diff --git a/crates/citygen/src/generate/mod.rs b/crates/citygen/src/generate/mod.rs
new file mode 100644
index 0000000..03ebe6b
--- /dev/null
+++ b/crates/citygen/src/generate/mod.rs
@@ -0,0 +1,383 @@
+//! The resumable generator: eleven chunked stages over a growing `City`.
+
+pub mod terrain;
+
+use crate::flavor::FlavorPack;
+use crate::geom::{Grid, Pos, Rect, Size};
+use crate::ids::{BuildingId, DistrictId};
+use crate::params::{GenError, GenParams};
+use crate::rng::stage_rng;
+use crate::tile::{Feature, Terrain, Tile};
+use crate::world::{BuildingKind, City, HeightClass, StratumKind};
+use rand_chacha::ChaCha8Rng;
+
+/// Generation stages, in execution order.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum Stage {
+    Terrain,
+    Districts,
+    Arterials,
+    Streets,
+    Lots,
+    Verticals,
+    Interiors,
+    Connectivity,
+    Dressing,
+    Naming,
+    Net,
+}
+
+impl Stage {
+    pub const ALL: [Stage; 11] = [
+        Stage::Terrain,
+        Stage::Districts,
+        Stage::Arterials,
+        Stage::Streets,
+        Stage::Lots,
+        Stage::Verticals,
+        Stage::Interiors,
+        Stage::Connectivity,
+        Stage::Dressing,
+        Stage::Naming,
+        Stage::Net,
+    ];
+
+    pub fn label(self) -> &'static str {
+        match self {
+            Stage::Terrain => "Shaping terrain",
+            Stage::Districts => "Zoning districts",
+            Stage::Arterials => "Routing arterials",
+            Stage::Streets => "Carving streets",
+            Stage::Lots => "Raising buildings",
+            Stage::Verticals => "Stacking strata",
+            Stage::Interiors => "Fitting interiors",
+            Stage::Connectivity => "Linking levels",
+            Stage::Dressing => "Hanging neon",
+            Stage::Naming => "Naming places",
+            Stage::Net => "Wiring the Net",
+        }
+    }
+
+    pub fn index(self) -> u8 {
+        Stage::ALL.iter().position(|s| *s == self).unwrap_or(0) as u8
+    }
+}
+
+/// What `Generator::step` reports.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum Progress {
+    Running { stage: Stage, pct: f32 },
+    Done,
+}
+
+/// Output of one `StageRunner::step`.
+pub(crate) enum StepOut {
+    Continue(f32),
+    Finished,
+}
+
+/// One stage, run as many small steps.
+pub(crate) trait StageRunner {
+    fn begin(&mut self, ctx: &mut GenCtx) -> Result<(), GenError>;
+    fn step(&mut self, ctx: &mut GenCtx) -> Result<StepOut, GenError>;
+}
+
+// Scratch types are populated by stages that land in later tasks; the allows go when they do.
+/// A rectangular block between streets, owned by one district.
+#[allow(dead_code)]
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct Block {
+    pub rect: Rect,
+    pub district: DistrictId,
+}
+
+#[allow(dead_code)]
+#[derive(Clone, Copy, Debug)]
+pub(crate) enum LotUse {
+    Building {
+        kind: BuildingKind,
+        class: HeightClass,
+        basement: bool,
+    },
+    Plaza,
+    Parking,
+}
+
+#[allow(dead_code)]
+#[derive(Clone, Copy, Debug)]
+pub(crate) struct Lot {
+    pub rect: Rect,
+    pub district: DistrictId,
+    pub use_: LotUse,
+}
+
+/// Intermediate state shared between stages; never serialized.
+#[allow(dead_code)]
+pub(crate) struct Scratch {
+    pub land: Grid<bool>,
+    pub wealth: Grid<f32>,
+    pub decay: Grid<f32>,
+    pub arterial: Grid<bool>,
+    pub road: Grid<bool>,
+    pub blocks: Vec<Block>,
+    pub lots: Vec<Lot>,
+    pub lobbies: Vec<(BuildingId, Pos)>,
+    pub water_edge: Option<crate::geom::Dir>,
+}
+
+impl Scratch {
+    fn new(size: Size) -> Self {
+        Scratch {
+            land: Grid::filled(size, true),
+            wealth: Grid::filled(size, 0.5),
+            decay: Grid::filled(size, 0.5),
+            arterial: Grid::filled(size, false),
+            road: Grid::filled(size, false),
+            blocks: Vec::new(),
+            lots: Vec::new(),
+            lobbies: Vec::new(),
+            water_edge: None,
+        }
+    }
+}
+
+/// Everything a stage can see and mutate.
