note
	description: "Two-way lists, kept sorted"
	library: "Free implementation of ELKS library"
	legal: "See notice at end of class."
	status: "See notice at end of class."
	names: sorted_two_way_list, sorted_struct, sequence
	representation: linked
	access: index, cursor, membership, min, max
	contents: generic
	date: "$Date: 2018-11-14 15:15:17 +0000 (Wed, 14 Nov 2018) $"
	revision: "$Revision: 102463 $"

class 
	PART_SORTED_TWO_WAY_LIST [G -> PART_COMPARABLE]

create 
	make,
	make_from_iterable


create {PART_SORTED_TWO_WAY_LIST}
	make_sublist

feature {NONE} -- Initialization

	default_create
			-- Process instances of classes with no creation clause.
			-- (Default: do nothing.)
			-- (from ANY)
		do
		end

	make
			-- Create an empty list.
			-- (from LINKED_LIST)
		do
			before := True
		ensure -- from LINKED_LIST
			is_before: before
		end

	make_from_iterable (other: ITERABLE [G])
			-- Create a linked list with all items obtained from other.
			-- (from LINKED_LIST)
		do
			make
			across
				other as o
			loop
				extend (o.item)
			end
		end
	
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)
		local
			pos: CURSOR
		do
			pos := cursor
			go_i_th (i)
			Result := item
			go_to (pos)
		end

	cursor: TWO_WAY_LIST_CURSOR [G]
			-- Current cursor position
			-- (from TWO_WAY_LIST)
		require -- from  CURSOR_STRUCTURE
			True
		do
			create Result.make (active, after, before)
		ensure -- from CURSOR_STRUCTURE
			cursor_not_void: Result /= Void
		end

	first: like item
			-- Item at first position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_empty
		do
			check
					attached first_element as f
			then
				Result := f.item
			end
		end

	first_element: detachable like new_cell
			-- Head of list
			-- (Anchor redefinition)
			-- (from TWO_WAY_LIST)

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

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

	has (v: G): BOOLEAN
			-- Does structure include v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from PART_SORTED_LIST)
		local
			pos: CURSOR
		do
			if not is_empty then
				pos := cursor
				start
				search (v)
				Result := not after
				go_to (pos)
			end
		end

	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
		do
			start
			if not off then
				search (v)
			end
			Result := not exhausted
		ensure -- from CONTAINER
			not_found_in_empty: Result implies not is_empty
		end

	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)
		local
			pos: CURSOR
		do
			pos := cursor
			go_i_th (i)
			Result := item
			go_to (pos)
		end

	index: INTEGER_32
			-- Index of current position
			-- (from LINKED_LIST)
		require -- from  LINEAR
			True
		local
			l_active, l_active_iterator: like active
		do
			if after then
				Result := count + 1
			elseif not before then
				from
					Result := 1
					l_active := active
					l_active_iterator := first_element
				until
					l_active_iterator = l_active or else l_active_iterator = Void
				loop
					l_active_iterator := l_active_iterator.right
					Result := Result + 1
				end
			end
		end

	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)
		local
			pos: CURSOR
		do
			pos := cursor
			Result := sequential_index_of (v, i)
			go_to (pos)
		end

	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
		local
			occur, pos: INTEGER_32
		do
			if object_comparison and v /= Void then
				from
					start
					pos := 1
				until
					exhausted or (occur = i)
				loop
					if item ~ v then
						occur := occur + 1
					end
					forth
					pos := pos + 1
				end
			else
				from
					start
					pos := 1
				until
					exhausted or (occur = i)
				loop
					if item = v then
						occur := occur + 1
					end
					forth
					pos := pos + 1
				end
			end
			if occur = i then
				Result := pos - 1
			end
		ensure -- from LINEAR
			non_negative_result: Result >= 0
		end

	item: G
			-- Current item
			-- (from LINKED_LIST)
		require -- from ACTIVE
			readable: readable
		require -- from TRAVERSABLE
			not_off: not off
		do
			check
					attached active as a
			then
				Result := a.item
			end
		end

	item_for_iteration: G
			-- Item at current position
			-- (from LINEAR)
		require -- from LINEAR
			not_off: not off
		do
			Result := item
		end

	last: like item
			-- Item at last position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_empty
		do
			check
					attached last_element as l
			then
				Result := l.item
			end
		end

	last_element: like first_element
			-- Tail of the list
			-- (from TWO_WAY_LIST)

	new_cursor: TWO_WAY_LIST_ITERATION_CURSOR [G]
			-- Fresh cursor associated with current structure
			-- (from TWO_WAY_LIST)
		require -- from  ITERABLE
			True
		do
			create Result.make (Current);
			Result.start
		ensure -- from ITERABLE
			result_attached: Result /= Void
		end

