diff -r bc7b6aa5d67a -r 7030706266df rust/hwphysics/src/grid.rs
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/rust/hwphysics/src/grid.rs	Fri Dec 06 22:20:53 2019 +0100
@@ -0,0 +1,142 @@
+use crate::{
+    collision::{fppoint_round, CircleBounds, DetectedCollisions},
+    common::GearId,
+};
+
+use fpnum::FPPoint;
+use integral_geometry::{GridIndex, Point, Size};
+
+struct GridBin {
+    static_refs: Vec<GearId>,
+    static_entries: Vec<CircleBounds>,
+
+    dynamic_refs: Vec<GearId>,
+    dynamic_entries: Vec<CircleBounds>,
+}
+
+impl GridBin {
+    fn new() -> Self {
+        Self {
+            static_refs: vec![],
+            static_entries: vec![],
+            dynamic_refs: vec![],
+            dynamic_entries: vec![],
+        }
+    }
+}
+
+const GRID_BIN_SIZE: usize = 128;
+
+pub struct Grid {
+    bins: Vec<GridBin>,
+    space_size: Size,
+    bins_count: Size,
+    index: GridIndex,
+}
+
+impl Grid {
+    pub fn new(size: Size) -> Self {
+        assert!(size.is_power_of_two());
+        let bins_count = Size::new(size.width / GRID_BIN_SIZE, size.height / GRID_BIN_SIZE);
+
+        Self {
+            bins: (0..bins_count.area()).map(|_| GridBin::new()).collect(),
+            space_size: size,
+            bins_count,
+            index: Size::square(GRID_BIN_SIZE).to_grid_index(),
+        }
+    }
+
+    fn bin_index(&self, position: &FPPoint) -> Point {
+        self.index.map(fppoint_round(position))
+    }
+
+    fn get_bin(&mut self, index: Point) -> &mut GridBin {
+        &mut self.bins[index.x as usize * self.bins_count.width + index.y as usize]
+    }
+
+    fn lookup_bin(&mut self, position: &FPPoint) -> &mut GridBin {
+        self.get_bin(self.bin_index(position))
+    }
+
+    pub fn insert_static(&mut self, gear_id: GearId, bounds: &CircleBounds) {
+        let bin = self.lookup_bin(&bounds.center);
+        bin.static_refs.push(gear_id);
+        bin.static_entries.push(*bounds)
+    }
+
+    pub fn insert_dynamic(&mut self, gear_id: GearId, bounds: &CircleBounds) {
+        let bin = self.lookup_bin(&bounds.center);
+        bin.dynamic_refs.push(gear_id);
+        bin.dynamic_entries.push(*bounds);
+    }
+
+    pub fn remove(&mut self, gear_id: GearId) {}
+
+    pub fn update_position(
+        &mut self,
+        gear_id: GearId,
+        old_position: &FPPoint,
+        new_position: &FPPoint,
+    ) {
+        let old_bin_index = self.bin_index(old_position);
+        let new_bin_index = self.bin_index(new_position);
+
+        let old_bin = self.lookup_bin(old_position);
+        if let Some(index) = old_bin.dynamic_refs.iter().position(|id| *id == gear_id) {
+            if old_bin_index == new_bin_index {
+                old_bin.dynamic_entries[index].center = *new_position
+            } else {
+                let bounds = old_bin.dynamic_entries.swap_remove(index);
+                let new_bin = self.get_bin(new_bin_index);
+
+                new_bin.dynamic_refs.push(gear_id);
+                new_bin.dynamic_entries.push(CircleBounds {
+                    center: *new_position,
+                    ..bounds
+                });
+            }
+        } else if let Some(index) = old_bin.static_refs.iter().position(|id| *id == gear_id) {
+            let bounds = old_bin.static_entries.swap_remove(index);
+            old_bin.static_refs.swap_remove(index);
+
+            let new_bin = if old_bin_index == new_bin_index {
+                old_bin
+            } else {
+                self.get_bin(new_bin_index)
+            };
+
+            new_bin.dynamic_refs.push(gear_id);
+            new_bin.dynamic_entries.push(CircleBounds {
+                center: *new_position,
+                ..bounds
+            });
+        }
+    }
+
+    pub fn check_collisions(&self, collisions: &mut DetectedCollisions) {
+        for bin in &self.bins {
+            for (index, bounds) in bin.dynamic_entries.iter().enumerate() {
+                for (other_index, other) in bin.dynamic_entries.iter().enumerate().skip(index + 1) {
+                    if bounds.intersects(other) && bounds != other {
+                        collisions.push(
+                            bin.dynamic_refs[index],
+                            Some(bin.dynamic_refs[other_index]),
+                            &((bounds.center + other.center) / 2),
+                        )
+                    }
+                }
+
+                for (other_index, other) in bin.static_entries.iter().enumerate() {
+                    if bounds.intersects(other) {
+                        collisions.push(
+                            bin.dynamic_refs[index],
+                            Some(bin.static_refs[other_index]),
+                            &((bounds.center + other.center) / 2),
+                        )
+                    }
+                }
+            }
+        }
+    }
+}