cumulus/src/app/panes.rs

//! Panes: the screen split into rectangles, the way vim splits windows.
//!
//! The layout is a tree. A leaf is a pane; a split lays its parts side by
//! side or one above the other, each with a share of the room. Splitting a
//! pane halves its room, closing one gives its room to a neighbor, and
//! `Ctrl-w =` evens the shares out.
//!
//! [`Panes`] holds what each pane shows (`T`: a view and a document, in the
//! app) apart from the layout, so the layout logic stays plain and testable.

use std::collections::BTreeMap;

use ratatui::layout::{Position, Rect};

use crate::model::Direction;

/// A pane, while it's open. Ids aren't reused.
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct PaneId(u32);

/// How a split arranges its parts.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Arrange {
    /// Next to each other, left to right (`Ctrl-w v`, `:vsplit`).
    SideBySide,
    /// One above the other (`Ctrl-w s`, `:split`).
    Stacked,
}

/// A part's room when a split is made or evened out. Splitting halves a
/// share, so it starts big enough to be halved many times over.
const FULL_SHARE: u32 = 1 << 20;

#[derive(Debug, Clone, PartialEq, Eq)]
enum Layout {
    Pane(PaneId),
    Split { arrange: Arrange, parts: Vec<Part> },
}

#[derive(Debug, Clone, PartialEq, Eq)]
struct Part {
    layout: Layout,
    /// This part's room, relative to the other parts of its split.
    share: u32,
}

/// The panes, their layout, and which one has the focus.
#[derive(Debug, Clone)]
pub struct Panes<T> {
    layout: Layout,
    /// What each pane shows. Every pane in the layout is here and the other
    /// way round, and `focused` is always one of them: every method keeps it
    /// so.
    contents: BTreeMap<PaneId, T>,
    focused: PaneId,
    next_id: u32,
}

impl<T> Panes<T> {
    /// One pane, showing `first`.
    pub fn new(first: T) -> Panes<T> {
        let id = PaneId(0);
        Panes { layout: Layout::Pane(id), contents: BTreeMap::from([(id, first)]), focused: id, next_id: 1 }
    }

    /// The pane with the focus.
    pub fn focused(&self) -> PaneId {
        self.focused
    }

    /// What the focused pane shows.
    pub fn current(&self) -> &T {
        self.contents.get(&self.focused).expect("the focused pane is always open")
    }

    pub fn current_mut(&mut self) -> &mut T {
        self.contents.get_mut(&self.focused).expect("the focused pane is always open")
    }

    pub fn get(&self, id: PaneId) -> Option<&T> {
        self.contents.get(&id)
    }

    pub fn get_mut(&mut self, id: PaneId) -> Option<&mut T> {
        self.contents.get_mut(&id)
    }

    /// How many panes there are.
    pub fn count(&self) -> usize {
        self.contents.len()
    }

    /// Every pane and what it shows, in no particular order.
    pub fn iter_mut(&mut self) -> impl Iterator<Item = (PaneId, &mut T)> {
        self.contents.iter_mut().map(|(&id, content)| (id, content))
    }

    /// Splits the focused pane in two. The new pane, showing `content`,
    /// comes first (left or above, as in vim) and takes the focus.
    pub fn split(&mut self, arrange: Arrange, content: T) -> PaneId {
        let id = PaneId(self.next_id);
        self.next_id += 1;
        self.contents.insert(id, content);
        let focused = self.focused;
        split_in(&mut self.layout, focused, id, arrange);
        self.focused = id;
        id
    }

    /// Closes the focused pane and gives its room to a neighbor, which takes
    /// the focus. The last pane can't close: `None`.
    pub fn close(&mut self) -> Option<T> {
        if self.contents.len() < 2 {
            return None;
        }
        let closing = self.focused;
        let next = close_in(&mut self.layout, closing)?;
        self.focused = next;
        self.contents.remove(&closing)
    }

    /// Closes every pane but the focused one, and returns what they showed.
    pub fn only(&mut self) -> Vec<T> {
        self.layout = Layout::Pane(self.focused);
        let others: Vec<PaneId> = self.contents.keys().copied().filter(|&id| id != self.focused).collect();
        others.into_iter().filter_map(|id| self.contents.remove(&id)).collect()
    }

