1 files changed, 73 insertions, 0 deletions
diff --git a/src/bin/day3.rs b/src/bin/day3.rs
new file mode 100644
index 0000000..03a51fa
--- /dev/null
+++ b/src/bin/day3.rs
@@ -0,0 +1,73 @@
+use std::collections::HashMap;
+
+fn main() {
+    let input = advent::read_file(3);
+    println!("3a: {}", houses_visited(&input));
+    println!("3b: {}", houses_visited_robo(&input));
+}
+
+fn new_pos(pos: (i32, i32), c: char) -> (i32, i32) {
+    match c {
+        'v' => (pos.0, pos.1 - 1),
+        '^' => (pos.0, pos.1 + 1),
+        '<' => (pos.0 - 1, pos.1),
+        '>' => (pos.0 + 1, pos.1),
+        _ => panic!("invalid character"),
+    }
+}
+
+fn houses_visited(input: &str) -> usize {
+    let mut map = HashMap::new();
+    let mut pos = (0, 0);
+    map.insert(pos, 1);
+
+    for c in input.chars() {
+        pos = new_pos(pos, c);
+        let count = map.entry(pos).or_insert(0);
+        *count += 1;
+    }
+
+    map.keys().len()
+}
+
+fn houses_visited_robo(input: &str) -> usize {
+    let mut map = HashMap::new();
+    let mut pos_santa = (0, 0);
+    let mut pos_robo = (0, 0);
+
+    map.insert(pos_santa, 2);
+
+    for c in input.chars().step_by(2) {
+        pos_santa = new_pos(pos_santa, c);
+
+        let count = map.entry(pos_santa).or_insert(0);
+        *count += 1;
+    }
+    for c in input.chars().skip(1).step_by(2) {
+        pos_robo = new_pos(pos_robo, c);
+
+        let count = map.entry(pos_robo).or_insert(0);
+        *count += 1;
+    }
+
+    map.keys().len()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test() {
+        assert_eq!(houses_visited(">"), 2);
+        assert_eq!(houses_visited("^>v<"), 4);
+        assert_eq!(houses_visited("^v^v^v^v^v"), 2);
+    }
+
+    #[test]
+    fn test_robo() {
+        assert_eq!(houses_visited_robo("^v"), 3);
+        assert_eq!(houses_visited_robo("^>v<"), 3);
+        assert_eq!(houses_visited_robo("^v^v^v^v^v"), 11);
+    }
+}