hjkl 0.14.6

//! Window-tree data model for vim-style splits (Phase 1 + Phase 2).
//!
//! A [`LayoutTree`] holds either a single [`Leaf`] (one window) or a
//! [`Split`] that recursively divides space between two sub-trees.  The
//! [`Window`] struct carries per-window scroll state so that two windows
//! showing the same buffer slot can scroll independently.

/// Stable id into `App::windows`. Never reused — new windows get the next
/// value from `App::next_window_id`.
pub type WindowId = usize;

/// Per-tab layout + focus state.
///
/// Each tab owns one [`LayoutTree`] (the window arrangement within that tab)
/// and records which window in that tree currently has focus.  Windows and
/// slots are shared across tabs — a `WindowId` refers into `App::windows`
/// regardless of which tab it lives in.
#[derive(Debug, Clone)]
pub struct Tab {
    /// Spatial layout tree for this tab. Leaves reference [`WindowId`]s.
    pub layout: LayoutTree,
    /// The window that has focus within this tab.
    pub focused_window: WindowId,
}

/// Per-window scroll + geometry state.
#[derive(Debug, Clone)]
pub struct Window {
    /// Index into `App::slots` for the buffer this window displays.
    pub slot: usize,
    /// Per-window top scroll row. Synced into the editor host viewport
    /// before dispatch, and back out after, for the focused window only.
    pub top_row: usize,
    /// Per-window top scroll column (char index).
    pub top_col: usize,
    /// Per-window cursor row (0-based). Synced alongside scroll so two
    /// windows on the same slot keep independent cursors.
    pub cursor_row: usize,
    /// Per-window cursor column (0-based).
    pub cursor_col: usize,
    /// The rect this window occupied in the last rendered frame.  Written
    /// by the renderer every frame; used by direction-navigation in later
    /// phases.  `None` until the first render.
    pub last_rect: Option<ratatui::layout::Rect>,
}

/// Direction of a split.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum SplitDir {
    Horizontal,
    Vertical,
}

/// A binary spatial tree that partitions the editor area into windows.
#[derive(Debug, Clone)]
pub enum LayoutTree {
    Leaf(WindowId),
    Split {
        dir: SplitDir,
        /// Fraction of the available space allocated to `a`. `0.0 < ratio < 1.0`.
        ratio: f32,
        a: Box<LayoutTree>,
        b: Box<LayoutTree>,
        /// Rect this split last occupied. Filled by render_layout each frame;
        /// read by resize commands to convert line/col deltas to ratio updates.
        /// None before the first render.
        last_rect: Option<ratatui::layout::Rect>,
    },
}

/// Internal direction enum used by `neighbor_direction`.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum NavDir {
    Below,
    Above,
    Left,
    Right,
}

impl LayoutTree {
    /// Pre-order traversal — returns all leaf ids in the order they appear
    /// top-to-bottom / left-to-right in the layout.
    pub fn leaves(&self) -> Vec<WindowId> {
        let mut out = Vec::new();
        self.collect_leaves(&mut out);
        out
    }

    fn collect_leaves(&self, out: &mut Vec<WindowId>) {
        match self {
            LayoutTree::Leaf(id) => out.push(*id),
            LayoutTree::Split { a, b, .. } => {
                a.collect_leaves(out);
                b.collect_leaves(out);
            }
        }
    }

    /// Return the next leaf in pre-order traversal, wrapping around.
    ///
    /// Returns `None` only if `id` is not in the tree (shouldn't happen in
    /// practice).
    pub fn next_leaf(&self, id: WindowId) -> Option<WindowId> {
        let leaves = self.leaves();
        let pos = leaves.iter().position(|&l| l == id)?;
        Some(leaves[(pos + 1) % leaves.len()])
    }

    /// Return the previous leaf in pre-order traversal, wrapping around.
    ///
    /// Returns `None` only if `id` is not in the tree (shouldn't happen in
    /// practice).
    pub fn prev_leaf(&self, id: WindowId) -> Option<WindowId> {
        let leaves = self.leaves();
        let pos = leaves.iter().position(|&l| l == id)?;
        let len = leaves.len();
        Some(leaves[(pos + len - 1) % len])
    }

