settled-reach/server/src/perception/shadowcast.rs

//! Shadowcasting field-of-view algorithms
//!
//! This module implements two shadowcasting algorithms for FOV calculation:
//! 1. Albert Ford's Symmetric Shadowcasting (production algorithm)
//! 2. Traditional Recursive Shadowcasting (reference implementation)
//!
//! Coordinate system: Y-down (North = y-1, South = y+1)
//!
//! References:
//! - Symmetric: https://www.albertford.com/shadowcasting/
//! - Traditional: RogueBasin recursive shadowcasting

use std::collections::HashSet;

/// Rational fraction for precise slope calculations without float drift
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
struct Fraction {
    num: i32,
    den: i32,
}

impl Fraction {
    fn new(num: i32, den: i32) -> Self {
        Self { num, den }
    }

    /// Compare this fraction to another: returns true if self < other
    fn less_than(&self, other: &Fraction) -> bool {
        self.num * other.den < other.num * self.den
    }

    /// Compare this fraction to another: returns true if self > other
    fn greater_than(&self, other: &Fraction) -> bool {
        self.num * other.den > other.num * self.den
    }
}

/// Public production API: Visibility map for a single z-level
#[derive(Debug, Clone)]
pub struct VisibilityMap {
    visible: HashSet<(i32, i32)>,
    z_level: i32,
}

impl VisibilityMap {
    /// Check if a tile at (x, y) is visible
    pub fn is_visible(&self, x: i32, y: i32) -> bool {
        self.visible.contains(&(x, y))
    }

    /// Iterate over all visible tiles
    pub fn visible_tiles(&self) -> impl Iterator<Item = (i32, i32)> + '_ {
        self.visible.iter().copied()
    }

    /// Count of visible tiles
    pub fn count(&self) -> usize {
        self.visible.len()
    }

    /// Get the z-level this visibility map represents
    pub fn z_level(&self) -> i32 {
        self.z_level
    }
}

/// Production FOV function - computes field of view using symmetric shadowcasting
///
/// # Arguments
/// * `is_opaque` - Function returning true if tile at (x, y) blocks vision
/// * `origin_x`, `origin_y` - Observer position
/// * `range` - Maximum vision distance (using Chebyshev distance)
/// * `z_level` - Z-level for the visibility map
///
/// # Returns
/// A VisibilityMap containing all visible tiles (including the origin)
pub fn compute_fov(
    is_opaque: impl Fn(i32, i32) -> bool,
    origin_x: i32,
    origin_y: i32,
    range: i32,
    z_level: i32,
) -> VisibilityMap {
    let visible = symmetric_shadowcast(&is_opaque, origin_x, origin_y, range);
    VisibilityMap { visible, z_level }
}

/// Albert Ford's Symmetric Shadowcasting algorithm
///
/// Key property: if tile A sees tile B, then tile B sees tile A (symmetry)
/// A tile is visible if its CENTER is within the unblocked cone
///
/// Uses rational fractions to avoid floating-point drift
pub fn symmetric_shadowcast(
    is_opaque: &impl Fn(i32, i32) -> bool,
    origin_x: i32,
    origin_y: i32,
    range: i32,
) -> HashSet<(i32, i32)> {
    let mut visible = HashSet::new();
    visible.insert((origin_x, origin_y)); // Origin is always visible

    // Process 4 cardinal quadrants
    for &cardinal in &[Cardinal::North, Cardinal::East, Cardinal::South, Cardinal::West] {
        scan_quadrant(&mut visible, is_opaque, origin_x, origin_y, range, cardinal);
    }

    visible
}

/// Cardinal directions for quadrant processing
#[derive(Debug, Clone, Copy)]
enum Cardinal {
    North,
    East,
    South,
    West,
}

impl Cardinal {
    /// Transform row/col in quadrant space to world (x, y)
    fn transform(&self, origin_x: i32, origin_y: i32, row: i32, col: i32) -> (i32, i32) {
        match self {
            Cardinal::North => (origin_x + col, origin_y - row),
            Cardinal::East => (origin_x + row, origin_y + col),
            Cardinal::South => (origin_x + col, origin_y + row),
            Cardinal::West => (origin_x - row, origin_y + col),
        }
    }
}

