    indexing
    	description: "Unbounded stacks implemented as linked lists"
    	legal: "See notice at end of class."
    	status: "See notice at end of class."
    	names: linked_stack, dispenser, linked_list
    	representation: linked
    	access: fixed, lifo, membership
    	contents: generic
    	date: "$Date: 2006-01-22 18:25:44 -0800 (Sun, 22 Jan 2006) $"
    	revision: "$Revision: 56675 $"

    class interface
    	LINKED_STACK [G]

    create
    	make
    			-- Create an empty list.
    			-- (from LINKED_LIST)
    		ensure -- from LINKED_LIST
    			is_before: before

    feature {ANY} -- Initialization

    	make
    			-- Create an empty list.
    			-- (from LINKED_LIST)
    		ensure -- from LINKED_LIST
    			is_before: before

    feature -- Access

    	generating_type: STRING_8
    			-- Name of current object's generating type
    			-- (type of which it is a direct instance)
    			-- (from ANY)

    	generator: STRING_8
    			-- Name of current object's generating class
    			-- (base class of the type of which it is a direct instance)
    			-- (from ANY)

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

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

    feature -- Measurement

    	count: INTEGER_32
    			-- Number of items
    			-- (from LINKED_LIST)

    	occurrences (v: like ll_item): INTEGER_32
    			-- Number of times `v' appears.
    			-- (Reference or object equality,
    			-- based on object_comparison.)
    			-- (from CHAIN)
    		require -- from  BAG
    			True
    		require -- from  LINEAR
    			True
    		ensure -- from BAG
    			non_negative_occurrences: Result >= 0

    feature -- Comparison

    	frozen deep_equal (some: ANY; other: like arg #1): BOOLEAN
    			-- Are `some' and `other' either both void
    			-- or attached to isomorphic object structures?
    			-- (from ANY)
    		ensure -- from ANY
    			shallow_implies_deep: standard_equal (some, other) implies Result
    			both_or_none_void: (some = Void) implies (Result = (other = Void))
    			same_type: (Result and (some /= Void)) implies some.same_type (other)
    			symmetric: Result implies deep_equal (other, some)

    	frozen equal (some: ANY; other: like arg #1): BOOLEAN
    			-- Are `some' and `other' either both void or attached
    			-- to objects considered equal?
    			-- (from ANY)
    		ensure -- from ANY
    			definition: Result = (some = Void and other = Void) or else ((some /= Void and other /= Void) and then some.is_equal (other))

    	is_equal (other: like Current): BOOLEAN
    			-- Does `other' contain the same elements?
    			-- (from LIST)
    		require -- from ANY
    			other_not_void: other /= Void
    		ensure -- from ANY
    			symmetric: Result implies other.is_equal (Current)
    			consistent: standard_is_equal (other) implies Result
    		ensure then -- from LIST
    			indices_unchanged: index = old index and other.index = old other.index
    			true_implies_same_size: Result implies count = other.count

    	frozen standard_equal (some: ANY; other: like arg #1): BOOLEAN
    			-- Are `some' and `other' either both void or attached to
    			-- field-by-field identical objects of the same type?
    			-- Always uses default object comparison criterion.
    			-- (from ANY)
    		ensure -- from ANY
    			definition: Result = (some = Void and other = Void) or else ((some /= Void and other /= Void) and then some.standard_is_equal (other))

    	frozen standard_is_equal (other: like Current): BOOLEAN
    			-- Is `other' attached to an object of the same type
    			-- as current object, and field-by-field identical to it?
    			-- (from ANY)
    		require -- from ANY
    			other_not_void: other /= Void
    		ensure -- from ANY
    			same_type: Result implies same_type (other)
    			symmetric: Result implies other.standard_is_equal (Current)

    feature -- Status report

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

    	conforms_to (other: ANY): BOOLEAN
    			-- Does type of current object conform to type
    			-- of `other' (as per Eiffel: The Language, chapter 13)?
    			-- (from ANY)
    		require -- from ANY
    			other_not_void: other /= Void

    	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)

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

    	is_inserted (v: G): BOOLEAN
    			-- Has `v' been inserted by the most recent insertion?
    			-- (By default, the value returned is equivalent to calling
    			-- `has (v)'. However, descendants might be able to provide more
    			-- efficient implementations.)
    			-- (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? (Answer: yes.)
    			-- (from DYNAMIC_CHAIN)
    		require -- from  COLLECTION
    			True

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

    	same_type (other: ANY): BOOLEAN
    			-- Is type of current object identical to type of `other'?
    			-- (from ANY)
    		require -- from ANY
    			other_not_void: other /= Void
    		ensure -- from ANY
    			definition: Result = (conforms_to (other) and other.conforms_to (Current))

    	writable: BOOLEAN
    			-- Is there a current item that may be modified?
    			-- (from DISPENSER)
    		require -- from  ACTIVE
    			True

    feature {ANY} -- Status report

    	after: BOOLEAN
    			-- Is there no valid cursor position to the right of cursor?
    			-- (from LINKED_LIST)

    	off: BOOLEAN
    			-- Is there no current item?
    			-- (from LINKED_LIST)
    		require -- from  TRAVERSABLE
    			True

    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: 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: 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: is_inserted (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: extendible

