tasks a-s

Task/Permutations/Common-Lisp/permutations-2.lisp

@@ -0,0 +1,18 @@
+(defun next-perm (vec cmp)  ; modify vector
+  (declare (type (simple-array * (*)) vec))
+  (macrolet ((el (i) `(aref vec ,i))
+             (cmp (i j) `(funcall cmp (el ,i) (el ,j))))
+    (loop with len = (1- (length vec))
+       for i from (1- len) downto 0
+       when (cmp i (1+ i)) do
+         (loop for k from len downto i
+            when (cmp i k) do
+              (rotatef (el i) (el k))
+              (setf k (1+ len))
+              (loop while (< (incf i) (decf k)) do
+                   (rotatef (el i) (el k)))
+              (return-from next-perm vec)))))
+
+;;; test code
+(loop for a = "1234" then (next-perm a #'char<) while a do
+     (write-line a))