Faster recursive solution

Author: maneatingape

README.md

@@ -88,7 +88,7 @@ Performance is reasonable even on older hardware, for example a 2011 MacBook Pro
 | 14 | [Restroom Redoubt](https://adventofcode.com/2024/day/14) | [Source](src/year2024/day14.rs) | 74 |
 | 15 | [Warehouse Woes](https://adventofcode.com/2024/day/15) | [Source](src/year2024/day15.rs) | 303 |
 | 16 | [Reindeer Maze](https://adventofcode.com/2024/day/16) | [Source](src/year2024/day16.rs) | 390 |
-| 17 | [Chronospatial Computer](https://adventofcode.com/2024/day/17) | [Source](src/year2024/day17.rs) | 6 |
+| 17 | [Chronospatial Computer](https://adventofcode.com/2024/day/17) | [Source](src/year2024/day17.rs) | 2 |
 
 ## 2023
 

src/year2024/day17.rs

@@ -29,6 +29,7 @@
 //!
 //! [`Intcode`]: crate::year2019::intcode
 use crate::util::parse::*;
+use std::ops::ControlFlow;
 
 pub fn parse(input: &str) -> Vec<u64> {
     input.iter_unsigned().collect()
@@ -51,28 +52,25 @@ pub fn part1(input: &[u64]) -> String {
 
 pub fn part2(input: &[u64]) -> u64 {
     // Start with known final value of `a`.
-    let mut todo = vec![0];
+    helper(input, input.len() - 1, 0).break_value().unwrap()
+}
 
-    for &valid in input.iter().skip(3).rev() {
-        let mut next = Vec::new();
+fn helper(program: &[u64], index: usize, a: u64) -> ControlFlow<u64> {
+    if index == 2 {
+        return ControlFlow::Break(a);
+    }
 
-        // Try all 8 combination of lower 3 bits for each possible valid value.
-        for i in todo {
-            for j in 0..8 {
-                let a = (i << 3) | j;
-                let mut computer = Computer::new(input, a);
+    // Try all 8 combination of lower 3 bits.
+    for i in 0..8 {
+        let next_a = (a << 3) | i;
+        let out = Computer::new(program, next_a).run().unwrap();
 
-                if computer.run().is_some_and(|out| out == valid) {
-                    next.push(a);
-                }
-            }
+        if out == program[index] {
+            helper(program, index - 1, next_a)?;
         }
-
-        todo = next;
     }
 
-    // Lowest possible initial value.
-    todo[0]
+    ControlFlow::Continue(())
 }
 
 struct Computer<'a> {