diff -r b4809653f9d6 -r 6e22f4390b7e rust/landgen/src/wavefront_collapse.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/landgen/src/wavefront_collapse.rs	Mon Jan 30 15:50:14 2023 +0100
@@ -0,0 +1,150 @@
+use integral_geometry::Size;
+use land2d::Land2D;
+use std::collections::HashMap;
+
+#[derive(PartialEq, Eq, Hash, Clone, Copy, Debug)]
+enum Tile {
+    Empty,
+    Outside,
+    Numbered(u32),
+}
+
+impl Tile {
+    fn is(&self, i: u32) -> bool {
+        *self == Tile::Numbered(i)
+    }
+
+    fn is_empty(&self) -> bool {
+        match self {
+            Tile::Empty => true,
+            Tile::Outside => true,
+            _ => false,
+        }
+    }
+
+    fn is_empty_or(&self, i: u32) -> bool {
+        match self {
+            Tile::Numbered(n) => *n == i,
+            Tile::Empty => true,
+            _ => false,
+        }
+    }
+
+    fn is_void_or(&self, i: u32) -> bool {
+        match self {
+            Tile::Numbered(n) => *n == i,
+            _ => true,
+        }
+    }
+}
+
+impl Default for Tile {
+    fn default() -> Self {
+        Tile::Outside
+    }
+}
+
+struct CollapseRule {
+    to_tile: Tile,
+    condition: fn([Tile; 4]) -> bool,
+}
+
+#[derive(Default)]
+struct WavefrontCollapse {
+    rules: HashMap<Tile, Vec<CollapseRule>>,
+}
+
+impl WavefrontCollapse {
+    pub fn generate_map<I: Iterator<Item = u32>, F: FnOnce(&mut Land2D<Tile>)>(
+        &mut self,
+        map_size: &Size,
+        seed_fn: F,
+        random_numbers: &mut I,
+    ) -> Land2D<Tile> {
+        let mut land = Land2D::new(map_size, Tile::Empty);
+
+        seed_fn(&mut land);
+
+        while self.collapse_step(&mut land, random_numbers) {}
+
+        land
+    }
+
+    fn add_rule(&mut self, from_tile: Tile, to_tile: Tile, condition: fn([Tile; 4]) -> bool) {
+        let rule = CollapseRule { to_tile, condition };
+        self.rules
+            .entry(from_tile)
+            .or_insert_with(Vec::new)
+            .push(rule);
+    }
+
+    fn collapse_step<I: Iterator<Item = u32>>(
+        &self,
+        land: &mut Land2D<Tile>,
+        random_numbers: &mut I,
+    ) -> bool {
+        let mut collapse_occurred = false;
+        for x in 0..land.width() {
+            for y in 0..land.height() {
+                let current_tile = land.map(y as i32, x as i32, |p| *p);
+
+                if let Some(rules) = self.rules.get(&current_tile) {
+                    for rule in rules
+                        .iter()
+                        .cycle()
+                        .skip(
+                            random_numbers.next().unwrap_or_default() as usize % (rules.len() + 1),
+                        )
+                        .take(rules.len())
+                    {
+                        let neighbors = self.get_neighbors(&land, x, y);
+                        let have_neighbors = neighbors.iter().any(|t| !t.is_empty());
+                        if have_neighbors && (rule.condition)(neighbors) {
+                            land.map(y as i32, x as i32, |p| *p = rule.to_tile);
+                            collapse_occurred = true;
+                            break;
+                        }
+                    }
+                }
+            }
+        }
+
+        collapse_occurred
+    }
+
+    fn get_neighbors(&self, land: &Land2D<Tile>, x: usize, y: usize) -> [Tile; 4] {
+        [
+            land.get(y as i32, x as i32 + 1),
+            land.get(y as i32 + 1, x as i32),
+            land.get(y as i32, x as i32 - 1),
+            land.get(y as i32 - 1, x as i32),
+        ]
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::{CollapseRule, Tile, WavefrontCollapse};
+    use integral_geometry::Size;
+    use land2d::Land2D;
+    use std::collections::HashMap;
+
+    #[test]
+    fn test_wavefront_collapse() {
+        let size = Size::new(4, 4);
+        let mut rnd = [0u32; 64].into_iter();
+        let mut wfc = WavefrontCollapse::default();
+
+        let empty_land = Land2D::new(&size, Tile::Empty);
+        let no_rules_land = wfc.generate_map(&size, |l| {}, &mut rnd);
+
+        assert_eq!(empty_land.raw_pixels(), no_rules_land.raw_pixels());
+
+        wfc.add_rule(Tile::Empty, Tile::Numbered(0), |neighbors| {
+            neighbors.iter().filter(|&n| *n == Tile::Empty).count() >= 2
+        });
+        let ruled_map = wfc.generate_map(&size, |l| {}, &mut rnd);
+
+        assert_eq!(ruled_map.raw_pixels(), empty_land.raw_pixels());
+    }
+}