add day 19 part 2

coded by claude

day19/src/main.rs

@@ -1,12 +1,57 @@
+use std::thread::Builder;
+
 #[cfg(debug_assertions)]
 const LENGTH: usize = 5;
 #[cfg(not(debug_assertions))]
 const LENGTH: usize = 3_017_957;
 
 fn main() {
-    let builder = std::thread::Builder::new().stack_size(100_000_000);
+    let builder = new_thread_builder();
     let handle = builder.spawn(part_1).unwrap();
     handle.join().unwrap();
+
+    let builder = new_thread_builder();
+    let handle = builder.spawn(part_2).unwrap();
+    handle.join().unwrap();
+}
+
+fn part_2() {
+    let mut next = [0usize; LENGTH];
+    for (idx, val) in next.iter_mut().enumerate() {
+        *val = (idx + 1) % LENGTH;
+    }
+
+    let mut pointer = 0;
+    let mut length = LENGTH;
+
+    // walk to the elf across from pointer, tracking its predecessor
+    let mut before_across = 0;
+    for _ in 0..(length / 2 - 1) {
+        before_across = next[before_across];
+    }
+    let mut across = next[before_across];
+
+    loop {
+        let hops = if length % 2 == 1 { 2 } else { 1 };
+
+        // remove `across` by relinking through its predecessor
+        next[before_across] = next[across];
+        length -= 1;
+
+        if length == 1 {
+            println!("{}", pointer + 1);
+            break;
+        }
+
+        pointer = next[pointer];
+
+        for i in 0..hops {
+            across = next[before_across];
+            if i + 1 < hops {
+                before_across = across;
+            }
+        }
+    }
 }
 
 fn part_1() {
@@ -30,3 +75,7 @@ fn part_1() {
         }
     }
 }
+
+fn new_thread_builder() -> Builder {
+    std::thread::Builder::new().stack_size(100_000_000)
+}