Implements an abstract output page.

+/* GNU Mailutils -- a suite of utilities for electronic mail
+   Copyright (C) 2005 Free Software Foundation, Inc.
+
+   GNU Mailutils is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 2, or (at your option)
+   any later version.
+
+   GNU Mailutils is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with GNU Mailutils; if not, write to the Free Software
+   Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
+   MA 02110-1301 USA */
+
+#include "mail.h"
+
+size_t top_of_page = 1; /* Number of the topmost message on the page */
+size_t cursor;          /* Number of current message */
+
+size_t *page_map;       /* Array of message numbers. page_map[N] holds
+			   number of the message occupying Nth line on
+			   the screen */
+unsigned page_size;     /* Capacity of page_map */
+unsigned page_avail;     /* First non-used entry in page map. Can be
+			   equal to page_size */
+
+/* Auxiliary function: Store number of message from mspec into page_map */
+static int
+_fill_map (msgset_t *mspec, message_t msg, void *data)
+{
+  unsigned *pos = data;
+  page_map[*pos] = mspec->msg_part[0];
+  ++*pos;
+  return 0;
+}
+
+/* Fill page_map.
+   page_avail must be set to zero before calling this function */
+static void
+fill_page_map ()
+{
+  util_range_msg (top_of_page, page_size,
+		  MSG_COUNT|MSG_NODELETED|MSG_SILENT, _fill_map,
+		  &page_avail);
+}
+
+/* Check if the page_map is valid. If not, fill it.
+   Page_avail can be set to zero to force re-filling page_map. In particular
+   this happens when deleting current message or when SIGWINCH is delivered
+   to the program */
+static void
+check_page_map ()
+{
+  if (!page_map)
+    {
+      page_size = util_screen_lines ();
+      page_map = xrealloc (page_map, sizeof (page_map[0]) * page_size);
+      page_avail = 0;
+    }
+  if (page_avail == 0)
+    fill_page_map ();
+}  
+
+/* Invalidate page_map. HARD=1 means 'hard invalidation', implying
+   the need to reallocate the array */
+void
+page_invalidate (int hard)
+{
+  page_avail = 0;
+  if (hard)
+    {
+      free (page_map);
+      page_map = NULL;
+    }
+}
+
+/* Invalidate page_map if VALUE is number of the current message */
+void
+cond_page_invalidate (size_t value)
+{
+  unsigned i;
+
+  if (page_map == NULL || page_avail == 0)
+    return;
+  for (i = 0; i < page_avail-1; i++)
+    if (page_map[i] >= value && value <= page_map[i+1])
+      {
+	page_invalidate (0);
+	return;
+      }
+}
+
+/* Return a 1-based index of page_map entry occupied by number VALUE.
+   Return 0 if VALUE is not found in page_map */
+static int
+page_line (size_t value)
+{
+  unsigned i;
+
+  for (i = 0; i < page_avail; i++)
+    if (page_map[i] == value)
+      return i+1;
+  return 0;
+}
+
+/* Return number of the current message.
+   Zero is returned if page_map is empty (i.e. mailbox is empy) */
+size_t
+get_cursor ()
+{
+  check_page_map ();
+  if (page_avail == 0)
+    return 0;
+  return page_map[cursor];
+}
+
+/* Move cursor to message number VALUE */
+void
+set_cursor (unsigned value)
+{
+  int n;
+  
+  if (total == 0)
+    {
+      cursor = 0;
+      return;
+    }
+  
+  check_page_map ();
+  n = page_line (value);
+  if (n == 0)
+    {
+      top_of_page = value;
+      cursor = 0;
+      page_avail = 0;
+      fill_page_map ();
+    }
+  else
+    cursor = n - 1;
+}
+
+/* Return T if the cursor points to message number N */
+int
+is_current_message (size_t n)
+{
+  check_page_map ();
+  return page_map[cursor] == n;
+}
+
+/* Apply function FUNC to each message from the page. DATA supplies
+   call-specific data to the function. */
+void
+page_do (msg_handler_t func, void *data)
+{
+  unsigned i;
+  
+  check_page_map ();
+  for (i = 0; i < page_avail; i++)
+    {
+      message_t msg;
+      msgset_t set;
+      
+      set.next = NULL;
+      set.npart = 1;
+      set.msg_part = page_map + i;
+      mailbox_get_message (mbox, page_map[i], &msg);
+      func (&set, msg, data);
+    }
+}
+
+/* Move current page OFFSET lines forward, if OFFSET > 0, or backward,
+   if OFFSET < 0.
+   Return number of items that will be displayed */
+size_t
+page_move (off_t offset)
+{
+  unsigned n;
+  size_t start;
+  size_t count = 0;
+  
+  check_page_map ();
+
+  if (offset > 0)
+    start = page_map[cursor] + offset;
+  else if (-offset > page_map[cursor])
+    start = 1;
+  else
+    start = page_map[cursor] + offset;
+  
+  util_range_msg (start, page_size,
+		  MSG_COUNT|MSG_NODELETED|MSG_SILENT, _fill_map, &count);
+  
+  if (offset < 0 && top_of_page == page_map[0])
+    {
+      page_avail = count;
+      return 0;
+    }
+  
+  if (count)
+    {
+      top_of_page = page_map[0];
+
+      if (count < page_size && top_of_page > 1)
+	{
+	  for (start = top_of_page - 1; count < page_size && start > 1;
+	       start--)
+	    {
+	      if (!util_isdeleted (start))
+		{
+		  top_of_page = start;
+		  cursor++;
+		  count++;
+		}
+	    }
+	  page_avail = 0;
+	  fill_page_map ();
+	}
+      else
+	page_avail = count;
+    }
+  return count;
+}