(defvar test-string
  (concat "q"
          "#0;2;0;0;0#1;2;100;100;0#2;2;0;100;0"
          "#1~~@@vv@@~~@@~~$"
          "#2??GG????-"
          "#1!14@"))


(defun sixel-get-params (string)
  "Retrieve the sixel parameters."
  (car (split-string string "q")))

(defun sixel-get-data (string)
  "Retrieve data string."
  (substring string (1+ (string-match "q" string))))

(defun sixel-compute-row-length (string)
  (apply 'max
         (mapcar
          (lambda (substr)
            (apply 'max (mapcar
                         (lambda (subsubstr)
                           (length (subsubstr)))
                         (split-string substr "$"))))
          (split-string string -))))

(defun sixel-tag-bits (sixel n tag)
  "Set bits of SIXEL corresponding to N with to the value TAG."
  (dotimes (i 6)
    (if (= (% n 2) 1)
        (aset sixel i tag))
    (setq n (/ n 2))))

(defun sixel-tag-sixel-in-row (row-variable index char tag)
  "Tag the bits of the sixel at INDEX in the list identified by
the variable ROW-VARIABLE corresponding to input character CHAR
with TAG."
  (while (not (< index (length (symbol-value row-variable))))
    (add-to-list row-variable (make-vector 6 nil)))
  (let ((sixel (elt (symbol-value row-variable) index)))
    (sixel-tag-bits sixel (- char 63) tag)))

(defun sixel-process-data (string)
  "Convert sixel string into a list of lists representing individual sixels."
  (with-temp-buffer
    (insert string)
    (goto-char (point-min))
    (let ((idx-out 0)
          this-row rows
          current-colour colour-map
          finished)
      (while  (not finished)
        (cond
         ((looking-at "#\\([0-9]+\\);\\([0-9]+\\);\\([0-9]+\\);\\([0-9]+\\);\\([0-9]+\\)")
          (let ((tag (intern (match-string 1)))
                (mode (match-string 2))
                (r (string-to-number (match-string 3)))
                (g (string-to-number (match-string 4)))
                (b (string-to-number (match-string 5))))
            (push (list tag r g b) colour-map)))

         ((looking-at "#\\([0-9]+\\)")
          (let ((tag (intern (match-string 1))))
            (setq current-colour tag)))

         ((looking-at "$")
          (setq idx-out 0))

         ((looking-at "^-")
          (push this-row 'rows)
          (setq this-row nil)
          (setq idx-out 0))

         ((looking-at "^!\\([0-9]+\\)\\([?-~]\\)")
          (let ((repeat-count (string-to-number (match-string 1)))
                (char (elt (match-string 2) 0)))
            (dotimes (i repeat-count)
              (sixel-tag-sixel-in-row 'result idx-out char current-colour))))

         ((looking-at "^\\([?-~]\\)")
          (let ((char (elt (match-string 1) 0)))
            (sixel-tag-sixel-in-row 'result idx-out char current-colour)))

         ((= (point) (point-max))
          (setq finished t))

         (t (error "Invalid characters found in input string.")))

        (goto-char (match-end 0)))
      (push this-row 'rows))))