	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
		do
			from
				start
				search (v)
			until
				exhausted
			loop
				Result := Result + 1
				forth
				search (v)
			end
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
		end

	search_after (v: like item)
			-- Go to first position with item greater
			-- than or equal to v.
			-- (from PART_SORTED_LIST)
		do
			from
				start
			until
				after or else v <= item
			loop
				forth
			end
		ensure -- from PART_SORTED_LIST
			argument_less_than_item: (not after) implies (v <= item)
		end

	search_before (v: like item)
			-- Go to last position with item less
			-- than or equal to v.
			-- (from PART_SORTED_LIST)
		do
			from
				finish
			until
				before or else v >= item
			loop
				back
			end
		ensure -- from PART_SORTED_LIST
				(not off) implies (item <= v)
		end

	sublist: detachable PART_SORTED_TWO_WAY_LIST [G]
			-- Result produced by last split
			-- (from TWO_WAY_LIST)
	
feature -- Measurement

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

	index_set: INTEGER_INTERVAL
		obsolete "Use `lower' and `upper' instead. [2017-05-31]"
			-- Range of acceptable indexes.
			-- (from READABLE_INDEXABLE)
		do
			create Result.make (Lower, count)
		ensure -- from READABLE_INDEXABLE
			not_void: Result /= Void
			empty_if_not_in_order: (Lower > count) implies Result.is_empty
			same_lower_if_not_empty: (Lower <= count) implies Result.lower = Lower
			same_upper_if_not_empty: (Lower <= count) implies Result.upper = count
		end

	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)
		local
			pos: CURSOR
		do
			pos := cursor
			Result := sequential_occurrences (v)
			go_to (pos)
		end
	
feature {NONE} -- Measurement

	estimated_count_of (other: ITERABLE [G]): INTEGER_32
			-- Estimated number of elements in other.
			-- (from CONTAINER)
		do
			if attached {FINITE [G]} other as f then
				Result := f.count
			elseif attached {READABLE_INDEXABLE [G]} other as r then
				Result := r.upper - r.lower + 1
			end
		ensure -- from CONTAINER
			instance_free: class
			non_negative_result: Result >= 0
		end
	
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)
		do
			if a = Void then
				Result := b = Void
			else
				Result := b /= Void and then a.is_deep_equal (b)
			end
		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)
		end

	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)
		do
			if a = Void then
				Result := b = Void
			else
				Result := b /= Void and then a.is_equal (b)
			end
		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))
		end

	frozen is_deep_equal alias "≡≡≡" (other: PART_SORTED_TWO_WAY_LIST [G]): BOOLEAN
			-- Are Current and other attached to isomorphic object structures?
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		external
			"built_in"
		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)
		end

	is_equal (other: PART_SORTED_TWO_WAY_LIST [G]): BOOLEAN
			-- Does other contain the same elements?
			-- (from LIST)
		require -- from ANY
			other_not_void: other /= Void
		do
			if Current = other then
				Result := True
			else
				Result := (is_empty = other.is_empty) and (object_comparison = other.object_comparison) and (count = other.count)
				if Result and not is_empty then
					if attached cursor as c1 and then attached other.cursor as c2 then
						from
							start;
							other.start
						until
							after or not Result
						loop
							if object_comparison then
								Result := item ~ other.item
							else
								Result := item = other.item
							end
							forth;
							other.forth
						end
						go_to (c1);
						other.go_to (c2)
					else
						check
							cursors_exist: False
						end
					end
				elseif is_empty and other.is_empty and object_comparison = other.object_comparison then
					Result := True
				end
			end
		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
		end

	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)
		do
			if a = Void then
				Result := b = Void
			else
				Result := b /= Void and then a.standard_is_equal (b)
			end
		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))
		end

	frozen standard_is_equal alias "≜" (other: PART_SORTED_TWO_WAY_LIST [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
		external
			"built_in"
		ensure -- from ANY
			same_type: Result implies same_type (other)
			symmetric: Result implies other.standard_is_equal (Current)
		end
	
feature -- Status report

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

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

	changeable_comparison_criterion: BOOLEAN
			-- May object_comparison be changed?
			-- (Answer: yes by default.)
			-- (from CONTAINER)
		do
			Result := True
		end

	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
		external
			"built_in"
		end

	empty: BOOLEAN
		obsolete "ELKS 2000: Use `is_empty' instead. [2017-05-31]"
			-- Is there no element?
			-- (from CONTAINER)
		do
			Result := is_empty
		end

	exhausted: BOOLEAN
			-- Has structure been completely explored?
			-- (from LINEAR)
		do
			Result := off
		ensure -- from LINEAR
			exhausted_when_off: off implies Result
		end

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

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

	is_empty: BOOLEAN
			-- Is structure empty?
			-- (from FINITE)
		require -- from  CONTAINER
			True
		do
			Result := count = 0
		end

	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)
		do
			Result := has (v)
		end

	isfirst: BOOLEAN
			-- Is cursor at first position?
			-- (from LINKED_LIST)
		require -- from  CHAIN
			True
		do
			Result := not after and not before and (active = first_element)
		ensure -- from CHAIN
			valid_position: Result implies not is_empty
		end

	islast: BOOLEAN
			-- Is cursor at last position?
			-- (from TWO_WAY_LIST)
		require -- from  CHAIN
			True
		do
			Result := (active = last_element) and then not after and then not before
		ensure -- from CHAIN
			valid_position: Result implies not is_empty
		end

