `Collapse`.
            Command::Confirm => {
                self.sel_anchor = None;
                self.no_op()
            }
            Command::MenuSelect(_) => self.no_op(),

            Command::InsertAtom(sym) => {
                let before = self.snapshot();
                self.replace_selection();
                let c = self.tree.insert_atom(self.cursor, sym);
                self.after_edit(before, c)
            }
            Command::InsertText(text) => {
                let before = self.snapshot();
                self.replace_selection();
                let mut c = self.cursor;
                for ch in text.chars() {
                    c = self.tree.insert_atom(c, Symbol::from_char(ch));
                }
                self.after_edit(before, c)
            }
            Command::InsertFraction(style) => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_fraction(self.cursor, style, sel);
                self.after_edit(before, c)
            }
            Command::InsertScript(slot) => {
                // `_` and `^` next to a big operator navigate into limits instead of editing.
                if let Some(c) = self.tree.bigop_limit_target(self.cursor, slot) {
                    self.moved(c)
                } else {
                    let before = self.snapshot();
                    let c = self.tree.attach_script(self.cursor, slot);
                    self.after_edit(before, c)
                }
            }
            Command::InsertBigOp(op) => {
                let before = self.snapshot();
                let c = self.tree.insert_big_op(self.cursor, op);
                self.after_edit(before, c)
            }
            Command::InsertSqrt => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_sqrt(self.cursor, sel);
                self.after_edit(before, c)
            }
            Command::InsertDelimiters { open, close } => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_delimiters(self.cursor, open, close, sel);
                self.after_edit(before, c)
            }
            Command::InsertAccent(mark) => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_accent(self.cursor, mark, sel);
                self.after_edit(before, c)
            }
            Command::InsertUnderOver(spec) => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_under_over(self.cursor, spec, sel);
                self.after_edit(before, c)
            }
            Command::InsertStyled(variant) | Command::ApplyStyle(variant) => {
                let before = self.snapshot();
                let sel = self.current_selection();
                let c = self.tree.insert_styled(self.cursor, variant, sel);
                self.after_edit(before, c)
            }
            Command::InsertMatrix { env, rows, cols } => {
                let before = self.snapshot();
                self.replace_selection();
                let c = self.tree.insert_matrix(self.cursor, env, rows, cols);
                self.after_edit(before, c)
            }
            Command::InsertFragment(frag) => {
                let before = self.snapshot();
                self.replace_selection();
                let c = self.tree.insert_fragment(self.cursor, &frag);
                self.after_edit(before, c)
            }

            Command::DeleteBackward => {
                if let Some(s) = self.current_selection() {
                    let before = self.snapshot();
                    let c = self.tree.delete_selection(s);
                    self.after_edit(before, c)
                } else if let Some(node) = nav::adjacent_structure_backward(&self.tree, self.cursor) {
                    // First Backspace opens a swap or delete menu, or selects for the next Backspace.
                    self.trigger_structure_backspace(node)
                } else if self.cursor.seq == self.tree.root() && self.tree.is_empty(self.cursor.seq)
                {
                    self.close_result()
                } else {
                    let before = self.snapshot();
                    let c = self.tree.delete_backward(self.cursor);
                    self.after_edit(before, c)
                }
            }
            Command::DeleteForward => {
                if let Some(s) = self.current_selection() {
                    let before = self.snapshot();
                    let c = self.tree.delete_selection(s);
                    self.after_edit(before, c)
                } else if let Some(node) = nav::adjacent_structure_forward(&self.tree, self.cursor) {
                    self.trigger_structure_backspace(node)
                } else if self.cursor.seq == self.tree.root() && self.tree.is_empty(self.cursor.seq)
                {
                    self.close_result()
                } else {
                    let before = self.snapshot();
                    let c = self.tree.delete_forward(self.cursor);
                    self.after_edit(before, c)
                }
            }