    /// Return `true` if `id` appears anywhere in the tree.
    pub fn contains(&self, id: WindowId) -> bool {
        match self {
            LayoutTree::Leaf(leaf_id) => *leaf_id == id,
            LayoutTree::Split { a, b, .. } => a.contains(id) || b.contains(id),
        }
    }

    /// Find the leaf for `id` and replace it in-place with `f(id)`.
    /// Returns `true` if the leaf was found and replaced.
    pub fn replace_leaf<F: FnOnce(WindowId) -> LayoutTree + 'static>(
        &mut self,
        id: WindowId,
        f: F,
    ) -> bool {
        self.replace_leaf_boxed(id, Box::new(f))
    }

    fn replace_leaf_boxed(
        &mut self,
        id: WindowId,
        f: Box<dyn FnOnce(WindowId) -> LayoutTree>,
    ) -> bool {
        match self {
            LayoutTree::Leaf(leaf_id) if *leaf_id == id => {
                *self = f(id);
                true
            }
            LayoutTree::Leaf(_) => false,
            LayoutTree::Split { a, b, .. } => {
                // We need to check `a` first; if not found, check `b`.
                // Because `Box<dyn FnOnce>` is not `Copy`, we do this by
                // checking containment first, then calling.
                if a.contains(id) {
                    a.replace_leaf_boxed(id, f)
                } else {
                    b.replace_leaf_boxed(id, f)
                }
            }
        }
    }

    /// Return the id of the next leaf below `id` in a `Horizontal` split,
    /// using pre-order traversal semantics.
    ///
    /// "Below" means: walking up from `id`, find the innermost enclosing
    /// `Horizontal` split where `id` lives in `a`; the answer is then the
    /// first (leftmost) leaf of `b`.  If `id` is the bottom-most window
    /// (or there are no horizontal splits above it), returns `None`.
    pub fn neighbor_below(&self, id: WindowId) -> Option<WindowId> {
        self.neighbor_direction(id, NavDir::Below)
    }

    /// Return the id of the next leaf above `id` in a `Horizontal` split.
    pub fn neighbor_above(&self, id: WindowId) -> Option<WindowId> {
        self.neighbor_direction(id, NavDir::Above)
    }

    /// Return the id of the next leaf to the left of `id` in a `Vertical`
    /// split.  Horizontal splits are passed through.
    pub fn neighbor_left(&self, id: WindowId) -> Option<WindowId> {
        self.neighbor_direction(id, NavDir::Left)
    }

    /// Return the id of the next leaf to the right of `id` in a `Vertical`
    /// split.  Horizontal splits are passed through.
    pub fn neighbor_right(&self, id: WindowId) -> Option<WindowId> {
        self.neighbor_direction(id, NavDir::Right)
    }

    /// Internal unified helper for directional navigation.
    ///
    /// - `Below` / `Above` act on `Horizontal` splits; `Vertical` is a pass-through.
    /// - `Left` / `Right` act on `Vertical` splits; `Horizontal` is a pass-through.
    ///
    /// In each "active" split direction:
    /// - For the "forward" direction (Below / Right), when `id` is in `a`:
    ///   try to find a deeper neighbour inside `a` first; failing that, cross to `b`.
    ///   When `id` is in `b`: recurse into `b` only (no cross available).
    /// - For the "backward" direction (Above / Left), symmetric.
    fn neighbor_direction(&self, id: WindowId, dir: NavDir) -> Option<WindowId> {
        match self {
            LayoutTree::Leaf(_) => None,
            LayoutTree::Split {
                dir: split_dir,
                a,
                b,
                ..
            } => {
                // Which split direction is "active" for this nav direction?
                let active_split = match dir {
                    NavDir::Below | NavDir::Above => SplitDir::Horizontal,
                    NavDir::Left | NavDir::Right => SplitDir::Vertical,
                };
                // Is this a "forward" traversal (a→b) or "backward" (b→a)?
                let forward = matches!(dir, NavDir::Below | NavDir::Right);