    /// Gives every part of every split the same room (`Ctrl-w =`).
    pub fn equalize(&mut self) {
        equalize(&mut self.layout);
    }

    /// Where each pane is in `area`, in reading order: left to right, top to
    /// bottom, the way the layout nests.
    pub fn areas(&self, area: Rect) -> Vec<(PaneId, Rect)> {
        let mut found = Vec::new();
        areas(&self.layout, area, &mut found);
        found
    }

    /// The pane at a position on screen, if there's one.
    pub fn at(&self, area: Rect, position: Position) -> Option<PaneId> {
        self.areas(area).into_iter().find(|(_, rect)| rect.contains(position)).map(|(id, _)| id)
    }

    /// Gives the focus to a pane. Returns false if there's no such pane.
    pub fn focus(&mut self, id: PaneId) -> bool {
        let open = self.contents.contains_key(&id);
        if open {
            self.focused = id;
        }
        open
    }

    /// `Ctrl-w h j k l`: the focus goes to the nearest pane that way, the
    /// one beside the focused pane's top-left corner first. Returns false if
    /// there's none.
    pub fn focus_toward(&mut self, direction: Direction, area: Rect) -> bool {
        let areas = self.areas(area);
        let Some(&(_, from)) = areas.iter().find(|(id, _)| *id == self.focused) else { return false };
        let best = areas
            .iter()
            .filter(|(id, _)| *id != self.focused)
            .filter_map(|&(id, rect)| Some((distance(from, rect, direction)?, id)))
            .min_by_key(|&(key, _)| key);
        match best {
            Some((_, id)) => self.focus(id),
            None => false,
        }
    }

    /// `Ctrl-w w`: the next pane in reading order, back to the first after
    /// the last.
    pub fn focus_next(&mut self) {
        let mut order = Vec::new();
        panes_in(&self.layout, &mut order);
        let here = order.iter().position(|&id| id == self.focused).unwrap_or(0);
        if let Some(&next) = order.get((here + 1) % order.len().max(1)) {
            self.focused = next;
        }
    }
}

/// How far `to` is from `from` going `direction`, for picking the pane to
/// focus: `None` unless it's that way and level with `from`. Nearer wins,
/// then the one beside `from`'s top-left corner, then the top-most or
/// left-most.
fn distance(from: Rect, to: Rect, direction: Direction) -> Option<(u16, bool, u16)> {
    let level_rows = to.top() < from.bottom() && from.top() < to.bottom();
    let level_cols = to.left() < from.right() && from.left() < to.right();
    let (gap, level, beside, along) = match direction {
        Direction::Left => {
            (from.left().checked_sub(to.right())?, level_rows, row_in(from.top(), to), to.top())
        }
        Direction::Right => {
            (to.left().checked_sub(from.right())?, level_rows, row_in(from.top(), to), to.top())
        }
        Direction::Up => {
            (from.top().checked_sub(to.bottom())?, level_cols, col_in(from.left(), to), to.left())
        }
        Direction::Down => {
            (to.top().checked_sub(from.bottom())?, level_cols, col_in(from.left(), to), to.left())
        }
    };
    level.then_some((gap, !beside, along))
}

fn row_in(row: u16, rect: Rect) -> bool {
    rect.top() <= row && row < rect.bottom()
}

fn col_in(col: u16, rect: Rect) -> bool {
    rect.left() <= col && col < rect.right()
}

/// Splits pane `target` in `layout`, putting `new` before it.
fn split_in(layout: &mut Layout, target: PaneId, new: PaneId, arrange: Arrange) -> bool {
    match layout {
        Layout::Pane(id) if *id == target => {
            let old = Part { layout: Layout::Pane(target), share: FULL_SHARE };
            let part = Part { layout: Layout::Pane(new), share: FULL_SHARE };
            *layout = Layout::Split { arrange, parts: vec![part, old] };
            true
        }
        Layout::Pane(_) => false,
        Layout::Split { arrange: own, parts } => {
            let same_way = *own == arrange;
            for index in 0..parts.len() {
                let is_target = parts[index].layout == Layout::Pane(target);
                if is_target && same_way {
                    // Same way as this split: the new pane joins it, and the
                    // two share the old pane's room.
                    let share = parts[index].share;
                    parts[index].share = share - share / 2;
                    let part = Part { layout: Layout::Pane(new), share: (share / 2).max(1) };
                    parts.insert(index, part);
                    return true;
                }
                if split_in(&mut parts[index].layout, target, new, arrange) {
                    return true;
                }
            }
            false
        }
    }
}