+#[allow(dead_code)]
+pub(crate) struct GenCtx {
+    pub city: City,
+    pub flavor: FlavorPack,
+    pub seed: u64,
+    pub scratch: Scratch,
+}
+
+#[allow(dead_code)]
+impl GenCtx {
+    pub fn rng(&self, stage: Stage) -> ChaCha8Rng {
+        stage_rng(self.seed, stage.index())
+    }
+
+    pub fn size(&self) -> Size {
+        self.city.size
+    }
+
+    pub fn params(&self) -> &GenParams {
+        &self.city.params
+    }
+
+    pub fn terrain(&self, k: StratumKind, p: Pos) -> Terrain {
+        self.city
+            .tile(k, p)
+            .map(|t| t.terrain)
+            .unwrap_or(Terrain::Void)
+    }
+
+    pub fn set_terrain(&mut self, k: StratumKind, p: Pos, t: Terrain) -> bool {
+        match self.city.tile_mut(k, p) {
+            Some(tile) => {
+                tile.terrain = t;
+                true
+            }
+            None => false,
+        }
+    }
+
+    pub fn set_feature(&mut self, k: StratumKind, p: Pos, f: Option<Feature>) -> bool {
+        match self.city.tile_mut(k, p) {
+            Some(tile) => {
+                tile.feature = f;
+                true
+            }
+            None => false,
+        }
+    }
+
+    pub fn tile(&self, k: StratumKind, p: Pos) -> Option<&Tile> {
+        self.city.tile(k, p)
+    }
+
+    pub fn in_district(&self, p: Pos, id: DistrictId) -> bool {
+        self.city
+            .tile(StratumKind::Street, p)
+            .is_some_and(|t| t.district == Some(id))
+    }
+
+    pub fn is_land(&self, p: Pos) -> bool {
+        self.scratch.land.get(p).copied().unwrap_or(false)
+    }
+}
+
+/// A stage that does nothing; placeholder until the real stage lands.
+struct NoopStage;
+
+impl StageRunner for NoopStage {
+    fn begin(&mut self, _ctx: &mut GenCtx) -> Result<(), GenError> {
+        Ok(())
+    }
+    fn step(&mut self, _ctx: &mut GenCtx) -> Result<StepOut, GenError> {
+        Ok(StepOut::Finished)
+    }
+}
+
+pub(crate) fn runner_for(stage: Stage) -> Box<dyn StageRunner> {
+    match stage {
+        Stage::Terrain => Box::new(terrain::TerrainStage::default()),
+        _ => Box::new(NoopStage),
+    }
+}
+
+/// Drives the stages one small step at a time so a UI can draw between steps.
+pub struct Generator {
+    ctx: GenCtx,
+    stage_idx: usize,
+    runner: Option<Box<dyn StageRunner>>,
+    error: Option<GenError>,
+    done: bool,
+}
+
+impl Generator {
+    pub fn new(params: GenParams) -> Result<Self, GenError> {
+        params.validate()?;
+        let flavor = FlavorPack::load(&params.flavor)?;
+        let city = City::empty(params.clone());
+        let scratch = Scratch::new(city.size);
+        Ok(Generator {
+            ctx: GenCtx {
+                seed: params.seed,
+                city,
+                flavor,
+                scratch,
+            },
+            stage_idx: 0,
+            runner: None,
+            error: None,
+            done: false,
+        })
+    }
+
+    /// Runs one small unit of work.
+    pub fn step(&mut self) -> Progress {
+        if self.done || self.error.is_some() {
+            return Progress::Done;
+        }
+        let stage = Stage::ALL[self.stage_idx];
+        if self.runner.is_none() {
+            let mut r = runner_for(stage);
+            if let Err(e) = r.begin(&mut self.ctx) {
+                self.error = Some(e);
+                return Progress::Done;
+            }
+            self.runner = Some(r);
+        }
+        let out = self
+            .runner
+            .as_mut()
+            .expect("runner set above")
+            .step(&mut self.ctx);
+        match out {
+            Ok(StepOut::Continue(pct)) => Progress::Running {
+                stage,
+                pct: pct.clamp(0.0, 1.0),
+            },
+            Ok(StepOut::Finished) => {
+                self.runner = None;
+                self.stage_idx += 1;
+                if self.stage_idx == Stage::ALL.len() {
+                    self.done = true;
+                    Progress::Done
+                } else {
+                    Progress::Running {
+                        stage: Stage::ALL[self.stage_idx],
+                        pct: 0.0,
+                    }
+                }
+            }
+            Err(e) => {
+                self.error = Some(e);
+                Progress::Done
+            }
+        }
+    }
+
+    /// The city so far; partial while running.
