-- Copyright 1993-1998, by the Cecil Project
-- Department of Computer Science and Engineering, University of Washington
-- See the LICENSE file for license information.

--DOC Stream views of lists are not positionable or extensible but are
--DOC insertable and removable.

-- Note: insertion at front of stream doesn't affect underlying list

template object list_stream_view[T] isa m_stream[T],
			      removable_stream[T], insertable_stream[T];

--DOCSKIP keep predicates out of manual
predicate list_stream_view_at_end[T] isa list_stream_view[T], stream_at_end[T];
predicate list_stream_view_before_end[T] isa list_stream_view[T],
					     stream_before_end[T];

-- list stream views aren't fully reversible,
-- but they can test for the beginning of the stream
predicate list_stream_view_at_start[T] isa list_stream_view[T]
			when list_stream_view.is_at_start;
predicate list_stream_view_after_start[T] isa list_stream_view[T]
			when list_stream_view.is_at_start.not;
divide list_stream_view[T] into list_stream_view_at_start[T],
				list_stream_view_after_start[T];
--DOCENDSKIP

private var field prev_cell(@:list_stream_view[`T]):simple_list[T] := nil[T];
private var field next_cell(@:list_stream_view[`T]):simple_list[T] := nil[T];

method view_stream(l@:simple_list[`T]):list_stream_view[T] {
    concrete object isa list_stream_view[T] { next_cell := l } }

method is_at_end(s@:list_stream_view[`T]):bool { s.next_cell.is_empty }
method is_at_start(s@:list_stream_view[`T]):bool {
    s.prev_cell.is_empty }

method forward(s@:list_stream_view_before_end[`T], at_end:&():void):void {
    s.prev_cell := s.next_cell;
    s.next_cell := s.next_cell.rest;
}

method peek_next(s@:list_stream_view_before_end[`T], at_end:&():T):T {
    s.next_cell.first }

method set_peek_next(s@:list_stream_view_before_end[`T], x:T):void {
    s.next_cell.first := x; }

method remove_next(s@:list_stream_view_before_end[`T]):void {
    s.next_cell := s.next_cell.rest;
    s.prev_cell.rest := s.next_cell;
}

method insert(s@:list_stream_view[`T], x:T):void {
    s.next_cell := cons(x, s.next_cell);
    if(s.is_at_start.not, { s.prev_cell.rest := s.next_cell; });
    s.forward;
}


-- view of a mutable list.
-- insertion at front of stream also inserts at from of list

template representation m_list_stream_view[T] isa list_stream_view[T];

--DOCSKIP those pesky predicates....
predicate m_list_stream_view_at_end[T] isa m_list_stream_view[T],
  					   stream_at_end[T];
predicate m_list_stream_view_before_end[T] isa m_list_stream_view[T],
					       stream_before_end[T];

predicate m_list_stream_view_at_start[T] isa m_list_stream_view[T],
					     list_stream_view_at_start[T];
predicate m_list_stream_view_after_start[T] isa m_list_stream_view[T],
				        list_stream_view_after_start[T];
--DOCENDSKIP

private field contents(@:m_list_stream_view[`T]):m_list[T];

method view_stream(l@:m_list[`T]):m_list_stream_view[T] {
    concrete object isa m_list_stream_view[T] {
	next_cell := l.front, contents := l } }

-- treat inserting at beginning specially
method insert(s@:m_list_stream_view_at_start[`T], x:T):void {
    resend;
    s.contents.front := s.next_cell;
}