//! The line layer: where lines and box borders run on screen, cell by cell.
//! Each cell remembers the arms that meet in it, so where lines cross or join
//! they get the right character (`├ ┬ ┼ ┨`) instead of overwriting each other.

use ratatui::layout::Size;
use ratatui::style::Color;

use super::camera::{ScreenPos, ScreenRect};
use crate::model::Direction;
use crate::model::{Border, LinkKey, NodeId, Stroke};
use crate::ui::glyphs::{Arms, Dash, Glyphs, Weight};

/// What a line cell belongs to, so a click on a line knows what it hit.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum LineOwner {
    /// The border of a node's box.
    Border(NodeId),
    /// The branch line from a node's parent down to the node.
    Branch(NodeId),
    /// A cross-link.
    Link(LinkKey),
    /// The line connect mode shows from a node to the cursor's, before
    /// Enter makes it.
    Preview,
}

/// The color and owner given to the cells a line or border passes through.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Paint {
    pub color: Color,
    pub owner: LineOwner,
}

/// How the straight runs through a cell are dashed. Lines that disagree
/// make it solid.
#[derive(Debug, Clone, Copy, Default, PartialEq, Eq)]
enum DashMix {
    #[default]
    Unset,
    Solid,
    Dashed(Dash),
}

#[derive(Debug, Clone, Copy, Default)]
struct Cell {
    arms: Arms,
    dash: DashMix,
    rounded: bool,
    paint: Option<Paint>,
}

/// The lines and borders of one frame, over a view of a given size. Cells
/// outside the view are dropped as they're added.
#[derive(Debug, Clone, Default)]
pub struct LineLayer {
    width: i32,
    height: i32,
    cells: Vec<Cell>,
}

impl LineLayer {
    /// An empty layer for a view of this size.
    pub fn new(size: Size) -> LineLayer {
        let (width, height) = (i32::from(size.width), i32::from(size.height));
        LineLayer { width, height, cells: vec![Cell::default(); size.area() as usize] }
    }

    /// Adds a box's border. `shown` tells which of its cells are visible:
    /// another box drawn on top can hide part of it.
    pub fn add_border(
        &mut self,
        rect: ScreenRect,
        border: Border,
        paint: Paint,
        shown: impl Fn(ScreenPos) -> bool,
    ) {
        let Some(stroke) = border.stroke() else { return };
        if rect.width < 2 || rect.height < 2 {
            return;
        }
        let (left, top, right, bottom) = (rect.left(), rect.top(), rect.right() - 1, rect.bottom() - 1);
        let corners = [
            (ScreenPos::new(left, top), Direction::Right, Direction::Down),
            (ScreenPos::new(right, top), Direction::Left, Direction::Down),
            (ScreenPos::new(left, bottom), Direction::Right, Direction::Up),
            (ScreenPos::new(right, bottom), Direction::Left, Direction::Up),
        ];
        for (at, one, other) in corners {
            if shown(at) {
                self.add_arm(at, one, stroke, paint);
                self.add_arm(at, other, stroke, paint);
                if border == Border::Rounded {
                    self.round(at);
                }
            }
        }
        for col in left + 1..right {
            for row in [top, bottom] {
                let at = ScreenPos::new(col, row);
                if shown(at) {
                    self.add_arm(at, Direction::Left, stroke, paint);
                    self.add_arm(at, Direction::Right, stroke, paint);
                }
            }
        }
        for row in top + 1..bottom {
            for col in [left, right] {
                let at = ScreenPos::new(col, row);
                if shown(at) {
                    self.add_arm(at, Direction::Up, stroke, paint);
                    self.add_arm(at, Direction::Down, stroke, paint);
                }
            }
        }
    }

    /// Adds a line through `points`, joined by straight runs (each pair of
    /// points shares a row or a column). Cells for which `skip` is true are
    /// left out: that's where the line passes under a box.
    pub fn add_path(
        &mut self,
        points: &[ScreenPos],
        stroke: Stroke,
        paint: Paint,
        skip: impl Fn(ScreenPos) -> bool,
    ) {
        for pair in points.windows(2) {
            let [from, to] = [pair[0], pair[1]];
            let Some(direction) = direction_between(from, to) else { continue };
            let mut at = from;
            loop {
                if !skip(at) {
                    if at != to {
                        self.add_arm(at, direction, stroke, paint);
                    }
                    if at != from {
                        self.add_arm(at, direction.opposite(), stroke, paint);
                    }
                }
                if at == to {
                    break;
                }
                at = step(at, direction);
            }
        }
    }

    /// Adds one arm to a cell, as where a line meets a border. A border's
    /// color and owner stay: the line only adds its arm.
    pub fn add_arm(&mut self, at: ScreenPos, direction: Direction, stroke: Stroke, paint: Paint) {
        let Some(cell) = self.cell_mut(at) else { return };
        cell.arms.add(direction, Weight::of(stroke));
        let dash = match stroke {
            Stroke::Dashed => DashMix::Dashed(Dash::Dashed),
            Stroke::Dotted => DashMix::Dashed(Dash::Dotted),
            Stroke::Solid | Stroke::Thick | Stroke::Double => DashMix::Solid,
        };
        cell.dash = match cell.dash {
            DashMix::Unset => dash,
            same if same == dash => same,
            _ => DashMix::Solid,
        };
        let is_border = matches!(cell.paint, Some(Paint { owner: LineOwner::Border(_), .. }));
        if !is_border {
            cell.paint = Some(paint);
        }
    }