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

	off: BOOLEAN
			-- Is there no current item?
			-- (from LINKED_LIST)
		require -- from  TRAVERSABLE
			True
		do
			Result := after or before
		end

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

	readable: BOOLEAN
			-- Is there a current item that may be read?
			-- (from LINKED_LIST)
		require -- from  ACTIVE
			True
		do
			Result := not off
		end

	replaceable: BOOLEAN
			-- Can current item be replaced?
			-- (from ACTIVE)
		do
			Result := True
		end

	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
		external
			"built_in"
		ensure -- from ANY
			definition: Result = (conforms_to (other) and other.conforms_to (Current))
		end

	sorted: BOOLEAN
			-- is the structures topologically sorted
			-- i.e i < j implies not i_th (i) > i_th (j)
		local
			c: CURSOR
			prev: like item
		do
			Result := True
			if count > 1 then
				from
					c := cursor
					start
					check
							not off
					end
					prev := item
					forth
				until
					after or not Result
				loop
					Result := prev <= item
					prev := item
					forth
				end
				go_to (c)
			end
		end

	valid_cursor (p: CURSOR): BOOLEAN
			-- Can the cursor be moved to position p?
			-- (from LINKED_LIST)
		require -- from  CURSOR_STRUCTURE
			True
		local
			temp, sought: like first_element
		do
			if attached {like cursor} p as ll_c then
				from
					temp := first_element
					sought := ll_c.active
					Result := ll_c.after or else ll_c.before
				until
					Result or else temp = Void
				loop
					Result := temp = sought
					temp := temp.right
				end
			end
		end

	valid_cursor_index (i: INTEGER_32): BOOLEAN
			-- Is i correctly bounded for cursor movement?
			-- (from CHAIN)
		do
			Result := (i >= 0) and (i <= count + 1)
		ensure -- from CHAIN
			valid_cursor_index_definition: Result = ((i >= 0) and (i <= count + 1))
		end

	valid_index (i: INTEGER_32): BOOLEAN
			-- Is i within allowable bounds?
			-- (from CHAIN)
		require -- from  READABLE_INDEXABLE
			True
		require -- from  TABLE
			True
		do
			Result := (i >= 1) and (i <= count)
		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))
		end

	writable: BOOLEAN
			-- Is there a current item that may be modified?
			-- (from SEQUENCE)
		require -- from  ACTIVE
			True
		do
			Result := not off
		end
	
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
		do
			object_comparison := True
		ensure -- from CONTAINER
				object_comparison
		end

	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
		do
			object_comparison := False
		ensure -- from CONTAINER
			reference_comparison: not object_comparison
		end
	
feature -- Cursor movement

	back
			-- Move cursor to previous position, if any.
			-- (from TWO_WAY_LIST)
		require -- from BILINEAR
			not_before: not before
		local
			a: like active
		do
			if after then
				after := False
				if is_empty then
					before := True
				end
			else
				a := active
				if a /= Void then
					a := a.left
					if a = Void then
						active := first_element
						before := True
					else
						active := a
					end
				end
			end
		end

	finish
			-- Move cursor to last position.
			-- (Go before if empty)
			-- (from TWO_WAY_LIST)
		require -- from  LINEAR
			True
		do
			if not is_empty then
				active := last_element
				after := False
				before := False
			else
				after := False
				before := True
			end
		ensure then -- from CHAIN
			at_last: not is_empty implies islast
		ensure then -- from LINKED_LIST
			empty_convention: is_empty implies before
		ensure then -- from TWO_WAY_LIST
			not_after: not after
		end

	forth
			-- Move cursor to next position, if any.
			-- (from TWO_WAY_LIST)
		require -- from LINEAR
			not_after: not after
		local
			a: like active
		do
			if before then
				before := False
				if is_empty then
					after := True
				end
			else
				a := active
				if a /= Void then
					a := a.right
					if a = Void then
						active := last_element
						after := True
					else
						active := a
					end
				end
			end
		ensure then -- from LIST
			moved_forth: index = old index + 1
		end

	go_i_th (i: INTEGER_32)
			-- Move cursor to i-th position.
			-- (from LINKED_LIST)
		require -- from CHAIN
			valid_cursor_index: valid_cursor_index (i)
		do
			if i = 0 then
				before := True
				after := False
				active := first_element
			elseif i = count + 1 then
				before := False
				after := True
				active := last_element
			else
				move (i - index)
			end
		ensure -- from CHAIN
			position_expected: index = i
		end

	go_to (p: CURSOR)
			-- Move cursor to position p.
			-- (from LINKED_LIST)
		require -- from CURSOR_STRUCTURE
			cursor_position_valid: valid_cursor (p)
		do
			if attached {like cursor} p as ll_c then
				after := ll_c.after
				before := ll_c.before
				if before then
					active := first_element
				elseif after then
					active := last_element
				else
					active := ll_c.active
				end
			else
				check
					correct_cursor_type: False
				end
			end
		end