                if *split_dir == active_split {
                    if a.contains(id) {
                        if forward {
                            // Try deeper forward-neighbour inside `a`.
                            let inner = a.neighbor_direction(id, dir);
                            if inner.is_some() {
                                return inner;
                            }
                            // Cross to `b`.
                            Some(first_leaf(b))
                        } else {
                            // Backward, `id` in `a` (the "first" half) — recurse only.
                            a.neighbor_direction(id, dir)
                        }
                    } else if b.contains(id) {
                        if forward {
                            // Forward, `id` in `b` (the "second" half) — recurse only.
                            b.neighbor_direction(id, dir)
                        } else {
                            // Try deeper backward-neighbour inside `b`.
                            let inner = b.neighbor_direction(id, dir);
                            if inner.is_some() {
                                return inner;
                            }
                            // Cross to `a`.
                            Some(last_leaf(a))
                        }
                    } else {
                        None
                    }
                } else {
                    // Pass-through: this split axis is orthogonal — recurse without offering a sibling.
                    if a.contains(id) {
                        a.neighbor_direction(id, dir)
                    } else if b.contains(id) {
                        b.neighbor_direction(id, dir)
                    } else {
                        None
                    }
                }
            }
        }
    }

    /// Walk the tree looking for the innermost enclosing Split with matching
    /// `dir` that contains `id`. Returns a mutable reference to the ratio,
    /// a copy of the last_rect, and whether the focused leaf is in `a`.
    /// Returns None if no such enclosing Split exists.
    pub fn enclosing_split_mut(
        &mut self,
        id: WindowId,
        dir: SplitDir,
    ) -> Option<(&mut f32, Option<ratatui::layout::Rect>, bool)> {
        match self {
            LayoutTree::Leaf(_) => None,
            LayoutTree::Split {
                dir: my_dir,
                ratio,
                a,
                b,
                last_rect,
            } => {
                let in_a = a.contains(id);
                let in_b = b.contains(id);
                if !in_a && !in_b {
                    return None;
                }

                let my_dir = *my_dir;
                let saved_rect = *last_rect;

                // Try deeper first (innermost wins).
                let inner = if in_a {
                    a.enclosing_split_mut(id, dir)
                } else {
                    b.enclosing_split_mut(id, dir)
                };
                if inner.is_some() {
                    return inner;
                }

                // No deeper match — am I a candidate?
                if my_dir == dir {
                    Some((ratio, saved_rect, in_a))
                } else {
                    None
                }
            }
        }
    }

    /// Reset all splits in the tree to 0.5 ratio.
    pub fn equalize_all(&mut self) {
        if let LayoutTree::Split { ratio, a, b, .. } = self {
            *ratio = 0.5;
            a.equalize_all();
            b.equalize_all();
        }
    }

    /// For each enclosing Split on the path from root to leaf `id`, invoke
    /// `f` with the split's mutable state. Order: outermost first.
    pub fn for_each_ancestor<F>(&mut self, id: WindowId, f: &mut F)
    where
        F: FnMut(SplitDir, &mut f32, bool, Option<ratatui::layout::Rect>),
    {
        if let LayoutTree::Split {
            dir,
            ratio,
            a,
            b,
            last_rect,
        } = self
        {
            let in_a = a.contains(id);
            let in_b = b.contains(id);
            if !in_a && !in_b {
                return;
            }
            // Outermost first: call f on this node before recursing.
            f(*dir, ratio, in_a, *last_rect);
            if in_a {
                a.for_each_ancestor(id, f);
            } else {
                b.for_each_ancestor(id, f);
            }
        }
    }

    /// Swap the two children of the deepest Split that directly contains
    /// `Leaf(id)` as one of its `a` or `b` children.
    ///
    /// Returns `true` if the swap was applied (i.e. there is an enclosing
    /// Split — `false` when `id` is the only window).
    pub fn swap_with_sibling(&mut self, id: WindowId) -> bool {
        match self {
            LayoutTree::Leaf(_) => false,
            LayoutTree::Split { a, b, .. } => {
                let a_is_focused_leaf = matches!(a.as_ref(), LayoutTree::Leaf(leaf) if *leaf == id);
                let b_is_focused_leaf = matches!(b.as_ref(), LayoutTree::Leaf(leaf) if *leaf == id);
                if a_is_focused_leaf || b_is_focused_leaf {
                    std::mem::swap(a, b);
                    return true;
                }
                // Recurse into whichever side contains id.
                if a.contains(id) {
                    return a.swap_with_sibling(id);
                }
                if b.contains(id) {
                    return b.swap_with_sibling(id);
                }
                false
            }
        }
    }