            Command::MatrixInsertRow(side) => {
                let before = self.snapshot();
                let c = self.tree.matrix_insert_row(self.cursor, side);
                self.after_edit(before, c)
            }
            Command::MatrixDeleteRow => {
                let before = self.snapshot();
                let c = self.tree.matrix_delete_row(self.cursor);
                self.after_edit(before, c)
            }
            Command::MatrixInsertCol(side) => {
                let before = self.snapshot();
                let c = self.tree.matrix_insert_col(self.cursor, side);
                self.after_edit(before, c)
            }
            Command::MatrixDeleteCol => {
                let before = self.snapshot();
                let c = self.tree.matrix_delete_col(self.cursor);
                self.after_edit(before, c)
            }
        }
    }

    /// Produce the paintable render output with IR plus caret and selection rects.
    pub fn render(&self) -> RenderOutput {
        let (latex, spans) = self.tree.export_latex();
        let ir = self.host.typeset(&latex);
        crate::matcher::render(
            &self.tree,
            self.cursor,
            self.current_selection(),
            &spans,
            ir,
            self.pending_menu.as_ref(),
        )
    }

    /// Serialize the current selection for the host's clipboard.
    pub fn selection_latex(&self) -> String {
        match self.current_selection() {
            Some(s) => self.tree.selection_latex(s),
            None => String::new(),
        }
    }
    /// Serialize the current selection as a document fragment.
    pub fn selection_fragment(&self) -> DocFragment {
        match self.current_selection() {
            Some(s) => {
                let lo = s.anchor.min(s.focus);
                let hi = s.anchor.max(s.focus);
                self.tree
                    .to_doc()
                    .root()
                    .get(lo..hi)
                    .map(<[_]>::to_vec)
                    .unwrap_or_default()
            }
            None => Vec::new(),
        }
    }

    /// Apply an undo entry from the host stack and return the inverse redo entry.
    pub fn apply_undo(&mut self, entry: UndoEntry) -> UndoEntry {
        let current = self.tree.to_doc();
        self.tree = crate::model::Tree::from_doc(&entry.0);
        self.cursor = Cursor {
            seq: self.tree.root(),
            index: self.tree.len(self.tree.root()),
        };
        self.sel_anchor = None;
        self.host.on_render();
        UndoEntry(current)
    }

    /// Place the caret at the root edge nearest `side` and clear selection.
    pub fn enter_from(&mut self, side: ExitDir) {
        let root = self.tree.root();
        let index = match side {
            ExitDir::Left | ExitDir::Up => 0,
            ExitDir::Right | ExitDir::Down => self.tree.len(root),
        };
        self.cursor = Cursor { seq: root, index };
        self.sel_anchor = None;
        self.pending_menu = None;
        self.host.on_render();
    }

    /// Return the whole document for host persistence.
    pub fn document(&self) -> Document {
        self.tree.to_doc()
    }

    /// Return clean LaTeX for display and external copy, dropping empty placeholders.
    pub fn latex(&self) -> String {
        self.tree.export_display()
    }

    fn snapshot(&self) -> Document {
        self.tree.to_doc()
    }

    /// Delete the active selection if any and collapse the cursor to its start.
    fn replace_selection(&mut self) {
        if let Some(s) = self.current_selection() {
            self.cursor = self.tree.delete_selection(s);
            self.sel_anchor = None;
        }
    }

    /// Select a whole structure before Backspace or Delete removes or replaces it.
    fn select_node(&mut self, node: NodeId) -> CommandResult {
        let Some((seq, idx)) = self.tree.index_in_parent(node) else {
            return self.no_op();
        };
        self.cursor = Cursor { seq, index: idx + 1 };
        self.sel_anchor = Some(Cursor { seq, index: idx });
        self.no_op()
    }

    /// Open the swap or delete menu for swappable structures, else select the node.
    fn trigger_structure_backspace(&mut self, node: NodeId) -> CommandResult {
        match self.tree.swap_variants(node) {
            Some(variants) => {
                self.pending_menu = Some(Menu::for_node(node, variants));
                self.no_op()
            }
            None => self.select_node(node),
        }
    }

