2 files changed, 102 insertions, 0 deletions
diff --git a/input/day13.txt b/input/day13.txt
new file mode 100644
index 0000000..cdd1bbd
--- /dev/null
+++ b/input/day13.txt
@@ -0,0 +1 @@
+1352
diff --git a/src/bin/day13.rs b/src/bin/day13.rs
new file mode 100644
index 0000000..a953b28
--- /dev/null
+++ b/src/bin/day13.rs
@@ -0,0 +1,101 @@
+use std::collections::{HashMap, VecDeque};
+use std::io::{self, Read};
+
+#[derive(Clone, Copy, PartialEq, Eq, Hash)]
+struct Point {
+    x: u32,
+    y: u32,
+}
+
+const START: Point = Point { x: 1, y: 1 };
+
+impl Point {
+    fn north(self) -> Option<Point> {
+        if self.y == 0 {
+            None
+        } else {
+            Some(Point { x: self.x, y: self.y - 1 })
+        }
+    }
+
+    fn east(self) -> Point {
+        Point { x: self.x + 1, y: self.y }
+    }
+
+    fn south(self) -> Point {
+        Point { x: self.x, y: self.y + 1 }
+    }
+
+    fn west(self) -> Option<Point> {
+        if self.x == 0 {
+            None
+        } else {
+            Some(Point { x: self.x - 1, y: self.y })
+        }
+    }
+
+    fn neighbors(self) -> [Option<Point>; 4] {
+        [self.north(), Some(self.east()), Some(self.south()), self.west()]
+    }
+}
+
+#[derive(Clone, Copy)]
+struct Layout {
+    constant: u32,
+}
+
+impl Layout {
+    fn is_open(&self, point: Point) -> bool {
+        let Point { x, y } = point;
+        let sum = x * x + 3 * x + 2 * x * y + y + y * y + self.constant;
+        sum.count_ones() % 2 == 0
+    }
+}
+
+fn solve(goal: Point, constant: u32) -> u32 {
+    let layout = Layout { constant: constant };
+
+    let mut frontier = VecDeque::new();
+    frontier.push_back(START);
+
+    let mut previous = HashMap::new();
+    previous.insert(START, START);
+
+    while !frontier.is_empty() {
+        let current = frontier.pop_front().unwrap();
+        if current == goal { break; }
+
+        let neighbors = current.neighbors();
+        let iter = neighbors.iter()
+            .filter_map(|&p| p)
+            .filter(|&p| layout.is_open(p));
+
+        for next in iter {
+            if !previous.contains_key(&next) {
+                frontier.push_back(next);
+                previous.insert(next, current);
+            }
+        }
+    }
+
+    let mut steps = 0;
+    let mut current = goal;
+    while current != START {
+        current = *previous.get(&current).unwrap();
+        steps += 1;
+    }
+
+    steps
+}
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    println!("Part 1: {}", solve(Point { x: 31, y: 39 }, input.trim().parse().unwrap()));
+}
+
+#[test]
+fn part1() {
+    assert_eq!(11, solve(Point { x: 7, y: 4 }, 10));
+}