1 files changed, 37 insertions, 0 deletions
diff --git a/year2021/day21/part2.clj b/year2021/day21/part2.clj
new file mode 100644
index 0000000..0042aaa
--- /dev/null
+++ b/year2021/day21/part2.clj
@@ -0,0 +1,37 @@
+(defn dice-probability-spread [sides rolls]
+  (let [sr (range 1 (inc sides))]
+    (->> sr
+         (iterate #(flatten (for [i sr] (map (partial + i) %))))
+         (drop (dec rolls))
+         ((comp frequencies first)))))
+
+(def rolls (dice-probability-spread 3 3))
+
+(defn advance-pos [pos roll] (let [n (+ pos roll)] (if (> n 10) (- n 10) n)))
+
+(defn add-roll [state roll p1-turn]
+  (let [pos   (if p1-turn :pos1 :pos2)
+        score (if p1-turn :score1 :score2)
+        new-pos (advance-pos (state pos) roll)]
+    (-> state
+        (assoc pos new-pos)
+        (update score + new-pos))))
+
+(defonce wins (atom [0 0]))
+
+(loop [turn 0 states {{:pos1 4 :score1 0 :pos2 8 :score2 0} 1}]
+  (if (empty? states)
+    (println (apply max @wins))
+    (recur
+      (if (= 0 turn) 1 0)
+      (reduce
+        #(let [kvp (first %2)]
+           (if (< ((key kvp) (if (= 0 turn) :score1 :score2)) 21)
+             (if (contains? %1 (key kvp))
+               (update %1 (key kvp) +' (val kvp))
+               (conj %1 kvp))
+             (do (swap! wins update turn +' (val kvp)) %1)))
+        {}
+        (for [s states r rolls]
+          {(add-roll (first s) (key r) (= 0 turn)) (*' (second s) (val r))})))))
+