//! Frontend-independent binary pane layout and spatial focus navigation.

#[derive(Clone, Copy, Debug, Eq, PartialEq, serde::Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum SplitAxis {
    /// Above/below.
    Horizontal,
    /// Left/right.
    Vertical,
}

#[derive(Clone, Copy, Debug, Eq, PartialEq)]
pub enum PaneDirection {
    Left,
    Down,
    Up,
    Right,
}

#[derive(Clone, Copy, Debug, Default, Eq, PartialEq)]
pub struct PaneRect {
    pub x: u16,
    pub y: u16,
    pub width: u16,
    pub height: u16,
}

impl PaneRect {
    fn is_empty(self) -> bool {
        self.width == 0 || self.height == 0
    }
}

/// Partition a rectangle into balanced halves, with the odd cell in the second.
pub fn split_rect(bounds: PaneRect, axis: SplitAxis) -> [PaneRect; 2] {
    let mut first = bounds;
    let mut second = bounds;
    match axis {
        SplitAxis::Horizontal => {
            first.height = bounds.height / 2;
            second.y = bounds.y.saturating_add(first.height);
            second.height -= first.height;
        }
        SplitAxis::Vertical => {
            first.width = bounds.width / 2;
            second.x = bounds.x.saturating_add(first.width);
            second.width -= first.width;
        }
    }
    [first, second]
}

#[derive(Clone, Debug)]
enum Node {
    Leaf(usize),
    Split {
        axis: SplitAxis,
        children: Box<[Node; 2]>,
    },
}

impl Node {
    fn contains(&self, id: usize) -> bool {
        match self {
            Self::Leaf(leaf) => *leaf == id,
            Self::Split { children, .. } => children.iter().any(|child| child.contains(id)),
        }
    }

    fn first_id(&self) -> usize {
        match self {
            Self::Leaf(id) => *id,
            Self::Split { children, .. } => children[0].first_id(),
        }
    }

    fn split(&mut self, id: usize, new_id: usize, axis: SplitAxis) -> bool {
        match self {
            Self::Leaf(leaf) if *leaf == id => {
                *self = Self::Split {
                    axis,
                    children: Box::new([Self::Leaf(id), Self::Leaf(new_id)]),
                };
                true
            }
            Self::Leaf(_) => false,
            Self::Split { children, .. } => children
                .iter_mut()
                .any(|child| child.split(id, new_id, axis)),
        }
    }

    fn close(&mut self, id: usize) -> Option<usize> {
        let Self::Split { children, .. } = self else {
            return None;
        };
        for index in 0..2 {
            if matches!(children[index], Self::Leaf(leaf) if leaf == id) {
                let sibling = 1 - index;
                let focus = children[sibling].first_id();
                *self = std::mem::replace(&mut children[sibling], Self::Leaf(id));
                return Some(focus);
            }
        }
        children.iter_mut().find_map(|child| child.close(id))
    }

    fn visit_rects(&self, bounds: PaneRect, visitor: &mut impl FnMut(usize, PaneRect)) {
        if bounds.is_empty() {
            return;
        }
        match self {
            Self::Leaf(id) => visitor(*id, bounds),
            Self::Split { axis, children } => {
                let halves = split_rect(bounds, *axis);
                children[0].visit_rects(halves[0], visitor);
                children[1].visit_rects(halves[1], visitor);
            }
        }
    }
}

#[derive(Clone, Debug)]
pub struct PaneTree {
    root: Node,
    len: usize,
}

impl PaneTree {
    pub fn new(id: usize) -> Self {
        Self {
            root: Node::Leaf(id),
            len: 1,
        }
    }

    /// Keep `id` first and insert `new_id` second. IDs must be unique.
    pub fn split(&mut self, id: usize, new_id: usize, axis: SplitAxis) -> bool {
        if self.contains(new_id) || !self.root.split(id, new_id, axis) {
            return false;
        }
        self.len += 1;
        true
    }

    /// Close a leaf and expand its sibling; return the sibling's first leaf.
    /// Unknown IDs and closing the last leaf leave the tree unchanged.
    pub fn close(&mut self, id: usize) -> Option<usize> {
        let focus = self.root.close(id)?;
        self.len -= 1;
        Some(focus)
    }

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

    pub fn is_empty(&self) -> bool {
        false
    }

    pub fn contains(&self, id: usize) -> bool {
        self.root.contains(id)
    }

    /// Visit nonempty leaves in tree order, without allocating.
    pub fn visit_rects(&self, bounds: PaneRect, mut visitor: impl FnMut(usize, PaneRect)) {
        self.root.visit_rects(bounds, &mut visitor);
    }

    pub fn neighbor(&self, id: usize, direction: PaneDirection, bounds: PaneRect) -> Option<usize> {
        let mut current = None;
        self.visit_rects(bounds, |leaf, rect| {
            if leaf == id {
                current = Some(rect);
            }
        });
        let current = current?;
        let mut best = None;
        self.visit_rects(bounds, |leaf, rect| {
            if leaf != id {
                consider_neighbor(current, direction, leaf, rect, &mut best);
            }
        });
        best.map(|(_, leaf)| leaf)
    }
}

// Score: prefer perpendicular overlap, then the nearest forward edge,
// perpendicular gap, center alignment, and finally the stable ID.
type NeighborScore = (bool, u32, u32, u32, usize);

fn neighbor_score(
    current: PaneRect,
    direction: PaneDirection,
    id: usize,
    candidate: PaneRect,
) -> Option<NeighborScore> {
    if current.is_empty() || candidate.is_empty() {
        return None;
    }
    let edges = |rect: PaneRect| {
        let x = u32::from(rect.x);
        let y = u32::from(rect.y);
        (x, y, x + u32::from(rect.width), y + u32::from(rect.height))
    };
    let (x, y, right, bottom) = edges(current);
    let (cx, cy, cright, cbottom) = edges(candidate);
    let (distance, start, end, cstart, cend) = match direction {
        PaneDirection::Left if cright <= x => (x - cright, y, bottom, cy, cbottom),
        PaneDirection::Right if cx >= right => (cx - right, y, bottom, cy, cbottom),
        PaneDirection::Up if cbottom <= y => (y - cbottom, x, right, cx, cright),
        PaneDirection::Down if cy >= bottom => (cy - bottom, x, right, cx, cright),
        _ => return None,
    };
    let overlap = start.max(cstart) < end.min(cend);
    let gap = start.max(cstart).saturating_sub(end.min(cend));
    let alignment = (start + end).abs_diff(cstart + cend);
    Some((!overlap, distance, gap, alignment, id))
}

fn consider_neighbor(
    current: PaneRect,
    direction: PaneDirection,
    id: usize,
    candidate: PaneRect,
    best: &mut Option<(NeighborScore, usize)>,
) {
    if let Some(score) = neighbor_score(current, direction, id, candidate)
        && best.as_ref().is_none_or(|(previous, _)| score < *previous)
    {
        *best = Some((score, id));
    }
}

/// Find a forward rectangle, favoring overlap and the closest edge.
/// Empty rectangles are ignored; ties are independent of iterator order.
pub fn directional_neighbor(
    current: PaneRect,
    direction: PaneDirection,
    candidates: impl Iterator<Item = (usize, PaneRect)>,
) -> Option<usize> {
    let mut best = None;
    for (id, candidate) in candidates {
        consider_neighbor(current, direction, id, candidate, &mut best);
    }
    best.map(|(_, id)| id)
}

#[cfg(test)]
#[path = "../tests/unit/panes.rs"]
mod tests;