(defparameter *movements-input* "./input.txt")
(ql:quickload :cl-strings)
(use-package :cl-strings)

(defun read-movements ()
  "Read input file, returning a list of its depth readings."
  (let ((input (open *movements-input* :if-does-not-exist nil))
        (input-movements '()))
    (when input
      (loop with direction and distance
            for line = (read-line input nil)
            while line do
            (setf direction (car (cl-strings:split line)))
            (setf distance (parse-integer (cadr (cl-strings:split line))))
            (setq input-movements (cons (cons direction distance) input-movements)))
      (close input))

    (reverse input-movements)))

(defun calculate-depths (input)
  "Calculate trajectory thru the deep."
  (defun iter-movements (movements horizontal depth)
    (cond ((eq movements '())
           (cons horizontal depth))
          ('true
           (let* ((movement (car movements))
                  (direction (car movement))
                  (distance (cdr movement)))
             (cond ((string= direction "up")
                    (iter-movements
                     (cdr movements)
                     horizontal
                     (- depth distance)))
                   ((string= direction "down")
                    (iter-movements
                     (cdr movements)
                     horizontal
                     (+ depth distance)))
                   ((string= direction "forward")
                    (iter-movements
                     (cdr movements)
                     (+ horizontal distance)
                     depth)))))))
  (iter-movements input 0 0))

;; get measurements!!
(calculate-depths (read-movements)) ;; (1868 . 1090)

(let* ((calculations
         (calculate-depths (read-movements)))
       (horizontal (car calculations))
       (depth (cdr calculations)))
  (* horizontal depth)) ;; results: 2036120

(defun calculate-depths-with-aim (input)
  "Calculate trajectory thru the deep, understanding aim."
  (defun iter-movements (movements horizontal depth aim)
    (if ((eq movements '())
           (cons horizontal depth))
        (let* ((movement (car movements))
               (direction (car movement))
               (units (cdr movement)))
          (cond ((string= direction "up")
                 (iter-movements
                  (cdr movements)
                  horizontal
                  depth
                  (- aim units)))
                ((string= direction "down")
                 (iter-movements
                  (cdr movements)
                  horizontal
                  depth
                  (+ aim units)))
                ((string= direction "forward")
                 (iter-movements
                  (cdr movements)
                  (+ horizontal units)
                  (+ depth (* units aim))
                  aim))))))
  (iter-movements input 0 0 0))

;; get measurements!!
(calculate-depths-with-aim (read-movements)) ;; (1868 . 1078987)

(let* ((calculations
         (calculate-depths-with-aim (read-movements)))
       (horizontal (car calculations))
       (depth (cdr calculations)))
  (* horizontal depth)) ;; results: 2015547716