/// Scan a single quadrant using symmetric shadowcasting
fn scan_quadrant(
    visible: &mut HashSet<(i32, i32)>,
    is_opaque: &impl Fn(i32, i32) -> bool,
    origin_x: i32,
    origin_y: i32,
    range: i32,
    cardinal: Cardinal,
) {
    let first_row = Row {
        depth: 1,
        start_slope: Fraction::new(-1, 1),
        end_slope: Fraction::new(1, 1),
    };

    scan_row(
        visible, is_opaque, origin_x, origin_y, range, cardinal, first_row,
    );
}

#[derive(Debug, Clone, Copy)]
struct Row {
    depth: i32,
    start_slope: Fraction,
    end_slope: Fraction,
}

/// Recursively scan a row in the quadrant
fn scan_row(
    visible: &mut HashSet<(i32, i32)>,
    is_opaque: &impl Fn(i32, i32) -> bool,
    origin_x: i32,
    origin_y: i32,
    range: i32,
    cardinal: Cardinal,
    mut row: Row,
) {
    if row.depth > range {
        return;
    }

    let mut prev_tile_opaque = None;
    let min_col = row.start_slope.num * row.depth / row.start_slope.den;
    let max_col = row.end_slope.num * row.depth / row.end_slope.den;

    for col in min_col..=max_col {
        let (x, y) = cardinal.transform(origin_x, origin_y, row.depth, col);

        // Check Chebyshev distance (max of absolute differences)
        let dx = (x - origin_x).abs();
        let dy = (y - origin_y).abs();
        if dx.max(dy) > range {
            continue;
        }

        // Check if tile center is within the view cone
        let tile_center_slope = Fraction::new(2 * col, 2 * row.depth);
        if is_visible_from_center(&row, &tile_center_slope) {
            visible.insert((x, y));
        }

        let is_opaque_tile = is_opaque(x, y);

        // Handle wall-to-floor transition
        if prev_tile_opaque == Some(true) && !is_opaque_tile {
            // Exiting shadow - update start slope for this row
            row.start_slope = Fraction::new(2 * col - 1, 2 * row.depth);
        }

        // Handle floor-to-wall transition
        if prev_tile_opaque == Some(false) && is_opaque_tile {
            // Entering shadow - recursively scan next row with narrowed end slope
            let mut next_row = row;
            next_row.depth = row.depth + 1;
            next_row.end_slope = Fraction::new(2 * col - 1, 2 * row.depth);
            scan_row(
                visible, is_opaque, origin_x, origin_y, range, cardinal, next_row,
            );
        }

        prev_tile_opaque = Some(is_opaque_tile);
    }

    // Continue to next row if the last tile wasn't opaque
    if prev_tile_opaque != Some(true) {
        let mut next_row = row;
        next_row.depth = row.depth + 1;
        scan_row(
            visible, is_opaque, origin_x, origin_y, range, cardinal, next_row,
        );
    }
}

/// Check if a tile center is visible given the current row's slope bounds
fn is_visible_from_center(row: &Row, tile_center_slope: &Fraction) -> bool {
    !tile_center_slope.less_than(&row.start_slope)
        && !tile_center_slope.greater_than(&row.end_slope)
}

/// Traditional recursive shadowcasting algorithm (8 octants, float slopes)
///
/// This is a simpler reference implementation using iterative distance-based scanning
pub fn recursive_shadowcast(
    is_opaque: &impl Fn(i32, i32) -> bool,
    origin_x: i32,
    origin_y: i32,
    range: i32,
) -> HashSet<(i32, i32)> {
    let mut visible = HashSet::new();
    visible.insert((origin_x, origin_y));

    // Simple approach: scan all tiles in range, use basic line-of-sight check
    for dx in -range..=range {
        for dy in -range..=range {
            let x = origin_x + dx;
            let y = origin_y + dy;

            // Skip origin (already added)
            if dx == 0 && dy == 0 {
                continue;
            }

            // Check Chebyshev distance (max of abs values)
            if dx.abs().max(dy.abs()) > range {
                continue;
            }

            // Check line of sight using simple raycast
            if has_line_of_sight(is_opaque, origin_x, origin_y, x, y) {
                visible.insert((x, y));
            }
        }
    }

    visible
}

/// Simple line-of-sight check using DDA-style line traversal
/// Returns true if target is visible (either no obstacles, or target itself is first obstacle)
fn has_line_of_sight(
    is_opaque: &impl Fn(i32, i32) -> bool,
    x0: i32,
    y0: i32,
    x1: i32,
    y1: i32,
) -> bool {
    let dx = (x1 - x0).abs();
    let dy = (y1 - y0).abs();
    let sx = if x0 < x1 { 1 } else { -1 };
    let sy = if y0 < y1 { 1 } else { -1 };
    let mut err = dx - dy;

    let mut x = x0;
    let mut y = y0;

    loop {
        // Check if we hit a blocking tile BEFORE reaching target
        if (x != x0 || y != y0) && (x != x1 || y != y1) {
            if is_opaque(x, y) {
                // Hit an obstacle before reaching target - blocked
                return false;
            }
        }

