ratatui-spatial-splits 0.1.1

//! Split manager: binary tree of splits with area caching.

use ratatui::layout::Rect;

use crate::navigation;
use crate::types::{AreaId, CloseResult, SplitNode, SplitResult};

/// A computed area for a leaf in the split tree.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct SplitArea {
    /// The unique identifier for this area.
    pub id: AreaId,
    /// The computed pixel rectangle.
    pub rect: Rect,
}

/// Pure geometry engine for managing spatial splits.
///
/// Manages a binary tree of horizontal and vertical splits, computing
/// pixel-accurate [`Rect`] areas for each leaf. The tree is mutated via
/// [`split_horizontal`](SplitManager::split_horizontal),
/// [`split_vertical`](SplitManager::split_vertical),
/// [`close`](SplitManager::close), and
/// [`resize`](SplitManager::resize).
///
/// Areas are cached and automatically recalculated when the tree is mutated
/// or the viewport changes.
#[derive(Debug, Clone)]
pub struct SplitManager {
    /// Root of the split tree.
    root: SplitNode,
    /// Next ID counter for new AreaIds.
    next_id: u64,
    /// Cached areas (recalculated when dirty).
    cached_areas: Vec<SplitArea>,
    /// Cached viewport rect.
    total_area: Rect,
    /// Whether the cache is dirty and needs recalculation.
    dirty: bool,
}

impl Default for SplitManager {
    fn default() -> Self {
        Self::new()
    }
}

impl SplitManager {
    /// Creates a new split manager with a single leaf.
    ///
    /// The initial leaf gets `AreaId(1)`. The viewport defaults to zero-sized,
    /// so you must call [`set_viewport`](Self::set_viewport) before areas are meaningful.
    #[must_use]
    pub fn new() -> Self {
        let initial_id = AreaId(1);
        Self {
            root: SplitNode::Leaf { id: initial_id },
            next_id: 2,
            cached_areas: Vec::new(),
            total_area: Rect::default(),
            dirty: true,
        }
    }

    // ── Geometry ──────────────────────────────────────────────────────

    /// Returns the computed areas for all leaves.
    ///
    /// Triggers a recalculation if the cache is dirty.
    pub fn areas(&mut self) -> &[SplitArea] {
        self.recalculate_if_dirty();
        &self.cached_areas
    }

    /// Returns the area for a specific leaf ID, if it exists.
    pub fn area_for_id(&mut self, id: AreaId) -> Option<Rect> {
        self.recalculate_if_dirty();
        self.cached_areas
            .iter()
            .find(|a| a.id == id)
            .map(|a| a.rect)
    }

    /// Updates the viewport rect and marks the cache as dirty.
    pub fn set_viewport(&mut self, rect: Rect) -> &[SplitArea] {
        if rect != self.total_area {
            self.total_area = rect;
            self.dirty = true;
        }
        self.areas()
    }

    // ── Navigation ────────────────────────────────────────────────────

    /// Finds the neighboring leaf in the given direction from `current_id`.
    ///
    /// Uses beam/raycasting against cached areas.
    pub fn navigate(
        &mut self,
        current_id: AreaId,
        direction: crate::navigation::Direction,
    ) -> Option<AreaId> {
        self.recalculate_if_dirty();
        navigation::navigate(&self.cached_areas, current_id, direction)
    }

    // ── Mutations ─────────────────────────────────────────────────────

    /// Splits the leaf with the given ID horizontally (top/bottom).
    ///
    /// The original leaf becomes the top child, and a new leaf is created
    /// as the bottom child. Returns `None` if the ID is not a leaf.
    pub fn split_horizontal(&mut self, id: AreaId) -> Option<SplitResult> {
        let new_id = AreaId(self.next_id);
        self.next_id += 1;

        let new_leaf = SplitNode::Leaf { id: new_id };
        let found = self.replace_leaf(id, |old| SplitNode::Horizontal {
            top: Box::new(old),
            bottom: Box::new(new_leaf.clone()),
            ratio: 0.5,
        });

        if found {
            self.dirty = true;
            Some(SplitResult {
                original: id,
                new: new_id,
            })
        } else {
            None
        }
    }

