diff --git a/src/day10/main.rs b/src/day10/main.rs
index 8cf7c01..23f70db 100644
--- a/src/day10/main.rs
+++ b/src/day10/main.rs
@@ -61,20 +61,15 @@ impl XY {
 
 struct MonitoringStation {
     map: HashMap<XY, Position>,
-    size_x: usize,
-    size_y: usize,
 }
 
 impl MonitoringStation {
     fn find(&self) -> (usize, XY) {
-        let mut best = (0, XY::from((0, 0)));
-        for x in 0..self.size_x {
-            for y in 0..self.size_y {
-                let root = XY::from((x, y));
-                // we only can build on astroids
-                if *self.map.get(&root).unwrap() == Position::Empty {
-                    continue;
-                }
+        let best = (0, XY::from((0, 0)));
+        self.map
+            .iter()
+            .filter(|(_, pos)| **pos == Position::Astroid)
+            .map(|(root, _)| {
                 let insight = self
                     .map
                     .iter()
@@ -83,7 +78,7 @@ impl MonitoringStation {
                         if *pos == Position::Empty {
                             return None;
                         }
-                        let rel = xy - &root;
+                        let rel = xy - root;
                         Some(rel.direction())
                     })
                     // and keep only one in each direction
@@ -93,12 +88,14 @@ impl MonitoringStation {
                     })
                     .len()
                     - 1;
-                if insight > best.0 {
-                    best = (insight, root)
+                (insight, root)
+            })
+            .fold(best, |acc, astroid| {
+                if astroid.0 > acc.0 {
+                    return (astroid.0, astroid.1.clone());
                 }
-            }
-        }
-        best
+                acc
+            })
     }
 }
 
@@ -108,12 +105,10 @@ where
 {
     fn from(reader: T) -> Self {
         let map: HashMap<XY, Position> = HashMap::new();
-        let mut size_y = 0;
         let map = reader
             .split(b'\n')
             .enumerate()
             .flat_map(|(y, line)| {
-                size_y += 1;
                 line.unwrap()
                     .iter()
                     .enumerate()
@@ -128,12 +123,7 @@ where
                 map.insert(xy.clone(), p.clone());
                 map
             });
-        let size_x = map.iter().count() / size_y;
-        MonitoringStation {
-            map,
-            size_x,
-            size_y,
-        }
+        MonitoringStation { map }
     }
 }