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

    	put (v: like item)
    		require -- from COLLECTION
    			extendible: extendible
    		ensure -- from COLLECTION
    			item_inserted: is_inserted (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 -- from COLLECTION
    			prunable: prunable
    		ensure -- from COLLECTION
    			wiped_out: is_empty
    		ensure then -- from DYNAMIC_LIST
    			is_before: before

    feature -- Conversion

    	linear_representation: ARRAYED_LIST [G]
    			-- Representation as a linear structure
    			-- (order is reverse of original order of insertion)
    		require -- from  CONTAINER
    			True

    feature -- Duplication

    	copy (other: like Current)
    			-- Update current object using fields of object attached
    			-- to `other', so as to yield equal objects.
    			-- (from LINKED_LIST)
    		require -- from ANY
    			other_not_void: other /= Void
    			type_identity: same_type (other)
    		ensure -- from ANY
    			is_equal: is_equal (other)

    	frozen deep_copy (other: like Current)
    			-- Effect equivalent to that of:
    			--		copy (`other' . deep_twin)
    			-- (from ANY)
    		require -- from ANY
    			other_not_void: other /= Void
    		ensure -- from ANY
    			deep_equal: deep_equal (Current, other)

    	frozen deep_twin: like Current
    			-- New object structure recursively duplicated from Current.
    			-- (from ANY)
    		ensure -- from ANY
    			deep_equal: deep_equal (Current, Result)

    	duplicate (n: INTEGER_32): 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

    	frozen standard_copy (other: like Current)
    			-- Copy every field of `other' onto corresponding field
    			-- of current object.
    			-- (from ANY)
    		require -- from ANY
    			other_not_void: other /= Void
    			type_identity: same_type (other)
    		ensure -- from ANY
    			is_standard_equal: standard_is_equal (other)

    	frozen standard_twin: like Current
    			-- New object field-by-field identical to `other'.
    			-- Always uses default copying semantics.
    			-- (from ANY)
    		ensure -- from ANY
    			standard_twin_not_void: Result /= Void
    			equal: standard_equal (Result, Current)

    	frozen twin: like Current
    			-- New object equal to `Current'
    			-- twin calls copy; to change copying/twining semantics, redefine copy.
    			-- (from ANY)
    		ensure -- from ANY
    			twin_not_void: Result /= Void
    			is_equal: Result.is_equal (Current)

    feature -- Basic operations

    	frozen default: like Current
    			-- Default value of object's type
    			-- (from ANY)

    	frozen default_pointer: POINTER
    			-- Default value of type `POINTER'
    			-- (Avoid the need to write `p'.default for
    			-- some `p' of type `POINTER'.)
    			-- (from ANY)

    	default_rescue
    			-- Process exception for routines with no Rescue clause.
    			-- (Default: do nothing.)
    			-- (from ANY)

    	frozen do_nothing
    			-- Execute a null action.
    			-- (from ANY)

    feature -- Output

    	io: STD_FILES
    			-- Handle to standard file setup
    			-- (from ANY)

    	out: STRING_8
    			-- New string containing terse printable representation
    			-- of current object
    			-- Was declared in ANY as synonym of tagged_out.
    			-- (from ANY)

    	print (some: ANY)
    			-- Write terse external representation of `some'
    			-- on standard output.
    			-- (from ANY)

    	frozen tagged_out: STRING_8
    			-- New string containing terse printable representation
    			-- of current object
    			-- Was declared in ANY as synonym of out.
    			-- (from ANY)

    feature -- Platform

    	operating_environment: OPERATING_ENVIRONMENT
    			-- Objects available from the operating system
    			-- (from ANY)

    invariant
    		-- from DISPENSER
    	readable_definition: readable = not is_empty
    	writable_definition: writable = not is_empty

    		-- from ACTIVE
    	writable_constraint: writable implies readable
    	empty_constraint: is_empty implies (not readable) and (not writable)

    		-- from ANY
    	reflexive_equality: standard_is_equal (Current)
    	reflexive_conformance: conforms_to (Current)

    		-- from FINITE
    	empty_definition: is_empty = (count = 0)
    	non_negative_count: count >= 0

    		-- from LINKED_LIST
    	prunable: prunable
    	empty_constraint: is_empty implies ((first_element = Void) and (active = Void))
    	not_void_unless_empty: (active = Void) implies is_empty
    	before_constraint: before implies (active = first_element)
    	after_constraint: after implies (active = last_element)

    		-- from LIST
    	before_definition: before = (index = 0)
    	after_definition: after = (index = count + 1)

    		-- from CHAIN
    	non_negative_index: index >= 0
    	index_small_enough: index <= count + 1
    	off_definition: off = ((index = 0) or (index = count + 1))
    	isfirst_definition: isfirst = ((not is_empty) and (index = 1))
    	islast_definition: islast = ((not is_empty) and (index = count))
    	item_corresponds_to_index: (not off) implies (ll_item = i_th (index))
    	index_set_has_same_count: index_set.count = count

    		-- from INDEXABLE
    	index_set_not_void: index_set /= Void

    		-- from BILINEAR
    	not_both: not (after and before)
    	before_constraint: before implies off

    		-- from LINEAR
    	after_constraint: after implies off

    		-- from TRAVERSABLE
    	empty_constraint: is_empty implies off

    		-- from DYNAMIC_CHAIN
    	extendible: extendible

    indexing
    	library: "EiffelBase: Library of reusable components for Eiffel."
    	copyright: "Copyright (c) 1984-2006, Eiffel Software and others"
    	license: "Eiffel Forum License v2 (see http://www.eiffel.com/licensing/forum.txt)"
    	source: "[
    		Eiffel Software
    		356 Storke Road, Goleta, CA 93117 USA
    		Telephone 805-685-1006, Fax 805-685-6869
    		Website http://www.eiffel.com
    		Customer support http://support.eiffel.com
    	]"

    end -- class LINKED_STACK