day 17 part 2

src/day17.rs

@@ -1,4 +1,4 @@
-use std::{fmt::Write, fs::read_to_string, io::BufRead, ops::BitXor};
+use std::{fs::read_to_string, ops::BitXor};
 
 use itertools::Itertools;
 
@@ -12,30 +12,21 @@ pub fn day_main() {
 type RiddleResult = String;
 
 fn part1(input: &str) -> RiddleResult {
-    let (first, second) = input.split_once("\n\n").unwrap();
-    dbg!(first);
-    dbg!(second);
-    let mut registers = first.lines().map(|line| line.split_once(": ").unwrap().1);
+    let (a, b, c, program) = parse_input(input);
 
-    let mut a = registers.next().unwrap().parse::<i64>().unwrap();
-    let mut b = registers.next().unwrap().parse::<i64>().unwrap();
-    let mut c = registers.next().unwrap().parse::<i64>().unwrap();
-
-    let program = second
-        .split_once(": ")
-        .unwrap()
-        .1
-        .split(",")
-        .map(|v| v.parse::<i64>().unwrap())
-        .collect_vec();
+    run(&program, a, b, c)
+        .into_iter()
+        .map(|it| it.to_string())
+        .join(",")
+}
 
+fn run(program: &Vec<i64>, mut a: i64, mut b: i64, mut c: i64) -> Vec<i64> {
     let mut ip = 0;
-    let mut output = String::new();
+    let mut output = vec![];
 
     while ip < program.len() {
         let opcode = program[ip];
         let operand = program[(ip + 1) % program.len()];
-        // println!("a: {a}, b: {b}, c: {c}, op={opcode} {operand} (ip: {ip})");
         match opcode {
             0 => a = a / (2i64.pow(val(a, b, c, operand) as u32)),
             1 => b = b.bitxor(operand),
@@ -48,26 +39,32 @@ fn part1(input: &str) -> RiddleResult {
                 }
             }
             4 => b = b.bitxor(c),
-            5 => output
-                .write_fmt(format_args!("{},", val(a, b, c, operand) % 8))
-                .unwrap(),
+            5 => output.push(val(a, b, c, operand) % 8),
             6 => b = a / (2i64.pow(val(a, b, c, operand) as u32)),
             7 => c = a / (2i64.pow(val(a, b, c, operand) as u32)),
             _ => panic!("illegal op code {}", opcode),
         }
         ip += 2;
-        // ip %= program.len();
-        // wait();
-    }
-    if output.ends_with(",") {
-        output.remove(output.len() - 1);
     }
     output
 }
 
-fn wait() {
-    let stdin = std::io::stdin();
-    let _line1 = stdin.lock().lines().next().unwrap().unwrap();
+fn parse_input(input: &str) -> (i64, i64, i64, Vec<i64>) {
+    let (first, second) = input.split_once("\n\n").unwrap();
+    let mut registers = first.lines().map(|line| line.split_once(": ").unwrap().1);
+
+    let a = registers.next().unwrap().parse::<i64>().unwrap();
+    let b = registers.next().unwrap().parse::<i64>().unwrap();
+    let c = registers.next().unwrap().parse::<i64>().unwrap();
+
+    let program = second
+        .split_once(": ")
+        .unwrap()
+        .1
+        .split(",")
+        .map(|v| v.parse::<i64>().unwrap())
+        .collect_vec();
+    (a, b, c, program)
 }
 
 fn val(a: i64, b: i64, c: i64, operand: i64) -> i64 {
@@ -80,8 +77,34 @@ fn val(a: i64, b: i64, c: i64, operand: i64) -> i64 {
     }
 }
 
-fn part2(_input: &str) -> RiddleResult {
-    String::new()
+fn part2(input: &str) -> RiddleResult {
+    let (_, _, _, program) = parse_input(input);
+    solve(&program, program.len() - 1, 0).unwrap().to_string()
+}
+
+fn solve(program: &Vec<i64>, index: usize, a: i64) -> Option<i64> {
+    // if index == 0 {
+    //     return if &run(program, a, 0, 0) == program {
+    //         Some(a)
+    //     } else {
+    //         None
+    //     };
+    // }
+    for i in 0..8 {
+        let next_a = a * 8 + i;
+        let o = run(program, next_a, 0, 0);
+        if o[0] == program[index] {
+            if index == 0 {
+                return Some(next_a);
+            } else {
+                let m = solve(program, index - 1, next_a);
+                if m.is_some() {
+                    return m;
+                }
+            }
+        }
+    }
+    None
 }
 
 #[cfg(test)]
@@ -103,6 +126,17 @@ Program: 0,1,5,4,3,0
 
     #[test]
     fn test2() {
-        assert_eq!(part2(TEST_INPUT.trim()), "".to_string());
+        assert_eq!(
+            part2(
+                "Register A: 2024
+Register B: 0
+Register C: 0
+
+Program: 0,3,5,4,3,0
+"
+                .trim()
+            ),
+            "117440".to_string()
+        );
     }
 }