note
	description: "Circular chains implemented as two-way linked lists"
	library: "Free implementation of ELKS library"
	legal: "See notice at end of class."
	status: "See notice at end of class."
	names: two_way_circular, ring, sequence
	representation: linked
	access: index, cursor, membership
	contents: generic
	date: "$Date: 2012-07-23 21:02:19 +0000 (Mon, 23 Jul 2012) $"
	revision: "$Revision: 91989 $"

class interface
	TWO_WAY_CIRCULAR [G]

create 
	make
			-- Create an empty list
			-- (from LINKED_CIRCULAR)

feature -- Access

	at alias "@" (i: INTEGER_32): like item assign put_i_th
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (i)

	cursor: CIRCULAR_CURSOR
			-- Current cursor position
			-- (from LINKED_CIRCULAR)
		ensure -- from CURSOR_STRUCTURE
			cursor_not_void: Result /= Void

	first: G
			-- Item at position currently defined as first
			-- (from CIRCULAR)
		require -- from CHAIN
			not_empty: not is_empty

	l_first: like item
			-- Item at first position
			-- (from CHAIN)
		require -- from CHAIN
			not_empty: not is_empty

	generating_type: TYPE [detachable TWO_WAY_CIRCULAR [G]]
			-- Type of current object
			-- (type of which it is a direct instance)
			-- (from ANY)
		ensure -- from ANY
			generating_type_not_void: Result /= Void

	generator: STRING_8
			-- Name of current object's generating class
			-- (base class of the type of which it is a direct instance)
			-- (from ANY)
		ensure -- from ANY
			generator_not_void: Result /= Void
			generator_not_empty: not Result.is_empty

	sequential_has (v: like item): BOOLEAN
			-- Does structure include an occurrence of v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		require -- from  CONTAINER
			True
		ensure -- from CONTAINER
			not_found_in_empty: Result implies not is_empty

	has (v: like item): BOOLEAN
			-- Does chain include v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)

	i_th alias "[]" (i: INTEGER_32): like item assign put_i_th
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of at.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (i)
		require -- from READABLE_INDEXABLE
			valid_index: valid_index (i)

	index: INTEGER_32
			-- Current cursor index, with respect to position
			-- currently defined as first
			-- (from CIRCULAR)
		require -- from  LINEAR
			True

	sequential_index_of (v: like item; i: INTEGER_32): INTEGER_32
			-- Index of i-th occurrence of v.
			-- 0 if none.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		require -- from LINEAR
			positive_occurrences: i > 0
		ensure -- from LINEAR
			non_negative_result: Result >= 0

	index_of (v: like item; i: INTEGER_32): INTEGER_32
			-- Index of i-th occurrence of item identical to v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- 0 if none.
			-- (from CHAIN)

	item: G
			-- Current item
			-- (from LINKED_CIRCULAR)
		require -- from ACTIVE
			readable: readable
		require -- from TRAVERSABLE
			not_off: not off

	item_for_iteration: G
			-- Item at current position
			-- (from LINEAR)
		require -- from LINEAR
			not_off: not off

	last: like first
			-- Item at position currently defined as last
			-- (from CIRCULAR)
		require -- from CHAIN
			not_empty: not is_empty

	new_cursor: INDEXABLE_ITERATION_CURSOR [G]
			-- Fresh cursor associated with current structure
			-- (from READABLE_INDEXABLE)
		require -- from  ITERABLE
			True
		ensure -- from ITERABLE
			result_attached: Result /= Void

	sequential_occurrences (v: like item): INTEGER_32
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		require -- from  BAG
			True
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
	
feature -- Measurement

	count: INTEGER_32
			-- Number of items
			-- (from LINKED_CIRCULAR)
		require -- from  FINITE
			True
		require -- from  READABLE_INDEXABLE
			True
		ensure -- from FINITE
			count_non_negative: Result >= 0

