diff --git a/src/bin/day9.rs b/src/bin/day9.rs
index 79e3b4c..8c4af39 100644
--- a/src/bin/day9.rs
+++ b/src/bin/day9.rs
@@ -1,5 +1,6 @@
 use std::{
     collections::{HashMap, HashSet},
+    hash::Hash,
     ops::RangeInclusive,
     time::Instant,
 };
@@ -183,11 +184,27 @@ impl Polygon {
     }
 }
 
+struct InsertGuard<'a, K, V>(K, V, &'a mut HashMap<K, V>)
+where
+    K: Hash + Eq + Copy,
+    V: Copy;
+
+impl<'a, K, V> Drop for InsertGuard<'a, K, V>
+where
+    K: Hash + Eq + Copy,
+    V: Copy,
+{
+    fn drop(&mut self) {
+        self.2.insert(self.0, self.1);
+    }
+}
+
 fn horizontal_edges_contained(
     poly: &Polygon,
     a: Point2,
     b: Point2,
     cache: &mut HashMap<Point2, bool>,
+    edge_cache: &mut HashMap<(u64, (u64, u64)), bool>,
 ) -> bool {
     assert_ne!(a.x, b.x);
 
@@ -206,6 +223,13 @@ fn horizontal_edges_contained(
     };
 
     for &y in y_values {
+        let key = (y, (min_x, max_x));
+        if let Some(&v) = edge_cache.get(&key) {
+            return v;
+        }
+
+        let guard = InsertGuard(key, false, edge_cache);
+
         // Left corner
 
         if !contains(Point2 { x: min_x, y }) {
@@ -241,6 +265,9 @@ fn horizontal_edges_contained(
         if !contains(Point2 { x: max_x - 1, y }) {
             return false;
         }
+
+        std::mem::forget(guard);
+        edge_cache.insert(key, true);
     }
 
     true
@@ -251,6 +278,7 @@ fn vertical_edges_contain(
     a: Point2,
     b: Point2,
     cache: &mut HashMap<Point2, bool>,
+    edge_cache: &mut HashMap<(u64, (u64, u64)), bool>,
 ) -> bool {
     assert_ne!(a.y, b.y);
 
@@ -269,6 +297,13 @@ fn vertical_edges_contain(
     };
 
     for &x in x_values {
+        let key = (x, (min_y, max_y));
+        if let Some(&v) = edge_cache.get(&key) {
+            return v;
+        }
+
+        let guard = InsertGuard(key, false, edge_cache);
+
         // Bottom corner
 
         if !contains(Point2 { x, y: min_y }) {
@@ -304,6 +339,9 @@ fn vertical_edges_contain(
         if !contains(Point2 { x, y: max_y - 1 }) {
             return false;
         }
+
+        std::mem::forget(guard);
+        edge_cache.insert(key, true);
     }
 
     true
@@ -316,6 +354,8 @@ pub fn part2(input: Parsed) {
     let mut max_area = 0;
 
     let mut contains_cache = HashMap::new();
+    let mut h_edge_cache = HashMap::new();
+    let mut v_edge_cache = HashMap::new();
 
     for (ia, &a) in poly.points().iter().enumerate() {
         for &b in poly.points().iter().skip(ia + 1) {
@@ -327,11 +367,15 @@ pub fn part2(input: Parsed) {
 
             assert!(area > 1);
 
-            if a.x != b.x && !horizontal_edges_contained(&poly, a, b, &mut contains_cache) {
+            if a.x != b.x
+                && !horizontal_edges_contained(&poly, a, b, &mut contains_cache, &mut h_edge_cache)
+            {
                 continue;
             }
 
-            if a.y != b.y && !vertical_edges_contain(&poly, a, b, &mut contains_cache) {
+            if a.y != b.y
+                && !vertical_edges_contain(&poly, a, b, &mut contains_cache, &mut v_edge_cache)
+            {
                 continue;
             }