March 2014 update

Task/Universal-Turing-machine/Clojure/universal-turing-machine-1.clj

@@ -0,0 +1,26 @@
+(defn tape
+  "Creates a new tape with given blank character and tape contents"
+  ([blank] (tape () blank () blank))
+  ([right blank] (tape () (first right) (rest right) blank))
+  ([left head right blank] [(reverse left) (or head blank) (into () right) blank]))
+
+; Tape operations
+(defn- left  [[[l & ls] _ rs       b] c] [ls          (or l b) (conj rs c) b])
+(defn- right [[ls       _ [r & rs] b] c] [(conj ls c) (or r b) rs          b])
+(defn- stay  [[ls       _ rs       b] c] [ls          c        rs          b])
+(defn- head [[_ c _ b]] (or c b))
+(defn- pretty [[ls c rs b]] (concat (reverse ls) [[(or c b)]] rs))
+
+(defn new-machine
+ "Returns a function that takes a tape as input, and returns the tape
+  after running the machine specified in `machine`."
+  [machine]
+  (let [rules (into {} (for [[s c c' a s'] (:rules machine)]
+                         [[s c] [c' (-> a name symbol resolve) s']]))
+        finished? (into #{} (:terminating machine))]
+    (fn [input-tape]
+      (loop [state (:initial machine) tape input-tape]
+        (if (finished? state)
+          (pretty tape)
+          (let [[out action new-state] (get rules [state (head tape)])]
+            (recur new-state (action tape out))))))))