From c4291595d24b1ca19db14a675268cd87d168983b Mon Sep 17 00:00:00 2001
From: Tim Vaughan <tgvaughan@gmail.com>
Date: Mon, 13 May 2019 23:42:29 +0200
Subject: [PATCH] Drafted row processing procedure.

---
 sixel.el | 50 ++++++++++++++++++++++++--------------------------
 1 file changed, 24 insertions(+), 26 deletions(-)

diff --git a/sixel.el b/sixel.el
index 6bcef4e..cd7fd31 100644
--- a/sixel.el
+++ b/sixel.el
@@ -30,27 +30,21 @@
                          (split-string substr "$"))))
           (split-string string -))))
 
-(defun sixel-tag-bits (n tag bit-count)
-  "Create a list of length BIT-COUNT with the elements corresponding to the
-true bits of N set to TAG."
-  (if (= bit-count 0)
-      nil
-    (cons (if (= (% n 2) 1)
-              tag
-            nil)
-          (sixel-tag-bits (/ n 2) tag (1- bit-count)))))
+(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-make-sixel-from-char (char tag)
-  "Retrieve sequence of bits to be set."
-  (sixel-tag-bits (- char 63) tag 6))
-
-(defun sixel-union (sixel1 sixel2)
-  "Returns a union of the two sixels, SIXEL1 and SIXEL2.
-When both sixels specify a tag for the same pixel, the tag
-from SIXEL2 wins."
-  (seq-mapn (lambda (tag1 tag2)
-              (or tag2 tag1))
-            sixel1 sixel2))
+(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-row (string)
   (let ((idx-in 0)
@@ -70,18 +64,22 @@ from SIXEL2 wins."
             (setq idx-in (match-end 0))))
          ((string-match "^#\\([0-9]+\\)" trunc-string)
           (let ((tag (intern (match-string 1 trunc-string))))
-            (setq current-colour tag)
+            (setq sixel-current-colour tag)
             (setq idx-in (match-end 0))))
+         ((string-match "^$" trunc-string)
+          (setq idx-out 0)
+          (setq idx-in (match-end 0 trunc-string)))
          ((string-match "^!\\([0-9]+\\)\\([?-~]\\)" trunc-string)
           (let ((repeat-count (string-to-number (match-string 1 trunc-string)))
                 (char (elt (match-string 2 trunc-string) 0)))
             (dotimes (i repeat-count)
-              (unless (< idx-out (length result))
-                (add-to-list result (make-vector 6 nil)))
-              (let ((sixel (elt result idx-out))))))))))))
-                
+              (sixel-tag-sixel-in-row 'result idx-out char sixel-current-colour)
+              (setq idx-out (+1 idx-out)))))
+         ((string-match "^\\([?-~]\\)" trunc-string)
+          (let ((char (elt (match-string 1 trunc-string) 0)))
+            (sixel-tag-sixel-in-row 'result idx-out char sixel-current-colour))
+          (setq idx-out (+1 idx-out))))))))
               
-
 (sixel-get-rows test-string)
 
 (defun sixel-to-xpm (string)
-- 
2.20.1