	Lower: INTEGER_32 = 1
			-- Minimum index.
			-- (from CHAIN)

	occurrences (v: like item): INTEGER_32
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
	
feature -- Comparison

	frozen deep_equal (a: detachable ANY; b: like arg #1): BOOLEAN
			-- Are a and b either both void
			-- or attached to isomorphic object structures?
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			shallow_implies_deep: standard_equal (a, b) implies Result
			both_or_none_void: (a = Void) implies (Result = (b = Void))
			same_type: (Result and (a /= Void)) implies (b /= Void and then a.same_type (b))
			symmetric: Result implies deep_equal (b, a)

	frozen equal (a: detachable ANY; b: like arg #1): BOOLEAN
			-- Are a and b either both void or attached
			-- to objects considered equal?
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			definition: Result = (a = Void and b = Void) or else ((a /= Void and b /= Void) and then a.is_equal (b))

	frozen is_deep_equal alias "≡≡≡" (other: TWO_WAY_CIRCULAR [G]): BOOLEAN
			-- Are Current and other attached to isomorphic object structures?
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		ensure -- from ANY
			shallow_implies_deep: standard_is_equal (other) implies Result
			same_type: Result implies same_type (other)
			symmetric: Result implies other.is_deep_equal (Current)

	is_equal (other: TWO_WAY_CIRCULAR [G]): BOOLEAN
			-- Does other contain the same elements?
			-- (from LIST)
		require -- from ANY
			other_not_void: other /= Void
		ensure -- from ANY
			symmetric: Result implies other ~ 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 (a: detachable ANY; b: like arg #1): BOOLEAN
			-- Are a and b 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
			instance_free: class
			definition: Result = (a = Void and b = Void) or else ((a /= Void and b /= Void) and then a.standard_is_equal (b))

	frozen standard_is_equal alias "≜" (other: TWO_WAY_CIRCULAR [G]): 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

	after: BOOLEAN
			-- Is there no valid cursor position to the right of cursor?
			-- (from CIRCULAR)
		require -- from  LINEAR
			True
		ensure then -- from CIRCULAR
			empty_and_std_after: Result = (is_empty and standard_after)

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

	before: BOOLEAN
			-- Is there no valid cursor position to the right of cursor?
			-- (from CIRCULAR)
		require -- from  BILINEAR
			True
		ensure then -- from CIRCULAR
			empty_and_std_before: Result = (is_empty and standard_before)

	l_before: BOOLEAN
			-- Is there no valid cursor position to the left of cursor?
			-- (from LIST)

	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

	exhausted: BOOLEAN
			-- Has structure been completely explored?
			-- (from CIRCULAR)
		require -- from  LINEAR
			True
		ensure -- from LINEAR
			exhausted_when_off: off implies Result

	extendible: BOOLEAN
			-- May new items be added? (Answer: yes.)
			-- (from DYNAMIC_CHAIN)
		require -- from  COLLECTION
			True

	Full: BOOLEAN = False
			-- Is structured filled to capacity? (Answer: no.)
			-- (from LINKED_CIRCULAR)

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

	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)

	isfirst: BOOLEAN
			-- Is cursor on first item?
			-- (from LINKED_CIRCULAR)
		ensure -- from CHAIN
			valid_position: Result implies not is_empty

	islast: BOOLEAN
			-- Is cursor on last item?
			-- (from LINKED_CIRCULAR)
		ensure -- from CHAIN
			valid_position: Result implies not is_empty

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

	l_off: BOOLEAN
			-- Is there no current item?
			-- (from CHAIN)
		require -- from  TRAVERSABLE
			True

	off: BOOLEAN
			-- Is there no current item?
			-- (from CIRCULAR)
		require -- from  TRAVERSABLE
			True
		ensure then -- from CIRCULAR
			only_when_empty: Result = is_empty

	prunable: BOOLEAN
			-- May items be removed? (Answer: yes.)
			-- (from DYNAMIC_TABLE)
		require -- from  COLLECTION
			True

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

	replaceable: BOOLEAN
			-- Can current item be replaced?
			-- (from ACTIVE)

	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))