	move (i: INTEGER_32)
			-- Move cursor i positions. The cursor
			-- may end up off if the offset is to big.
			-- (from TWO_WAY_LIST)
		require -- from  CHAIN
			True
		local
			counter: INTEGER_32
			p: like first_element
		do
			if i > 0 then
				ll_move (i)
			elseif i < 0 then
				if after then
					after := False
					counter := -1
				end
				from
					p := active
				until
					(counter = i) or else (p = Void)
				loop
					p := p.left
					counter := counter - 1
				end
				if p = Void then
					before := True
					active := first_element
				else
					active := p
				end
			end
		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)
		end

	ll_move (i: INTEGER_32)
			-- Move cursor i positions. The cursor
			-- may end up off if the offset is too big.
			-- (from LINKED_LIST)
		require -- from  CHAIN
			True
		local
			counter, new_index: INTEGER_32
			p: like first_element
		do
			if i > 0 then
				if before then
					before := False
					counter := 1
				end
				from
					p := active
				until
					(counter = i) or else (p = Void)
				loop
					active := p
					p := p.right
					counter := counter + 1
				end
				if p = Void then
					after := True
				else
					active := p
				end
			elseif i < 0 then
				new_index := index + i
				before := True
				after := False
				active := first_element
				if new_index > 0 then
					move (new_index)
				end
			end
		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)
		ensure then -- from LINKED_LIST
			moved_if_inbounds: ((old index + i) >= 0 and (old index + i) <= (count + 1)) implies index = (old index + i)
			before_set: (old index + i) <= 0 implies before
			after_set: (old index + i) >= (count + 1) implies after
		end

	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
		do
			if before and not is_empty then
				forth
			end
			Precursor (v)
		ensure -- from LINEAR
			object_found: (not exhausted and object_comparison) implies v ~ item
			item_found: (not exhausted and not object_comparison) implies v = item
		end

	start
			-- Move cursor to first position.
			-- (from LINKED_LIST)
		require -- from  TRAVERSABLE
			True
		do
			if first_element /= Void then
				active := first_element
				after := False
			else
				after := True
			end
			before := False
		ensure then -- from CHAIN
			at_first: not is_empty implies isfirst
		ensure then -- from LINKED_LIST
			empty_convention: is_empty implies after
		end
	
feature -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
			-- (from CHAIN)
		require -- from SEQUENCE
			argument_not_void: s /= Void
		local
			l: SEQUENCE [G]
			l_cursor: CURSOR
		do
			l := s
			if s = Current then
				l := twin
			end
			from
				l_cursor := cursor;
				l.start
			until
				l.exhausted
			loop
				extend (l.item)
				finish;
				l.forth
			end
			go_to (l_cursor)
		ensure -- from SEQUENCE
			new_count: count >= old count
		end

	extend (v: like item)
			-- Put v at proper position in list.
			-- The cursor ends up on the newly inserted
			-- item.
		require -- from COLLECTION
			extendible: extendible
		do
			search_after (v)
			put_left (v)
			back
		ensure -- from COLLECTION
			item_inserted: is_inserted (v)
		ensure then -- from PART_SORTED_LIST
			remains_sorted: (old sorted) implies sorted
			item_inserted: item = v
		end

	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
		local
			lin_rep: LINEAR [G]
			l_cursor: CURSOR
		do
			lin_rep := other.linear_representation
			from
				l_cursor := cursor;
				lin_rep.start
			until
				not extendible or else lin_rep.off
			loop
				extend (lin_rep.item)
				finish;
				lin_rep.forth
			end
			go_to (l_cursor)
		end

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

	merge (other: LINEAR [G])
			-- Add all items from other at their proper positions.
			-- (from PART_SORTED_LIST)
		do
			from
				other.start
			until
				other.off
			loop
				extend (other.item);
				other.forth
			end
		ensure then -- from PART_SORTED_LIST
			remains_sorted: (old sorted) implies sorted
		end

	merge_left (other: PART_SORTED_TWO_WAY_LIST [G])
			-- Merge other into current structure before cursor
			-- position. Do not move cursor. Empty other.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_before: not before
			other_exists: other /= Void
			not_current: other /= Current
		local
			other_count: INTEGER_32
		do
			if not other.is_empty then
				other_count := other.count
				check
						attached other.first_element as other_first_element
						attached other.last_element as other_last_element
				then
					other.wipe_out
					if is_empty then
						last_element := other_last_element
						first_element := other_first_element
						if before then
							active := first_element
						else
							active := last_element
						end
					elseif isfirst then
						other_last_element.put_right (first_element)
						first_element := other_first_element
					elseif after then
						other_first_element.put_left (last_element)
						last_element := other_last_element
						active := last_element
					elseif attached active as a then
						other_first_element.put_left (a.left);
						a.put_left (other_last_element)
					end
				end
				count := count + other_count
			end
		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
		end

	merge_right (other: PART_SORTED_TWO_WAY_LIST [G])
			-- Merge other into current structure after cursor
			-- position. Do not move cursor. Empty other.
			-- (from TWO_WAY_LIST)
		do
			if is_empty or else islast then
				last_element := other.last_element
			end
			ll_merge_right (other)
		end