    /// Splits the leaf with the given ID vertically (left/right).
    ///
    /// The original leaf becomes the left child, and a new leaf is created
    /// as the right child. Returns `None` if the ID is not a leaf.
    pub fn split_vertical(&mut self, id: AreaId) -> Option<SplitResult> {
        let new_id = AreaId(self.next_id);
        self.next_id += 1;

        let new_leaf = SplitNode::Leaf { id: new_id };
        let found = self.replace_leaf(id, |old| SplitNode::Vertical {
            left: Box::new(old),
            right: Box::new(new_leaf.clone()),
            ratio: 0.5,
        });

        if found {
            self.dirty = true;
            Some(SplitResult {
                original: id,
                new: new_id,
            })
        } else {
            None
        }
    }

    /// Closes (removes) the leaf with the given ID.
    ///
    /// The sibling of the closed leaf replaces their parent node.
    /// Returns `None` if the ID is not found or is the only leaf.
    pub fn close(&mut self, id: AreaId) -> Option<CloseResult> {
        if self.is_single_leaf() {
            return None;
        }

        let sibling_id = self.find_sibling_id(id)?;
        let found = self.remove_leaf(id);

        if found {
            self.dirty = true;
            Some(CloseResult {
                removed: id,
                surviving: sibling_id,
            })
        } else {
            None
        }
    }

    /// Resizes the split containing the given leaf by adjusting the ratio.
    ///
    /// `amount` is in character units (positive = grow first child, negative = shrink).
    /// Returns `true` if a resize was actually performed.
    pub fn resize(
        &mut self,
        id: AreaId,
        direction: crate::navigation::Direction,
        amount: i16,
    ) -> bool {
        let total_size = match direction {
            crate::navigation::Direction::Left | crate::navigation::Direction::Right => {
                self.total_area.width
            }
            crate::navigation::Direction::Up | crate::navigation::Direction::Down => {
                self.total_area.height
            }
        };

        if total_size == 0 {
            return false;
        }

        let adjusted = self.adjust_parent_ratio(id, direction, amount, total_size);
        if adjusted {
            self.dirty = true;
        }
        adjusted
    }

    // ── Queries ───────────────────────────────────────────────────────

    /// Returns all leaf AreaIds in the tree.
    pub fn leaves(&self) -> Vec<AreaId> {
        let mut result = Vec::new();
        Self::collect_leaves(&self.root, &mut result);
        result
    }

    /// Returns `true` if the tree contains a leaf with the given ID.
    pub fn contains(&self, id: AreaId) -> bool {
        Self::contains_recursive(&self.root, id)
    }

    /// Returns `true` if the tree consists of a single leaf.
    pub fn is_single_leaf(&self) -> bool {
        self.root.is_leaf()
    }

    /// Returns the AreaId of the root leaf (only valid for single-leaf trees).
    ///
    /// # Panics
    ///
    /// Panics if the tree is not a single leaf.
    pub fn root_id(&self) -> AreaId {
        self.root
            .leaf_id()
            .expect("root should always be a leaf when called on single-leaf tree")
    }

    // ── Private helpers ───────────────────────────────────────────────

    fn recalculate_if_dirty(&mut self) {
        if !self.dirty {
            return;
        }
        self.cached_areas.clear();
        Self::compute_areas(&self.root, self.total_area, &mut self.cached_areas);
        self.dirty = false;
    }

