2 files changed, 59 insertions, 0 deletions
diff --git a/year2021/day21/part1.clj b/year2021/day21/part1.clj
new file mode 100644
index 0000000..174f5a1
--- /dev/null
+++ b/year2021/day21/part1.clj
@@ -0,0 +1,22 @@
+(require 'clojure.string)
+
+(loop [player-positions   (->> (slurp "./in")
+                               clojure.string/split-lines
+                               (map (comp read-string second #(clojure.string/split % #": ")))
+                               (mapv #(drop (dec %) (cycle (range 1 11)))))
+       player-scores      (into [] (repeat (count player-positions) 0))
+       player-turn        (cycle (range 0 (count player-scores)))
+       deterministic-dice (cycle (range 1 101))
+       dice-roll-count    0]
+  (if-not (every? #(< % 1000) player-scores)
+    (println (* dice-roll-count (apply min player-scores)))
+    (let [roll         (reduce + (take 3 deterministic-dice))
+          turn         (first player-turn)
+          new-position (drop roll (get player-positions turn))]
+      (recur
+        (assoc player-positions turn new-position)
+        (update player-scores turn + (first new-position))
+        (next player-turn)
+        (drop 3 deterministic-dice)
+        (+ dice-roll-count 3)))))
+
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))})))))
+