My solution for day 02 of advent of code 2021.

day02.lisp

(defun read-input (filename)
  (with-open-file (stream filename)
    (loop for line = (read-line stream nil)
          while line
          collect line)))

(defun move-command (command x y)
  "A command has two parts. the direction and the count."
  (let* ((dir-and-steps (cl-ppcre:split "\\s+" command))
         (dir (first dir-and-steps))
         (steps (parse-integer (second dir-and-steps))))
    (cond ((string-equal dir "forward") (values (+ x steps) y))
          ((string-equal dir "up") (values x (- y steps)))
          ((string-equal dir "down") (values x (+ y steps)))
          (t (error "I give up.")))))

(defun updated-move-command (command x y aim)
  "A command has two parts. the direction and the count."
  (let* ((dir-and-steps (cl-ppcre:split "\\s+" command))
         (dir (first dir-and-steps))
         (steps (parse-integer (second dir-and-steps))))
    (cond ((string-equal dir "forward") (values (+ x steps) (+ y (* steps aim)) aim))
          ((string-equal dir "up") (values x y (- aim steps)))
          ((string-equal dir "down") (values x y (+ aim steps)))
          (t (error "I give up.")))))


(defun process-input-commands (commands)
  (let ((x 0)
        (y 0)
        (aim 0))
    (loop for command in commands do
      (and
       (setf (values x y aim) (updated-move-command command x y aim))
       (format t "x: ~a, y: ~a, aim: ~a~%" x y aim)))
    (* x y)))

(process-input-commands (read-input "day02.txt"))