+    pub fn city(&self) -> &City {
+        &self.ctx.city
+    }
+
+    pub fn error(&self) -> Option<&GenError> {
+        self.error.as_ref()
+    }
+
+    pub fn is_done(&self) -> bool {
+        self.done || self.error.is_some()
+    }
+
+    /// Validates and returns the finished city.
+    pub fn finish(self) -> Result<City, GenError> {
+        if let Some(e) = self.error {
+            return Err(e);
+        }
+        self.ctx.city.validate().map_err(GenError::Invalid)?;
+        Ok(self.ctx.city)
+    }
+
+    /// Runs every stage to completion.
+    pub fn run(params: GenParams) -> Result<City, GenError> {
+        let mut g = Generator::new(params)?;
+        while let Progress::Running { .. } = g.step() {}
+        g.finish()
+    }
+
+    /// Test helper: run until `stage` has finished and hand back the context.
+    #[cfg(test)]
+    #[allow(dead_code)]
+    pub(crate) fn run_until(params: GenParams, stage: Stage) -> GenCtx {
+        let mut g = Generator::new(params).expect("valid params");
+        loop {
+            match g.step() {
+                Progress::Running { stage: s, .. } if s.index() > stage.index() => break,
+                Progress::Running { .. } => {}
+                Progress::Done => break,
+            }
+        }
+        assert!(g.error.is_none(), "{:?}", g.error);
+        g.ctx
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::params::{GenParams, SizePreset};
+
+    #[test]
+    fn generator_walks_all_stages_in_order() {
+        let mut g = Generator::new(GenParams {
+            seed: 1,
+            size: SizePreset::Custom { w: 80, h: 40 },
+            districts: 4,
+            ..Default::default()
+        })
+        .unwrap();
+        let mut seen = vec![];
+        while let Progress::Running { stage, pct } = g.step() {
+            assert!((0.0..=1.0).contains(&pct));
+            if seen.last() != Some(&stage) {
+                seen.push(stage);
+            }
+        }
+        assert_eq!(seen, Stage::ALL.to_vec());
+        assert!(g.error().is_none());
+        assert!(g.finish().is_ok());
+    }
+
+    #[test]
+    fn generator_rejects_invalid_params() {
+        assert!(
+            Generator::new(GenParams {
+                districts: 99,
+                ..Default::default()
+            })
+            .is_err()
+        );
+    }
+}
diff --git a/crates/citygen/src/generate/terrain.rs b/crates/citygen/src/generate/terrain.rs
new file mode 100644
index 0000000..280a752
--- /dev/null
+++ b/crates/citygen/src/generate/terrain.rs
@@ -0,0 +1,135 @@
+//! Stage 1: water and land.
+
+use super::{GenCtx, Stage, StageRunner, StepOut};
+use crate::geom::{Dir, Pos};
+use crate::params::{GenError, WaterMode};
+use crate::tile::Terrain;
+use crate::world::StratumKind;
+use noise::{NoiseFn, OpenSimplex};
+use rand::Rng;
+use rand::seq::IndexedRandom;
+
+#[derive(Default)]
+pub(crate) struct TerrainStage {
+    row: i32,
+    noise: Option<OpenSimplex>,
+    edge: Option<Dir>,
+    river_axis_horizontal: bool,
+    river_offset: f64,
+}
+
+impl TerrainStage {
+    fn is_water(&self, ctx: &GenCtx, p: Pos) -> bool {
+        let size = ctx.size();
+        let (w, h) = (size.w as f64, size.h as f64);
+        let noise = self.noise.as_ref().expect("begin sets noise");
+        match ctx.params().water {
+            WaterMode::None => false,
+            WaterMode::Bay => {
+                let edge = self.edge.expect("begin sets edge");
+                // `t` runs along the edge, `d` is the distance from it.
