2016 Update

Task/Rep-string/Common-Lisp/rep-string-1.lisp

@@ -0,0 +1,18 @@
+(ql:quickload :alexandria)
+(defun rep-stringv (a-str &optional (max-rotation (floor (/ (length a-str) 2))))
+  ;; Exit condition if no repetition found.
+  (cond ((< max-rotation 1) "Not a repeating string")
+        ;; Two checks:
+        ;; 1. Truncated string must be equal to rotation by repetion size.
+        ;; 2. Remaining chars (rest-str) are identical to starting chars (beg-str)
+        ((let* ((trunc (* max-rotation (truncate (length a-str) max-rotation)))
+                (truncated-str (subseq a-str 0 trunc))
+                (rest-str (subseq a-str trunc))
+                (beg-str (subseq a-str 0 (rem (length a-str) max-rotation))))
+           (and (string= beg-str rest-str)
+                (string= (alexandria:rotate (copy-seq truncated-str) max-rotation)
+                         truncated-str)))
+         ;; If both checks pass, return the repeting string.
+         (subseq a-str 0 max-rotation))
+        ;; Recurse function reducing length of rotation.
+        (t (rep-stringv a-str (1- max-rotation)))))

Task/Rep-string/Common-Lisp/rep-string-2.lisp

@@ -0,0 +1,15 @@
+(setf test-strings '("1001110011"
+                     "1110111011"
+                     "0010010010"
+                     "1010101010"
+                     "1111111111"
+                     "0100101101"
+                     "0100100"
+                     "101"
+                     "11"
+                     "00"
+                     "1"
+                     ))
+
+(loop for item in test-strings
+      collecting (cons item (rep-stringv item)))