indexing
	description: "Cursors for structures that can be traversed forward"
	library: "Gobo Eiffel Structure Library"
	author: "Eric Bezault <ericb@gobo.demon.co.uk>"
	copyright: "Copyright (c) 1997, Eric Bezault"

deferred class interface
	DS_LINEAR_CURSOR [G]

feature -- Access

	container: DS_LINEAR [G]
			-- Structure traversed

	index: INTEGER
			-- Index of current position
			-- (from DS_INDEXED_CURSOR)
		require -- from DS_INDEXED_CURSOR
			valid_cursor: is_valid
		ensure -- from DS_INDEXED_CURSOR
			valid_index: valid_index (Result)

	item: G
			-- Item at cursor position
			-- (from DS_CURSOR)
		require -- from DS_CURSOR
			valid_cursor: is_valid;
			not_off: not off
	
feature -- Status report

	after: BOOLEAN
			-- Is there no valid position to right of cursor?
		require
			valid_cursor: is_valid

	is_first: BOOLEAN
			-- Is cursor on first item?
		require
			valid_cursor: is_valid
		ensure
			not_empty: Result implies not container.is_empty;
			not_off: Result implies not off;
			definition: Result implies (item = container.first)

	is_valid: BOOLEAN
			-- Is cursor valid?
			-- (A cursor might become invalid if container
			-- has been modified during traversal.)
			-- (from DS_CURSOR)

	off: BOOLEAN
			-- Is there no item at cursor position?
		require -- from DS_CURSOR
			valid_cursor: is_valid

	valid_index (i: INTEGER): BOOLEAN
			-- Is i a valid index value?
	
feature -- Cursor movement

	forth
			-- Move cursor to next position.
		require
			valid_cursor: is_valid;
			not_after: not after

	go_to (i: INTEGER)
			-- Move cursor to i-th position.
			-- (from DS_INDEXED_CURSOR)
		require -- from DS_INDEXED_CURSOR
			valid_cursor: is_valid;
			valid_index: valid_index (i)
		ensure -- from DS_INDEXED_CURSOR
			moved: index = i

	search_forth (v: G)
			-- Move to first position at or after current
			-- position where item and v are equal.
			-- (Use searcher's comparison criterion from container.)
			-- Move after if not found.
		require
			valid_cursor: is_valid;
			not_off: not off or after

	start
			-- Move cursor to first position.
		require
			valid_cursor: is_valid
		ensure
			empty_behavior: container.is_empty implies after;
			not_empty_behavior: not container.is_empty implies is_first
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
	after_constraint: after implies off;
	empty_property: container.is_empty implies off;
		-- from DS_CURSOR
	container_not_void: container /= void;
	empty_constraint: container.is_empty implies off;

end -- class DS_LINEAR_CURSOR