    /// Remove the leaf `id` from the tree.  When its parent `Split` is left
    /// with only the sibling, that split is replaced by the sibling subtree
    /// (collapse).
    ///
    /// Returns the `WindowId` of the leaf that should receive focus after
    /// removal (the sibling that survived the collapse), or `Err` if `id` is
    /// the only remaining leaf.
    pub fn remove_leaf(&mut self, id: WindowId) -> Result<WindowId, &'static str> {
        if matches!(self, LayoutTree::Leaf(_)) {
            return Err("E444: Cannot close last window");
        }
        match self.try_remove_leaf(id) {
            Some(focus) => Ok(focus),
            None => Err("E444: Cannot close last window"),
        }
    }

    /// Recursive helper for `remove_leaf`.  Returns `Some(new_focus)` when
    /// `id` was found and removed (or the caller needs to collapse this node),
    /// `None` when `id` was not in this subtree.
    fn try_remove_leaf(&mut self, id: WindowId) -> Option<WindowId> {
        match self {
            LayoutTree::Leaf(_) => None, // can't remove the only leaf
            LayoutTree::Split { a, b, .. } => {
                // Case 1: `a` is the leaf we want to remove.
                if matches!(a.as_ref(), LayoutTree::Leaf(leaf) if *leaf == id) {
                    let new_focus = first_leaf(b);
                    // Collapse: replace self with b.
                    *self = *b.clone();
                    return Some(new_focus);
                }
                // Case 2: `b` is the leaf we want to remove.
                if matches!(b.as_ref(), LayoutTree::Leaf(leaf) if *leaf == id) {
                    let new_focus = last_leaf(a);
                    // Collapse: replace self with a.
                    *self = *a.clone();
                    return Some(new_focus);
                }
                // Case 3: recurse into `a`.
                if a.contains(id) {
                    return a.try_remove_leaf(id);
                }
                // Case 4: recurse into `b`.
                if b.contains(id) {
                    return b.try_remove_leaf(id);
                }
                None
            }
        }
    }
}

/// First (top / left) leaf in a subtree.
fn first_leaf(tree: &LayoutTree) -> WindowId {
    match tree {
        LayoutTree::Leaf(id) => *id,
        LayoutTree::Split { a, .. } => first_leaf(a),
    }
}

/// Last (bottom / right) leaf in a subtree.
fn last_leaf(tree: &LayoutTree) -> WindowId {
    match tree {
        LayoutTree::Leaf(id) => *id,
        LayoutTree::Split { b, .. } => last_leaf(b),
    }
}

// ── Unit tests ────────────────────────────────────────────────────────────────

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

    #[test]
    fn tab_struct_constructs_with_layout_and_focus() {
        let layout = LayoutTree::Leaf(0);
        let tab = Tab {
            layout,
            focused_window: 0,
        };
        assert_eq!(tab.focused_window, 0);
        assert_eq!(tab.layout.leaves(), vec![0]);
    }
}

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

    fn leaf(id: WindowId) -> LayoutTree {
        LayoutTree::Leaf(id)
    }

    fn hsplit(ratio: f32, a: LayoutTree, b: LayoutTree) -> LayoutTree {
        LayoutTree::Split {
            dir: SplitDir::Horizontal,
            ratio,
            a: Box::new(a),
            b: Box::new(b),
            last_rect: None,
        }
    }

    fn vsplit(ratio: f32, a: LayoutTree, b: LayoutTree) -> LayoutTree {
        LayoutTree::Split {
            dir: SplitDir::Vertical,
            ratio,
            a: Box::new(a),
            b: Box::new(b),
            last_rect: None,
        }
    }

