diff -r 0135e64c6c66 -r c4fd2813b127 rust/hwphysics/src/collision.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hwphysics/src/collision.rs	Wed Jul 31 23:14:27 2019 +0200
@@ -0,0 +1,154 @@
+use std::ops::RangeInclusive;
+
+use crate::{
+    common::{GearData, GearDataProcessor, GearId},
+    grid::Grid,
+    physics::PhysicsData,
+};
+
+use fpnum::*;
+use integral_geometry::{GridIndex, Point, Size};
+use land2d::Land2D;
+
+pub fn fppoint_round(point: &FPPoint) -> Point {
+    Point::new(point.x().round(), point.y().round())
+}
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct CircleBounds {
+    pub center: FPPoint,
+    pub radius: FPNum,
+}
+
+impl CircleBounds {
+    pub fn intersects(&self, other: &CircleBounds) -> bool {
+        (other.center - self.center).is_in_range(self.radius + other.radius)
+    }
+
+    pub fn rows(&self) -> impl Iterator<Item = (usize, RangeInclusive<usize>)> {
+        let radius = self.radius.abs_round() as usize;
+        let center = Point::from_fppoint(&self.center);
+        (center.y as usize - radius..=center.y as usize + radius)
+            .map(move |row| (row, center.x as usize - radius..=center.x as usize + radius))
+    }
+}
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct CollisionData {
+    pub bounds: CircleBounds,
+}
+
+impl GearData for CollisionData {}
+
+#[derive(PartialEq, Eq, Clone, Copy, Debug)]
+pub struct ContactData {
+    pub elasticity: FPNum,
+    pub friction: FPNum,
+}
+
+impl GearData for ContactData {}
+
+struct EnabledCollisionsCollection {
+    gear_ids: Vec<GearId>,
+    collisions: Vec<CollisionData>,
+}
+
+impl EnabledCollisionsCollection {
+    fn new() -> Self {
+        Self {
+            gear_ids: Vec::new(),
+            collisions: Vec::new(),
+        }
+    }
+
+    fn push(&mut self, gear_id: GearId, collision: CollisionData) {
+        self.gear_ids.push(gear_id);
+        self.collisions.push(collision);
+    }
+
+    fn iter(&self) -> impl Iterator<Item = (GearId, &CollisionData)> {
+        self.gear_ids.iter().cloned().zip(self.collisions.iter())
+    }
+}
+
+pub struct CollisionProcessor {
+    grid: Grid,
+    enabled_collisions: EnabledCollisionsCollection,
+
+    detected_collisions: DetectedCollisions,
+}
+
+pub struct DetectedCollisions {
+    pub pairs: Vec<(GearId, Option<GearId>)>,
+    pub positions: Vec<Point>,
+}
+
+impl DetectedCollisions {
+    pub fn new(capacity: usize) -> Self {
+        Self {
+            pairs: Vec::with_capacity(capacity),
+            positions: Vec::with_capacity(capacity),
+        }
+    }
+
+    pub fn push(
+        &mut self,
+        contact_gear_id1: GearId,
+        contact_gear_id2: Option<GearId>,
+        position: &FPPoint,
+    ) {
+        self.pairs.push((contact_gear_id1, contact_gear_id2));
+        self.positions.push(fppoint_round(&position));
+    }
+
+    pub fn clear(&mut self) {
+        self.pairs.clear();
+        self.positions.clear()
+    }
+}
+
+impl CollisionProcessor {
+    pub fn new(size: Size) -> Self {
+        Self {
+            grid: Grid::new(size),
+            enabled_collisions: EnabledCollisionsCollection::new(),
+            detected_collisions: DetectedCollisions::new(0),
+        }
+    }
+
+    pub fn process(
+        &mut self,
+        land: &Land2D<u32>,
+        updates: &crate::physics::PositionUpdates,
+    ) -> &DetectedCollisions {
+        self.detected_collisions.clear();
+        for (id, old_position, new_position) in updates.iter() {
+            self.grid.update_position(id, old_position, new_position)
+        }
+
+        self.grid.check_collisions(&mut self.detected_collisions);
+
+        for (gear_id, collision) in self.enabled_collisions.iter() {
+            if collision
+                .bounds
+                .rows()
+                .any(|(y, r)| (&land[y][r]).iter().any(|v| *v != 0))
+            {
+                self.detected_collisions
+                    .push(gear_id, None, &collision.bounds.center)
+            }
+        }
+
+        &self.detected_collisions
+    }
+}
+
+impl GearDataProcessor<CollisionData> for CollisionProcessor {
+    fn add(&mut self, gear_id: GearId, gear_data: CollisionData) {
+        self.grid.insert_static(gear_id, &gear_data.bounds);
+    }
+
+    fn remove(&mut self, gear_id: GearId) {
+        self.grid.remove(gear_id);
+    }
+}