+                let (t, d, extent) = match edge {
+                    Dir::N => (p.x as f64, p.y as f64, h),
+                    Dir::S => (p.x as f64, h - 1.0 - p.y as f64, h),
+                    Dir::W => (p.y as f64, p.x as f64, w),
+                    _ => (p.y as f64, w - 1.0 - p.x as f64, w),
+                };
+                let depth = 0.15 * extent + noise.get([t * 0.04, 7.3]) * 0.06 * extent;
+                d < depth
+            }
+            WaterMode::River => {
+                let (t, c, extent) = if self.river_axis_horizontal {
+                    (p.x as f64, p.y as f64, h)
+                } else {
+                    (p.y as f64, p.x as f64, w)
+                };
+                let centre =
+                    extent / 2.0 + self.river_offset + noise.get([t * 0.02, 3.1]) * 0.25 * extent;
+                let half = 3.0 + (noise.get([t * 0.05, 9.7]) + 1.0);
+                (c - centre).abs() < half
+            }
+        }
+    }
+}
+
+impl StageRunner for TerrainStage {
+    fn begin(&mut self, ctx: &mut GenCtx) -> Result<(), GenError> {
+        let mut rng = ctx.rng(Stage::Terrain);
+        self.noise = Some(OpenSimplex::new(ctx.seed as u32));
+        self.edge = Dir::CARDINAL.choose(&mut rng).copied();
+        self.river_axis_horizontal = rng.random_bool(0.5);
+        self.river_offset = rng.random_range(-0.1..0.1) * ctx.size().h as f64;
+        ctx.scratch.water_edge = self.edge;
+        self.row = 0;
+        Ok(())
+    }
+
+    fn step(&mut self, ctx: &mut GenCtx) -> Result<StepOut, GenError> {
+        let size = ctx.size();
+        if self.row >= size.h as i32 {
+            return Ok(StepOut::Finished);
+        }
+        let y = self.row;
+        for x in 0..size.w as i32 {
+            let p = Pos::new(x, y);
+            if self.is_water(ctx, p) {
+                ctx.scratch.land.set(p, false);
+                ctx.set_terrain(StratumKind::Street, p, Terrain::Water);
+            }
+        }
+        self.row += 1;
+        Ok(StepOut::Continue(self.row as f32 / size.h as f32))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::generate::Generator;
+    use crate::params::{GenParams, SizePreset, WaterMode};
+    use crate::tile::Terrain;
+    use crate::world::StratumKind;
+
+    #[test]
+    fn bay_covers_ten_to_twenty_percent_of_street() {
+        for seed in 0..5 {
+            let c = Generator::run(GenParams {
+                seed,
+                water: WaterMode::Bay,
+                size: SizePreset::Custom { w: 120, h: 60 },
+                districts: 4,
+                ..Default::default()
+            })
+            .unwrap();
+            let st = c.stratum(StratumKind::Street).unwrap();
+            let water = st.tiles.count(|t| t.terrain == Terrain::Water) as f32
+                / st.tiles.size().area() as f32;
+            assert!((0.08..=0.25).contains(&water), "seed {seed}: {water}");
+        }
+        let c = Generator::run(GenParams {
+            seed: 3,
+            water: WaterMode::None,
+            size: SizePreset::Custom { w: 120, h: 60 },
+            districts: 4,
+            ..Default::default()
+        })
+        .unwrap();
+        assert_eq!(
+            c.stratum(StratumKind::Street)
+                .unwrap()
+                .tiles
+                .count(|t| t.terrain == Terrain::Water),
+            0
+        );
+        let c = Generator::run(GenParams {
+            seed: 3,
+            water: WaterMode::River,
+            size: SizePreset::Custom { w: 120, h: 60 },
+            districts: 4,
+            ..Default::default()
+        })
+        .unwrap();
+        let st = c.stratum(StratumKind::Street).unwrap();
+        let water = st.tiles.count(|t| t.terrain == Terrain::Water);
+        assert!(water > 300 && water < 1500, "river tiles {water}");
+    }
+}
diff --git a/crates/citygen/src/lib.rs b/crates/citygen/src/lib.rs
index 93893fd..335bc70 100644
--- a/crates/citygen/src/lib.rs
+++ b/crates/citygen/src/lib.rs
@@ -1,10 +1,12 @@
 //! World model and generator for the cyberpunk city.