	ll_merge_right (other: PART_SORTED_TWO_WAY_LIST [G])
			-- Merge other into current structure after cursor
			-- position. Do not move cursor. Empty other.
			-- (from LINKED_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_after: not after
			other_exists: other /= Void
			not_current: other /= Current
		local
			other_first_element: like first_element
			other_count: INTEGER_32
			a: like active
		do
			if attached other.last_element as other_last_element then
				other_first_element := other.first_element
				other_count := other.count;
				other.wipe_out
				check
						other_first_element /= Void
				end
				a := active
				if a = Void then
					first_element := other_first_element
					active := first_element
				else
					if not islast then
						other_last_element.put_right (a.right)
					end;
					a.put_right (other_first_element)
				end
				count := count + other_count
			end
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + old other.count
			same_index: index = old index
			other_is_empty: other.is_empty
		end

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

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

	put_front (v: like item)
			-- Add v to beginning.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		do
			ll_put_front (v)
			if count = 1 then
				last_element := first_element
			end
		end

	ll_put_front (v: like item)
			-- Add v to beginning.
			-- Do not move cursor.
			-- (from LINKED_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
		local
			p: like new_cell
		do
			p := new_cell (v);
			p.put_right (first_element)
			first_element := p
			if before or is_empty then
				active := p
			end
			count := count + 1
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			item_inserted: first = v
		end

	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)
		local
			pos: CURSOR
		do
			pos := cursor
			go_i_th (i)
			replace (v)
			go_to (pos)
		ensure -- from TABLE
			inserted: i_th (i) = v
		end

	put_left (v: like item)
			-- Add v to the left of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_before: not before
		local
			p: like first_element
			a: like active
		do
			p := new_cell (v)
			if is_empty then
				first_element := p
				last_element := p
				active := p
				before := False
			elseif after then
				p.put_left (last_element)
				last_element := p
				active := p
			elseif isfirst then
				p.put_right (active)
				first_element := p
			else
				a := active
				if a /= Void then
					p.put_left (a.left)
				end;
				p.put_right (active)
			end
			count := count + 1
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			new_index: index = old index + 1
		ensure then -- from LINKED_LIST
			previous_exists: previous /= Void
			item_inserted: attached previous as q and then q.item = v
		end

	put_right (v: like item)
			-- Add v to the right of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		local
			was_last: BOOLEAN
			a: like active
		do
			was_last := islast
			ll_put_right (v)
			if count = 1 then
				last_element := active
			elseif was_last then
				a := active
				if a /= Void then
					last_element := a.right
				end
			end
		end

	ll_put_right (v: like item)
			-- Add v to the right of cursor position.
			-- Do not move cursor.
			-- (from LINKED_LIST)
		require -- from DYNAMIC_CHAIN
			extendible: extendible
			not_after: not after
		local
			p: like new_cell
			a: like active
		do
			p := new_cell (v)
			check
					is_empty implies before
			end
			if before then
				p.put_right (first_element)
				first_element := p
				active := p
			else
				a := active
				if a /= Void then
					p.put_right (a.right);
					a.put_right (p)
				end
			end
			count := count + 1
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + 1
			same_index: index = old index
		ensure then -- from LINKED_LIST
			next_exists: next /= Void
			item_inserted: not old before implies (attached next as n and then n.item = v)
			item_inserted_before: old before implies (attached active as c and then c.item = v)
		end

	replace (v: like item)
			-- Replace current item by v.
			-- (from LINKED_LIST)
		require -- from ACTIVE
			writable: writable
			replaceable: replaceable
		local
			a: like active
		do
			a := active
			if a /= Void then
				a.put (v)
			end
		ensure -- from ACTIVE
			item_replaced: item = v
		end
	
feature -- Removal

	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
		do
			search (v)
			if not exhausted then
				remove
			end
		end

	prune_all (v: like item)
			-- Remove all items identical to v.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- Leave cursor off.
		require -- from COLLECTION
			prunable: prunable
		do
			from
				start
				search (v)
			until
				off or else v < item
			loop
				remove
			end
			if not off then
				finish
				forth
			end
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted
		end

	remove
			-- Remove current item.
			-- Move cursor to right neighbor
			-- (or after if no right neighbor).
			-- (from TWO_WAY_LIST)
		require -- from ACTIVE
			prunable: prunable
			writable: writable
		local
			succ, pred, removed: like first_element
			a: like active
		do
			removed := active
			if isfirst then
				a := first_element
				if a /= Void then
					active := a.right;
					a.forget_right
				end
				first_element := active
				if count = 1 then
					check
						no_active: active = Void
					end
					after := True
					last_element := Void
				end
			elseif islast then
				a := last_element
				if a /= Void then
					active := a.left;
					a.forget_left
				end
				last_element := active
				after := True
			else
				a := active
				if a /= Void then
					pred := a.left
					succ := a.right
				end
				if pred /= Void and then succ /= Void then
					pred.forget_right;
					succ.forget_left;
					pred.put_right (succ)
				end
				active := succ
			end
			count := count - 1
			cleanup_after_remove (removed)
		ensure then -- from DYNAMIC_LIST
			after_when_empty: is_empty implies after
		end

