%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Read-article-trail
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% How many articles to maintain in the trail
variable max_read_history = 50;
variable read_history = String_Type [max_read_history];
variable next_slot_to_use = 0;
variable slot_being_shown = 0;
variable slots_in_use = 0;
%
% read_history: [ N N N N N X X X X X N N N ] (circular buffer, X are valid)
%                                     ^
%                                     \- next_slot_to_use points here
% slot_being_shown must point at an X. slots_in_use is 5 here.

define show_article_in_slot()
{
   %fprintf(stderr, "showing article %d (%s)\n",slot_being_shown, read_history[slot_being_shown]);
   () = locate_header_by_msgid (read_history[slot_being_shown],0);
   call ("article_pagedn");
}

define go_back_one_read_article()
{
   %fprintf(stderr, "backward\n");
   variable d = next_slot_to_use - slot_being_shown;
   if (d < 0) {
      d += max_read_history;
   }
   % ASSERT: 0 <= d <= slots_in_use
   % (and d == 0 if and only if slots_in_use == 0)
   if (d == slots_in_use) {
      % can't go any further backward
      return;
   }

   if (slot_being_shown == 0) {
      slot_being_shown = max_read_history;
   }
   slot_being_shown--;
   show_article_in_slot();
}

define go_forward_one_read_article()
{
   %fprintf(stderr, "forward\n");
   variable d = next_slot_to_use - slot_being_shown;
   if (d < 0) {
      d += max_read_history;
   }
   % ASSERT: 0 <= d <= slots_in_use
   % (and d == 0 if and only if slots_in_use == 0)
   if (d <= 1) {
      % can't go any further forward
      return;
   }

   slot_being_shown++;
   if (slot_being_shown == max_read_history) {
      slot_being_shown = 0;
   }
   show_article_in_slot();
}

define add_article_to_read_history(art)
{
  % Just add this article to the end of the history list.
  %fprintf(stderr, "adding article %s\n", art); 
  read_history[next_slot_to_use] = art;
  slot_being_shown = next_slot_to_use;
  
  next_slot_to_use++;
  if (next_slot_to_use == max_read_history) {
     next_slot_to_use = 0;
  }

  if (slots_in_use < max_read_history) {
     slots_in_use++;
  }
}

define maybe_add_article_to_read_history()
{
   % Add this article to the history only if it isn't there
   % already. [The most common case being that we've just hit '<'
   % to go back in the history; we definitely don't want to re-add
   % the page to the end of the list!]
   variable this_article = extract_article_header ("Message-Id");
   %fprintf(stderr,"thinking about adding article %s\n", this_article);
   variable h = next_slot_to_use;
   variable i = 0;
   while (i < slots_in_use) {
      i++;
      if (h == 0) {
         h = max_read_history;
      }
      h--;
      if (read_history[h] == this_article) {
         return;
      }
   }
   add_article_to_read_history(this_article);
}

define clear_read_history()
{
   next_slot_to_use = 0;
   slot_being_shown = 0;
   slots_in_use = 0;
}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% To use:
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

define read_article_hook()
{
   maybe_add_article_to_read_history();
}

define article_mode_hook()
{
   clear_read_history();
}

definekey ("go_back_one_read_article", "<", "article");
definekey ("go_forward_one_read_article", ">", "article");