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

	valid_cursor_index (i: INTEGER_32): BOOLEAN
			-- Is i a possible cursor position?
			-- (from CIRCULAR)
		require -- from  CHAIN
			True
		ensure -- from CHAIN
			valid_cursor_index_definition: Result = ((i >= 0) and (i <= count + 1))
		ensure then -- from CIRCULAR
			valid_cursor_index_definition: Result = ((i >= 0) and (i <= count))

	valid_index (i: INTEGER_32): BOOLEAN
			-- Is i within allowable bounds?
			-- (from CHAIN)
		require -- from  READABLE_INDEXABLE
			True
		require -- from  TABLE
			True
		ensure -- from READABLE_INDEXABLE
			only_if_in_index_set: Result implies (Lower <= i and i <= count)
		ensure then -- from CHAIN
			valid_index_definition: Result = ((i >= 1) and (i <= count))

	writable: BOOLEAN
			-- Is there a current item that may be written?
			-- (from LINKED_CIRCULAR)
		require -- from  ACTIVE
			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 -- Cursor movement

	back
			-- Move cursor to previous item, cyclically.
			-- (from CIRCULAR)
		require -- from BILINEAR
			not_before: not before

	finish
			-- Move cursor to last position.
			-- (No effect if empty)
			-- (from CHAIN)
		require -- from  LINEAR
			True
		ensure then -- from CHAIN
			at_last: not is_empty implies islast

	forth
			-- Move cursor to next item, cyclically.
			-- (from CIRCULAR)
		require -- from LINEAR
			not_after: not after
		ensure then -- from CIRCULAR
			moved_forth_at_end: (old index = count) implies (index = 1)

	go_i_th (i: INTEGER_32)
			-- Move cursor to i-th position from current start, cyclically.
			-- (from CIRCULAR)
		require -- from CHAIN
			valid_cursor_index: valid_cursor_index (i)
		require else -- from CIRCULAR
			index_big_enough: i >= 1
			not_empty: not is_empty
		ensure -- from CHAIN
			position_expected: index = i

	l_go_i_th (i: INTEGER_32)
			-- Move cursor to i-th position.
			-- (from CHAIN)
		require -- from CHAIN
			valid_cursor_index: valid_cursor_index (i)
		ensure -- from CHAIN
			position_expected: index = i

	go_to (p: CURSOR)
			-- Move cursor to position p.
			-- (from LINKED_CIRCULAR)
		require -- from CURSOR_STRUCTURE
			cursor_position_valid: valid_cursor (p)

	l_forth
			-- (from LINKED_CIRCULAR)

	move (i: INTEGER_32)
			-- Move cursor to i-th item from current position,
			-- cyclically.
			-- (from CIRCULAR)
		require -- from  CHAIN
			True
		ensure -- from CHAIN
			too_far_right: (old index + i > count) implies exhausted
			too_far_left: (old index + i < 1) implies exhausted
			expected_index: (not exhausted) implies (index = old index + i)

	search (v: like item)
			-- Move to first position (at or after current
			-- position) where item and v are equal.
			-- If structure does not include v ensure that
			-- exhausted will be true.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from BILINEAR)
		require -- from  LINEAR
			True
		ensure -- from LINEAR
			object_found: (not exhausted and object_comparison) implies v ~ item
			item_found: (not exhausted and not object_comparison) implies v = item

	set_start
			-- Select current item as the first.
			-- (from LINKED_CIRCULAR)
		require -- from CIRCULAR
			not_empty: not is_empty

	start
			-- Move to position currently selected as first.
			-- (from LINKED_CIRCULAR)
		require -- from  TRAVERSABLE
			True
		ensure then -- from CHAIN
			at_first: not is_empty implies isfirst
	
feature -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
			-- (from CHAIN)
		require -- from SEQUENCE
			argument_not_void: s /= Void
		ensure -- from SEQUENCE
			new_count: count >= old count

	extend (v: like item)
			-- Add v to end.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: is_inserted (v)

	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 CHAIN)
		require -- from COLLECTION
			other_not_void: other /= Void
			extendible: extendible

	force (v: like item)
			-- Add v to end.
			-- (from SEQUENCE)
		require -- from SEQUENCE
			extendible: extendible
		ensure then -- from SEQUENCE
			new_count: count = old count + 1
			item_inserted: has (v)

	merge_left (other: TWO_WAY_CIRCULAR [G])
			-- Merge other into current structure before cursor
			-- position. Do not move cursor. Empty other.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_before: not before
			other_exists: other /= Void
			not_current: other /= Current
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + old other.count
			new_index: index = old index + old other.count
			other_is_empty: other.is_empty

	merge_right (other: TWO_WAY_CIRCULAR [G])
			-- Merge other into current structure after cursor
			-- position. Do not move cursor. Empty other.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_after: not after
			other_exists: other /= Void
			not_current: other /= Current
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + old other.count
			same_index: index = old index
			other_is_empty: other.is_empty