	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)
		local
			new_index: like index
		do
			new_index := index
			if new_index > i then
				new_index := new_index - 1
			end
			go_i_th (i)
			remove
			go_i_th (new_index)
		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
		end

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

	remove_right
			-- Remove item to the right of cursor position.
			-- Do not move cursor.
			-- (from TWO_WAY_LIST)
		require -- from DYNAMIC_CHAIN
			right_exists: index < count
		do
			forth
			remove
			back
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count - 1
			same_index: index = old index
		end

	remove_sublist
			-- (from TWO_WAY_LIST)
		do
			sublist := Void
		end

	split (n: INTEGER_32)
			-- Remove from current list
			-- min (n, count - index - 1) items
			-- starting at cursor position.
			-- Move cursor right one position.
			-- Make extracted sublist accessible
			-- through attribute sublist.
			-- (from TWO_WAY_LIST)
		require -- from TWO_WAY_LIST
			not_off: not off
			valid_sublist: n >= 0
		local
			actual_number, active_index: INTEGER_32
			p_elem, s_elem, e_elem, n_elem: like first_element
		do
			if n = 0 then
				create sublist.make
			else
				active_index := index
				if active_index + n > count + 1 then
					actual_number := count + 1 - active_index
				else
					actual_number := n
				end
				s_elem := active
				p_elem := previous
				move (actual_number - 1)
				e_elem := active
				n_elem := next
				if s_elem /= Void then
					s_elem.forget_left
				end
				if e_elem /= Void then
					e_elem.forget_right
				end
				create sublist.make_sublist (s_elem, e_elem, actual_number)
				count := count - actual_number
				if p_elem /= Void then
					p_elem.put_right (n_elem)
				else
					first_element := n_elem
				end
				if n_elem /= Void then
					active := n_elem
				else
					last_element := p_elem
					active := p_elem
					after := True
				end
			end
		end

	wipe_out
			-- Remove all items.
			-- (from TWO_WAY_LIST)
		require -- from COLLECTION
			prunable: prunable
		do
			ll_wipe_out
			last_element := Void
		ensure -- from COLLECTION
			wiped_out: is_empty
		end

	ll_wipe_out
			-- Remove all items.
			-- (from LINKED_LIST)
		do
			internal_wipe_out
		ensure then -- from DYNAMIC_LIST
			is_before: before
		end

	chain_wipe_out
			-- Remove all items.
			-- (from DYNAMIC_CHAIN)
		require -- from COLLECTION
			prunable: prunable
		do
			from
				start
			until
				is_empty
			loop
				remove
			end
		ensure -- from COLLECTION
			wiped_out: is_empty
		end
	
feature -- Transformation

	sort
			-- sort the list
			-- Has O(count * log (count)) complexity.
		local
			no_change: BOOLEAN
			gap: INTEGER_32
			left_cell, cell: like first_element
			left_cell_item, cell_item: like item
		do
			if not is_empty then
				from
					gap := count * 10 // 13
				until
					gap = 0
				loop
					from
						no_change := False
						go_i_th (1 + gap)
					until
						no_change
					loop
						no_change := True
						from
							left_cell := first_element
							cell := active
						until
							cell = Void
						loop
							if left_cell /= Void then
								left_cell_item := left_cell.item
								cell_item := cell.item
								if cell_item < left_cell_item then
									no_change := False;
									cell.put (left_cell_item);
									left_cell.put (cell_item)
								end
								left_cell := left_cell.right
							end
							cell := cell.right
						end
					end
					gap := gap * 10 // 13
				end
			end
		end

	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)
		local
			old_item, new_item: like item
			pos: CURSOR
		do
			pos := cursor
			old_item := item
			go_i_th (i)
			new_item := item
			replace (old_item)
			go_to (pos)
			replace (new_item)
		ensure -- from CHAIN
			swapped_to_item: item = old i_th (i)
			swapped_from_item: i_th (i) = old item
		end
	
feature -- Conversion

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

	frozen clone (other: detachable ANY): like other
		obsolete "Use `twin' instead. [2017-05-31]"
			-- Void if other is void; otherwise new object
			-- equal to other
			--
			-- For non-void other, clone calls copy;
			-- to change copying/cloning semantics, redefine copy.
			-- (from ANY)
		do
			if other /= Void then
				Result := other.twin
			end
		ensure -- from ANY
			instance_free: class
			equal: Result ~ other
		end

	copy (other: PART_SORTED_TWO_WAY_LIST [G])
			-- 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)
		local
			cur: LINKED_LIST_CURSOR [G]
		do
			if other /= Current then
				standard_copy (other)
				if not other.is_empty then
					internal_wipe_out
					cur := other.cursor
					from
						other.start
					until
						other.off
					loop
						extend (other.item)
						finish;
						other.forth
					end
					if cur /= Void then
						other.go_to (cur)
					end
				end
			end
		ensure -- from ANY
			is_equal: Current ~ other
		end

