day9

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diff --git a/inputs/day9 b/inputs/day9
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+U 9
+R 12
+L 1
+U 9
+L 14
+U 11
+L 8
+U 10
+L 7
+U 1
+L 11
+R 2
+D 12
+U 3
+R 2
+L 14
+D 1
+L 12
+R 16
+D 16
+R 8
+L 15
+R 3
+L 10
+D 14
+R 9
+U 2
+D 4
+U 7
+D 2
+R 11
+D 6
+U 2
+R 4
+L 2
+R 6
+L 11
+R 9
+L 1
+R 8
+D 3
+R 4
+U 14
+D 7
+L 11
+R 14
+L 8
+D 9
+L 6
+D 15
+R 4
+D 9
+U 7
+R 6
+L 13
+D 11
+R 10
+L 17
+R 17
+D 17
+R 7
+U 15
+D 14
+R 6
+U 14
+R 5
+U 17
+R 7
+L 17
+U 14
+L 14
+R 13
+U 4
+R 14
+U 10
+L 16
+U 8
+R 2
+U 4
+D 16
+U 4
+L 14
+R 7
+D 13
+U 10
+L 10
+R 3
+L 6
+D 7
+R 13
+L 6
+U 7
+R 16
+L 10
+U 6
+L 6
+U 11
+D 6
+U 5
+L 4
+U 17
+R 6
+L 3
+D 5
+R 4
+U 1
+L 14
+R 10
+L 4
+U 9
+R 7
+D 2
+R 6
+U 12
+D 15
+R 15
+L 13
+U 5
+L 12
+D 3
+U 15
+L 9
+U 3
+D 12
+R 15
+U 7
+L 7
+R 11
+L 14
+R 2
+D 6
+R 6
+D 9
+R 2
+L 12
+R 2
+U 12
+R 8
+L 2
+U 12
+R 2
+D 17
+L 8
+D 1
+U 16
+D 1
+R 5
+L 9
+U 4
+L 1
+U 9
+L 11
+R 9
+D 7
+R 16
+D 12
+R 3
+U 7
+L 4
+R 7
+L 8
+D 9
+U 10
+D 7
+U 10
+D 5
+U 14
+D 5
+U 11
+R 15
+U 8
+D 12
+U 8
+L 18
+U 16
+D 16
+U 3
+R 5
+D 2
+U 10
+L 17
+R 5
+U 15
+R 9
+D 5
+U 9
+L 12
+D 17
+R 14
+L 4
+R 14
+L 17
+D 9
+L 10
+D 17
+R 1
+U 2
+R 7
+U 13
+R 1
+U 17
+R 4
+L 3
+D 14
+U 11
+L 15
+R 6
+U 4
+D 8
+L 17
+D 3
+R 13
+U 10
+R 4
+U 5
+R 3
+U 16
+D 6
+L 5
+R 18
+L 11
+D 16
+U 7
+R 7
+U 1
+R 14
+U 14
+R 4
+D 3
+R 12
+D 13
+U 5
+D 8
+R 17
+L 9
+D 4
+L 13
+U 8
+R 13
+D 5
+U 17
+L 11
+U 13
+D 9
+L 8
+R 14
+L 15
+D 10
+R 11
+D 8
+L 10
+U 3
+L 3
+R 12
+L 9
+R 17
+U 10
+L 17
+U 13
+L 12
+U 1
+D 2
+R 1
+L 16
+R 11
+D 15
+L 8
+R 10
+L 12
+U 3
+L 17
+U 7
+R 6
+U 16
+R 1
+L 3
+R 5
+L 11
+R 4
+D 10
+L 1
+R 4
+U 12
+L 18
+R 8
+L 7
+R 16
+U 19
+R 10
+U 11
+D 12
+R 6
+U 17
+L 16
+D 19
+L 9
+U 1
+R 12
+U 12
+R 15
+D 4
+L 15
+R 4
+L 9
+R 15
+L 10
+R 7
+L 16
+U 16
+L 19
+U 5
+L 4
+U 12
+L 6
+U 5
+R 17
+U 11
+R 6
+U 15
+L 9
+D 4
+R 2
+L 7
+R 16
+L 13
+U 19
+R 4
+U 13
+L 18
+D 19
+U 15
+R 13
+L 11
+U 18
+D 2
+U 9
+D 18
+L 13
+D 13
+L 11
+U 3
+D 11
+U 16
+R 14
+D 6
+L 12
+D 9
+R 2
+D 14
+L 12
+R 15
+D 2
+R 11
+U 7
+D 9
+L 16
+R 12
+L 12
+D 14
+U 2
+D 12
+R 8
+U 9
+D 10
+U 12
+L 17
+U 19
+L 4
+U 15
+R 12
+L 11
+D 12
+L 8
+U 3
+R 7
+L 2
+D 4
+U 17
diff --git a/src/bin/day9.rs b/src/bin/day9.rs
new file mode 100644
index 0000000..b4c834e
--- /dev/null
+++ b/src/bin/day9.rs
@@ -0,0 +1,156 @@
+use std::collections::HashSet;
+
+static DAY: u8 = 9;
+
+fn main() {
+    let input = advent::read_lines(DAY);
+    println!("{DAY}a: {}", tail_positions(&input, 2));
+    println!("{DAY}b: {}", tail_positions(&input, 10));
+}
+
+enum Direction {
+    Up,
+    Down,
+    Left,
+    Right,
+}
+
+impl Direction {
+    fn new(input: &str) -> Direction {
+        match input {
+            "U" => Direction::Up,
+            "D" => Direction::Down,
+            "L" => Direction::Left,
+            "R" => Direction::Right,
+            _ => unimplemented!(),
+        }
+    }
+}
+
+struct Movement {
+    direction: Direction,
+    distance: isize,
