Extract map data for BBC micro game Castle Quest into a recognisable emacs buffer

extract-cq.el

;
; Extracting Castle Quest map data; the file is 2K from BBC Micro memory 0x400
; See https://en.wikipedia.org/wiki/Castle_Quest_(1985_video_game)#Development
;
(defun extract-cq-map()
  (interactive)
  (let ((map-data (f-read-bytes "C:/Path/To/Raw/CastleQuestMap")))
    (switch-to-buffer (get-buffer-create "*CQ*"))
    (erase-buffer)
    (ruler-mode 1)
    (linum-mode 1)
    (let ((this-offset 0)) ; 0, 350, 700
      (loop for scr from 0 to 4 do ; 4, 4, 3
            (loop for i from 0 to 9 do ; 10 across
                  (loop for j from 0 to 6 do ; 7 down
                        (let ((this-byte (string-to-char (substring map-data this-offset (1+ this-offset)))))
                          (insert (propertize (format "%02d" (lsh this-byte -4)) 'face `(:foreground ,(if (= (lsh this-byte -4) 0) "yellow" "red"))))
                          (insert (propertize (format "%02d" (logand this-byte 15)) 'face `(:foreground ,(if (= (logand this-byte 15) 0) "yellow" "red"))))
                          (when (= (forward-line 1) 1) (newline))
                          (end-of-line)
                          (setq this-offset (1+ this-offset))))
                  (goto-char (point-min))
                  (end-of-line)))
      (reverse-region (point-min) (point-max))
      (message "Done. Offset ends at %d" this-offset))))