/// Takes pane `target` out of `layout`, giving its room to the part before
/// it (or after it, for a first part). Returns the pane that gets the
/// focus.
fn close_in(layout: &mut Layout, target: PaneId) -> Option<PaneId> {
    let Layout::Split { parts, .. } = layout else { return None };
    let index = parts.iter().position(|part| part.layout == Layout::Pane(target));
    let next = match index {
        Some(index) => {
            let removed = parts.remove(index);
            let neighbor = index.saturating_sub(1).min(parts.len().saturating_sub(1));
            let part = parts.get_mut(neighbor)?;
            part.share = part.share.saturating_add(removed.share);
            first_pane(&part.layout)
        }
        None => parts.iter_mut().find_map(|part| close_in(&mut part.layout, target))?,
    };
    // A split left with one part is just that part.
    if parts.len() == 1
        && let Some(only) = parts.pop()
    {
        *layout = only.layout;
    }
    flatten(layout);
    Some(next)
}

/// A split inside a split that arranges the same way joins its parent, so
/// `Ctrl-w v` twice gives three panes side by side, not a nest.
fn flatten(layout: &mut Layout) {
    let Layout::Split { arrange, parts } = layout else { return };
    let mut flat = Vec::with_capacity(parts.len());
    for mut part in std::mem::take(parts) {
        flatten(&mut part.layout);
        match part.layout {
            Layout::Split { arrange: inner, parts: inner_parts } if inner == *arrange => {
                let total: u64 = inner_parts.iter().map(|inner| u64::from(inner.share)).sum::<u64>().max(1);
                for mut inner_part in inner_parts {
                    let share = u64::from(part.share) * u64::from(inner_part.share) / total;
                    inner_part.share = u32::try_from(share).unwrap_or(u32::MAX).max(1);
                    flat.push(inner_part);
                }
            }
            layout => flat.push(Part { layout, ..part }),
        }
    }
    *parts = flat;
}

fn equalize(layout: &mut Layout) {
    if let Layout::Split { parts, .. } = layout {
        for part in parts {
            part.share = FULL_SHARE;
            equalize(&mut part.layout);
        }
    }
}

fn first_pane(layout: &Layout) -> PaneId {
    match layout {
        Layout::Pane(id) => *id,
        Layout::Split { parts, .. } => parts.first().map_or(PaneId(0), |part| first_pane(&part.layout)),
    }
}

fn panes_in(layout: &Layout, found: &mut Vec<PaneId>) {
    match layout {
        Layout::Pane(id) => found.push(*id),
        Layout::Split { parts, .. } => parts.iter().for_each(|part| panes_in(&part.layout, found)),
    }
}

/// Lays `layout` out in `area`: each part gets room in proportion to its
/// share, and the last one takes what rounding leaves.
fn areas(layout: &Layout, area: Rect, found: &mut Vec<(PaneId, Rect)>) {
    let (arrange, parts) = match layout {
        Layout::Pane(id) => return found.push((*id, area)),
        Layout::Split { arrange, parts } => (*arrange, parts),
    };
    let length = match arrange {
        Arrange::SideBySide => area.width,
        Arrange::Stacked => area.height,
    };
    let total: u64 = parts.iter().map(|part| u64::from(part.share)).sum::<u64>().max(1);
    let mut start = 0u16;
    for (index, part) in parts.iter().enumerate() {
        let size = if index + 1 == parts.len() {
            length - start
        } else {
            let size = u64::from(length) * u64::from(part.share) / total;
            u16::try_from(size).unwrap_or(length).min(length - start)
        };
        let rect = match arrange {
            Arrange::SideBySide => Rect::new(area.x + start, area.y, size, area.height),
            Arrange::Stacked => Rect::new(area.x, area.y + start, area.width, size),
        };
        areas(&part.layout, rect, found);
        start += size;
    }
}