	sequence_put (v: like item)
			-- Add v to end.
			-- (from SEQUENCE)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: is_inserted (v)
		ensure then -- from SEQUENCE
			new_count: count = old count + 1

	put (v: like item)
			-- Replace current item by v.
			-- (Synonym for replace)
			-- (from CHAIN)
		require -- from CHAIN
			writeable: writable
			replaceable: replaceable
		ensure -- from CHAIN
			same_count: count = old count
			is_inserted: is_inserted (v)

	put_front (v: like item)
			-- Add v to beginning.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			item_inserted: first = v

	put_i_th (v: like item; i: INTEGER_32)
			-- Put v at i-th position.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (i)
		require -- from TABLE
			valid_key: valid_index (i)
		ensure -- from TABLE
			inserted: i_th (i) = v

	put_left (v: like item)
			-- Add v to the left of cursor position.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_before: not before
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			new_index: index = old index + 1

	put_right (v: like item)
			-- Add v to the right of cursor position.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_after: not after
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			same_index: index = old index

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

	l_prune (v: like item)
			-- Remove the first occurrence of v if any.
			-- If no such occurrence go off.
			-- (from SEQUENCE)
		require -- from COLLECTION
			prunable: prunable

	prune (v: like item)
			-- Remove first occurrence of v, if any,
			-- after cursor position.
			-- If found, move cursor to right neighbor;
			-- if not, make structure exhausted.
			-- (from DYNAMIC_CHAIN)
		require -- from COLLECTION
			prunable: prunable

	l_prune_all (v: like item)
			-- Remove all occurrences of v; go off.
			-- (from SEQUENCE)
		require -- from COLLECTION
			prunable: prunable
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)

	prune_all (v: like item)
			-- Remove all occurrences of v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- Leave structure exhausted.
			-- (from DYNAMIC_CHAIN)
		require -- from COLLECTION
			prunable: prunable
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted

	remove
			-- Remove item at cursor position.
			-- Move cursor to right neighbor (cyclically).
			-- If removed item was at current starting position,
			-- move starting position to right neighbor.
			-- (from CIRCULAR)
		require -- from ACTIVE
			prunable: prunable
			writable: writable

	remove_i_th (i: INTEGER_32)
			-- Remove item at index i.
			-- Move cursor to next neighbor (or after if no next neighbor) if it is at i-th position.
			-- Do not change cursor otherwise.
			-- (from DYNAMIC_CHAIN)
		require -- from DYNAMIC_TABLE
			prunable: prunable
			valid_key: valid_index (i)
		ensure then -- from DYNAMIC_CHAIN
			new_count: count = old count - 1
			same_index_if_below: index <= i implies index = old index
			new_index_if_above: index > i implies index = old index - 1
			same_leading_items: across
					old twin as c
				all
					c.target_index < i implies c.item = i_th (c.target_index)
				end
			same_trailing_items: across
					old twin as c
				all
					c.target_index > i implies c.item = i_th (c.target_index - 1)
				end

	remove_left
			-- Remove item to the left of cursor position.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			left_exists: index > 1
		require else -- from LINKED_CIRCULAR
				count > 1
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count - 1
			new_index: index = old index - 1

	remove_right
			-- Remove item to the right of cursor position.
			-- Do not move cursor.
			-- (from LINKED_CIRCULAR)
		require -- from DYNAMIC_CHAIN
			right_exists: index < count
		require else -- from LINKED_CIRCULAR
				count > 1
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count - 1
			same_index: index = old index

	wipe_out
			-- Remove all items.
			-- (from DYNAMIC_CHAIN)
		require -- from COLLECTION
			prunable: prunable
		ensure -- from COLLECTION
			wiped_out: is_empty
	
feature -- Transformation

	swap (i: INTEGER_32)
			-- Exchange item at i-th position with item
			-- at cursor position.
			-- (from CHAIN)
		require -- from CHAIN
			not_off: not off
			valid_index: valid_index (i)
		ensure -- from CHAIN
			swapped_to_item: item = old i_th (i)
			swapped_from_item: i_th (i) = old item
	
feature -- Conversion

	linear_representation: LINEAR [G]
			-- Representation as a linear structure
			-- (from LINEAR)
		require -- from  CONTAINER
			True
	