	frozen deep_clone (other: detachable ANY): like other
		obsolete "Use `deep_twin' instead. [2017-05-31]"
			-- Void if other is void: otherwise, new object structure
			-- recursively duplicated from the one attached to other
			-- (from ANY)
		do
			if other /= Void then
				Result := other.deep_twin
			end
		ensure -- from ANY
			instance_free: class
			deep_equal: deep_equal (other, Result)
		end

	frozen deep_copy (other: PART_SORTED_TWO_WAY_LIST [G])
			-- Effect equivalent to that of:
			--		copy (other . deep_twin)
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		do
			copy (other.deep_twin)
		ensure -- from ANY
			deep_equal: deep_equal (Current, other)
		end

	frozen deep_twin: PART_SORTED_TWO_WAY_LIST [G]
			-- New object structure recursively duplicated from Current.
			-- (from ANY)
		external
			"built_in"
		ensure -- from ANY
			deep_twin_not_void: Result /= Void
			deep_equal: deep_equal (Current, Result)
		end

	duplicate (n: INTEGER_32): PART_SORTED_TWO_WAY_LIST [G]
		obsolete "[
			Create a new container explicitly using `make_from_iterable` if available.
			Otherwise, replace a call to the feature with code that creates and initializes container.
			[2018-11-30]
		]"
			-- Copy of sub-chain beginning at current position
			-- and having min (n, from_here) items,
			-- where from_here is the number of items
			-- at or to the right of current position.
			-- (from DYNAMIC_CHAIN)
		require -- from CHAIN
			not_off_unless_after: off implies after
			valid_subchain: n >= 0
		local
			pos: CURSOR
			counter: INTEGER_32
		do
			from
				Result := new_chain
				if object_comparison then
					Result.compare_objects
				end
				pos := cursor
			until
				(counter = n) or else exhausted
			loop
				Result.extend (item)
				forth
				counter := counter + 1
			end
			go_to (pos)
		end

	frozen standard_clone (other: detachable ANY): like other
		obsolete "Use `standard_twin' instead. [2017-05-31]"
			-- Void if other is void; otherwise new object
			-- field-by-field identical to other.
			-- Always uses default copying semantics.
			-- (from ANY)
		do
			if other /= Void then
				Result := other.standard_twin
			end
		ensure -- from ANY
			instance_free: class
			equal: standard_equal (Result, other)
		end

	frozen standard_copy (other: PART_SORTED_TWO_WAY_LIST [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)
		external
			"built_in"
		ensure -- from ANY
			is_standard_equal: standard_is_equal (other)
		end

	frozen standard_twin: PART_SORTED_TWO_WAY_LIST [G]
			-- New object field-by-field identical to other.
			-- Always uses default copying semantics.
			-- (from ANY)
		external
			"built_in"
		ensure -- from ANY
			standard_twin_not_void: Result /= Void
			equal: standard_equal (Result, Current)
		end

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

	frozen as_attached: attached PART_SORTED_TWO_WAY_LIST [G]
		obsolete "Remove calls to this feature. [2017-05-31]"
			-- Attached version of Current.
			-- (Can be used during transitional period to convert
			-- non-void-safe classes to void-safe ones.)
			-- (from ANY)
		do
			Result := Current
		end

	frozen default: detachable PART_SORTED_TWO_WAY_LIST [G]
			-- Default value of object's type
			-- (from ANY)
		do
		end

	frozen default_pointer: POINTER
			-- Default value of type POINTER
			-- (Avoid the need to write p.default for
			-- some p of type POINTER.)
			-- (from ANY)
		do
		ensure -- from ANY
			instance_free: class
		end

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

	frozen do_nothing
			-- Execute a null action.
			-- (from ANY)
		do
		ensure -- from ANY
			instance_free: class
		end
	
feature {PART_SORTED_TWO_WAY_LIST}{TWO_WAY_LIST}{LINKED_LIST}{DYNAMIC_CHAIN} -- Implementation

	new_chain: like Current
		obsolete "Use explicit creation instead. See also explanations for `duplicate`. [2018-11-30]"
			-- A newly created instance of the same type.
			-- This feature may be redefined in descendants so as to
			-- produce an adequately allocated and initialized object.
		require -- from  DYNAMIC_CHAIN
			True
		do
			create Result.make
		ensure -- from DYNAMIC_CHAIN
			result_exists: Result /= Void
		end
	
feature {TWO_WAY_LIST} -- Implementation

	make_sublist (first_item, last_item: like first_element; n: INTEGER_32)
			-- Create sublist
			-- (from TWO_WAY_LIST)
		do
			make
			first_element := first_item
			last_element := last_item
			active := first_element
			count := n
		end
	
feature {TWO_WAY_LIST}{LINKED_LIST} -- Implementation

	new_cell (v: like item): BI_LINKABLE [G]
			-- A newly created instance of the type of first_element.
			-- (from TWO_WAY_LIST)
		do
			create Result.put (v)
		ensure -- from LINKED_LIST
			result_exists: Result /= Void
		end

