helix/helix-tui/src/layout.rs

use std::cell::RefCell;
use std::collections::HashMap;

use cassowary::strength::{REQUIRED, WEAK};
use cassowary::WeightedRelation::*;
use cassowary::{Constraint as CassowaryConstraint, Expression, Solver, Variable};

use helix_view::graphics::{Margin, Rect};

#[derive(Debug, Hash, Clone, Copy, PartialEq, Eq)]
pub enum Corner {
    TopLeft,
    TopRight,
    BottomRight,
    BottomLeft,
}

#[derive(Debug, Hash, Clone, PartialEq, Eq)]
pub enum Direction {
    Horizontal,
    Vertical,
}

#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
pub enum Constraint {
    // TODO: enforce range 0 - 100
    Percentage(u16),
    Ratio(u32, u32),
    Length(u16),
    Max(u16),
    Min(u16),
}

impl Constraint {
    pub fn apply(&self, length: u16) -> u16 {
        match *self {
            Constraint::Percentage(p) => length * p / 100,
            Constraint::Ratio(num, den) => {
                let r = num * u32::from(length) / den;
                r as u16
            }
            Constraint::Length(l) => length.min(l),
            Constraint::Max(m) => length.min(m),
            Constraint::Min(m) => length.max(m),
        }
    }
}

#[derive(Debug, Clone, Copy, PartialEq)]
pub enum Alignment {
    Left,
    Center,
    Right,
}

#[derive(Debug, Clone, PartialEq, Eq, Hash)]
pub struct Layout {
    direction: Direction,
    margin: Margin,
    constraints: Vec<Constraint>,
}

thread_local! {
    static LAYOUT_CACHE: RefCell<HashMap<(Rect, Layout), Vec<Rect>>> = RefCell::new(HashMap::new());
}

impl Default for Layout {
    fn default() -> Layout {
        Layout {
            direction: Direction::Vertical,
            margin: Margin::none(),
            constraints: Vec::new(),
        }
    }
}

impl Layout {
    pub fn constraints<C>(mut self, constraints: C) -> Layout
    where
        C: Into<Vec<Constraint>>,
    {
        self.constraints = constraints.into();
        self
    }

    pub fn margin(mut self, margin: u16) -> Layout {
        self.margin = Margin::all(margin);
        self
    }

    pub fn horizontal_margin(mut self, horizontal: u16) -> Layout {
        self.margin.left = horizontal;
        self.margin.right = horizontal;
        self
    }

    pub fn vertical_margin(mut self, vertical: u16) -> Layout {
        self.margin.top = vertical;
        self.margin.bottom = vertical;
        self
    }

    pub fn direction(mut self, direction: Direction) -> Layout {
        self.direction = direction;
        self
    }

    /// Wrapper function around the cassowary-rs solver to be able to split a given
    /// area into smaller ones based on the preferred widths or heights and the direction.
    ///
    /// # Examples
    /// ```
    /// # use helix_tui::layout::{Constraint, Direction, Layout};
    /// # use helix_view::graphics::Rect;
    /// let chunks = Layout::default()
    ///     .direction(Direction::Vertical)
    ///     .constraints([Constraint::Length(5), Constraint::Min(0)].as_ref())
    ///     .split(Rect {
    ///         x: 2,
    ///         y: 2,
    ///         width: 10,
    ///         height: 10,
    ///     });
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect {
    ///             x: 2,
    ///             y: 2,
    ///             width: 10,
    ///             height: 5
    ///         },
    ///         Rect {
    ///             x: 2,
    ///             y: 7,
    ///             width: 10,
    ///             height: 5
    ///         }
    ///     ]
    /// );
    ///
    /// let chunks = Layout::default()
    ///     .direction(Direction::Horizontal)
    ///     .constraints([Constraint::Ratio(1, 3), Constraint::Ratio(2, 3)].as_ref())
    ///     .split(Rect {
    ///         x: 0,
    ///         y: 0,
    ///         width: 9,
    ///         height: 2,
    ///     });
    /// assert_eq!(
    ///     chunks,
    ///     vec![
    ///         Rect {
    ///             x: 0,
    ///             y: 0,
    ///             width: 3,
    ///             height: 2
    ///         },
    ///         Rect {
    ///             x: 3,
    ///             y: 0,
    ///             width: 6,
    ///             height: 2
    ///         }
    ///     ]
    /// );
    /// ```
    pub fn split(&self, area: Rect) -> Vec<Rect> {
        // TODO: Maybe use a fixed size cache ?
        LAYOUT_CACHE.with(|c| {
            c.borrow_mut()
                .entry((area, self.clone()))
                .or_insert_with(|| split(area, self))
                .clone()
        })
    }
}

fn split(area: Rect, layout: &Layout) -> Vec<Rect> {
    let mut solver = Solver::new();
    let mut vars: HashMap<Variable, (usize, usize)> = HashMap::new();
    let elements = layout
        .constraints
        .iter()
        .map(|_| Element::new())
        .collect::<Vec<Element>>();
    let mut results = layout
        .constraints
        .iter()
        .map(|_| Rect::default())
        .collect::<Vec<Rect>>();