feature -- Duplication

	copy (other: TWO_WAY_CIRCULAR [G])
			-- Update current object using fields of object attached
			-- to other, so as to yield equal objects.
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
			type_identity: same_type (other)
		ensure -- from ANY
			is_equal: Current ~ other

	frozen deep_copy (other: TWO_WAY_CIRCULAR [G])
			-- 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: TWO_WAY_CIRCULAR [G]
			-- New object structure recursively duplicated from Current.
			-- (from ANY)
		ensure -- from ANY
			deep_twin_not_void: Result /= Void
			deep_equal: deep_equal (Current, Result)

	frozen standard_copy (other: TWO_WAY_CIRCULAR [G])
			-- 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: TWO_WAY_CIRCULAR [G]
			-- 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: TWO_WAY_CIRCULAR [G]
			-- New object equal to Current
			-- twin calls copy; to change copying/twinning semantics, redefine copy.
			-- (from ANY)
		ensure -- from ANY
			twin_not_void: Result /= Void
			is_equal: Result ~ Current
	
feature -- Basic operations

	frozen default: detachable TWO_WAY_CIRCULAR [G]
			-- 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)
		ensure -- from ANY
			instance_free: class

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

	frozen do_nothing
			-- Execute a null action.
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
	
feature -- Inapplicable

	l_remove
			-- Remove current item.
			-- (from CHAIN)
		require -- from ACTIVE
			prunable: prunable
			writable: writable
	
feature -- Implementation

	list: TWO_WAY_LIST [G]
	
feature -- Iteration

	do_all (action: PROCEDURE [G])
			-- Apply action to every item.
			-- Semantics not guaranteed if action changes the structure;
			-- in such a case, apply iterator to clone of structure instead.
			-- (from LINEAR)
		require -- from TRAVERSABLE
			action_exists: action /= Void

	do_if (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item that satisfies test.
			-- Semantics not guaranteed if action or test changes the structure;
			-- in such a case, apply iterator to clone of structure instead.
			-- (from LINEAR)
		require -- from TRAVERSABLE
			action_exists: action /= Void
			test_exists: test /= Void

	for_all (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Is test true for all items?
			-- Semantics not guaranteed if test changes the structure;
			-- in such a case, apply iterator to clone of structure instead.
			-- (from LINEAR)
		require -- from TRAVERSABLE
			test_exists: test /= Void
		ensure then -- from LINEAR
			empty: is_empty implies Result

	there_exists (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Is test true for at least one item?
			-- Semantics not guaranteed if test changes the structure;
			-- in such a case, apply iterator to clone of structure instead.
			-- (from LINEAR)
		require -- from TRAVERSABLE
			test_exists: test /= Void
	
feature -- Output

	Io: STD_FILES
			-- Handle to standard file setup
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			io_not_void: Result /= Void

	out: STRING_8
			-- New string containing terse printable representation
			-- of current object
			-- (from ANY)
		ensure -- from ANY
			out_not_void: Result /= Void

	print (o: detachable ANY)
			-- Write terse external representation of o
			-- on standard output.
			-- (from ANY)
		ensure -- from ANY
			instance_free: class

	frozen tagged_out: STRING_8
			-- New string containing terse printable representation
			-- of current object
			-- (from ANY)
		ensure -- from ANY
			tagged_out_not_void: Result /= Void
	
feature -- Platform

	Operating_environment: OPERATING_ENVIRONMENT
			-- Objects available from the operating system
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			operating_environment_not_void: Result /= Void
	
invariant
		-- from CIRCULAR
	not_before_unless_empty: before implies is_empty
	not_after_unless_empty: after implies is_empty
	not_off_unless_empty: off implies is_empty

		-- 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 (item = i_th (index))

		-- 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 READABLE_INDEXABLE
	consistent_boundaries: Lower <= count or else Lower = count + 1

		-- 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 FINITE
	empty_definition: is_empty = (count = 0)

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

note
	copyright: "Copyright (c) 1984-2012, Eiffel Software and others"
	license: "Eiffel Forum License v2 (see http://www.eiffel.com/licensing/forum.txt)"
	source: "[
		Eiffel Software
		5949 Hollister Ave., 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 TWO_WAY_CIRCULAR

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