    /// Build a horizontal split with a pre-filled last_rect for resize tests.
    fn hsplit_with_rect(
        ratio: f32,
        a: LayoutTree,
        b: LayoutTree,
        rect: ratatui::layout::Rect,
    ) -> LayoutTree {
        LayoutTree::Split {
            dir: SplitDir::Horizontal,
            ratio,
            a: Box::new(a),
            b: Box::new(b),
            last_rect: Some(rect),
        }
    }

    /// Build a vertical split with a pre-filled last_rect for resize tests.
    fn vsplit_with_rect(
        ratio: f32,
        a: LayoutTree,
        b: LayoutTree,
        rect: ratatui::layout::Rect,
    ) -> LayoutTree {
        LayoutTree::Split {
            dir: SplitDir::Vertical,
            ratio,
            a: Box::new(a),
            b: Box::new(b),
            last_rect: Some(rect),
        }
    }

    // ── leaves() ─────────────────────────────────────────────────────────────

    #[test]
    fn leaves_single_leaf() {
        let tree = leaf(0);
        assert_eq!(tree.leaves(), vec![0]);
    }

    #[test]
    fn leaves_two_leaf_split() {
        let tree = hsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.leaves(), vec![0, 1]);
    }

    #[test]
    fn leaves_nested_horizontal_splits() {
        // 0
        // ──
        // 1
        // ──
        // 2
        let tree = hsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.leaves(), vec![0, 1, 2]);
    }

    #[test]
    fn leaves_nested_left_split() {
        let tree = hsplit(0.5, hsplit(0.5, leaf(0), leaf(1)), leaf(2));
        assert_eq!(tree.leaves(), vec![0, 1, 2]);
    }

    // ── replace_leaf() ───────────────────────────────────────────────────────

    #[test]
    fn replace_leaf_on_single_leaf() {
        let mut tree = leaf(0);
        let replaced = tree.replace_leaf(0, |_| leaf(99));
        assert!(replaced);
        assert_eq!(tree.leaves(), vec![99]);
    }

    #[test]
    fn replace_leaf_in_split_left() {
        let mut tree = hsplit(0.5, leaf(0), leaf(1));
        let replaced = tree.replace_leaf(0, |id| hsplit(0.5, leaf(id + 10), leaf(id)));
        assert!(replaced);
        assert_eq!(tree.leaves(), vec![10, 0, 1]);
    }

    #[test]
    fn replace_leaf_not_found_returns_false() {
        let mut tree = hsplit(0.5, leaf(0), leaf(1));
        let replaced = tree.replace_leaf(99, |_| leaf(99));
        assert!(!replaced);
        assert_eq!(tree.leaves(), vec![0, 1]);
    }

    // ── neighbor_below() / neighbor_above() ──────────────────────────────────

    #[test]
    fn neighbor_below_two_leaf() {
        let tree = hsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.neighbor_below(0), Some(1));
        assert_eq!(tree.neighbor_below(1), None);
    }

    #[test]
    fn neighbor_above_two_leaf() {
        let tree = hsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.neighbor_above(0), None);
        assert_eq!(tree.neighbor_above(1), Some(0));
    }

    #[test]
    fn neighbor_below_three_leaf_nested_bottom() {
        // 0
        // ─
        // 1
        // ─
        // 2
        let tree = hsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.neighbor_below(0), Some(1));
        assert_eq!(tree.neighbor_below(1), Some(2));
        assert_eq!(tree.neighbor_below(2), None);
    }

    #[test]
    fn neighbor_above_three_leaf_nested_bottom() {
        let tree = hsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.neighbor_above(0), None);
        assert_eq!(tree.neighbor_above(1), Some(0));
        assert_eq!(tree.neighbor_above(2), Some(1));
    }

    #[test]
    fn neighbor_below_three_leaf_nested_top() {
        // 0
        // ─
        // 1
        // ─
        // 2   (layout: (0|1)/2)
        let tree = hsplit(0.5, hsplit(0.5, leaf(0), leaf(1)), leaf(2));
        assert_eq!(tree.neighbor_below(0), Some(1));
        assert_eq!(tree.neighbor_below(1), Some(2));
        assert_eq!(tree.neighbor_below(2), None);
    }