    fn compute_areas(node: &SplitNode, rect: Rect, out: &mut Vec<SplitArea>) {
        match node {
            SplitNode::Leaf { id } => {
                out.push(SplitArea { id: *id, rect });
            }
            SplitNode::Horizontal { top, bottom, ratio } => {
                let total_height = f64::from(rect.height);
                #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
                let top_height = (total_height * ratio).round() as u16;
                let bottom_height = rect.height.saturating_sub(top_height);
                let top_rect = Rect::new(rect.x, rect.y, rect.width, top_height);
                let bottom_rect = Rect::new(
                    rect.x,
                    rect.y.saturating_add(top_height),
                    rect.width,
                    bottom_height,
                );
                Self::compute_areas(top, top_rect, out);
                Self::compute_areas(bottom, bottom_rect, out);
            }
            SplitNode::Vertical { left, right, ratio } => {
                let total_width = f64::from(rect.width);
                #[allow(clippy::cast_possible_truncation, clippy::cast_sign_loss)]
                let left_width = (total_width * ratio).round() as u16;
                let right_width = rect.width.saturating_sub(left_width);
                let left_rect = Rect::new(rect.x, rect.y, left_width, rect.height);
                let right_rect = Rect::new(
                    rect.x.saturating_add(left_width),
                    rect.y,
                    right_width,
                    rect.height,
                );
                Self::compute_areas(left, left_rect, out);
                Self::compute_areas(right, right_rect, out);
            }
        }
    }

    fn collect_leaves(node: &SplitNode, out: &mut Vec<AreaId>) {
        match node {
            SplitNode::Leaf { id } => out.push(*id),
            SplitNode::Horizontal { top, bottom, .. } => {
                Self::collect_leaves(top, out);
                Self::collect_leaves(bottom, out);
            }
            SplitNode::Vertical { left, right, .. } => {
                Self::collect_leaves(left, out);
                Self::collect_leaves(right, out);
            }
        }
    }

    fn contains_recursive(node: &SplitNode, id: AreaId) -> bool {
        match node {
            SplitNode::Leaf { id: leaf_id } => *leaf_id == id,
            SplitNode::Horizontal { top, bottom, .. } => {
                Self::contains_recursive(top, id) || Self::contains_recursive(bottom, id)
            }
            SplitNode::Vertical { left, right, .. } => {
                Self::contains_recursive(left, id) || Self::contains_recursive(right, id)
            }
        }
    }

    /// Replaces a leaf node identified by `id` with the result of `f`.
    /// Returns `true` if a replacement was made.
    fn replace_leaf(&mut self, id: AreaId, mut f: impl FnMut(SplitNode) -> SplitNode) -> bool {
        Self::replace_leaf_recursive(&mut self.root, id, &mut f)
    }

    fn replace_leaf_recursive(
        node: &mut SplitNode,
        id: AreaId,
        f: &mut dyn FnMut(SplitNode) -> SplitNode,
    ) -> bool {
        match node {
            SplitNode::Leaf { id: leaf_id } if *leaf_id == id => {
                let old = std::mem::replace(node, SplitNode::Leaf { id: AreaId(0) });
                *node = f(old);
                true
            }
            SplitNode::Horizontal { top, bottom, .. } => {
                Self::replace_leaf_recursive(top, id, f)
                    || Self::replace_leaf_recursive(bottom, id, f)
            }
            SplitNode::Vertical { left, right, .. } => {
                Self::replace_leaf_recursive(left, id, f)
                    || Self::replace_leaf_recursive(right, id, f)
            }
            _ => false,
        }
    }

    /// Finds the sibling AreaId of the given leaf.
    fn find_sibling_id(&self, id: AreaId) -> Option<AreaId> {
        Self::find_sibling_recursive(&self.root, id)
    }

    fn find_sibling_recursive(node: &SplitNode, id: AreaId) -> Option<AreaId> {
        match node {
            SplitNode::Leaf { .. } => None,
            SplitNode::Horizontal { top, bottom, .. } => {
                if Self::contains_recursive(top, id) {
                    if top.is_leaf() && top.leaf_id() == Some(id) {
                        Self::first_leaf(bottom)
                    } else {
                        Self::find_sibling_recursive(top, id)
                    }
                } else if bottom.is_leaf() && bottom.leaf_id() == Some(id) {
                    Self::first_leaf(top)
                } else {
                    Self::find_sibling_recursive(bottom, id)
                }
            }
            SplitNode::Vertical { left, right, .. } => {
                if Self::contains_recursive(left, id) {
                    if left.is_leaf() && left.leaf_id() == Some(id) {
                        Self::first_leaf(right)
                    } else {
                        Self::find_sibling_recursive(left, id)
                    }
                } else if right.is_leaf() && right.leaf_id() == Some(id) {
                    Self::first_leaf(left)
                } else {
                    Self::find_sibling_recursive(right, id)
                }
            }
        }
    }

