(fill-common-string-prefix): New function.

(fill-context-prefix): Use the longest common substring of both
prefixes, if there is one.

diff --git a/lisp/textmodes/fill.el b/lisp/textmodes/fill.el
index e4bdb9db65e294bad9db205f24f981cecb996286..f8eb668a83b251774eadabbffa4c9883f06f1e0d 100644 (file)
--- a/lisp/textmodes/fill.el
+++ b/lisp/textmodes/fill.el
@@ -185,6 +185,15 @@ Remove indentation from each line."
       ;; of the line and wants it to stay at the end of the line.
       (insert-before-markers-and-inherit ? )))))
 
+(defun fill-common-string-prefix (s1 s2)
+  "Return the longest common prefix of strings S1 and S2, or nil if none."
+  (let ((cmp (compare-strings s1 nil nil s2 nil nil)))
+    (if (eq cmp t) 
+       s1
+      (setq cmp (1- (abs cmp)))
+      (unless (zerop cmp)
+       (substring s1 0 cmp)))))
+    
 (defun fill-context-prefix (from to &optional first-line-regexp)
   "Compute a fill prefix from the text between FROM and TO.
 This uses the variables `adaptive-fill-regexp' and `adaptive-fill-function'
@@ -233,13 +242,24 @@ act as a paragraph-separator."
                       ;; If the second line prefix is whitespace, use it.
                       (string-match "\\`[ \t]+\\'" second-line-prefix))
                   second-line-prefix
-                ;; If the second line has the first line prefix,
-                ;; plus whitespace, use the part that the first line shares.
-                (if (string-match (concat "\\`"
-                                            (regexp-quote first-line-prefix)
-                                            "[ \t]*\\'")
-                                  second-line-prefix)
-                    first-line-prefix)))
+
+                ;; If using the common prefix of first-line-prefix
+                ;; and second-line-prefix leads to problems, consider
+                ;; to restore the code below that's commented out,
+                ;; and document why a common prefix cannot be used.
+                
+;               ;; If the second line has the first line prefix,
+;               ;; plus whitespace, use the part that the first line shares.
+;               (if (string-match (concat "\\`"
+;                                           (regexp-quote first-line-prefix)
+;                                           "[ \t]*\\'")
+;                                 second-line-prefix)
+;                   first-line-prefix)))
+
+                ;; Use the longest common substring of both prefixes,
+                ;; if there is one.
+                (fill-common-string-prefix first-line-prefix
+                                           second-line-prefix)))
        ;; If we get a fill prefix from a one-line paragraph,
        ;; maybe change it to whitespace,
        ;; and check that it isn't a paragraph starter.