d9p1

day9/src/main.rs

@@ -0,0 +1,39 @@
+use std::env;
+use std::fs::File;
+use std::io::{self, BufRead};
+use std::collections::HashSet;
+
+fn main() {
+    let file = File::open(&env::args().nth(1).unwrap()).unwrap();
+    let reader = io::BufReader::new(file);
+    let (mut hx, mut hy, mut tx, mut ty) = (0, 0, 0, 0);
+    let mut visited: HashSet<(i64, i64)> = vec![(0, 0)].into_iter().collect();
+
+    for line in reader.lines().flatten() {
+        let (movx, movy) = match line.chars().nth(0) {
+            Some('L') => (-1, 0),
+            Some('R') => (1, 0),
+            Some('U') => (0, -1),
+            Some('D') => (0, 1),
+            _ => (999, 999),
+        };
+        let count = line[2..].parse::<i64>().unwrap();
+        for _ in 0..count {
+            (hx, hy) = (hx + movx, hy + movy);
+            (tx, ty) = move_tail(hx, hy, tx, ty).unwrap();
+            visited.insert((tx, ty));
+        }
+    }
+    println!("{:?}", visited.len());
+}
+
+fn move_tail(hx: i64, hy: i64, tx: i64, ty: i64) -> Option<(i64, i64)> {
+    match (hx - tx, hy - ty) {
+        (-1..=1, -1..=1) => Some((tx, ty)),
+        (-1..=1, 2) => Some((hx, ty + 1)),
+        (-1..=1, -2) => Some((hx, ty - 1)),
+        (2, -1..=1) => Some((tx + 1, hy)),
+        (-2, -1..=1) => Some((tx - 1, hy)),
+        _ => None
+    }
+}