indexing
	"Unbounded stacks implemented as linked lists"
	names: linked_stack, dispenser, linked_list
	representation: linked
	access: fixed, lifo, membership
	contents: generic

class interface
	LINKED_STACK [G]

create 

	make
			-- Create an empty list.
			-- (from LINKED_LIST)

feature {ANY} -- Initialization

	make
			-- Create an empty list.
			-- (from LINKED_LIST)

feature -- Access

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

	item: G
			-- Item at the first position
		require -- from ACTIVE
			readable: readable

	occurrences (v: like ll_item): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0

	occurrences (v: like ll_item): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
	
feature -- Measurement

	count: INTEGER
			-- Number of items
			-- (from LINKED_LIST)
	
feature -- Status report

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

	empty: BOOLEAN
			-- Is structure empty?
			-- (from FINITE)

	Extendible: BOOLEAN is true
			-- May new items be added? (Answer: yes.)
			-- (from DYNAMIC_CHAIN)

	Full: BOOLEAN is false
			-- Is structured filled to capacity? (Answer: no.)
			-- (from LINKED_LIST)

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

	prunable: BOOLEAN
			-- May items be removed? (Answer: yes.)
			-- (from DYNAMIC_CHAIN)

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

	writable: BOOLEAN
			-- Is there a current item that may be modified?
			-- (from DISPENSER)
	
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 -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
			-- (Synonym for fill)
			-- (from DISPENSER)
		require -- from SEQUENCE
			argument_not_void: s /= void
		ensure -- from SEQUENCE
			new_count: count >= old count

	extend (v: like item)
			-- Push v onto top.
		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
		ensure then -- from STACK
			item_pushed: item = v

	fill (other: LINEAR [G])
			-- Fill with as many items of other as possible.
			-- Fill items with greatest index from other first.
			-- Items inserted with lowest index (from other) will
			-- always be on the top of stack.
			-- The representations of other and current structure
			-- need not be the same.
			-- (from STACK)
		require -- from COLLECTION
			other_not_void: other /= void;
			extendible

	force (v: like item)
			-- Push v onto top.
		require -- from SEQUENCE
			extendible: extendible
		ensure then -- from SEQUENCE
			new_count: count = old count + 1;
			item_inserted: has (v)
		ensure then -- from STACK
			item_pushed: item = v

	put (v: like item)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from STACK
			item_pushed: item = v

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

	remove
			-- Remove item on top.
		require -- from ACTIVE
			prunable: prunable;
			writable: writable

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

	linear_representation: ARRAYED_LIST [G]
			-- Representation as a linear structure
			-- (order is reverse of original order of insertion)
	
feature -- Duplication

	duplicate (n: INTEGER): like Current
			-- New stack containing the n latest items inserted
			-- in current stack.
			-- If n is greater than count, identical to current stack.
		require -- from CHAIN
			not_off_unless_after: off implies after;
			valid_subchain: n >= 0
		require else
			positive_argument: n > 0
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
		-- from DISPENSER
	readable_definition: readable = not empty;
	writable_definition: writable = not empty;
		-- from ACTIVE
	writable_constraint: writable implies readable;
	empty_constraint: empty implies (not readable) and (not writable);
		-- from FINITE
	empty_definition: empty = (count = 0);
	non_negative_count: count >= 0;
		-- from LINKED_LIST
	prunable: prunable;
		-- from DYNAMIC_CHAIN
	extendible: extendible;

end -- class LINKED_STACK