    /// Consume recognized menu commands and let other commands fall through.
    fn exec_menu(&mut self, cmd: &Command) -> Option<CommandResult> {
        match cmd {
            Command::InsertAtom(sym) if sym.latex.chars().count() == 1 => {
                let menu = self.pending_menu.as_mut()?;
                menu.query.push_str(&sym.latex);
                menu.selected = 0;
                Some(self.no_op())
            }
            Command::InsertText(text) => {
                let menu = self.pending_menu.as_mut()?;
                menu.query.push_str(text);
                menu.selected = 0;
                Some(self.no_op())
            }
            Command::DeleteBackward => {
                let menu = self.pending_menu.as_mut()?;
                if menu.query.is_empty() {
                    let selected = menu.selected;
                    Some(self.commit_menu_row(selected))
                } else {
                    menu.query.pop();
                    menu.selected = 0;
                    Some(self.no_op())
                }
            }
            Command::Move(Dir::Up) => {
                let menu = self.pending_menu.as_mut()?;
                menu.selected = menu.selected.saturating_sub(1);
                Some(self.no_op())
            }
            Command::Move(Dir::Down) => {
                let menu = self.pending_menu.as_mut()?;
                let max = menu.visible().len().saturating_sub(1);
                menu.selected = (menu.selected + 1).min(max);
                Some(self.no_op())
            }
            Command::Confirm => {
                let selected = self.pending_menu.as_ref()?.selected;
                Some(self.commit_menu_row(selected))
            }
            Command::MenuSelect(i) => Some(self.commit_menu_row(*i)),
            Command::Collapse => {
                self.pending_menu = None;
                Some(self.no_op())
            }
            _ => None,
        }
    }

    /// Commit the visible menu row by deleting or swapping the anchored structure.
    fn commit_menu_row(&mut self, visible_idx: usize) -> CommandResult {
        let Some(menu) = self.pending_menu.take() else {
            return self.no_op();
        };
        let visible = menu.visible();
        let Some(row) = visible.get(visible_idx) else {
            return self.no_op();
        };
        match row {
            RowEffect::Delete => {
                let before = self.snapshot();
                let Some((seq, idx)) = self.tree.index_in_parent(menu.anchor) else {
                    return self.no_op();
                };
                let c = self.tree.delete_selection(Selection { seq, anchor: idx, focus: idx + 1 });
                self.after_edit(before, c)
            }
            RowEffect::Swap(variant) => {
                let before = self.snapshot();
                self.tree.apply_swap(menu.anchor, variant);
                self.after_edit(before, self.cursor)
            }
        }
    }

    fn extend(&mut self, right: bool) -> CommandResult {
        let sel = self.current_selection().unwrap_or(Selection {
            seq: self.cursor.seq,
            anchor: self.cursor.index,
            focus: self.cursor.index,
        });
        let ns = selection::extend(&self.tree, sel, right);
        self.cursor = Cursor {
            seq: ns.seq,
            index: ns.focus,
        };
        self.sel_anchor = Some(Cursor { seq: ns.seq, index: ns.anchor });
        self.no_op()
    }

    fn next_fill_target(&self, forward: bool) -> Option<Cursor> {
        // Tab fills big operator limits in reading order from lower to upper.
        if forward {
            // From the lower limit, go to the empty upper limit next.
            if let Some(parent) = self.tree.seq_parent(self.cursor.seq) {
                if let Some(crate::model::Kind::BigOp { lower, upper, .. }) =
                    self.tree.kind(parent)
                {
                    if self.cursor.seq == *lower && self.tree.is_empty(*upper) {
                        return Some(Cursor {
                            seq: *upper,
                            index: 0,
                        });
                    }
                }
            }
            // Just after a big operator, go to the first empty limit.
            if self.cursor.index > 0 {
                let prev = self.tree.items(self.cursor.seq)[self.cursor.index - 1];
                if let Some(crate::model::Kind::BigOp { lower, upper, .. }) =
                    self.tree.kind(prev)
                {
                    for s in [*lower, *upper] {
                        if self.tree.is_empty(s) {
                            return Some(Cursor { seq: s, index: 0 });
                        }
                    }
                }
            }
            // From a radical radicand, visit the empty degree before continuing.
            if let Some(parent) = self.tree.seq_parent(self.cursor.seq) {
                if let Some(crate::model::Kind::Sqrt { index, radicand }) = self.tree.kind(parent) {
                    if self.cursor.seq == *radicand && self.tree.is_empty(*index) {
                        return Some(Cursor {
                            seq: *index,
                            index: 0,
                        });
                    }
                }
            }
        }
        let mut c = self.cursor;
        for _ in 0..100_000 {
            c = if forward {
                nav::move_right(&self.tree, c)?
            } else {
                nav::move_left(&self.tree, c)?
            };
            if self.tree.is_empty(c.seq) {
                return Some(Cursor {
                    seq: c.seq,
                    index: 0,
                });
            }
        }
        None
    }

