done with part2

2022-aoc/src/d9.rs

@@ -2,17 +2,16 @@ use std::collections::HashSet;
 
 use aoc_runner_derive::{aoc as aoc_run, aoc_generator};
 
-type Position = (i32, i32);
+type Coord = (i32, i32);
 
 #[aoc_generator(day9)]
-fn parse_input(input: &str) -> Vec<Position> {
+fn parse_input(input: &str) -> Vec<Coord> {
     let mut out = Vec::new();
     let (mut x, mut y) = (0, 0);
     out.push((x, y));
     for line in input.lines() {
-        let mut split = line.split_ascii_whitespace();
-        let dir = split.next().unwrap();
-        let num: i32 = split.next().unwrap().parse().unwrap();
+        let (dir, num) = line.split_once(' ').unwrap();
+        let num: i32 = num.parse().unwrap();
         for _ in 0..num {
             match dir {
                 "U" => y += 1,
@@ -28,7 +27,7 @@ fn parse_input(input: &str) -> Vec<Position> {
 }
 
 #[aoc_run(day9, part1)]
-fn part1(input: &[Position]) -> usize {
+fn part1(input: &[Coord]) -> usize {
     let mut tail = (0, 0);
     let mut set = HashSet::new();
     set.insert(tail);
@@ -36,22 +35,30 @@ fn part1(input: &[Position]) -> usize {
         tail = new_tail(mv, &tail);
         set.insert(tail);
     }
-
     set.len()
 }
 
 #[aoc_run(day9, part2)]
-fn part2(input: &[Position]) -> u32 {
-    0
+fn part2(input: &[Coord]) -> usize {
+    let mut knots = Vec::from_iter((0..9).map(|_| (0, 0)));
+    let mut set = HashSet::new();
+    set.insert((0, 0));
+    for mv in input {
+        let mut head = *mv;
+        for knot in knots.iter_mut() {
+            *knot = new_tail(&head, knot);
+            head = *knot;
+        }
+        set.insert(*knots.last().unwrap());
+    }
+    set.len()
 }
 
-fn new_tail(head: &Position, tail: &Position) -> Position {
+fn new_tail(head: &Coord, tail: &Coord) -> Coord {
     let dy = head.1 - tail.1;
     let dx = head.0 - tail.0;
     match (dx, dy) {
         (m, n) if m.abs() < 2 && n.abs() < 2 => *tail,
-        (m, _) if m.abs() == 2 => (tail.0 + m / 2, tail.1 + dy),
-        (_, m) if m.abs() == 2 => (tail.0 + dx, tail.1 + m / 2),
-        _ => unreachable!(),
+        _ => (tail.0 + dx.signum(), tail.1 + dy.signum()),
     }
 }