diff --git a/day03/src/main.rs b/day03/src/main.rs
index b6a6afa..0d543d6 100644
--- a/day03/src/main.rs
+++ b/day03/src/main.rs
@@ -1,4 +1,5 @@
 mod geom;
+mod point_set;
 
 fn main() {
     println!("Hello, world!");
diff --git a/day03/src/point_set.rs b/day03/src/point_set.rs
new file mode 100644
index 0000000..c6419c0
--- /dev/null
+++ b/day03/src/point_set.rs
@@ -0,0 +1,45 @@
+use std::collections::HashMap;
+
+use crate::geom::{Point, Coord};
+
+type BitSet = u64;
+
+fn split(p: Point) -> (Point, BitSet) {
+    let xh = p.x >> 3;
+    let yh = p.y >> 3;
+    let xl = p.x & 7;
+    let yl = p.y & 7;
+    let b = (yl << 3) + xl;
+    (Point { x: xh, y: yh }, 1 << b)
+}
+
+pub struct PointSet {
+    inner: HashMap<Point, BitSet>
+}
+
+impl PointSet {
+    pub fn new() -> Self {
+        Self { inner: HashMap::new() }
+    }
+
+    pub fn contains(&self, p: Point) -> bool {
+        let (key, mask) = split(p);
+        self.inner.get(&key).cloned().unwrap_or(0) & mask != 0
+    }
+
+    pub fn insert(&mut self, p: Point) -> bool {
+        let (key, mask) = split(p);
+        let grid_ptr = self.inner.entry(key).or_insert(0);
+        let old = *grid_ptr & mask != 0;
+        *grid_ptr |= mask;
+        return old;
+    }
+    
+    pub fn remove(&mut self, p: Point) -> bool {
+        let (key, mask) = split(p);
+        let grid_ptr = self.inner.entry(key).or_insert(0);
+        let old = *grid_ptr & mask != 0;
+        *grid_ptr &= !mask;
+        return old;
+    }
+}