Implement bidi-sensitive movement with arrow keys.

 src/bidi.c (bidi_paragraph_init): Don't leave alone garbage values
 of bidi_it->paragraph_dir.  Call bidi_initialize if needed.
 src/xdisp.c (Fcurrent_bidi_paragraph_direction): New function.
 (syms_of_xdisp): Defsubr it.
 src/cmds.c (Fforward_char, Fbackward_char): Doc fix.
 src/subr.el (right-arrow-command, left-arrow-command): New functions.
 src/bindings.el (global-map): Bind them to right and left arrow keys.
 etc/NEWS: Mention current-bidi-paragraph-direction

4 files changed, 101 insertions, 5 deletions

diff --git a/src/ChangeLog b/src/ChangeLog
index a9757cd02b4..32413ae11c3 100644
--- a/src/ChangeLog
+++ b/src/ChangeLog
@@ -1,5 +1,11 @@
 2010-05-15  Eli Zaretskii  <eliz@gnu.org>
 
+        * bidi.c (bidi_paragraph_init): Don't leave alone garbage values
+        of bidi_it->paragraph_dir.  Call bidi_initialize if needed.
+
+        * xdisp.c (Fcurrent_bidi_paragraph_direction): New function.
+        (syms_of_xdisp): Defsubr it.
+
         * Makefile.in: Fix MSDOS-related comments.
 
 2010-05-15  Glenn Morris  <rgm@gnu.org>
diff --git a/src/bidi.c b/src/bidi.c
index c4cb4c599df..6939e51159f 100644
--- a/src/bidi.c
+++ b/src/bidi.c
@@ -890,6 +890,9 @@ bidi_paragraph_init (bidi_dir_t dir, struct bidi_it *bidi_it)
       EMACS_INT pos;
       bidi_type_t type;
 
+      if (!bidi_initialized)
+        bidi_initialize ();
+
       /* If we are inside a paragraph separator, we are just waiting
          for the separator to be exhausted; use the previous paragraph
          direction.  But don't do that if we have been just reseated,
@@ -957,7 +960,7 @@ bidi_paragraph_init (bidi_dir_t dir, struct bidi_it *bidi_it)
   /* Contrary to UAX#9 clause P3, we only default the paragraph
      direction to L2R if we have no previous usable paragraph
      direction.  */
-  if (bidi_it->paragraph_dir == NEUTRAL_DIR)
+  if (bidi_it->paragraph_dir != L2R && bidi_it->paragraph_dir != R2L)
     bidi_it->paragraph_dir = L2R; /* P3 and ``higher protocols'' */
   if (bidi_it->paragraph_dir == R2L)
     bidi_it->level_stack[0].level = 1;
diff --git a/src/cmds.c b/src/cmds.c
index 5d450fe9a13..b8a65324e9f 100644
--- a/src/cmds.c
+++ b/src/cmds.c
@@ -57,8 +57,12 @@ DEFUN ("forward-point", Fforward_point, Sforward_point, 1, 1, 0,
 }
 
 DEFUN ("forward-char", Fforward_char, Sforward_char, 0, 1, "^p",
-       doc: /* Move point right N characters (left if N is negative).
-On reaching end of buffer, stop and signal error.  */)
+       doc: /* Move point N characters forward (backward if N is negative).
+On reaching end or beginning of buffer, stop and signal error.
+
+Depending on the bidirectional context, the movement may be to the
+right or to the left on the screen.  This is in contrast with
+\\[right-arrow-command], which see.  */)
      (n)
      Lisp_Object n;
 {
@@ -93,8 +97,12 @@ On reaching end of buffer, stop and signal error.  */)
 }
 
 DEFUN ("backward-char", Fbackward_char, Sbackward_char, 0, 1, "^p",
-       doc: /* Move point left N characters (right if N is negative).
-On attempt to pass beginning or end of buffer, stop and signal error.  */)
+       doc: /* Move point N characters backward (forward if N is negative).
+On attempt to pass beginning or end of buffer, stop and signal error.
+
+Depending on the bidirectional context, the movement may be to the
+right or to the left on the screen.  This is in contrast with
+\\[left-arrow-command], which see.  */)
      (n)
      Lisp_Object n;
 {
diff --git a/src/xdisp.c b/src/xdisp.c
index fd1b1eaab21..6b3097c9a1a 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -18241,6 +18241,84 @@ display_line (it)
   return row->displays_text_p;
 }
 
+DEFUN ("current-bidi-paragraph-direction", Fcurrent_bidi_paragraph_direction,
+       Scurrent_bidi_paragraph_direction, 0, 1, 0,
+       doc: /* Return paragraph direction at point in BUFFER.
+Value is either `left-to-right' or `right-to-left'.
+If BUFFER is omitted or nil, it defaults to the current buffer.
+
+Paragraph direction determines how the text in the paragraph is displayed.
+In left-to-right paragraphs, text begins at the left margin of the window
+and the reading direction is generally left to right.  In right-to-left
+paragraphs, text begins at the right margin and is read from right to left.
+
+See also `bidi-paragraph-direction'.  */)
+     (buffer)
+     Lisp_Object buffer;
+{
+  struct buffer *buf;
+  struct buffer *old;
+
+  if (NILP (buffer))
+    buf = current_buffer;
+  else
+    {
+      CHECK_BUFFER (buffer);
+      buf = XBUFFER (buffer);
+      old = current_buffer;
+    }
+
+  if (NILP (buf->bidi_display_reordering))
+    return Qleft_to_right;
+  else if (!NILP (buf->bidi_paragraph_direction))
+    return buf->bidi_paragraph_direction;
+  else
+    {
+      /* Determine the direction from buffer text.  We could try to
+         use current_matrix if it is up to date, but this seems fast
+         enough as it is.  */
+      struct bidi_it itb;
+      EMACS_INT pos = BUF_PT (buf);
+      EMACS_INT bytepos = BUF_PT_BYTE (buf);
+
+      if (buf != current_buffer)
+        set_buffer_temp (buf);
+      /* Find previous non-empty line.  */
+      if (pos >= ZV && pos > BEGV)
+        {
+          pos--;
+          bytepos = CHAR_TO_BYTE (pos);
+        }
+      while (FETCH_BYTE (bytepos) == '\n')
+        {
+          if (bytepos <= BEGV_BYTE)
+            break;
+          bytepos--;
+          pos--;
+        }
+      while (!CHAR_HEAD_P (FETCH_BYTE (bytepos)))
+        bytepos--;
+      itb.charpos = pos;
+      itb.bytepos = bytepos;
+      itb.first_elt = 1;
+
+      bidi_paragraph_init (NEUTRAL_DIR, &itb);
+      if (buf != current_buffer)
+        set_buffer_temp (old);
+      switch (itb.paragraph_dir)
+        {
+        case L2R:
+          return Qleft_to_right;
+          break;
+        case R2L:
+          return Qright_to_left;
+          break;
+        default:
+          abort ();
+        }
+    }
+}
+
 
 
 /***********************************************************************
@@ -25940,6 +26018,7 @@ syms_of_xdisp ()
 #endif
   defsubr (&Sformat_mode_line);
   defsubr (&Sinvisible_p);
+  defsubr (&Scurrent_bidi_paragraph_direction);
 
   staticpro (&Qmenu_bar_update_hook);
   Qmenu_bar_update_hook = intern_c_string ("menu-bar-update-hook");