+}
+
+impl Movement {
+    fn new(input: &str) -> Movement {
+        let mut it = input.split_whitespace();
+        let direction = Direction::new(it.next().unwrap());
+        let distance = it.next().unwrap().parse().unwrap();
+        Movement { direction, distance }
+    }
+}
+
+#[derive(Copy,Clone,PartialEq,Eq,Hash)]
+struct Position {
+    x: isize,
+    y: isize,
+}
+
+impl Position {
+    fn is_adjacent(&self, other: &Position) -> bool {
+        let dx = self.x.abs_diff(other.x);
+        let dy = self.y.abs_diff(other.y);
+        dx <= 1 && dy <= 1
+    }
+
+    fn step(&mut self, direction: &Direction) {
+        match direction {
+            Direction::Up => self.y += 1,
+            Direction::Down => self.y -= 1,
+            Direction::Left => self.x -= 1,
+            Direction::Right => self.x += 1,
+        }
+    }
+
+    fn follow(&mut self, other: &Position) {
+        if self.is_adjacent(other) {
+            return;
+        }
+
+        if self.x == other.x {
+            if self.y > other.y {
+                self.y -= 1;
+            } else {
+                self.y += 1;
+            }
+            return;
+        }
+
+        if self.y == other.y {
+            if self.x > other.x {
+                self.x -= 1;
+            } else {
+                self.x += 1;
+            }
+            return;
+        }
+
+        if self.x > other.x {
+            self.x -= 1;
+        } else {
+            self.x += 1;
+        }
+        if self.y > other.y {
+            self.y -= 1;
+        } else {
+            self.y += 1;
+        }
+    }
+}
+
+fn tail_positions(input: &[String], knot_count: usize) -> usize {
+    let movements = input.iter()
+                         .map(|x| Movement::new(x))
+                         .collect::<Vec<_>>();
+
+    let mut visited = HashSet::new();
+    let mut knots = vec![Position { x: 0, y: 0 }; knot_count];
+    visited.insert(knots[0]);
+    for movement in movements {
+        for _ in 0 .. movement.distance {
+            knots[0].step(&movement.direction);
+            for idx in 1 .. knot_count {
+                let new_pos = knots[idx-1];
+                knots[idx].follow(&new_pos);
+            }
+            visited.insert(knots[knot_count-1]);
+        }
+    }
+    visited.len()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        let input = [
+            "R 4",
+            "U 4",
+            "L 3",
+            "D 1",
+            "R 4",
+            "D 1",
+            "L 5",
+            "R 2",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+
+        assert_eq!(tail_positions(&input, 2), 13);
+        assert_eq!(tail_positions(&input, 10), 1);
+
+        let input = [
+            "R 5",
+            "U 8",
+            "L 8",
+            "D 3",
+            "R 17",
+            "D 10",
+            "L 25",
+            "U 20",
+        ].iter().map(|&x| String::from(x)).collect::<Vec<_>>();
+
+        assert_eq!(tail_positions(&input, 10), 36);
+    }
+}