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

    const SCREEN: Rect = Rect { x: 0, y: 0, width: 80, height: 20 };

    fn rects<'a>(panes: &Panes<&'a str>) -> Vec<(&'a str, Rect)> {
        panes.areas(SCREEN).into_iter().map(|(id, rect)| (*panes.get(id).unwrap(), rect)).collect()
    }

    #[test]
    fn splits_halve_the_pane_and_the_new_one_comes_first() {
        let mut panes = Panes::new("canvas");
        panes.split(Arrange::SideBySide, "outline");
        assert_eq!(
            rects(&panes),
            [("outline", Rect::new(0, 0, 40, 20)), ("canvas", Rect::new(40, 0, 40, 20))]
        );
        assert_eq!(*panes.current(), "outline", "the new pane has the focus");
        panes.split(Arrange::Stacked, "note");
        assert_eq!(
            rects(&panes),
            [
                ("note", Rect::new(0, 0, 40, 10)),
                ("outline", Rect::new(0, 10, 40, 10)),
                ("canvas", Rect::new(40, 0, 40, 20)),
            ]
        );
        // Side by side again, from the canvas: it joins the top split.
        panes.focus_toward(Direction::Right, SCREEN);
        panes.split(Arrange::SideBySide, "board");
        let widths: Vec<(&str, u16)> =
            rects(&panes).into_iter().map(|(name, rect)| (name, rect.width)).collect();
        assert_eq!(widths, [("note", 40), ("outline", 40), ("board", 20), ("canvas", 20)]);
    }

    #[test]
    fn focus_moves_to_the_pane_that_way() {
        let mut panes = Panes::new("right");
        panes.split(Arrange::SideBySide, "top left");
        panes.split(Arrange::Stacked, "top left 2");
        // top left 2 above top left, both left of right.
        assert_eq!(*panes.current(), "top left 2");
        assert!(panes.focus_toward(Direction::Down, SCREEN));
        assert_eq!(*panes.current(), "top left");
        assert!(panes.focus_toward(Direction::Right, SCREEN));
        assert_eq!(*panes.current(), "right");
        assert!(panes.focus_toward(Direction::Left, SCREEN));
        assert_eq!(*panes.current(), "top left 2", "level with the top: the top one");
        assert!(!panes.focus_toward(Direction::Up, SCREEN), "nothing above");
        panes.focus_next();
        assert_eq!(*panes.current(), "top left");
        panes.focus_next();
        panes.focus_next();
        assert_eq!(*panes.current(), "top left 2", "after the last, the first");
    }

    #[test]
    fn closing_gives_the_room_to_a_neighbor() {
        let mut panes = Panes::new("canvas");
        panes.split(Arrange::SideBySide, "outline");
        panes.split(Arrange::Stacked, "note");
        assert_eq!(panes.close(), Some("note"));
        assert_eq!(*panes.current(), "outline", "the neighbor takes the focus");
        assert_eq!(
            rects(&panes),
            [("outline", Rect::new(0, 0, 40, 20)), ("canvas", Rect::new(40, 0, 40, 20))]
        );
        assert_eq!(panes.close(), Some("outline"));
        assert_eq!(panes.close(), None, "the last pane stays");
        assert_eq!(rects(&panes), [("canvas", SCREEN)]);
    }

    #[test]
    fn only_and_even_sizes() {
        let mut panes = Panes::new("a");
        panes.split(Arrange::SideBySide, "b");
        panes.split(Arrange::SideBySide, "c");
        let widths =
            |panes: &Panes<&str>| rects(panes).into_iter().map(|(_, rect)| rect.width).collect::<Vec<_>>();
        assert_eq!(widths(&panes), [20, 20, 40]);
        panes.equalize();
        assert_eq!(widths(&panes), [26, 26, 28]);
        let mut closed = panes.only();
        closed.sort_unstable();
        assert_eq!(closed, ["a", "b"]);
        assert_eq!(rects(&panes), [("c", SCREEN)]);
        assert_eq!(panes.at(SCREEN, Position::new(70, 3)), Some(panes.focused()));
    }
}