    /// Finish a mutating command by setting the cursor and notifying the host.
    fn after_edit(&mut self, before: Document, cursor: Cursor) -> CommandResult {
        self.cursor = cursor;
        self.sel_anchor = None;
        let doc = self.tree.to_doc();
        self.host.on_change(&doc);
        self.host.on_render();
        CommandResult {
            cursor,
            undo: Some(UndoEntry(before)),
            exit: None,
        }
    }

    /// Extend selection toward `target`, preserving the original drag anchor across seq changes.
    fn extend_to(&mut self, target: Cursor) -> CommandResult {
        let anchor = self.sel_anchor.unwrap_or(self.cursor);
        let sel = selection::extend_to(&self.tree, anchor, target);
        self.sel_anchor = Some(Cursor { seq: sel.seq, index: sel.anchor });
        self.cursor = Cursor { seq: sel.seq, index: sel.focus };
        self.no_op()
    }

    /// Return a motion result and clear selection.
    fn moved(&mut self, cursor: Cursor) -> CommandResult {
        self.cursor = cursor;
        self.sel_anchor = None;
        CommandResult {
            cursor,
            undo: None,
            exit: None,
        }
    }

    /// Return a result that keeps current cursor and selection state.
    fn no_op(&self) -> CommandResult {
        CommandResult {
            cursor: self.cursor,
            undo: None,
            exit: None,
        }
    }

    /// Collapse active selection at a boundary, or exit toward `dir` for a plain caret.
    fn boundary(&mut self, dir: ExitDir) -> CommandResult {
        if self.current_selection().is_some() {
            return self.moved(self.cursor);
        }
        self.exit_result(dir)
    }

    /// Exit vertical motion only when no ancestor slot can keep the caret inside math.
    fn vertical_boundary(&mut self, up: bool) -> CommandResult {
        if self.current_selection().is_some() {
            return self.moved(self.cursor);
        }
        let dir = if up { ExitDir::Up } else { ExitDir::Down };
        let mut seq = self.cursor.seq;
        loop {
            let Some(parent) = self.tree.seq_parent(seq) else {
                return self.exit_result(dir);
            };
            let Some((pseq, _)) = self.tree.index_in_parent(parent) else {
                return self.exit_result(dir);
            };
            if nav::vertical(&self.tree, Cursor { seq: pseq, index: 0 }, up).is_some() {
                return self.moved(self.cursor);
            }
            seq = pseq;
        }
    }

    fn exit_result(&self, dir: ExitDir) -> CommandResult {
        self.host.on_exit(dir);
        CommandResult {
            cursor: self.cursor,
            undo: None,
            exit: Some(dir),
        }
    }

    /// Return a result that asks the host to end math mode without mutation.
    fn close_result(&self) -> CommandResult {
        self.host.request_close();
        CommandResult {
            cursor: self.cursor,
            undo: None,
            exit: None,
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::command::Command;
    use crate::host::Host;
    use crate::model::{FracStyle, Kind, MathClass, ScriptSlot, Symbol, Variant};

    struct NullHost;
    impl Host for NullHost {
        fn typeset(&self, _latex: &str) -> mathtex_ir::Fragment {
            mathtex_ir::Fragment::default()
        }
        fn on_change(&self, _doc: &Document) {}
        fn on_exit(&self, _dir: ExitDir) {}
        fn request_close(&self) {}
        fn on_render(&self) {}
    }

