iterator-from-combos.lisp

(defun iterator-from-combos (&rest lists)
  "Takes a set of lists, and returns an iterator that iterates through every
possible combo of values where each value is from the list passed at its
index."
  (if (contains nil lists) ;; If any of the passed lists are nil, no combos can be made.
	  (lambda () (values nil t)) ;; There are no combos, return an empty iterator.
	  (labels ;; Otherwise, build the combos iterator.
		  ((shift (lists base) ;; Shifts the set of lists down by one. 
			 ;; Returns two values, the new lists, and whether we have hit the end
			 (if (not lists) ;; If no lists have been passed, we are done shifting.
				 (values nil t) ;; Return done value.
				 (let ((first (car lists)) (rest (cdr lists))) ;;Test if we need to ripple shift.
				   (if (not (cdr first)) ;; Ripple
					   (multiple-value-bind (result done) (shift (cdr lists) (cdr base))
						 (values (cons (car base) result) done)) ;; Otherwise, shift one and return.
					   (values (cons (cdr first) rest) nil)))))) ;; State variables.
		(let ((state lists)) ;;The actual iterator.
		  (lambda () ;;Check if last iteration.
			(if (not state) ;;Stop signal
				(values nil t) ;; Temporary storage of this result.
				(let ((result (mapcar #'car state))) ;; Shift.
				  (multiple-value-bind (new done) (shift state lists) 
					;; The shift function says we are done, set up so next iterations is last iteration.
					(if done
						(setf state nil)
						(setf state new))
					(values result nil)))))))))