 
 pub mod flavor;
+pub mod generate;
 pub mod geom;
 pub mod ids;
 pub mod net;
 pub mod params;
+pub mod pathing;
 pub mod rng;
 pub mod theme;
 pub mod tile;
@@ -12,6 +14,7 @@ pub mod validate;
 pub mod world;
 
 pub use flavor::FlavorPack;
+pub use generate::{Generator, Progress, Stage};
 pub use geom::{Dir, Grid, Pos, Rect, Size};
 pub use ids::*;
 pub use net::*;
diff --git a/crates/citygen/src/params.rs b/crates/citygen/src/params.rs
index 886c367..59fb082 100644
--- a/crates/citygen/src/params.rs
+++ b/crates/citygen/src/params.rs
@@ -111,13 +111,13 @@ impl GenParams {
                 format!("{} is outside 4..=12", self.districts),
             ));
         }
-        if (w as u64) * (h as u64) < self.districts as u64 * 1000 {
+        if (w as u64) * (h as u64) < self.districts as u64 * 800 {
             return Err(bad(
                 "districts",
                 format!(
                     "too many districts for map size ({} districts need at least {} tiles, map has {})",
                     self.districts,
-                    self.districts as u64 * 1000,
+                    self.districts as u64 * 800,
                     w as u64 * h as u64
                 ),
             ));
diff --git a/crates/citygen/src/pathing.rs b/crates/citygen/src/pathing.rs
new file mode 100644
index 0000000..22e2ea3
--- /dev/null
+++ b/crates/citygen/src/pathing.rs
@@ -0,0 +1,216 @@
+//! Grid search helpers shared by generation stages: flood fill, components, A*, BFS, Bresenham.
+
+use crate::geom::{Grid, Pos, Size};
+use std::cmp::Reverse;
+use std::collections::{BinaryHeap, VecDeque};
+
+/// All positions 4-connected to `start` through `passable`. Empty if `start` is not passable.
+pub fn flood_fill(size: Size, start: Pos, passable: impl Fn(Pos) -> bool) -> Vec<Pos> {
+    let mut seen = Grid::filled(size, false);
+    let mut out = Vec::new();
+    if !size.contains(start) || !passable(start) {
+        return out;
+    }
+    let mut queue = VecDeque::from([start]);
+    seen.set(start, true);
+    while let Some(p) = queue.pop_front() {
+        out.push(p);
+        for n in p.neighbors4() {
+            if size.contains(n) && !seen[n] && passable(n) {
+                seen.set(n, true);
+                queue.push_back(n);
+            }
+        }
+    }
+    out
+}
+
+/// Every 4-connected component of passable tiles, largest first (ties: earliest first cell).
+pub fn components(size: Size, passable: impl Fn(Pos) -> bool) -> Vec<Vec<Pos>> {
+    let mut seen = Grid::filled(size, false);
+    let mut comps: Vec<Vec<Pos>> = Vec::new();
+    for y in 0..size.h as i32 {
+        for x in 0..size.w as i32 {
+            let start = Pos::new(x, y);
+            if seen[start] || !passable(start) {
+                continue;
+            }
+            let mut comp = Vec::new();
+            let mut queue = VecDeque::from([start]);
+            seen.set(start, true);
+            while let Some(p) = queue.pop_front() {
+                comp.push(p);
+                for n in p.neighbors4() {
+                    if size.contains(n) && !seen[n] && passable(n) {
+                        seen.set(n, true);
+                        queue.push_back(n);
+                    }
+                }
+            }
+            comps.push(comp);
+        }
+    }
+    comps.sort_by_key(|c| Reverse(c.len()));
+    comps
+}
+
+/// 4-neighbour A*. `cost(from, to)` returns `None` for impassable steps. Path includes both ends.
+pub fn astar(
+    size: Size,
+    start: Pos,
+    goal: Pos,
+    cost: impl Fn(Pos, Pos) -> Option<u32>,
+) -> Option<Vec<Pos>> {
+    if !size.contains(start) || !size.contains(goal) {
+        return None;
+    }
+    let mut g = Grid::filled(size, u32::MAX);
+    let mut came: Grid<Option<Pos>> = Grid::filled(size, None);
+    let mut heap = BinaryHeap::new();
+    let mut counter = 0u32;
+    g.set(start, 0);
+    heap.push(Reverse((start.manhattan(goal) as u32, counter, start)));
+    while let Some(Reverse((_, _, p))) = heap.pop() {
+        if p == goal {
+            let mut path = vec![p];
+            let mut cur = p;
+            while let Some(prev) = came[cur] {
+                path.push(prev);
+                cur = prev;
+            }
+            path.reverse();
+            return Some(path);
+        }
+        let gp = g[p];
+        for n in p.neighbors4() {
+            if !size.contains(n) {
+                continue;
+            }
+            let Some(c) = cost(p, n) else { continue };
+            let ng = gp.saturating_add(c);
+            if ng < g[n] {
+                g.set(n, ng);
+                came.set(n, Some(p));
+                counter += 1;
+                heap.push(Reverse((ng + n.manhattan(goal) as u32, counter, n)));
+            }
+        }
+    }
+    None
+}
+
+/// Shortest 4-path from `start` through `passable` tiles to the nearest `target` tile,
+/// at most `max` steps long. The target tile itself need not be passable.