    fn first_leaf(node: &SplitNode) -> Option<AreaId> {
        match node {
            SplitNode::Leaf { id } => Some(*id),
            SplitNode::Horizontal { top, .. } => Self::first_leaf(top),
            SplitNode::Vertical { left, .. } => Self::first_leaf(left),
        }
    }

    /// Removes a leaf and promotes its sibling to replace the parent.
    fn remove_leaf(&mut self, id: AreaId) -> bool {
        Self::remove_leaf_recursive(&mut self.root, id)
    }

    fn remove_leaf_recursive(node: &mut SplitNode, id: AreaId) -> bool {
        match node {
            SplitNode::Leaf { .. } => false,
            SplitNode::Horizontal { top, bottom, .. } => {
                if top.is_leaf() && top.leaf_id() == Some(id) {
                    let sibling =
                        std::mem::replace(bottom.as_mut(), SplitNode::Leaf { id: AreaId(0) });
                    *node = sibling;
                    true
                } else if bottom.is_leaf() && bottom.leaf_id() == Some(id) {
                    let sibling =
                        std::mem::replace(top.as_mut(), SplitNode::Leaf { id: AreaId(0) });
                    *node = sibling;
                    true
                } else {
                    Self::remove_leaf_recursive(top.as_mut(), id)
                        || Self::remove_leaf_recursive(bottom.as_mut(), id)
                }
            }
            SplitNode::Vertical { left, right, .. } => {
                if left.is_leaf() && left.leaf_id() == Some(id) {
                    let sibling =
                        std::mem::replace(right.as_mut(), SplitNode::Leaf { id: AreaId(0) });
                    *node = sibling;
                    true
                } else if right.is_leaf() && right.leaf_id() == Some(id) {
                    let sibling =
                        std::mem::replace(left.as_mut(), SplitNode::Leaf { id: AreaId(0) });
                    *node = sibling;
                    true
                } else {
                    Self::remove_leaf_recursive(left.as_mut(), id)
                        || Self::remove_leaf_recursive(right.as_mut(), id)
                }
            }
        }
    }

    /// Adjusts the parent split ratio for the given leaf in the given direction.
    fn adjust_parent_ratio(
        &mut self,
        id: AreaId,
        direction: crate::navigation::Direction,
        amount: i16,
        total_size: u16,
    ) -> bool {
        Self::adjust_ratio_recursive(&mut self.root, id, direction, amount, total_size)
    }

