diff --git a/Cargo.toml b/Cargo.toml
index a5a2b2b..3ee5431 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -32,4 +32,8 @@ path = "src/day7/main.rs"
 
 [[bin]]
 name = "day9"
-path = "src/day9/main.rs"
\ No newline at end of file
+path = "src/day9/main.rs"
+
+[[bin]]
+name = "day10"
+path = "src/day10/main.rs"
\ No newline at end of file
diff --git a/input10 b/input10
new file mode 100644
index 0000000..51520ab
--- /dev/null
+++ b/input10
@@ -0,0 +1,24 @@
+.###.#...#.#.##.#.####..
+.#....#####...#.######..
+#.#.###.###.#.....#.####
+##.###..##..####.#.####.
+###########.#######.##.#
+##########.#########.##.
+.#.##.########.##...###.
+###.#.##.#####.#.###.###
+##.#####.##..###.#.##.#.
+.#.#.#####.####.#..#####
+.###.#####.#..#..##.#.##
+########.##.#...########
+.####..##..#.###.###.#.#
+....######.##.#.######.#
+###.####.######.#....###
+############.#.#.##.####
+##...##..####.####.#..##
+.###.#########.###..#.##
+#.##.#.#...##...#####..#
+##.#..###############.##
+##.###.#####.##.######..
+##.#####.#.#.##..#######
+...#######.######...####
+#....#.#.#.####.#.#.#.##
diff --git a/src/day10/main.rs b/src/day10/main.rs
new file mode 100644
index 0000000..8cf7c01
--- /dev/null
+++ b/src/day10/main.rs
@@ -0,0 +1,268 @@
+use std::collections::{HashMap, HashSet};
+use std::fs::File;
+use std::io::{BufRead, BufReader};
+use std::ops::Sub;
+
+#[derive(Clone, Debug, PartialEq)]
+enum Position {
+    Empty,
+    Astroid,
+}
+
+impl From<&u8> for Position {
+    fn from(c: &u8) -> Self {
+        match c {
+            b'.' => Position::Empty,
+            b'#' => Position::Astroid,
+            _ => panic!("invalid input"),
+        }
+    }
+}
+
+#[derive(Clone, Debug, Hash, PartialEq, Eq)]
+struct XY {
+    x: isize,
+    y: isize,
+}
+
+impl From<(usize, usize)> for XY {
+    fn from((x, y): (usize, usize)) -> Self {
+        XY {
+            x: x as isize,
+            y: y as isize,
+        }
+    }
+}
+
+impl Sub for &XY {
+    type Output = XY;
+    fn sub(self, other: &XY) -> Self::Output {
+        let xy = XY {
+            x: self.x - other.x,
+            y: self.y - other.y,
+        };
+        xy
+    }
+}
+
+impl XY {
+    fn direction(&self) -> Self {
+        let d = gcd(self.y, self.x).abs();
+        if d != 0 {
+            XY {
+                x: self.x / d,
+                y: self.y / d,
+            }
+        } else {
+            self.clone()
+        }
+    }
+}
+
+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 insight = self
+                    .map
+                    .iter()
+                    // get direction to any of the astroids
+                    .filter_map(|(xy, pos)| {
+                        if *pos == Position::Empty {
+                            return None;
+                        }
+                        let rel = xy - &root;
+                        Some(rel.direction())
+                    })
+                    // and keep only one in each direction
+                    .fold(HashSet::new(), |mut acc, xy| {
+                        acc.insert(xy);
+                        acc
+                    })
+                    .len()
+                    - 1;
+                if insight > best.0 {
+                    best = (insight, root)
+                }
+            }
+        }
+        best
+    }
+}
+
+impl<T> From<T> for MonitoringStation
+where
+    T: BufRead,
+{
+    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()
+                    .map(|(x, c)| {
+                        let xy = XY::from((x, y));
+                        let p = Position::from(c);
+                        (xy, p)
+                    })
+                    .collect::<Vec<(XY, Position)>>()
+            })
+            .fold(map, |mut map, (xy, p)| {
+                map.insert(xy.clone(), p.clone());
+                map
+            });
+        let size_x = map.iter().count() / size_y;
+        MonitoringStation {
+            map,
+            size_x,
+            size_y,
+        }
+    }
+}
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let f = File::open("input10")?;
+    let reader = BufReader::new(f);
+    let best = MonitoringStation::from(reader).find();
+    println!("best: {:?}", best);
+    Ok(())
+}
+
+fn gcd(x: isize, y: isize) -> isize {
+    let mut x = x;
+    let mut y = y;
+    while y != 0 {
+        let t = y;
+        y = x % y;
+        x = t;
+    }
+    x
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn example1() {
+        let reader = BufReader::new(".#..#\n.....\n#####\n....#\n...##".as_bytes());
+        let (count, xy) = MonitoringStation::from(reader).find();
+        assert_eq!(xy, XY { x: 3, y: 4 });
+        assert_eq!(count, 8);
+    }
+
+    #[test]
+    fn example2() {
+        let reader = BufReader::new(
+            "
+......#.#.
+#..#.#....
+..#######.
+.#.#.###..
+.#..#.....
+..#....#.#
+#..#....#.
+.##.#..###
+##...#..#.
+.#....####"
+                .trim()
+                .as_bytes(),
+        );
+        let (count, xy) = MonitoringStation::from(reader).find();
+        assert_eq!(xy, XY { x: 5, y: 8 });
+        assert_eq!(count, 33);
+    }
+
+    #[test]
+    fn example3() {
+        let reader = BufReader::new(
+            "
+#.#...#.#.
+.###....#.
+.#....#...
+##.#.#.#.#
+....#.#.#.
+.##..###.#
+..#...##..
+..##....##
+......#...
+.####.###."
+                .trim()
+                .as_bytes(),
+        );
+        let (count, xy) = MonitoringStation::from(reader).find();
+        assert_eq!(xy, XY { x: 1, y: 2 });
+        assert_eq!(count, 35);
+    }
+
+    #[test]
+    fn example4() {
+        let reader = BufReader::new(
+            "
+.#..#..###
+####.###.#
+....###.#.
+..###.##.#
+##.##.#.#.
+....###..#
+..#.#..#.#
+#..#.#.###
+.##...##.#
+.....#.#.."
+                .trim()
+                .as_bytes(),
+        );
+        let (count, xy) = MonitoringStation::from(reader).find();
+        assert_eq!(xy, XY { x: 6, y: 3 });
+        assert_eq!(count, 41);
+    }
+
+    #[test]
+    fn example5() {
+        let reader = BufReader::new(
+            "
+.#..##.###...#######
+##.############..##.
+.#.######.########.#
+.###.#######.####.#.
+#####.##.#.##.###.##
+..#####..#.#########
+####################
+#.####....###.#.#.##
+##.#################
+#####.##.###..####..
+..######..##.#######
+####.##.####...##..#
+.#####..#.######.###
+##...#.##########...
+#.##########.#######
+.####.#.###.###.#.##
+....##.##.###..#####
+.#.#.###########.###
+#.#.#.#####.####.###
+###.##.####.##.#..##"
+                .trim()
+                .as_bytes(),
+        );
+        let (count, xy) = MonitoringStation::from(reader).find();
+        assert_eq!(xy, XY { x: 11, y: 13 });
+        assert_eq!(count, 210);
+    }
+}