    #[derive(Default)]
    struct HostLog {
        exits: std::cell::RefCell<Vec<ExitDir>>,
        closes: std::cell::Cell<usize>,
        renders: std::cell::Cell<usize>,
    }

    struct RecordingHost(std::rc::Rc<HostLog>);
    impl Host for RecordingHost {
        fn typeset(&self, _latex: &str) -> mathtex_ir::Fragment {
            mathtex_ir::Fragment::default()
        }
        fn on_change(&self, _doc: &Document) {}
        fn on_exit(&self, dir: ExitDir) {
            self.0.exits.borrow_mut().push(dir);
        }
        fn request_close(&self) {
            self.0.closes.set(self.0.closes.get() + 1);
        }
        fn on_render(&self) {
            self.0.renders.set(self.0.renders.get() + 1);
        }
    }

    fn recording() -> (std::rc::Rc<HostLog>, Editor<RecordingHost>) {
        let log = std::rc::Rc::new(HostLog::default());
        let ed = Editor::new(RecordingHost(log.clone()), None);
        (log, ed)
    }

    fn sym(c: &str) -> Symbol {
        Symbol {
            latex: c.into(),
            class: MathClass::Ord,
        }
    }

    #[test]
    fn insert_fraction_and_navigate() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        // Cursor is in the empty numerator.
        assert!(ed.tree.is_empty(ed.cursor.seq));
        ed.exec(Command::InsertAtom(sym("x")));
        // Numerator now contains "x".
        assert_eq!(ed.tree.len(ed.cursor.seq), 1);
    }

    #[test]
    fn backspace_next_to_structure_selects_then_deletes() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        ed.exec(Command::InsertAtom(sym("a")));
        let root = ed.tree.root();
        let frac = ed.tree.items(root)[0];
        let den = ed.tree.child_seqs(frac)[1];
        ed.cursor = Cursor { seq: den, index: 0 };
        ed.exec(Command::InsertAtom(sym("b"))); // \frac{a}{b}
        // Move out to the root, right after the whole fraction.
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;

        // First Backspace selects the whole fraction without deleting it.
        ed.exec(Command::DeleteBackward);
        assert_eq!(ed.tree.len(root), 1);
        let sel = ed.current_selection().expect("first backspace should select the structure");
        assert_eq!((sel.seq, sel.anchor, sel.focus), (root, 0, 1));

        // Second Backspace deletes the selected fraction.
        ed.exec(Command::DeleteBackward);
        assert_eq!(ed.tree.len(root), 0);
    }

    #[test]
    fn backspace_next_to_delim_opens_swap_menu() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertDelimiters { open: '(', close: ')' });
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;

        ed.exec(Command::DeleteBackward);
        // Swappable structures open a menu instead of plain selection.
        assert!(ed.current_selection().is_none());
        let out = ed.render();
        let menu = out.menu.expect("swap menu should be open");
        assert_eq!(menu.items[0].label, "Delete"); // pinned first row
        assert!(menu.items.len() > 1); // plus bracket alternatives
        assert_eq!(ed.tree.len(root), 1); // nothing deleted yet
    }

    #[test]
    fn menu_filters_by_typed_query_but_keeps_delete_pinned() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertDelimiters { open: '(', close: ')' });
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        ed.exec(Command::DeleteBackward); // opens the menu

        ed.exec(Command::InsertText("brace".into()));
        let out = ed.render();
        let menu = out.menu.expect("menu should still be open while typing");
        assert!(menu.items.iter().any(|i| i.label == "Delete")); // still pinned
        assert!(menu.items.iter().any(|i| i.label.to_lowercase().contains("brace")));
        assert!(!menu.items.iter().any(|i| i.label.contains('[')));
    }

