Day 21 part 2. Shitty bitty.

src/tasks/day21.rs

@@ -1,8 +1,9 @@
+use std::collections::HashSet;
 use std::ops::{BitAnd, BitOr};
 
 pub fn task1() {
     let input = include_str!("../../input/day21.txt");
-    let result = run1(input, 0);
+    let result = run1(input, 3007673);
     println!("{result:?}");
 }
 
@@ -15,6 +16,8 @@ fn run1(input: &str, start_value: i64) -> (i64, usize) {
     let mut registers: Registers = [start_value, 0, 0, 0, 0, 0];
     let mut ip = 0;
     let mut count = 0usize;
+    let mut seen: HashSet<_> = HashSet::new();
+    let mut last = 0;
     loop {
         //print!("ip={ip}, {registers:?}");
 
@@ -31,14 +34,29 @@ fn run1(input: &str, start_value: i64) -> (i64, usize) {
         ip = registers[ip_register] as usize;
         ip += 1;
 
-        // uncomment this and print look at register 2 to find the value for register 0
+        // Part 1: uncomment this and print look at register 2 to find the value for register 0
         // that would cause the program to halt
+        // if ip == 28 {
+        //     println!("Value in register 2: {}", registers[2]);
+        // }
+        // Part 2:  9969507 too high
+        // 16774755 highest value
+        // Store all values of register 2 at the possible exit point. Once you see one you already
+        // saw: you've seen all (there is a loop). The one before that was the one it took the longest
+        // to reach without seeing anything twice.
         if ip == 28 {
-            println!("{registers:?}");
-            break;
+            if seen.contains(&registers[2]) {
+                println!("First double: {registers:?}");
+                println!("Last was {last}");
+                break;
+            } else {
+                seen.insert(registers[2]);
+                last = registers[2];
+            }
         }
 
         if program.get(ip).is_none() {
+            println!("exit by leaving instruction space");
             break;
         }
         count += 1;