    let dest_area = area.inner(&layout.margin);
    for (i, e) in elements.iter().enumerate() {
        vars.insert(e.x, (i, 0));
        vars.insert(e.y, (i, 1));
        vars.insert(e.width, (i, 2));
        vars.insert(e.height, (i, 3));
    }
    let mut ccs: Vec<CassowaryConstraint> =
        Vec::with_capacity(elements.len() * 4 + layout.constraints.len() * 6);
    for elt in &elements {
        ccs.push(elt.width | GE(REQUIRED) | 0f64);
        ccs.push(elt.height | GE(REQUIRED) | 0f64);
        ccs.push(elt.left() | GE(REQUIRED) | f64::from(dest_area.left()));
        ccs.push(elt.top() | GE(REQUIRED) | f64::from(dest_area.top()));
        ccs.push(elt.right() | LE(REQUIRED) | f64::from(dest_area.right()));
        ccs.push(elt.bottom() | LE(REQUIRED) | f64::from(dest_area.bottom()));
    }
    if let Some(first) = elements.first() {
        ccs.push(match layout.direction {
            Direction::Horizontal => first.left() | EQ(REQUIRED) | f64::from(dest_area.left()),
            Direction::Vertical => first.top() | EQ(REQUIRED) | f64::from(dest_area.top()),
        });
    }
    if let Some(last) = elements.last() {
        ccs.push(match layout.direction {
            Direction::Horizontal => last.right() | EQ(REQUIRED) | f64::from(dest_area.right()),
            Direction::Vertical => last.bottom() | EQ(REQUIRED) | f64::from(dest_area.bottom()),
        });
    }
    match layout.direction {
        Direction::Horizontal => {
            for pair in elements.windows(2) {
                ccs.push((pair[0].x + pair[0].width) | EQ(REQUIRED) | pair[1].x);
            }
            for (i, size) in layout.constraints.iter().enumerate() {
                ccs.push(elements[i].y | EQ(REQUIRED) | f64::from(dest_area.y));
                ccs.push(elements[i].height | EQ(REQUIRED) | f64::from(dest_area.height));
                ccs.push(match *size {
                    Constraint::Length(v) => elements[i].width | EQ(WEAK) | f64::from(v),
                    Constraint::Percentage(v) => {
                        elements[i].width | EQ(WEAK) | (f64::from(v * dest_area.width) / 100.0)
                    }
                    Constraint::Ratio(n, d) => {
                        elements[i].width
                            | EQ(WEAK)
                            | (f64::from(dest_area.width) * f64::from(n) / f64::from(d))
                    }
                    Constraint::Min(v) => elements[i].width | GE(WEAK) | f64::from(v),
                    Constraint::Max(v) => elements[i].width | LE(WEAK) | f64::from(v),
                });
            }
        }
        Direction::Vertical => {
            for pair in elements.windows(2) {
                ccs.push((pair[0].y + pair[0].height) | EQ(REQUIRED) | pair[1].y);
            }
            for (i, size) in layout.constraints.iter().enumerate() {
                ccs.push(elements[i].x | EQ(REQUIRED) | f64::from(dest_area.x));
                ccs.push(elements[i].width | EQ(REQUIRED) | f64::from(dest_area.width));
                ccs.push(match *size {
                    Constraint::Length(v) => elements[i].height | EQ(WEAK) | f64::from(v),
                    Constraint::Percentage(v) => {
                        elements[i].height | EQ(WEAK) | (f64::from(v * dest_area.height) / 100.0)
                    }
                    Constraint::Ratio(n, d) => {
                        elements[i].height
                            | EQ(WEAK)
                            | (f64::from(dest_area.height) * f64::from(n) / f64::from(d))
                    }
                    Constraint::Min(v) => elements[i].height | GE(WEAK) | f64::from(v),
                    Constraint::Max(v) => elements[i].height | LE(WEAK) | f64::from(v),
                });
            }
        }
    }
    solver.add_constraints(&ccs).unwrap();
    for &(var, value) in solver.fetch_changes() {
        let (index, attr) = vars[&var];
        let value = if value.is_sign_negative() {
            0
        } else {
            value as u16
        };
        match attr {
            0 => {
                results[index].x = value;
            }
            1 => {
                results[index].y = value;
            }
            2 => {
                results[index].width = value;
            }
            3 => {
                results[index].height = value;
            }
            _ => {}
        }
    }

    // Fix imprecision by extending the last item a bit if necessary
    if let Some(last) = results.last_mut() {
        match layout.direction {
            Direction::Vertical => {
                last.height = dest_area.bottom() - last.y;
            }
            Direction::Horizontal => {
                last.width = dest_area.right() - last.x;
            }
        }
    }
    results
}

/// A container used by the solver inside split
struct Element {
    x: Variable,
    y: Variable,
    width: Variable,
    height: Variable,
}

impl Element {
    fn new() -> Element {
        Element {
            x: Variable::new(),
            y: Variable::new(),
            width: Variable::new(),
            height: Variable::new(),
        }
    }

    fn left(&self) -> Variable {
        self.x
    }

    fn top(&self) -> Variable {
        self.y
    }

    fn right(&self) -> Expression {
        self.x + self.width
    }

    fn bottom(&self) -> Expression {
        self.y + self.height
    }
}

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

    #[test]
    fn test_vertical_split_by_height() {
        let target = Rect {
            x: 2,
            y: 2,
            width: 10,
            height: 10,
        };

        let chunks = Layout::default()
            .direction(Direction::Vertical)
            .constraints(
                [
                    Constraint::Percentage(10),
                    Constraint::Max(5),
                    Constraint::Min(1),
                ]
                .as_ref(),
            )
            .split(target);

        assert_eq!(target.height, chunks.iter().map(|r| r.height).sum::<u16>());
        chunks.windows(2).for_each(|w| assert!(w[0].y <= w[1].y));
    }
}