Finally discovered that pos != new_pos

2021/benches/days.rs

@@ -7,7 +7,7 @@ use criterion::criterion_main;
 use criterion::BenchmarkId;
 use criterion::Criterion;
 
-const DAYS_IMPLEMENTED: usize = 20;
+const DAYS_IMPLEMENTED: usize = 21;
 
 fn read_input(day: usize) -> Vec<u8> {
     let input_path = format!("inputs/{:02}.txt", day);

2021/src/day21.rs

@@ -96,8 +96,57 @@ pub fn part1(input: &mut dyn Read) -> String {
     (loser_score * rolls).to_string()
 }
 
-pub fn part2(_input: &mut dyn Read) -> String {
-    todo!()
+const MAX_TURNS: usize = 21; // Trivial upper bound, could be lower
+const ROLLS: [i32; 7] = [3, 4, 5, 6, 7, 8, 9];
+const FREQS: [u64; 7] = [1, 3, 6, 7, 6, 3, 1];
+
+fn multiverse_recursive(
+    remaining: &mut [u64],
+    wins: &mut [u64],
+    turn: usize,
+    alive: u64,
+    pos: i32,
+    score: i32,
+) {
+    for (roll, freq) in ROLLS.into_iter().zip(FREQS) {
+        let new_pos = (pos + roll - 1) % 10 + 1;
+        let new_score = score + new_pos;
+        let new_alive = alive * freq;
+
+        if new_score >= 21 {
+            wins[turn] += new_alive;
+        } else {
+            remaining[turn] += new_alive;
+            multiverse_recursive(remaining, wins, turn + 1, new_alive, new_pos, new_score);
+        }
+    }
+}
+
+fn multiverse(pos: i32) -> ([u64; MAX_TURNS], [u64; MAX_TURNS]) {
+    let mut alive = [0; MAX_TURNS];
+    let mut wins = [0; MAX_TURNS];
+
+    alive[0] = 1;
+
+    multiverse_recursive(&mut alive, &mut wins, 1, 1, pos, 0);
+
+    (wins, alive)
+}
+
+pub fn part2(input: &mut dyn Read) -> String {
+    let (a, b) = read_input(input);
+
+    let (a_wins, a_alive) = multiverse(a);
+    let (b_wins, b_alive) = multiverse(b);
+
+    let a_winner: u64 = a_wins[1..]
+        .into_iter()
+        .zip(b_alive)
+        .map(|(&a, b)| a * b)
+        .sum();
+    let b_winner: u64 = b_wins.into_iter().zip(a_alive).map(|(a, b)| a * b).sum();
+
+    a_winner.max(b_winner).to_string()
 }
 
 #[cfg(test)]
@@ -112,4 +161,9 @@ mod tests {
     fn sample_part1() {
         test_implementation(part1, SAMPLE, 739785);
     }
+
+    #[test]
+    fn sample_part2() {
+        test_implementation(part2, SAMPLE, 444356092776315u64);
+    }
 }