	previous: like first_element
			-- Element left of cursor
			-- (from TWO_WAY_LIST)
		local
			a: like active
		do
			if after then
				Result := active
			else
				a := active
				if a /= Void then
					Result := a.left
				end
			end
		end
	
feature {LINKED_LIST} -- Implementation

	active: like first_element
			-- Element at cursor position
			-- (from LINKED_LIST)

	cleanup_after_remove (v: like first_element)
			-- Clean-up a just removed cell.
			-- (from LINKED_LIST)
		require -- from LINKED_LIST
			non_void_cell: v /= Void
		do
		end

	next: like first_element
			-- Element right of cursor
			-- (from LINKED_LIST)
		local
			a: like active
		do
			if before then
				Result := active
			else
				a := active
				if a /= Void then
					Result := a.right
				end
			end
		end
	
feature {NONE} -- Implementation

	frozen internal_wipe_out
			-- Remove all items.
			-- (from LINKED_LIST)
		require -- from LINKED_LIST
				prunable
		do
			active := Void
			first_element := Void
			before := True
			after := False
			count := 0
		ensure -- from LINKED_LIST
			wiped_out: is_empty
			is_before: before
		end
	
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
		local
			c: detachable CURSOR
			cs: detachable CURSOR_STRUCTURE [G]
		do
			if attached {CURSOR_STRUCTURE [G]} Current as acs then
				cs := acs
				c := acs.cursor
			end
			from
				start
			until
				after
			loop
				action.call ([item])
				forth
			end
			if cs /= Void and c /= Void then
				cs.go_to (c)
			end
		end

	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
		local
			c: detachable CURSOR
			cs: detachable CURSOR_STRUCTURE [G]
		do
			if attached {CURSOR_STRUCTURE [G]} Current as acs then
				cs := acs
				c := acs.cursor
			end
			from
				start
			until
				after
			loop
				if test.item ([item]) then
					action.call ([item])
				end
				forth
			end
			if cs /= Void and c /= Void then
				cs.go_to (c)
			end
		end

	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
		local
			c: detachable CURSOR
			cs: detachable CURSOR_STRUCTURE [G]
		do
			if attached {CURSOR_STRUCTURE [G]} Current as acs then
				cs := acs
				c := acs.cursor
			end
			from
				start
				Result := True
			until
				after or not Result
			loop
				Result := test.item ([item])
				forth
			end
			if cs /= Void and c /= Void then
				cs.go_to (c)
			end
		ensure then -- from LINEAR
			empty: is_empty implies Result
		end

	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
		local
			c: detachable CURSOR
			cs: detachable CURSOR_STRUCTURE [G]
		do
			if attached {CURSOR_STRUCTURE [G]} Current as acs then
				cs := acs
				c := acs.cursor
			end
			from
				start
			until
				after or Result
			loop
				Result := test.item ([item])
				forth
			end
			if cs /= Void and c /= Void then
				cs.go_to (c)
			end
		end
	
feature -- Output

	Io: STD_FILES
			-- Handle to standard file setup
			-- (from ANY)
		once
			create Result;
			Result.set_output_default
		ensure -- from ANY
			instance_free: class
			io_not_void: Result /= Void
		end

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

	print (o: detachable ANY)
			-- Write terse external representation of o
			-- on standard output.
			-- (from ANY)
		local
			s: READABLE_STRING_8
		do
			if attached o then
				s := o.out
				if attached {READABLE_STRING_32} s as s32 then
					Io.put_string_32 (s32)
				elseif attached {READABLE_STRING_8} s as s8 then
					Io.put_string (s8)
				else
					Io.put_string_32 (s.as_string_32)
				end
			end
		ensure -- from ANY
			instance_free: class
		end

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

	Operating_environment: OPERATING_ENVIRONMENT
			-- Objects available from the operating system
			-- (from ANY)
		once
			create Result
		ensure -- from ANY
			instance_free: class
			operating_environment_not_void: Result /= Void
		end
	
feature {NONE} -- Retrieval

	frozen internal_correct_mismatch
			-- Called from runtime to perform a proper dynamic dispatch on correct_mismatch
			-- from MISMATCH_CORRECTOR.
			-- (from ANY)
		local
			l_msg: STRING_32
			l_exc: EXCEPTIONS
		do
			if attached {MISMATCH_CORRECTOR} Current as l_corrector then
				l_corrector.correct_mismatch
			else
				create l_msg.make_from_string ("Mismatch: ".as_string_32)
				create l_exc;
				l_msg.append (generating_type.name_32);
				l_exc.raise_retrieval_exception (l_msg)
			end
		end
	
invariant
		-- from TWO_WAY_LIST
	non_empty_list_has_two_endpoints: not is_empty implies (first_element /= Void and last_element /= Void)
	empty_list_has_no_endpoints: is_empty implies last_element = Void
	first_element_constraint: attached first_element as f implies f.left = Void
	last_element_constraint: attached last_element as l implies l.right = Void

		-- 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 (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)

note
	copyright: "Copyright (c) 1984-2018, 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 PART_SORTED_TWO_WAY_LIST