From 5180a1e9e16a5e786b40036a0fa5a0ed6950baaa Mon Sep 17 00:00:00 2001
From: kageru <kageru@encode.moe>
Date: Fri, 9 Dec 2022 12:12:03 +0100
Subject: [PATCH] Add 2022/09/2

---
 2022/src/bin/day09.rs | 50 ++++++++++++++++++++++++++++---------------
 1 file changed, 33 insertions(+), 17 deletions(-)

diff --git a/2022/src/bin/day09.rs b/2022/src/bin/day09.rs
index c4ac140..e84d18b 100644
--- a/2022/src/bin/day09.rs
+++ b/2022/src/bin/day09.rs
@@ -12,46 +12,52 @@ fn parse_input(raw: &str) -> Parsed {
 }
 
 fn part1(parsed: &Parsed) -> usize {
+    rope::<2>(parsed)
+}
+
+fn part2(parsed: &Parsed) -> usize {
+    rope::<10>(parsed)
+}
+
+fn rope<const KNOTS: usize>(parsed: &Parsed) -> usize {
+    let mut knots = [(0i32, 0i32); KNOTS];
     let mut positions = HashSet::from([(0, 0)]);
-    let mut head = (0, 0);
-    let mut tail = (0, 0);
     for &(dir, n) in parsed {
         for _ in 0..n {
             match dir {
-                b'U' => head.1 += 1,
-                b'D' => head.1 -= 1,
-                b'L' => head.0 -= 1,
-                b'R' => head.0 += 1,
+                b'U' => knots[0].1 += 1,
+                b'D' => knots[0].1 -= 1,
+                b'L' => knots[0].0 -= 1,
+                b'R' => knots[0].0 += 1,
                 _ => unreachable!(),
             }
-            if step_towards(&mut tail, &head) {
-                positions.insert(tail);
+            for i in 0..KNOTS - 1 {
+                let ro = knots[i];
+                if !step_towards(&mut knots[i + 1], ro) {
+                    break;
+                }
             }
+            positions.insert(knots[KNOTS - 1]);
         }
     }
     positions.len()
 }
 
-fn step_towards(tail: &mut (i32, i32), head: &(i32, i32)) -> bool {
+fn step_towards(tail: &mut (i32, i32), head: (i32, i32)) -> bool {
     let xdiff = head.0 - tail.0;
     let ydiff = head.1 - tail.1;
     match (xdiff.abs(), ydiff.abs()) {
         (0 | 1, 0 | 1) => return false,
         (_, 0) => tail.0 += xdiff.signum(),
         (0, _) => tail.1 += ydiff.signum(),
-        (1, _) | (_, 1) => {
+        _ => {
             tail.0 += xdiff.signum();
             tail.1 += ydiff.signum();
         }
-        _ => return false,
     }
     true
 }
 
-fn part2(parsed: &Parsed) -> usize {
-    unimplemented!()
-}
-
 boilerplate! {
     TEST_INPUT == "R 4
 U 4
@@ -63,9 +69,19 @@ L 5
 R 2",
     tests: {
         part1: { TEST_INPUT => 13 },
-        part2: { TEST_INPUT => 0 },
+        part2: {
+            TEST_INPUT => 1,
+            "R 5
+U 8
+L 8
+D 3
+R 17
+D 10
+L 25
+U 20" => 36,
+        },
     },
     bench1 == 6367,
-    bench2 == 0,
+    bench2 == 2536,
     bench_parse: Vec::len => 2000,
 }