    #[test]
    fn menu_backspace_with_empty_query_confirms_delete() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertDelimiters { open: '(', close: ')' });
        ed.exec(Command::InsertAtom(sym("x")));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        ed.exec(Command::DeleteBackward); // opens the menu, query empty
        ed.exec(Command::DeleteBackward); // empty query confirms delete
        assert_eq!(ed.tree.len(root), 0);
        assert!(ed.render().menu.is_none());
    }

    #[test]
    fn menu_select_swaps_the_bracket_in_place() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertDelimiters { open: '(', close: ')' });
        ed.exec(Command::InsertAtom(sym("x")));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        ed.exec(Command::DeleteBackward); // opens the menu

        let out = ed.render();
        let menu = out.menu.unwrap();
        let idx = menu
            .items
            .iter()
            .position(|i| i.label.starts_with("[ ]"))
            .expect("bracket alternative present");
        ed.exec(Command::MenuSelect(idx));

        assert!(ed.render().menu.is_none()); // menu closed after commit
        let delim = ed.tree.items(root)[0];
        match ed.tree.kind(delim) {
            Some(Kind::Delim { open, close, body }) => {
                assert_eq!((*open, *close), ('[', ']'));
                assert_eq!(ed.tree.len(*body), 1); // "x" preserved
            }
            _ => panic!("expected Delim"),
        }
    }

    #[test]
    fn menu_escape_cancels_without_changes() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertDelimiters { open: '(', close: ')' });
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        ed.exec(Command::DeleteBackward); // opens the menu
        ed.exec(Command::Collapse); // Escape
        assert!(ed.render().menu.is_none());
        assert_eq!(ed.tree.len(root), 1); // untouched
        match ed.tree.kind(ed.tree.items(root)[0]) {
            Some(Kind::Delim { open, close, .. }) => assert_eq!((*open, *close), ('(', ')')),
            _ => panic!("expected Delim"),
        }
    }

    #[test]
    fn backspace_next_to_text_block_enters_instead_of_selecting() {
        let mut ed = Editor::new(NullHost, None);
        let c = ed.exec(Command::InsertStyled(Variant::Text));
        let content = c.cursor.seq;
        ed.cursor = Cursor { seq: content, index: 0 };
        ed.exec(Command::InsertAtom(sym("x")));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;

        ed.exec(Command::DeleteBackward);
        // The \text{} carve out enters the text node without selecting or deleting it.
        assert!(ed.current_selection().is_none());
        assert_eq!(ed.cursor.seq, content);
        assert_eq!(ed.tree.len(root), 1);
        assert_eq!(ed.tree.len(content), 1);
    }

    #[test]
    fn typing_while_structure_selected_replaces_it() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        ed.exec(Command::DeleteBackward); // selects the empty fraction
        assert!(ed.current_selection().is_some());

        ed.exec(Command::InsertAtom(sym("z")));
        assert_eq!(ed.tree.len(root), 1);
        assert!(matches!(ed.tree.kind(ed.tree.items(root)[0]), Some(Kind::Atom(_))));
    }

    #[test]
    fn delete_forward_next_to_structure_selects_then_deletes() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 0 };
        ed.sel_anchor = None;

        ed.exec(Command::DeleteForward);
        assert_eq!(ed.tree.len(root), 1);
        let sel = ed.current_selection().expect("first delete-forward should select the structure");
        assert_eq!(
            (sel.seq, sel.anchor.min(sel.focus), sel.anchor.max(sel.focus)),
            (root, 0, 1)
        );

        ed.exec(Command::DeleteForward);
        assert_eq!(ed.tree.len(root), 0);
    }

    #[test]
    fn select_all_then_delete_clears() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertText("abc".into()));
        ed.exec(Command::SelectAll);
        assert!(ed.current_selection().is_some());
        ed.exec(Command::DeleteBackward);
        assert_eq!(ed.document().len(), 0);
    }

