X-Git-Url: https://git.distorted.org.uk/~mdw/sod/blobdiff_plain/9a3cb4610ab59001fd44c13dcdc6ea206acf0e50..00d59354c311fb28730b7c9b117b0d91aac092cc:/src/utilities.lisp

diff --git a/src/utilities.lisp b/src/utilities.lisp
index 1670f55..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.