Solve 2021 day 21

2021/21_dice.py

@@ -0,0 +1,42 @@
+#!/usr/bin/env python3
+import sys
+from itertools import cycle
+from functools import lru_cache
+
+def move(pos, score, amount):
+    pos = (pos + amount - 1) % 10 + 1
+    return pos, score + pos
+
+def practice(a, b):
+    players = [(a, 0), (b, 0)]
+    die = cycle(range(1, 101))
+    rolls = 0
+    while True:
+        for player, (pos, score) in enumerate(players):
+            roll = next(die) + next(die) + next(die)
+            rolls += 3
+            pos, score = players[player] = move(pos, score, roll)
+            if score >= 1000:
+                otherpos, otherscore = players[1 - player]
+                return rolls * otherscore
+
+def play(a, b):
+    @lru_cache(maxsize=None)
+    def wins(a, b):
+        awins = bwins = 0
+        for x in range(1, 4):
+            for y in range(1, 4):
+                for z in range(1, 4):
+                    pos, score = roll_a = move(*a, x + y + z)
+                    if score >= 21:
+                        awins += 1
+                    else:
+                        roll_bwins, roll_awins = wins(b, roll_a)
+                        awins += roll_awins
+                        bwins += roll_bwins
+        return awins, bwins
+    return max(wins((a, 0), (b, 0)))
+
+a, b = (int(line.split(': ')[-1]) for line in sys.stdin)
+print(practice(a, b))
+print(play(a, b))

2021/input/21

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+Player 1 starting position: 8
+Player 2 starting position: 7