    #[allow(clippy::too_many_lines)]
    fn adjust_ratio_recursive(
        node: &mut SplitNode,
        id: AreaId,
        direction: crate::navigation::Direction,
        amount: i16,
        total_size: u16,
    ) -> bool {
        match node {
            SplitNode::Leaf { .. } => false,
            SplitNode::Horizontal { top, bottom, ratio } => {
                // Check direct-leaf matches first, regardless of which subtree contains id
                if top.is_leaf() && top.leaf_id() == Some(id) {
                    match direction {
                        crate::navigation::Direction::Down => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio + delta).clamp(0.1, 0.9);
                            true
                        }
                        crate::navigation::Direction::Up => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio - delta).clamp(0.1, 0.9);
                            true
                        }
                        _ => Self::adjust_ratio_recursive(top, id, direction, amount, total_size),
                    }
                } else if bottom.is_leaf() && bottom.leaf_id() == Some(id) {
                    match direction {
                        crate::navigation::Direction::Up => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio + delta).clamp(0.1, 0.9);
                            true
                        }
                        crate::navigation::Direction::Down => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio - delta).clamp(0.1, 0.9);
                            true
                        }
                        _ => {
                            Self::adjust_ratio_recursive(bottom, id, direction, amount, total_size)
                        }
                    }
                } else if Self::contains_recursive(top, id) {
                    Self::adjust_ratio_recursive(top, id, direction, amount, total_size)
                } else {
                    Self::adjust_ratio_recursive(bottom, id, direction, amount, total_size)
                }
            }
            SplitNode::Vertical { left, right, ratio } => {
                // Check direct-leaf matches first, regardless of which subtree contains id
                if left.is_leaf() && left.leaf_id() == Some(id) {
                    match direction {
                        crate::navigation::Direction::Right => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio + delta).clamp(0.1, 0.9);
                            true
                        }
                        crate::navigation::Direction::Left => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio - delta).clamp(0.1, 0.9);
                            true
                        }
                        _ => Self::adjust_ratio_recursive(left, id, direction, amount, total_size),
                    }
                } else if right.is_leaf() && right.leaf_id() == Some(id) {
                    match direction {
                        crate::navigation::Direction::Left => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio + delta).clamp(0.1, 0.9);
                            true
                        }
                        crate::navigation::Direction::Right => {
                            let delta = f64::from(amount) / f64::from(total_size);
                            *ratio = (*ratio - delta).clamp(0.1, 0.9);
                            true
                        }
                        _ => Self::adjust_ratio_recursive(right, id, direction, amount, total_size),
                    }
                } else if Self::contains_recursive(left, id) {
                    Self::adjust_ratio_recursive(left, id, direction, amount, total_size)
                } else {
                    Self::adjust_ratio_recursive(right, id, direction, amount, total_size)
                }
            }
        }
    }
}

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

    #[test]
    fn new_manager_has_single_leaf_with_full_viewport() {
        // Given a new split manager with a viewport.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When querying areas.
        let areas = mgr.areas();

        // Then a single area fills the entire viewport.
        assert_eq!(areas.len(), 1);
        assert_eq!(areas[0].id, AreaId(1));
        assert_eq!(areas[0].rect, Rect::new(0, 0, 100, 100));
    }

    #[test]
    fn split_vertical_divides_area_into_left_and_right() {
        // Given a manager with a single leaf and 100×100 viewport.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When splitting vertically.
        let result = mgr.split_vertical(AreaId(1)).unwrap();

        // Then the original leaf stays on the left and a new leaf appears on the right.
        assert_eq!(result.original, AreaId(1));
        assert_eq!(result.new, AreaId(2));

        let areas = mgr.areas();
        assert_eq!(areas.len(), 2);
        assert_eq!(areas[0].id, AreaId(1));
        assert_eq!(areas[0].rect, Rect::new(0, 0, 50, 100));
        assert_eq!(areas[1].id, AreaId(2));
        assert_eq!(areas[1].rect, Rect::new(50, 0, 50, 100));
    }

    #[test]
    fn split_horizontal_divides_area_into_top_and_bottom() {
        // Given a manager with a single leaf and 100×100 viewport.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When splitting horizontally.
        let result = mgr.split_horizontal(AreaId(1)).unwrap();

        // Then the original leaf stays on top and a new leaf appears on the bottom.
        assert_eq!(result.new, AreaId(2));

        let areas = mgr.areas();
        assert_eq!(areas.len(), 2);
        assert_eq!(areas[0].rect, Rect::new(0, 0, 100, 50));
        assert_eq!(areas[1].rect, Rect::new(0, 50, 100, 50));
    }

    #[test]
    fn nested_splits_produce_correct_areas() {
        // Given a manager with a 100×100 viewport.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When splitting vertically then horizontally on the right child.
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // Then three areas are produced with correct rects.
        let areas = mgr.areas();
        assert_eq!(areas.len(), 3);

        assert_eq!(areas[0].id, AreaId(1));
        assert_eq!(areas[0].rect, Rect::new(0, 0, 50, 100));

        assert_eq!(areas[1].id, AreaId(2));
        assert_eq!(areas[1].rect, Rect::new(50, 0, 50, 50));

        assert_eq!(areas[2].id, AreaId(3));
        assert_eq!(areas[2].rect, Rect::new(50, 50, 50, 50));
    }

