Advent of Code 2015; Day 3.

day-3.lisp

(defun next-house (house i)
  (destructuring-bind (x y) house
    (case i
      (#\> (list (+ x 1) y))
      (#\< (list (- x 1) y))
      (#\^ (list x (+ y 1)))
      (t (list x (- y 1))))))

(defun count-unique-deliveries-by-santa (file)
  (with-open-file (stream file)
    (let ((instructions (read-line stream)))
      (loop for i across instructions
            for houses = (list (list 0 0)) then (cons (next-house (car houses)
                                                                  i)
                                                      houses)
            finally (return (length (delete-duplicates houses :test #'equal)))))))

(defun count-unique-deliveries-by-santa-and-robot (file)
  (with-open-file (stream file)
    (let ((instructions (read-line stream))
          (santa-houses (list (list 0 0)))
          (robot-houses (list (list 0 0))))
      (loop for i across instructions
            for idx from 0
            when (evenp idx) do (setf santa-houses
                                      (cons (next-house (car santa-houses)
                                                        i)
                                            santa-houses))
            when (oddp idx) do (setf robot-houses
                                     (cons (next-house (car robot-houses)
                                                       i)
                                           robot-houses)))
      (length (union santa-houses robot-houses :test #'equal)))))