Add 21/2

2021/src/bin/day21.rs

@@ -1,8 +1,8 @@
 #![feature(test)]
+use fnv::FnvHashMap;
 extern crate test;
 
 type Parsed = (u16, u16);
-
 const INPUT: Parsed = (7, 3);
 
 fn part1((p1, p2): Parsed) -> usize {
@@ -22,8 +22,48 @@ fn part1((p1, p2): Parsed) -> usize {
         .unwrap()
 }
 
+#[derive(Debug, Hash, PartialEq, Eq, Clone, Copy)]
+struct GameState {
+    odd_round: bool,
+    // Tuples are (position, total score)
+    scores:    [(u16, usize); 2],
+}
+
+// Just this, but cached and grouped because I don’t want to calc it in the loop
+// iproduct!(1..=3, 1..=3, 1..=3).map(|(a, b, c)| a + b + c)
+const POSSIBLE_ROLLS: [(u16, usize); 7] = [(6, 7), (5, 6), (7, 6), (4, 3), (8, 3), (3, 1), (9, 1)];
+
 fn part2((p1, p2): Parsed) -> usize {
-    unimplemented!()
+    let start = GameState { odd_round: false, scores: [(p1, 0), (p2, 0)] };
+    let mut games = FnvHashMap::default();
+    games.insert(start, 1);
+    let mut p1_wins = 0;
+    let mut p2_wins = 0;
+    while !games.is_empty() {
+        games = games
+            .iter()
+            .flat_map(|(start, count)| POSSIBLE_ROLLS.iter().map(move |(die, count2)| (start, count * count2, die)))
+            .map(|(start, count, die)| {
+                let mut scores = start.scores;
+                let mut points = scores[start.odd_round as usize].0 + die;
+                points -= (points - 1) / 10 * 10;
+                scores[start.odd_round as usize].0 = points;
+                scores[start.odd_round as usize].1 += points as usize;
+                (GameState { odd_round: !start.odd_round, scores }, count)
+            })
+            .fold(FnvHashMap::default(), |mut acc, (state, count)| {
+                // Remove done games and queue the rest for another round
+                if state.scores[0].1 >= 21 {
+                    p1_wins += count;
+                } else if state.scores[1].1 >= 21 {
+                    p2_wins += count;
+                } else {
+                    *acc.entry(state).or_insert(0) += count;
+                }
+                acc
+            })
+    }
+    p1_wins.max(p2_wins)
 }
 
 fn main() {
@@ -41,8 +81,18 @@ mod tests {
         assert_eq!(part1((4, 8)), 739785);
     }
 
+    #[test]
+    fn part2_test() {
+        assert_eq!(part2((4, 8)), 444356092776315);
+    }
+
     #[bench]
     fn part1_bench(b: &mut test::Bencher) {
         b.iter(|| assert_eq!(part1(test::black_box(INPUT)), 551901))
     }
+
+    #[bench]
+    fn part2_bench(b: &mut test::Bencher) {
+        b.iter(|| assert_eq!(part2(test::black_box(INPUT)), 272847859601291))
+    }
 }