day4.el

(defun substring2 (lists x1 y1 x2 y2)
  (let ((i x1)
        (j y1)
        (res ""))
    (while (and (or (/= i x2) (/= j y2))
                (>= i 0)
                (>= j 0)
                (< j (length lists))
                (< i (length (nth j lists))))
      (setq res (concat res (substring (nth j lists) i (1+ i))))
      (let ((progress nil))
        (when (> x2 i)
          (setq i (1+ i))
          (setq progress t))
        (when (> y2 j)
          (setq j (1+ j))
          (setq progress t))
        (when (< x2 i)
          (setq i (1- i))
          (setq progress t))
        (when (< y2 j)
          (setq j (1- j))
          (setq progress t))
        (unless progress
          (error "invalid arguments"))))
    res))

(defun is-xmas (word)
  (or (string= word "XMAS")
      (string= word "SAMX")))

(defun is-mas (word)
  (or (string= word "MAS")
      (string= word "SAM")))

(let ((lines (with-temp-buffer
               (insert-file-contents "day4.txt")
               (split-string (buffer-string) "\n")))
      (count 0))
  (dotimes (i (length lines))
    (let ((line (nth i lines)))
      (setq j 0)
      (dotimes (j (length line))
        ;; (when (is-xmas (substring2 lines j i (+ j 4) i)) (setq count (1+ count)))
        ;; (when (is-xmas (substring2 lines j i j (+ i 4))) (setq count (1+ count)))
        ;; (when (is-xmas (substring2 lines j i (+ j 4) (+ i 4))) (setq count (1+ count)))
        ;; (when (is-xmas (substring2 lines j i (- j 4) (+ i 4))) (setq count (1+ count)))
        (when (and (is-mas (substring2 lines (- j 1) (- i 1) (+ j 2) (+ i 2)))
                   (is-mas (substring2 lines (+ j 1) (- i 1) (- j 2) (+ i 2))))
          (setq count (1+ count))))))
  count)