From a443f736c77bf14240b6ec0c8e5c8c21447074c4 Mon Sep 17 00:00:00 2001
From: Tom Smeding <tom.smeding@gmail.com>
Date: Thu, 13 Dec 2018 19:31:25 +0100
Subject: Day 13 part 2

---
 2018/src/day13.rs | 47 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 37 insertions(+), 10 deletions(-)

diff --git a/2018/src/day13.rs b/2018/src/day13.rs
index 13ff3a4..92411c1 100644
--- a/2018/src/day13.rs
+++ b/2018/src/day13.rs
@@ -139,10 +139,18 @@ pub fn main<T: BufRead>(reader: T) -> io::Result<(String, String)> {
         }
     }
 
-    let part1;
+    let mut first_crash = None;
+    let part2;
+
+    loop {
+        let mut to_remove = Vec::new();
+
+        for (i, mut cart) in carts.iter_mut().enumerate() {
+            if !hascart[cart.pos.y][cart.pos.x] {
+                to_remove.push(i);
+                continue;
+            }
 
-    'outer: loop {
-        for mut cart in &mut carts {
             let pos2 = cart.nextpos();
             let c = grid[pos2.y][pos2.x];
 
@@ -156,15 +164,29 @@ pub fn main<T: BufRead>(reader: T) -> io::Result<(String, String)> {
                 dir2 = newdir(&cart.dir, c);
             }
 
+            hascart[cart.pos.y][cart.pos.x] = false;
+
             if hascart[pos2.y][pos2.x] {
-                part1 = format!("{},{}", pos2.x, pos2.y);
-                break 'outer;
+                if first_crash.is_none() {
+                    first_crash = Some(pos2.clone());
+                }
+
+                to_remove.push(i);
+                hascart[pos2.y][pos2.x] = false;
+            } else {
+                hascart[pos2.y][pos2.x] = true;
+                cart.pos = pos2;
+                cart.dir = dir2;
             }
+        }
 
-            hascart[cart.pos.y][cart.pos.x] = false;
-            hascart[pos2.y][pos2.x] = true;
-            cart.pos = pos2;
-            cart.dir = dir2;
+        for idx in to_remove {
+            carts.swap_remove(idx);
+        }
+
+        if carts.len() == 1 {
+            part2 = format!("{},{}", carts[0].pos.x, carts[0].pos.y);
+            break;
         }
 
         carts.sort_by_key(|cart| (cart.pos.y, cart.pos.x));
@@ -172,5 +194,10 @@ pub fn main<T: BufRead>(reader: T) -> io::Result<(String, String)> {
         // print_grid(&grid, &carts);
     }
 
-    Ok((part1, String::new()))
+    let part1 = match first_crash {
+        Some(pos) => format!("{},{}", pos.x, pos.y),
+        None => unreachable!()
+    };
+
+    Ok((part1, part2))
 }
-- 
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