(ql:quickload '(fiveam))
(defpackage :day21
  (:use :cl :fiveam))
(in-package :day21)
;;22:55
;;23:32

(defun find-start (field)
  (destructuring-bind (rows cols)
      (array-dimensions field)
    (loop for row below rows do
      (loop for col below cols do
        (when (eq (aref field row col) #\S)
          (return-from find-start (list row col)))))))

(defun solver1 (lines step-count)
  (let* ((field (make-array (list (length lines) (length (car lines)))
                            :initial-contents lines))
         pending
         (end-positions (make-hash-table :test #'equal))
         (visited (make-hash-table :test #'equal))
         (start (find-start field)))
    (destructuring-bind (rows cols)
        (array-dimensions field)

      (push (cons step-count start) pending)
      (setf (gethash start visited) step-count)

      (loop while pending
            for (steps-left row col) = (pop pending)
            ;; an even number of steps left means it is possible
            ;; to go and return to the place in the steps available
            ;; Thus count to the solution.
            when (evenp steps-left)
              do (setf (gethash (list row col) end-positions) t)
            unless (zerop steps-left)
              do (loop
                   for (dr dc) in '((-1 0) (0 -1) (1 0) (0 1))
                   for nr = (+ row dr)
                   for nc = (+ col dc)
                   for new-pos = (list nr nc)
                   when (and (< (gethash new-pos visited -1) (1- steps-left))
                             (< -1 nr rows)
                             (< -1 nc cols)
                             (not (eq #\# (aref field nr nc))))
                     do (setf (gethash new-pos visited) (1- steps-left))
                        (push (cons (1- steps-left) new-pos) pending))))
    (hash-table-count end-positions)))

(test solutions
  (is (= 16 (solver1 (uiop:read-file-lines "eg-in") 6)))
  (is (= 3574 (solver1 (uiop:read-file-lines "input") 64))))