Day 25, part 1

resources/twenty_five.txt

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+Begin in state A.
+Perform a diagnostic checksum after 12134527 steps.
+
+In state A:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state B.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state B:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state C.
+
+In state C:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 0.
+    - Move one slot to the left.
+    - Continue with state D.
+
+In state D:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state E.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the left.
+    - Continue with state C.
+
+In state E:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state F.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+
+In state F:
+  If the current value is 0:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state A.
+  If the current value is 1:
+    - Write the value 1.
+    - Move one slot to the right.
+    - Continue with state E.

src/advent/twenty_five.clj

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+(ns advent.twenty-five)
+
+(def blueprints {:A {0 {:write 1
+                        :move :right
+                        :next :B}
+                     1 {:write 0
+                        :move :left
+                        :next :C}}
+                 :B {0 {:write 1
+                        :move :left
+                        :next :A}
+                     1 {:write 1
+                        :move :right
+                        :next :C}}
+                 :C {0 {:write 1
+                        :move :right
+                        :next :A}
+                     1 {:write 0
+                        :move :left
+                        :next :D}}
+                 :D {0 {:write 1
+                        :move :left
+                        :next :E}
+                     1 {:write 1
+                        :move :left
+                        :next :C}}
+                 :E {0 {:write 1
+                        :move :right
+                        :next :F}
+                     1 {:write 1
+                        :move :right
+                        :next :A}}
+                 :F {0 {:write 1
+                        :move :right
+                        :next :A}
+                     1 {:write 1
+                        :move :right
+                        :next :E}}})
+
+(defn initial-machine [initial-state]
+  {:state initial-state
+   :position 0
+   :on #{}})
+
+(defmulti write (fn [_ value] value))
+
+(defmethod write 0 [{:keys [position] :as state} value]
+  (update state :on disj position))
+
+(defmethod write 1 [{:keys [position] :as state} value]
+  (update state :on conj position))
+
+(defmulti move (fn [_ value] value))
+
+(defmethod move :left [{:keys [position] :as state} value]
+  (update state :position dec))
+
+(defmethod move :right [{:keys [position] :as state} value]
+  (update state :position inc))
+
+(defn current-value [{:keys [on position]}]
+  (if (contains? on position)
+    1
+    0))
+
+(defn move-once [{:keys [state] :as machine}]
+  (let [value (current-value machine)
+        {to-write :write
+         to-move :move
+         next-state :next} (get-in blueprints [state value])]
+    (-> machine
+        (write to-write)
+        (move to-move)
+        (assoc :state next-state))))
+
+(defn diagnostic-checksum [{:keys [on]}]
+  (count on))
+
+(defn run []
+  (let [steps-needed 12134527]
+    (loop [machine (initial-machine :A)
+           steps-taken 0]
+      (if (= steps-taken steps-needed)
+        (diagnostic-checksum machine)
+        (recur (move-once machine)
+               (inc steps-taken))))))