indexing
	description: "Active structures, which always have a current position accessible through a cursor."
	names: cursor_structure, access
	access: cursor, membership
	contents: generic

deferred class interface
	CURSOR_STRUCTURE [G]

feature -- Access

	cursor: CURSOR
			-- Current cursor position

	has (v: G): BOOLEAN
			-- Does structure include v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CONTAINER)
		ensure -- from CONTAINER
			not_found_in_empty: Result implies not empty

	item: G
			-- Current item
			-- (from ACTIVE)
		require -- from ACTIVE
			readable: readable
	
feature -- Measurement

	occurrences (v: G): INTEGER
			-- Number of times v appears in structure
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from BAG)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
	
feature -- Status report

	changeable_comparison_criterion: BOOLEAN
			-- May object_comparison be changed?
			-- (Answer: yes by default.)
			-- (from CONTAINER)

	empty: BOOLEAN
			-- Is there no element?
			-- (from CONTAINER)

	extendible: BOOLEAN
			-- May new items be added?
			-- (from COLLECTION)

	object_comparison: BOOLEAN
			-- Must search operations use equal rather than =
			-- for comparing references? (Default: no, use =.)
			-- (from CONTAINER)

	prunable: BOOLEAN
			-- May items be removed?
			-- (from COLLECTION)

	readable: BOOLEAN
			-- Is there a current item that may be read?
			-- (from ACTIVE)

	valid_cursor (p: CURSOR): BOOLEAN
			-- Can the cursor be moved to position p?

	writable: BOOLEAN
			-- Is there a current item that may be modified?
			-- (from ACTIVE)
	
feature -- Status setting

	compare_objects
			-- Ensure that future search operations will use equal
			-- rather than = for comparing references.
			-- (from CONTAINER)
		require -- from CONTAINER
			changeable_comparison_criterion
		ensure -- from CONTAINER
			object_comparison

	compare_references
			-- Ensure that future search operations will use =
			-- rather than equal for comparing references.
			-- (from CONTAINER)
		require -- from CONTAINER
			changeable_comparison_criterion
		ensure -- from CONTAINER
			reference_comparison: not object_comparison
	
feature -- Cursor movement

	go_to (p: CURSOR)
			-- Move cursor to position p.
		require
			cursor_position_valid: valid_cursor (p)
	
feature -- Element change

	extend (v: G)
			-- Add a new occurrence of v.
			-- (from BAG)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from BAG
			one_more_occurrence: occurrences (v) = old (occurrences (v)) + 1

	fill (other: CONTAINER [G])
			-- Fill with as many items of other as possible.
			-- The representations of other and current structure
			-- need not be the same.
			-- (from COLLECTION)
		require -- from COLLECTION
			other_not_void: other /= void;
			extendible

	put (v: G)
			-- Ensure that structure includes v.
			-- Was declared in COLLECTION as synonym of put and extend.
			-- (from COLLECTION)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)

	replace (v: G)
			-- Replace current item by v.
			-- (from ACTIVE)
		require -- from ACTIVE
			writable: writable
		ensure -- from ACTIVE
			item_replaced: item = v
	
feature -- Removal

	prune (v: G)
			-- Remove one occurrence of v if any.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable: prunable

	prune_all (v: G)
			-- Remove all occurrences of v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)

	remove
			-- Remove current item.
			-- (from ACTIVE)
		require -- from ACTIVE
			prunable: prunable;
			writable: writable

	wipe_out
			-- Remove all items.
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			wiped_out: empty
	
feature -- Conversion

	linear_representation: LINEAR [G]
			-- Representation as a linear structure
			-- (from CONTAINER)
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
		-- from ACTIVE
	writable_constraint: writable implies readable;
	empty_constraint: empty implies (not readable) and (not writable);

end -- class CURSOR_STRUCTURE