        // If we reach the target, we can see it
        if x == x1 && y == y1 {
            return true;
        }

        let e2 = 2 * err;
        if e2 > -dy {
            err -= dy;
            x += sx;
        }
        if e2 < dx {
            err += dx;
            y += sy;
        }
    }
}

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

    /// Helper: create a simple wall map from a grid
    fn make_wall_fn(walls: HashSet<(i32, i32)>) -> impl Fn(i32, i32) -> bool {
        move |x, y| walls.contains(&(x, y))
    }

    #[test]
    fn test_open_field_symmetric() {
        // Open field: all tiles within range should be visible
        let no_walls = HashSet::new();
        let is_opaque = make_wall_fn(no_walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 5);

        // Should see at least the cross pattern + diagonals
        assert!(visible.contains(&(0, 0))); // origin
        assert!(visible.contains(&(1, 0))); // east
        assert!(visible.contains(&(0, 1))); // south
        assert!(visible.contains(&(-1, 0))); // west
        assert!(visible.contains(&(0, -1))); // north
        assert!(visible.contains(&(1, 1))); // SE diagonal
        assert!(visible.len() > 20); // Reasonable coverage
    }

    #[test]
    fn test_open_field_recursive() {
        let no_walls = HashSet::new();
        let is_opaque = make_wall_fn(no_walls);

        let visible = recursive_shadowcast(&is_opaque, 0, 0, 5);

        assert!(visible.contains(&(0, 0)));
        assert!(visible.contains(&(1, 0)));
        assert!(visible.contains(&(0, 1)));
        assert!(visible.len() > 20);
    }

    #[test]
    fn test_single_wall_blocks_vision() {
        // Wall at (1, 0) should block vision beyond it
        let mut walls = HashSet::new();
        walls.insert((1, 0));
        let is_opaque = make_wall_fn(walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 5);

        // Should see the wall
        assert!(visible.contains(&(1, 0)));
        // Should NOT see directly behind it
        assert!(!visible.contains(&(2, 0)));
    }

    #[test]
    fn test_origin_always_visible() {
        let mut walls = HashSet::new();
        // Even if origin is "opaque" it should be visible
        walls.insert((0, 0));
        let is_opaque = make_wall_fn(walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 5);
        assert!(visible.contains(&(0, 0)));
    }

    #[test]
    fn test_range_cutoff() {
        let no_walls = HashSet::new();
        let is_opaque = make_wall_fn(no_walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 3);

        // Should see (3, 0) but not (4, 0)
        assert!(visible.contains(&(3, 0)));
        assert!(!visible.contains(&(4, 0)));
    }

    #[test]
    fn test_corner_peek() {
        // Wall at (1, 1), can we peek around corners?
        let mut walls = HashSet::new();
        walls.insert((1, 1));
        let is_opaque = make_wall_fn(walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 5);

        // Should see the wall
        assert!(visible.contains(&(1, 1)));
        // Should still see adjacent tiles like (2, 1) and (1, 2)
        assert!(visible.contains(&(2, 1)));
        assert!(visible.contains(&(1, 2)));
    }

    #[test]
    fn test_pillar_casts_shadow() {
        // Pillar at (2, 0) should cast shadow
        let mut walls = HashSet::new();
        walls.insert((2, 0));
        let is_opaque = make_wall_fn(walls);

        let visible = symmetric_shadowcast(&is_opaque, 0, 0, 10);

        // See the pillar
        assert!(visible.contains(&(2, 0)));
        // Should NOT see far behind it
        assert!(!visible.contains(&(8, 0)));
    }

    #[test]
    fn test_production_api() {
        let no_walls = HashSet::new();
        let is_opaque = make_wall_fn(no_walls);

        let vis_map = compute_fov(is_opaque, 5, 5, 10, 0);

        assert_eq!(vis_map.z_level(), 0);
        assert!(vis_map.is_visible(5, 5));
        assert!(vis_map.is_visible(6, 5));
        assert!(vis_map.count() > 50);

        let tiles: Vec<_> = vis_map.visible_tiles().collect();
        assert!(!tiles.is_empty());
    }
}