The following Unicode's bidirectional control chars are modal so that
they push a new bidirectional rendering mode to a stack:
U+202A LEFT-TO-RIGHT EMBEDDING
U+202B RIGHT-TO-LEFT EMBEDDING
U+202D LEFT-TO-RIGHT OVERRIDE
U+202E RIGHT-TO-LEFT OVERRIDE
Every mode must be terminated with with character U+202C POP
DIRECTIONAL FORMATTING which pops the mode from the stack. The stack
is per paragraph. A new text paragraph resets the rendering mode
changed by these control characters.
This change adds a new function "notmuch-balance-bidi-ctrl-chars"
which reads its STRING argument and ensures that all push
characters (U+202A, U+202B, U+202D, U+202E) have a pop character
pair (U+202C). The function may add more U+202C characters at the end
of the returned string, or it may remove some U+202C characters. The
returned string is safe in the sense that it won't change the
surrounding bidirectional rendering mode. This function should be used
when sanitizing arbitrary input.
---
emacs/notmuch-lib.el | 54 ++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 54 insertions(+)
diff --git a/emacs/notmuch-lib.el b/emacs/notmuch-lib.el
index 118faf1e..e6252c6c 100644
--- a/emacs/notmuch-lib.el
+++ b/emacs/notmuch-lib.el
@@ -469,6 +469,60 @@ be displayed."
"[No Subject]"
subject)))
+
+(defun notmuch-balance-bidi-ctrl-chars (string)
+ "Balance bidirectional control chars in STRING.
+
+The following Unicode's bidirectional control chars are modal so
+that they push a new bidirectional rendering mode to a stack:
+U+202A LEFT-TO-RIGHT EMBEDDING, U+202B RIGHT-TO-LEFT EMBEDDING,
+U+202D LEFT-TO-RIGHT OVERRIDE and U+202E RIGHT-TO-LEFT OVERRIDE.
+Every mode must be terminated with with character U+202C POP
+DIRECTIONAL FORMATTING which pops the mode from the stack. The
+stack is per paragraph. A new text paragraph resets the rendering
+mode changed by these control characters.
+
+This function reads the STRING argument and ensures that all push
+characters (U+202A, U+202B, U+202D, U+202E) have a pop character
+pair (U+202C). The function may add more U+202C characters at the
+end of the returned string, or it may remove some U+202C
+characters. The returned string is safe in the sense that it
+won't change the surrounding bidirectional rendering mode. This
+function should be used when sanitizing arbitrary input."
+
+ (let ((new-string nil)
+ (stack-count 0))
+
+ (cl-flet ((push-char-p (c)
+ ;; U+202A LEFT-TO-RIGHT EMBEDDING
+ ;; U+202B RIGHT-TO-LEFT EMBEDDING
+ ;; U+202D LEFT-TO-RIGHT OVERRIDE
+ ;; U+202E RIGHT-TO-LEFT OVERRIDE
+ (cl-find c '(?\u202a ?\u202b ?\u202d ?\u202e)))
+ (pop-char-p (c)
+ ;; U+202C POP DIRECTIONAL FORMATTING
+ (eql c ?\u202c)))
+
+ (cl-loop for char across string
+ do (cond ((push-char-p char)
+ (cl-incf stack-count)
+ (push char new-string))
+ ((and (pop-char-p char)
+ (cl-plusp stack-count))
+ (cl-decf stack-count)
+ (push char new-string))
+ ((and (pop-char-p char)
+ (not (cl-plusp stack-count)))
+ ;; The stack is empty. Ignore this pop character.
+ )
+ (t (push char new-string)))))
+
+ ;; Add possible missing pop characters.
+ (cl-loop repeat stack-count
+ do (push ?\x202c new-string))
+
+ (seq-into (nreverse new-string) 'string)))
+
(defun notmuch-sanitize (str)
"Sanitize control character in STR.
--
2.20.1
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