    #[test]
    fn split_returns_none_for_nonexistent_leaf() {
        // Given a new manager with no such leaf.
        let mut mgr = SplitManager::new();

        // When splitting a nonexistent area.
        let result = mgr.split_vertical(AreaId(99));

        // Then no split occurs.
        assert!(result.is_none());
    }

    #[test]
    fn close_promotes_sibling_to_replace_parent() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When closing the right area.
        let result = mgr.close(AreaId(2)).unwrap();

        // Then the sibling expands to fill the entire viewport.
        assert_eq!(result.removed, AreaId(2));
        assert_eq!(result.surviving, AreaId(1));

        let areas = mgr.areas();
        assert_eq!(areas.len(), 1);
        assert_eq!(areas[0].rect, Rect::new(0, 0, 100, 100));
    }

    #[test]
    fn close_returns_none_when_single_leaf_remains() {
        // Given a manager with only one leaf.
        let mut mgr = SplitManager::new();

        // When attempting to close it.
        let result = mgr.close(AreaId(1));

        // Then no close occurs.
        assert!(result.is_none());
    }

    #[test]
    fn resize_from_left_child_grows_leftward() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When resizing the left child to the right.
        let resized = mgr.resize(AreaId(1), Direction::Right, 10);

        // Then the left area grows and the right area shrinks.
        assert!(resized);
        let areas = mgr.areas();
        assert!(areas[0].rect.width > 50);
        assert!(areas[1].rect.width < 50);
    }

    #[test]
    fn resize_from_right_child_grows_rightward() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When resizing the right child to the right.
        let resized = mgr.resize(AreaId(2), Direction::Right, 10);

        // Then the right area grows and the left area shrinks.
        assert!(resized);
        let areas = mgr.areas();
        assert!(areas[0].rect.width < 50);
        assert!(areas[1].rect.width > 50);
    }

    #[test]
    fn resize_from_bottom_child_grows_downward() {
        // Given a horizontally split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_horizontal(AreaId(1)).unwrap();

        // When resizing the bottom child downward.
        let resized = mgr.resize(AreaId(2), Direction::Down, 10);

        // Then the bottom area grows and the top area shrinks.
        assert!(resized);
        let areas = mgr.areas();
        assert!(areas[0].rect.height < 50);
        assert!(areas[1].rect.height > 50);
    }

    #[test]
    fn mutation_invalidates_area_cache() {
        // Given a manager with a single area.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When querying areas before and after a split.
        let areas_before = mgr.areas();
        assert_eq!(areas_before.len(), 1);

        mgr.split_vertical(AreaId(1)).unwrap();
        let areas_after = mgr.areas();

        // Then the cached areas reflect the new tree structure.
        assert_eq!(areas_after.len(), 2);
    }

    #[test]
    fn leaves_returns_all_leaf_ids() {
        // Given a manager with two splits.
        let mut mgr = SplitManager::new();
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // When querying all leaves.
        let mut leaves = mgr.leaves();
        leaves.sort();

        // Then all three leaf IDs are returned.
        assert_eq!(leaves, vec![AreaId(1), AreaId(2), AreaId(3)]);
    }

    #[test]
    fn contains_returns_true_for_existing_leaf() {
        // Given a new manager.
        let mgr = SplitManager::new();

        // When checking if the initial leaf exists.
        let found = mgr.contains(AreaId(1));

        // Then it is found.
        assert!(found);
    }

    #[test]
    fn contains_returns_false_for_missing_leaf() {
        // Given a new manager.
        let mgr = SplitManager::new();

        // When checking for a nonexistent leaf.
        let found = mgr.contains(AreaId(99));

        // Then it is not found.
        assert!(!found);
    }

    #[test]
    fn is_single_leaf_returns_true_before_split() {
        // Given a new manager.
        let mgr = SplitManager::new();

