X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/bb99b6957941ea4ea57835fad294baf7312b668c..d5fdd49e70b734b791eb907706f92da5775e2a8b:/src/utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index a496283..25733e8 100644
--- a/src/utilities.lisp
+++ b/src/utilities.lisp
@@ -451,6 +451,25 @@
    and return the result of appending all of the resulting lists."
   (reduce #'append (apply #'mapcar function list more-lists) :from-end t))
 
+(export 'cross-product)
+(defun cross-product (&rest pieces)
+  "Return the cross product of the PIECES.
+
+   Each arguments may be a list, or a (non-nil) atom, which is equivalent to
+   a singleton list containing just that atom.  Return a list of all possible
+   lists which can be constructed by taking one item from each argument list
+   in turn, in an arbitrary order."
+  (reduce (lambda (piece tails)
+	    (mapcan (lambda (tail)
+		      (mapcar (lambda (head)
+				(cons head tail))
+			      (if (listp piece) piece
+				  (list piece))))
+		    tails))
+	  pieces
+	  :from-end t
+	  :initial-value '(nil)))
+
 (export 'distinguished-point-shortest-paths)
 (defun distinguished-point-shortest-paths (root neighbours-func)
   "Moderately efficient shortest-paths-from-root computation.
@@ -717,18 +736,18 @@
 			      (setf (gethash k seen) item)))))
 		sequence)))
 	((listp sequence)
-	 (mapl (lambda (tail)
-		 (let* ((item (car tail))
-			(rest (cdr tail))
-			(match (member (funcall key item) rest
-				       :test test :key key)))
-		   (when match (funcall report item (car match)))))
-	       sequence))
+	 (do ((tail sequence (cdr tail))
+	      (i 0 (1+ i)))
+	     ((endp tail))
+	     (let* ((item (car tail))
+		    (match (find (funcall key item) sequence
+				 :test test :key key :end i)))
+	       (when match (funcall report item match)))))
 	((vectorp sequence)
 	 (dotimes (i (length sequence))
 	   (let* ((item (aref sequence i))
 		  (pos (position (funcall key item) sequence
-				 :key key :test test :start (1+ i))))
+				 :key key :test test :end i)))
 	     (when pos (funcall report item (aref sequence pos))))))
 	(t
 	 (error 'type-error :datum sequence :expected-type 'sequence))))