    #[test]
    fn pointer_drag_anchors_at_cursor_not_first_move_target() {
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertText("abcd".into()));
        let root = ed.tree.root();
        // Simulate `MoveTo` placing the cursor at drag start index 1.
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        // First drag move reaches index 3 while the anchor stays at drag start index 1.
        ed.extend_to(Cursor { seq: root, index: 3 });
        assert_eq!(ed.sel_anchor, Some(Cursor { seq: root, index: 1 }));
        let sel = ed.current_selection().unwrap();
        assert_eq!((sel.anchor, sel.focus), (1, 3));
        // Further drag moves keep the anchor fixed while focus follows.
        ed.extend_to(Cursor { seq: root, index: 2 });
        let sel = ed.current_selection().unwrap();
        assert_eq!((sel.anchor, sel.focus), (1, 2));
    }

    #[test]
    fn pointer_extend_with_existing_selection_keeps_its_anchor() {
        // Shift click extending an active selection must keep the original anchor.
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertText("abcd".into()));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = Some(Cursor { seq: root, index: 0 });
        ed.extend_to(Cursor { seq: root, index: 4 });
        assert_eq!(ed.sel_anchor, Some(Cursor { seq: root, index: 0 }));
        let sel = ed.current_selection().unwrap();
        assert_eq!((sel.anchor, sel.focus), (0, 4));
    }

    #[test]
    fn pointer_drag_through_a_child_seq_promotes_to_common_ancestor() {
        // Dragging through a child seq must promote selection to bracket the whole structure.
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        let root = ed.tree.root();
        let frac = ed.tree.items(root)[0];
        let num = ed.tree.child_seqs(frac)[0];
        // Mousedown before the fraction.
        ed.cursor = Cursor { seq: root, index: 0 };
        ed.sel_anchor = None;
        // Dragging into the numerator promotes selection to bracket the fraction.
        ed.extend_to(Cursor { seq: num, index: 0 });
        assert_eq!(ed.sel_anchor, Some(Cursor { seq: root, index: 0 }));
        let sel = ed.current_selection().unwrap();
        assert_eq!((sel.seq, sel.anchor, sel.focus), (root, 0, 1));
        // Dragging past the fraction keeps the exact bracketed selection.
        ed.extend_to(Cursor { seq: root, index: 1 });
        let sel = ed.current_selection().unwrap();
        assert_eq!((sel.seq, sel.anchor, sel.focus), (root, 0, 1));
    }

    #[test]
    fn pointer_drag_from_inside_a_script_base_past_the_end_selects_everything() {
        // Dragging from after "x" in the Script base to after final "2" selects the root sequence.
        let mut ed = Editor::new(NullHost, None);
        ed.exec(Command::InsertAtom(sym("x")));
        ed.exec(Command::InsertScript(ScriptSlot::Sup));
        ed.exec(Command::InsertAtom(sym("2")));
        let root = ed.tree.root();
        // Exit the superscript back to the root, then type "+2".
        ed.cursor = Cursor { seq: root, index: ed.tree.len(root) };
        ed.sel_anchor = None;
        ed.exec(Command::InsertAtom(sym("+")));
        ed.exec(Command::InsertAtom(sym("2")));
        assert_eq!(ed.latex(), "x^2+2");

        let script = ed.tree.items(root)[0];
        let base = match ed.tree.kind(script) {
            Some(Kind::Script { base, .. }) => *base,
            _ => panic!("expected a Script"),
        };
        // Mousedown right after "x", inside the base.
        ed.cursor = Cursor { seq: base, index: 1 };
        ed.sel_anchor = None;
        // Drag to the very end of the document.
        let end = ed.tree.len(root);
        ed.extend_to(Cursor { seq: root, index: end });
        let sel = ed.current_selection().unwrap();
        assert_eq!(sel.seq, root);
        assert_eq!((sel.anchor.min(sel.focus), sel.anchor.max(sel.focus)), (0, end));
    }

    #[test]
    fn arrow_left_at_root_start_exits_left() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertText("ab".into()));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 0 };
        ed.sel_anchor = None;
        let r = ed.exec(Command::Move(Dir::Left));
        assert_eq!(r.exit, Some(ExitDir::Left));
        assert_eq!(&*log.exits.borrow(), &[ExitDir::Left]);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 0 });
    }