        // When checking if it is a single leaf.
        let result = mgr.is_single_leaf();

        // Then it is.
        assert!(result);
    }

    #[test]
    fn is_single_leaf_returns_false_after_split() {
        // Given a manager that has been split.
        let mut mgr = SplitManager::new();
        mgr.split_vertical(AreaId(1)).unwrap();

        // When checking if it is a single leaf.
        let result = mgr.is_single_leaf();

        // Then it is not.
        assert!(!result);
    }

    #[test]
    fn navigate_returns_right_neighbor_across_split() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When navigating right from the left area.
        let result = mgr.navigate(AreaId(1), Direction::Right);

        // Then the right area is found.
        assert_eq!(result, Some(AreaId(2)));
    }

    #[test]
    fn navigate_returns_left_neighbor_across_split() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When navigating left from the right area.
        let result = mgr.navigate(AreaId(2), Direction::Left);

        // Then the left area is found.
        assert_eq!(result, Some(AreaId(1)));
    }

    #[test]
    fn navigate_returns_none_at_edge() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When navigating left from the leftmost area.
        let result = mgr.navigate(AreaId(1), Direction::Left);

        // Then no neighbor is found.
        assert_eq!(result, None);
    }

    #[test]
    fn navigate_finds_right_neighbor_in_nested_layout() {
        // Given a nested layout (vertical split, right child split horizontally).
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // When navigating right from the left column.
        let result = mgr.navigate(AreaId(1), Direction::Right);

        // Then the top-right area is found.
        assert_eq!(result, Some(AreaId(2)));
    }

    #[test]
    fn navigate_finds_left_neighbor_in_nested_layout() {
        // Given a nested layout (vertical split, right child split horizontally).
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // When navigating left from the top-right area.
        let result = mgr.navigate(AreaId(2), Direction::Left);

        // Then the left column is found.
        assert_eq!(result, Some(AreaId(1)));
    }

    #[test]
    fn navigate_finds_up_neighbor_in_nested_layout() {
        // Given a nested layout (vertical split, right child split horizontally).
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // When navigating up from the bottom-right area.
        let result = mgr.navigate(AreaId(3), Direction::Up);

        // Then the top-right area is found.
        assert_eq!(result, Some(AreaId(2)));
    }

    #[test]
    fn navigate_finds_down_neighbor_in_nested_layout() {
        // Given a nested layout (vertical split, right child split horizontally).
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // When navigating down from the top-right area.
        let result = mgr.navigate(AreaId(2), Direction::Down);

        // Then the bottom-right area is found.
        assert_eq!(result, Some(AreaId(3)));
    }

    #[test]
    fn area_for_id_returns_correct_rects_after_split() {
        // Given a vertically split layout.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));
        mgr.split_vertical(AreaId(1)).unwrap();

        // When looking up rects by area ID.
        let left = mgr.area_for_id(AreaId(1));
        let right = mgr.area_for_id(AreaId(2));

        // Then each area's rect matches its computed position.
        assert_eq!(left, Some(Rect::new(0, 0, 50, 100)));
        assert_eq!(right, Some(Rect::new(50, 0, 50, 100)));
    }

    #[test]
    fn area_for_id_returns_none_for_missing_id() {
        // Given a manager with a single area.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When looking up a nonexistent area ID.
        let result = mgr.area_for_id(AreaId(99));

        // Then none is returned.
        assert_eq!(result, None);
    }

    #[test]
    fn four_way_split_produces_nonzero_areas() {
        // Given a manager with a 100×100 viewport.
        let mut mgr = SplitManager::new();
        mgr.set_viewport(Rect::new(0, 0, 100, 100));

        // When creating a four-way grid of splits.
        mgr.split_vertical(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(1)).unwrap();
        mgr.split_horizontal(AreaId(2)).unwrap();

        // Then all four areas have nonzero dimensions.
        let areas = mgr.areas();
        assert_eq!(areas.len(), 4);
        for area in areas {
            assert!(area.rect.width > 0);
            assert!(area.rect.height > 0);
        }
    }
}