    /// Draws the light corner on a cell rounded, as a rounded box's corners
    /// are.
    pub fn round(&mut self, at: ScreenPos) {
        if let Some(cell) = self.cell_mut(at) {
            cell.rounded = true;
        }
    }

    /// Every cell a line runs through, with its character and color.
    pub fn cells(&self, glyphs: Glyphs) -> impl Iterator<Item = (ScreenPos, char, Color)> + '_ {
        self.cells.iter().enumerate().filter(|(_, cell)| !cell.arms.is_empty()).map(move |(index, cell)| {
            let index = index as i32;
            let at = ScreenPos::new(index % self.width, index / self.width);
            let dash = match cell.dash {
                DashMix::Dashed(dash) => Some(dash),
                DashMix::Unset | DashMix::Solid => None,
            };
            let color = cell.paint.map_or(Color::Reset, |paint| paint.color);
            (at, glyphs.line(cell.arms, dash, cell.rounded), color)
        })
    }

    /// What the line at a cell belongs to, if a line runs there.
    pub fn owner(&self, at: ScreenPos) -> Option<LineOwner> {
        let cell = self.cell(at)?;
        if cell.arms.is_empty() { None } else { cell.paint.map(|paint| paint.owner) }
    }

    /// Whether any line runs through a cell.
    pub fn is_used(&self, at: ScreenPos) -> bool {
        self.cell(at).is_some_and(|cell| !cell.arms.is_empty())
    }

    fn index(&self, at: ScreenPos) -> Option<usize> {
        let inside = (0..self.width).contains(&at.col) && (0..self.height).contains(&at.row);
        inside.then(|| (at.row * self.width + at.col) as usize)
    }

    fn cell(&self, at: ScreenPos) -> Option<&Cell> {
        self.index(at).and_then(|index| self.cells.get(index))
    }

    fn cell_mut(&mut self, at: ScreenPos) -> Option<&mut Cell> {
        self.index(at).and_then(|index| self.cells.get_mut(index))
    }
}

/// The direction from one cell to another on the same row or column.
pub fn direction_between(from: ScreenPos, to: ScreenPos) -> Option<Direction> {
    match (to.col - from.col, to.row - from.row) {
        (0, 0) => None,
        (0, rows) if rows < 0 => Some(Direction::Up),
        (0, _) => Some(Direction::Down),
        (cols, 0) if cols < 0 => Some(Direction::Left),
        (_, 0) => Some(Direction::Right),
        _ => None,
    }
}

/// The next cell in a direction.
pub fn step(at: ScreenPos, direction: Direction) -> ScreenPos {
    match direction {
        Direction::Up => ScreenPos::new(at.col, at.row - 1),
        Direction::Right => ScreenPos::new(at.col + 1, at.row),
        Direction::Down => ScreenPos::new(at.col, at.row + 1),
        Direction::Left => ScreenPos::new(at.col - 1, at.row),
    }
}

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

    fn text(layer: &LineLayer, size: Size) -> Vec<String> {
        let mut rows = vec![vec![' '; size.width as usize]; size.height as usize];
        for (at, glyph, _) in layer.cells(Glyphs::Unicode) {
            rows[at.row as usize][at.col as usize] = glyph;
        }
        rows.into_iter().map(|row| row.into_iter().collect()).collect()
    }

    #[test]
    fn lines_join_borders_and_each_other() {
        let size = Size::new(12, 5);
        let mut layer = LineLayer::new(size);
        let paint = Paint { color: Color::Reset, owner: LineOwner::Border(NodeId::new(1)) };
        let box_rect = ScreenRect::new(0, 0, 5, 5);
        layer.add_border(box_rect, Border::Thick, paint, |_| true);
        let line = Paint { color: Color::Reset, owner: LineOwner::Branch(NodeId::new(2)) };
        let path = [ScreenPos::new(4, 2), ScreenPos::new(7, 2), ScreenPos::new(7, 4), ScreenPos::new(11, 4)];
        layer.add_path(&path, Stroke::Solid, line, |at| box_rect.contains(at));
        layer.add_arm(ScreenPos::new(4, 2), Direction::Right, Stroke::Solid, line);
        layer.add_path(&[ScreenPos::new(9, 0), ScreenPos::new(9, 4)], Stroke::Dashed, line, |_| false);
        assert_eq!(
            text(&layer, size),
            vec![
                "┏━━━┓    ╷  ", //
                "┃   ┃    ╎  ",
                "┃   ┠──┐ ╎  ",
                "┃   ┃  │ ╎  ",
                "┗━━━┛  └─┴─╴",
            ]
        );
        assert_eq!(
            layer.owner(ScreenPos::new(4, 2)),
            Some(LineOwner::Border(NodeId::new(1))),
            "borders keep theirs"
        );
        assert_eq!(layer.owner(ScreenPos::new(5, 2)), Some(LineOwner::Branch(NodeId::new(2))));
    }
}