21

2021/21.input

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

2021/21.py

@@ -0,0 +1,91 @@
+import functools
+
+def parse_input(s):
+    p1, p2 = s.splitlines()
+    p1 = int(p1.split()[4])
+    p2 = int(p2.split()[4])
+
+    return p1, p2
+
+def gen_die():
+    while True:
+        for n in range(1, 101):
+            yield n
+
+def step(player, score, die):
+    r1 = next(die)
+    r2 = next(die)
+    r3 = next(die)
+    player += r1 + r2 + r3
+    while player > 10:
+        player -= 10
+    score += player
+
+    return player, score
+
+def part1(s):
+    p1, p2 = parse_input(s)
+    s1, s2 = 0, 0
+
+    die = gen_die()
+
+    rolls = 0
+
+    while True:
+        p1, s1 = step(p1, s1, die)
+        rolls += 3
+        if s1 >= 1000:
+            break
+        p2, s2 = step(p2, s2, die)
+        rolls += 3
+        if s2 >= 1000:
+            break
+
+    answer = rolls * min(s1, s2)
+
+    print(f'The answer to part one is {answer}')
+
+def part2(s):
+    p1, p2 = parse_input(s)
+
+    @functools.cache
+    def impl(p1, p2, s1=0, s2=0, active_player=1):
+        p1_wins = 0
+        p2_wins = 0
+        for r1 in (1, 2, 3):
+            for r2 in (1, 2, 3):
+                for r3 in (1, 2, 3):
+                    if active_player == 1:
+                        new_p1 = p1 + r1 + r2 + r3
+                        while new_p1 > 10:
+                            new_p1 -= 10
+                        new_s1 = s1 + new_p1
+                        if new_s1 >= 21:
+                            p1_wins += 1
+                        else:
+                            sub_p1_wins, sub_p2_wins = impl(new_p1, p2, new_s1, s2, 2)
+                            p1_wins += sub_p1_wins
+                            p2_wins += sub_p2_wins
+                    else:
+                        new_p2 = p2 + r1 + r2 + r3
+                        while new_p2 > 10:
+                            new_p2 -= 10
+                        new_s2 = s2 + new_p2
+                        if new_s2 >= 21:
+                            p2_wins += 1
+                        else:
+                            sub_p1_wins, sub_p2_wins = impl(p1, new_p2, s1, new_s2, 1)
+                            p1_wins += sub_p1_wins
+                            p2_wins += sub_p2_wins
+        return p1_wins, p2_wins
+
+    p1_wins, p2_wins = impl(p1, p2)
+
+    answer = max(p1_wins, p2_wins)
+
+    print(f'The answer to part two is {answer}')
+
+with open('./2021/21.input') as input_file:
+    INPUT = input_file.read()
+part1(INPUT)
+part2(INPUT)

2021/21.test

@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 8