    #[test]
    fn arrow_right_at_root_end_exits_right() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertText("ab".into()));
        let root = ed.tree.root();
        let r = ed.exec(Command::Move(Dir::Right));
        assert_eq!(r.exit, Some(ExitDir::Right));
        assert_eq!(&*log.exits.borrow(), &[ExitDir::Right]);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 2 });
    }

    #[test]
    fn arrows_in_flat_sequence_exit_up_and_down() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertText("ab".into()));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 1 };
        ed.sel_anchor = None;
        let r = ed.exec(Command::Move(Dir::Up));
        assert_eq!(r.exit, Some(ExitDir::Up));
        let r = ed.exec(Command::Move(Dir::Down));
        assert_eq!(r.exit, Some(ExitDir::Down));
        assert_eq!(&*log.exits.borrow(), &[ExitDir::Up, ExitDir::Down]);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 1 });
    }

    #[test]
    fn arrow_down_in_numerator_enters_denominator_not_exit() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertFraction(FracStyle::Bar));
        let frac = ed.tree.items(ed.tree.root())[0];
        let den = ed.tree.child_seqs(frac)[1];
        let r = ed.exec(Command::Move(Dir::Down));
        assert_eq!(r.exit, None);
        assert_eq!(ed.cursor.seq, den);
        assert!(log.exits.borrow().is_empty());
    }

    #[test]
    fn arrow_at_boundary_with_selection_collapses_without_exit() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertText("ab".into()));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 2 };
        ed.sel_anchor = Some(Cursor { seq: root, index: 0 });
        let r = ed.exec(Command::Move(Dir::Right));
        assert_eq!(r.exit, None);
        assert!(log.exits.borrow().is_empty());
        assert!(ed.current_selection().is_none());
        assert_eq!(ed.cursor, Cursor { seq: root, index: 2 });
    }

    #[test]
    fn delete_on_empty_root_requests_close_without_mutation() {
        let (log, mut ed) = recording();
        let r = ed.exec(Command::DeleteBackward);
        assert_eq!(log.closes.get(), 1);
        assert!(r.undo.is_none());
        assert!(r.exit.is_none());
        assert_eq!(ed.document().len(), 0);
        assert_eq!(ed.cursor, Cursor { seq: ed.tree.root(), index: 0 });
        let r = ed.exec(Command::DeleteForward);
        assert_eq!(log.closes.get(), 2);
        assert!(r.undo.is_none());
        assert_eq!(ed.document().len(), 0);
    }

    #[test]
    fn enter_from_places_caret_at_matching_edge() {
        let (log, mut ed) = recording();
        ed.exec(Command::InsertText("ab".into()));
        let root = ed.tree.root();
        ed.cursor = Cursor { seq: root, index: 2 };
        ed.sel_anchor = Some(Cursor { seq: root, index: 0 });
        let renders = log.renders.get();
        ed.enter_from(ExitDir::Left);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 0 });
        assert!(ed.sel_anchor.is_none());
        assert_eq!(log.renders.get(), renders + 1);
        ed.enter_from(ExitDir::Right);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 2 });
        ed.enter_from(ExitDir::Up);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 0 });
        ed.enter_from(ExitDir::Down);
        assert_eq!(ed.cursor, Cursor { seq: root, index: 2 });
    }

    #[test]
    fn undo_restores_previous_document() {
        let mut ed = Editor::new(NullHost, None);
        let r1 = ed.exec(Command::InsertAtom(sym("a")));
        let r2 = ed.exec(Command::InsertAtom(sym("b")));
        assert_eq!(ed.document().len(), 2);
        // host would stack r2.undo, apply it to undo "b"
        ed.apply_undo(r2.undo.unwrap());
        assert_eq!(ed.document().len(), 1);
        ed.apply_undo(r1.undo.unwrap());
        assert_eq!(ed.document().len(), 0);
    }
}