    #[test]
    fn neighbor_above_three_leaf_nested_top() {
        let tree = hsplit(0.5, hsplit(0.5, leaf(0), leaf(1)), leaf(2));
        assert_eq!(tree.neighbor_above(0), None);
        assert_eq!(tree.neighbor_above(1), Some(0));
        assert_eq!(tree.neighbor_above(2), Some(1));
    }

    // ── remove_leaf() ────────────────────────────────────────────────────────

    #[test]
    fn remove_leaf_only_leaf_errors() {
        let mut tree = leaf(0);
        assert!(tree.remove_leaf(0).is_err());
    }

    #[test]
    fn remove_leaf_collapses_parent_keeps_sibling() {
        let mut tree = hsplit(0.5, leaf(0), leaf(1));
        let focus = tree.remove_leaf(0).unwrap();
        assert_eq!(focus, 1);
        assert_eq!(tree.leaves(), vec![1]);
    }

    #[test]
    fn remove_leaf_b_side_collapses_to_a() {
        let mut tree = hsplit(0.5, leaf(0), leaf(1));
        let focus = tree.remove_leaf(1).unwrap();
        assert_eq!(focus, 0);
        assert_eq!(tree.leaves(), vec![0]);
    }

    #[test]
    fn remove_leaf_nested_middle() {
        // 0 / (1 / 2)  → remove 1 → 0 / 2
        let mut tree = hsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        let focus = tree.remove_leaf(1).unwrap();
        assert_eq!(focus, 2);
        assert_eq!(tree.leaves(), vec![0, 2]);
    }

    // ── neighbor_left() / neighbor_right() ───────────────────────────────────

    #[test]
    fn neighbor_left_in_vertical_split() {
        // vsplit: a=0 (left), b=1 (right)
        let tree = vsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.neighbor_left(0), None);
        assert_eq!(tree.neighbor_left(1), Some(0));
    }

    #[test]
    fn neighbor_right_in_vertical_split() {
        let tree = vsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.neighbor_right(0), Some(1));
        assert_eq!(tree.neighbor_right(1), None);
    }

    #[test]
    fn neighbor_left_no_op_in_horizontal_split() {
        // A pure horizontal split has no left/right neighbours.
        let tree = hsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.neighbor_left(0), None);
        assert_eq!(tree.neighbor_left(1), None);
        assert_eq!(tree.neighbor_right(0), None);
        assert_eq!(tree.neighbor_right(1), None);
    }

    #[test]
    fn neighbor_left_three_leaf_vertical() {
        // vsplit: 0 | (1 | 2)
        let tree = vsplit(0.5, leaf(0), vsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.neighbor_left(0), None);
        assert_eq!(tree.neighbor_left(1), Some(0));
        assert_eq!(tree.neighbor_left(2), Some(1));
    }

    #[test]
    fn neighbor_right_three_leaf_vertical() {
        let tree = vsplit(0.5, leaf(0), vsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.neighbor_right(0), Some(1));
        assert_eq!(tree.neighbor_right(1), Some(2));
        assert_eq!(tree.neighbor_right(2), None);
    }

    // ── next_leaf() / prev_leaf() ─────────────────────────────────────────────

    #[test]
    fn next_leaf_cycles_through_all_leaves() {
        // 0 | (1 / 2) — mix of vertical and horizontal
        let tree = vsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.next_leaf(0), Some(1));
        assert_eq!(tree.next_leaf(1), Some(2));
        // wraps around
        assert_eq!(tree.next_leaf(2), Some(0));
    }

    #[test]
    fn prev_leaf_wraps_around() {
        let tree = vsplit(0.5, leaf(0), hsplit(0.5, leaf(1), leaf(2)));
        assert_eq!(tree.prev_leaf(0), Some(2));
        assert_eq!(tree.prev_leaf(1), Some(0));
        assert_eq!(tree.prev_leaf(2), Some(1));
    }

