Day 21

Cargo.toml

@@ -82,3 +82,7 @@ path = "src/day19.rs"
 [[bin]]
 name = "day20"
 path = "src/day20.rs"
+
+[[bin]]
+name = "day21"
+path = "src/day21.rs"

inputs/day21.txt

@@ -0,0 +1,2 @@
+Player 1 starting position: 8
+Player 2 starting position: 7

src/day21.rs

@@ -0,0 +1,122 @@
+use std::collections::HashMap;
+
+struct DeterministicDice {
+    rolls: usize,
+    last: usize,
+}
+
+impl DeterministicDice {
+    fn new() -> DeterministicDice {
+        DeterministicDice { rolls: 0, last: 0 }
+    }
+}
+
+impl Iterator for DeterministicDice {
+    type Item = usize;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.rolls += 3;
+        let x = self.last % 100 + 1;
+        let y = (self.last + 1) % 100 + 1;
+        let z = (self.last + 2) % 100 + 1;
+        self.last = z;
+        Some(x + y + z)
+    }
+}
+
+fn turns_to_win(positions: (usize, usize)) -> (usize, usize) {
+    let mut dice = DeterministicDice::new();
+    let mut positions = positions;
+    let mut scores = (0, 0);
+
+    loop {
+        positions.0 = (positions.0 + dice.next().unwrap() - 1) % 10 + 1;
+        scores.0 += positions.0;
+        if scores.0 >= 1000 {
+            return (dice.rolls, scores.1);
+        }
+
+        positions.1 = (positions.1 + dice.next().unwrap() - 1) % 10 + 1;
+        scores.1 += positions.1;
+        if scores.1 >= 1000 {
+            return (dice.rolls, scores.0);
+        }
+    }
+}
+
+fn simulate_universes(positions: (usize, usize)) -> usize {
+    type GameState = ((usize, usize), (usize, usize));
+
+    let mut games: HashMap<GameState, usize> = HashMap::new();
+    let mut wins = (0, 0);
+
+    games.insert(((positions.0 - 1, positions.1 - 1), (0, 0)), 1);
+
+    let dice_rolls = [
+        3, 4, 5, 4, 5, 6, 5, 6, 7, 4, 5, 6, 5, 6, 7, 6, 7, 8, 5, 6, 7, 6, 7, 8, 7, 8, 9,
+    ];
+    loop {
+        if games.is_empty() {
+            break;
+        }
+        let mut next_turn = HashMap::new();
+        for ((positions, scores), count) in games {
+            for player1_dice in dice_rolls {
+                let player1_position = (positions.0 + player1_dice) % 10;
+                let player1_score = scores.0 + player1_position + 1;
+                if player1_score < 21 {
+                    for player2_dice in dice_rolls {
+                        let player2_position = (positions.1 + player2_dice) % 10;
+                        let player2_score = scores.1 + player2_position + 1;
+                        if player2_score < 21 {
+                            let new_count = next_turn
+                                .entry((
+                                    (player1_position, player2_position),
+                                    (player1_score, player2_score),
+                                ))
+                                .or_insert(0);
+                            *new_count += count;
+                        } else {
+                            wins.1 += count;
+                        }
+                    }
+                } else {
+                    wins.0 += count;
+                }
+            }
+        }
+        games = next_turn;
+    }
+
+    if wins.0 > wins.1 {
+        wins.0
+    } else {
+        wins.1
+    }
+}
+
+fn main() {
+    const INPUT: &str = include_str!("../inputs/day21.txt");
+    let positions = INPUT
+        .lines()
+        .map(|line| line.rsplit_once(' ').unwrap().1.parse().unwrap())
+        .collect::<Vec<usize>>();
+    let (turns, losing_score) = turns_to_win((positions[0], positions[1]));
+    println!("solution: {}", turns * losing_score);
+    println!(
+        "solution: {}",
+        simulate_universes((positions[0], positions[1]))
+    );
+}
+
+#[test]
+fn test_turns_to_win() {
+    let (turns, losing_score) = turns_to_win((4, 8));
+    assert_eq!(turns, 993);
+    assert_eq!(losing_score, 745);
+}
+
+#[test]
+fn test_simulate_universes() {
+    assert_eq!(simulate_universes((4, 8)), 444356092776315);
+}