+pub fn bfs_nearest(
+    size: Size,
+    start: Pos,
+    passable: impl Fn(Pos) -> bool,
+    target: impl Fn(Pos) -> bool,
+    max: usize,
+) -> Option<Vec<Pos>> {
+    if !size.contains(start) {
+        return None;
+    }
+    let mut came: Grid<Option<Pos>> = Grid::filled(size, None);
+    let mut dist = Grid::filled(size, usize::MAX);
+    let mut queue = VecDeque::from([start]);
+    dist.set(start, 0);
+    while let Some(p) = queue.pop_front() {
+        let d = dist[p];
+        if target(p) && p != start {
+            let mut path = vec![p];
+            let mut cur = p;
+            while let Some(prev) = came[cur] {
+                path.push(prev);
+                cur = prev;
+            }
+            path.reverse();
+            return Some(path);
+        }
+        if d >= max {
+            continue;
+        }
+        for n in p.neighbors4() {
+            if size.contains(n) && dist[n] == usize::MAX && (passable(n) || target(n)) {
+                dist.set(n, d + 1);
+                came.set(n, Some(p));
+                queue.push_back(n);
+            }
+        }
+    }
+    None
+}
+
+/// Integer line from `a` to `b`, both inclusive.
+pub fn bresenham(a: Pos, b: Pos) -> Vec<Pos> {
+    let mut out = Vec::new();
+    let (mut x, mut y) = (a.x, a.y);
+    let dx = (b.x - a.x).abs();
+    let dy = -(b.y - a.y).abs();
+    let sx = if a.x < b.x { 1 } else { -1 };
+    let sy = if a.y < b.y { 1 } else { -1 };
+    let mut err = dx + dy;
+    loop {
+        out.push(Pos::new(x, y));
+        if x == b.x && y == b.y {
+            break;
+        }
+        let e2 = 2 * err;
+        if e2 >= dy {
+            err += dy;
+            x += sx;
+        }
+        if e2 <= dx {
+            err += dx;
+            y += sy;
+        }
+    }
+    out
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::geom::{Pos, Size};
+
+    #[test]
+    fn astar_routes_around_block() {
+        let size = Size { w: 5, h: 5 };
+        let wall = |p: Pos| p.x == 2 && p.y != 4;
+        let path = astar(size, Pos::new(0, 0), Pos::new(4, 0), |_, to| {
+            if wall(to) { None } else { Some(1) }
+        })
+        .unwrap();
+        assert_eq!(path[0], Pos::new(0, 0));
+        assert_eq!(*path.last().unwrap(), Pos::new(4, 0));
+        assert!(path.iter().all(|p| !wall(*p)));
+        assert_eq!(path.len(), 13);
+    }
+
+    #[test]
+    fn components_are_largest_first() {
+        let size = Size { w: 6, h: 1 };
+        let pass = |p: Pos| p.x != 2;
+        let c = components(size, pass);
+        assert_eq!(c.len(), 2);
+        assert_eq!(c[0].len(), 3);
+        assert_eq!(c[1].len(), 2);
+        assert_eq!(flood_fill(size, Pos::new(0, 0), pass).len(), 2);
+    }
+
+    #[test]
+    fn bfs_nearest_finds_target() {
+        let size = Size { w: 10, h: 1 };
+        let p = bfs_nearest(size, Pos::new(0, 0), |_| true, |p| p.x == 7, 20).unwrap();
+        assert_eq!(p.len(), 8);
+        assert!(bfs_nearest(size, Pos::new(0, 0), |_| true, |p| p.x == 7, 3).is_none());
+    }
+
+    #[test]
+    fn bresenham_endpoints() {
+        let line = bresenham(Pos::new(0, 0), Pos::new(3, 1));
+        assert_eq!(line.first(), Some(&Pos::new(0, 0)));
+        assert_eq!(line.last(), Some(&Pos::new(3, 1)));
+        assert_eq!(line.len(), 4);
+    }
+}