    #[test]
    fn next_leaf_single_leaf_wraps_to_self() {
        let tree = leaf(0);
        assert_eq!(tree.next_leaf(0), Some(0));
    }

    #[test]
    fn next_prev_returns_none_for_unknown_id() {
        let tree = vsplit(0.5, leaf(0), leaf(1));
        assert_eq!(tree.next_leaf(99), None);
        assert_eq!(tree.prev_leaf(99), None);
    }

    // ── enclosing_split_mut() ────────────────────────────────────────────────

    #[test]
    fn enclosing_split_mut_returns_innermost() {
        // outer: hsplit 0 / inner: hsplit 1 / 2
        // Querying for id=1 should return the inner split (ratio=0.6), not the outer (ratio=0.4).
        // Pre-fill rects to verify last_rect is propagated correctly.
        let outer_rect = ratatui::layout::Rect {
            x: 0,
            y: 0,
            width: 80,
            height: 40,
        };
        let inner_rect = ratatui::layout::Rect {
            x: 0,
            y: 20,
            width: 80,
            height: 20,
        };
        let mut tree = hsplit_with_rect(
            0.4,
            leaf(0),
            hsplit_with_rect(0.6, leaf(1), leaf(2), inner_rect),
            outer_rect,
        );
        let result = tree.enclosing_split_mut(1, SplitDir::Horizontal);
        assert!(result.is_some(), "should find enclosing horizontal split");
        let (ratio, rect, in_a) = result.unwrap();
        assert!(
            (*ratio - 0.6).abs() < 1e-5,
            "innermost split ratio should be 0.6, got {ratio}"
        );
        assert_eq!(
            rect,
            Some(inner_rect),
            "should return inner rect, not outer"
        );
        assert!(
            in_a,
            "id=1 is in the 'a' (left/top) side of the inner split"
        );
    }

    #[test]
    fn enclosing_split_mut_skips_wrong_dir() {
        // vsplit at root containing a leaf — asking for Horizontal should find nothing.
        let mut tree = vsplit(0.5, leaf(0), leaf(1));
        let result = tree.enclosing_split_mut(0, SplitDir::Horizontal);
        assert!(
            result.is_none(),
            "should not match a Vertical split for Horizontal dir"
        );
    }

    #[test]
    fn enclosing_split_mut_returns_none_for_only_leaf() {
        let mut tree = leaf(0);
        let result = tree.enclosing_split_mut(0, SplitDir::Horizontal);
        assert!(result.is_none(), "single leaf has no enclosing split");
    }

    #[test]
    fn equalize_all_resets_nested_splits_to_half() {
        // Two nested hsplits with non-0.5 ratios — equalize must reset both.
        let mut tree = hsplit(0.3, leaf(0), hsplit(0.7, leaf(1), leaf(2)));
        tree.equalize_all();
        // Walk and verify every split is now 0.5.
        fn check_all_half(t: &LayoutTree) {
            if let LayoutTree::Split { ratio, a, b, .. } = t {
                assert!(
                    (ratio - 0.5).abs() < 1e-5,
                    "ratio should be 0.5, got {ratio}"
                );
                check_all_half(a);
                check_all_half(b);
            }
        }
        check_all_half(&tree);
    }

    #[test]
    fn for_each_ancestor_visits_outermost_first() {
        // outer vsplit (ratio=0.3) containing inner hsplit (ratio=0.7) at leaf 1 / leaf 2.
        // Asking from leaf 1 — should visit outer vsplit, then inner hsplit, in that order.
        let outer_rect = ratatui::layout::Rect {
            x: 0,
            y: 0,
            width: 80,
            height: 24,
        };
        let inner_rect = ratatui::layout::Rect {
            x: 24,
            y: 0,
            width: 56,
            height: 24,
        };
        let mut tree = vsplit_with_rect(
            0.3,
            leaf(0),
            hsplit_with_rect(0.7, leaf(1), leaf(2), inner_rect),
            outer_rect,
        );
        let mut visited_dirs: Vec<SplitDir> = Vec::new();
        let mut visited_ratios: Vec<f32> = Vec::new();
        tree.for_each_ancestor(1, &mut |dir, ratio, _in_a, _rect| {
            visited_dirs.push(dir);
            visited_ratios.push(*ratio);
        });
        assert_eq!(
            visited_dirs,
            vec![SplitDir::Vertical, SplitDir::Horizontal],
            "outermost (Vertical) should be visited first"
        );
        assert!(
            (visited_ratios[0] - 0.3).abs() < 1e-5,
            "outer ratio should be 0.3"
        );
        assert!(
            (visited_ratios[1] - 0.7).abs() < 1e-5,
            "inner ratio should be 0.7"
        );
    }

