Add day17 part2

day17/src/main.rs

@@ -1,4 +1,4 @@
-#[derive(Debug)]
+#[derive(Debug, Clone)]
 struct State {
     reg_a: usize,
     reg_b: usize,
@@ -67,19 +67,61 @@ fn parse(input: &str) -> State {
     }
 }
 
-fn part1(state: &mut State) -> String {
+fn get_output(state: &mut State) -> Vec<u8> {
     while state.pc < state.prog.len() - 1 {
         state.exec_instruction();
     }
-    state
-        .out
+    state.out.clone()
+}
+
+fn part1(state: &mut State) -> String {
+    get_output(state)
         .iter()
         .map(|n| n.to_string())
         .collect::<Vec<_>>()
         .join(",")
 }
 
-fn main() {
-    let mut input = parse(include_str!("../input.txt"));
-    println!("{}", part1(&mut input));
+// B <- A % 8
+// B <- B ^ 3
+// C <- A >> B
+// B <- B ^ C
+// A <- A >> 3
+// B <- B ^ 5
+// print B & 7
+// jmp 0 if A
+
+fn find_input(state: &State, instructions: &[u8], curr_a: usize) -> Vec<usize> {
+    let mut candidates = vec![];
+    if instructions.is_empty() {
+        return vec![curr_a];
+    }
+    let curr = instructions.last().unwrap();
+    for i in 0..8 {
+        let mut new_state = state.clone();
+        let new_a = (curr_a << 3) + i;
+        new_state.reg_a = new_a;
+        if get_output(&mut new_state)[0] == *curr {
+            candidates.push(new_a);
+        }
+    }
+    let mut results = vec![];
+    for candidate in candidates {
+        results.extend(find_input(
+            state,
+            &instructions[0..instructions.len() - 1],
+            candidate,
+        ));
+    }
+    results
+}
+
+fn part2(state: &State) -> usize {
+    *find_input(state, &state.prog, 0).iter().min().unwrap()
+}
+
+fn main() {
+    let input = parse(include_str!("../input.txt"));
+    println!("{}", part1(&mut input.clone()));
+    println!("{}", part2(&input));
 }