    // ── swap_with_sibling() ───────────────────────────────────────────────────

    #[test]
    fn swap_with_sibling_swaps_two_leaves() {
        // hsplit: a=0 (top), b=1 (bottom). Swap from 0 — should produce a=1, b=0.
        let mut tree = hsplit(0.5, leaf(0), leaf(1));
        let swapped = tree.swap_with_sibling(0);
        assert!(swapped, "swap should succeed in a two-leaf split");
        assert_eq!(tree.leaves(), vec![1, 0], "leaves should be swapped");
    }

    #[test]
    fn swap_with_sibling_in_nested_split_swaps_at_focused_parent() {
        // Layout: hsplit( leaf(0), vsplit( leaf(1), leaf(2) ) )
        // Focused = 1. Its direct parent is the inner vsplit.
        // Swap should swap 1 and 2 within the vsplit, not the outer hsplit.
        let mut tree = hsplit(0.5, leaf(0), vsplit(0.5, leaf(1), leaf(2)));
        let swapped = tree.swap_with_sibling(1);
        assert!(swapped, "swap should succeed");
        // Pre-order after: 0, then inner vsplit now has a=2, b=1 → [0, 2, 1]
        assert_eq!(
            tree.leaves(),
            vec![0, 2, 1],
            "inner leaves should be swapped"
        );
    }

    #[test]
    fn swap_with_sibling_returns_false_for_only_leaf() {
        let mut tree = leaf(0);
        let swapped = tree.swap_with_sibling(0);
        assert!(!swapped, "single leaf has no sibling to swap with");
    }

    // ── mixed_layout_navigation ───────────────────────────────────────────────

    #[test]
    fn mixed_layout_navigation() {
        // Layout:
        //   Horizontal split:
        //     a = Vertical split { A=0, B=1 }   (top row, two columns)
        //     b = Leaf(2)                         (bottom row, full width)
        //
        // Visual:
        //   ┌───┬───┐
        //   │ 0 │ 1 │
        //   ├───┴───┤
        //   │   2   │
        //   └───────┘
        let tree = hsplit(0.5, vsplit(0.5, leaf(0), leaf(1)), leaf(2));

        // Left/right within the top vsplit.
        assert_eq!(tree.neighbor_right(0), Some(1));
        assert_eq!(tree.neighbor_left(1), Some(0));

        // No right neighbour for 1 (rightmost in vsplit, hsplit passthrough).
        assert_eq!(tree.neighbor_right(1), None);
        // No left neighbour for 0 (leftmost in vsplit).
        assert_eq!(tree.neighbor_left(0), None);

        // Above/below across the horizontal split.
        // 0 and 1 are both in `a`; below from either reaches 2.
        assert_eq!(tree.neighbor_below(0), Some(2));
        assert_eq!(tree.neighbor_below(1), Some(2));
        assert_eq!(tree.neighbor_below(2), None);
        assert_eq!(tree.neighbor_above(2), Some(1)); // last_leaf of `a` = last_leaf(vsplit(0,1)) = 1
        assert_eq!(tree.neighbor_above(0), None);
        assert_eq!(tree.neighbor_above(1), None);

        // Cycle: pre-order is 0, 1, 2.
        assert_eq!(tree.next_leaf(0), Some(1));
        assert_eq!(tree.next_leaf(1), Some(2));
        assert_eq!(tree.next_leaf(2), Some(0));
        assert_eq!(tree.prev_leaf(0), Some(2));
        assert_eq!(tree.prev_leaf(2), Some(1));
    }
}