indexing
	description: "Sequential, two-way linked lists"
	names: two_way_list, sequence
	representation: linked
	access: index, cursor, membership
	contents: generic

class interface
	TWO_WAY_LIST [G]

create 

	make_sublist (first_item, last_item: like first_element; n: INTEGER)

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

feature -- Initialization

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

	cursor: CURSOR
			-- Current cursor position
			-- (from LINKED_LIST)

	first: like item
			-- Item at first position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not empty

	first_element: BI_LINKABLE [G]
			-- Head of list
			-- (Anchor redefinition)

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

	i_th (i: INTEGER): like item
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th and @.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)

	index: INTEGER
			-- Index of current position
			-- (from LINKED_LIST)

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

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

	last: like item
			-- Item at last position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not empty

	last_element: like first_element
			-- Tail of the list

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

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

	sequential_occurrences (v: G): INTEGER
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0

	sublist: like Current
			-- Result produced by last split

	infix "@" (i: INTEGER): like item
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th and @.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
	
feature -- Measurement

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

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

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

	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)

	isfirst: BOOLEAN
			-- Is cursor at first position?
			-- (from LINKED_LIST)
		ensure -- from CHAIN
			valid_position: Result implies not empty

	islast: BOOLEAN
			-- Is cursor at last position?
		ensure -- from CHAIN
			valid_position: Result implies not empty

	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)

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

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

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

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

	valid_index (i: INTEGER): BOOLEAN
			-- Is i within allowable bounds?
			-- (from CHAIN)
		ensure then -- from CHAIN
			valid_index_definition: Result = ((i >= 1) and (i <= count))

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

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

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

	back
			-- Move cursor to previous position, if any.
		require -- from BILINEAR
			not_before: not before
		ensure then -- from BILINEAR
			moved_back: index = old index - 1

	finish
			-- Move cursor to last position.
			-- (Go before if empty)
		ensure then -- from CHAIN
			at_last: not empty implies islast
		ensure then -- from LINKED_LIST
			empty_convention: empty implies before
		ensure then
			not_after: not after

	forth
			-- Move cursor to next position, if any.
		require -- LIST
			precursor: True
		require -- from LINEAR
			not_after: not after
		ensure then -- from LIST
			moved_forth: index = old index + 1

	go_i_th (i: INTEGER)
			-- Move cursor to i-th position.
			-- (from LINKED_LIST)
		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_LIST)
		require -- from CURSOR_STRUCTURE
			cursor_position_valid: valid_cursor (p)

	move (i: INTEGER)
			-- Move cursor i positions. The cursor
			-- may end up off if the offset is to big.
		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

	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)
		ensure -- from LINEAR
			object_found: (not exhausted and object_comparison) implies equal (v, item);
			item_found: (not exhausted and not object_comparison) implies v = item

	start
			-- Move cursor to first position.
			-- (from LINKED_LIST)
		ensure then -- from CHAIN
			at_first: not empty implies isfirst
		ensure then -- from LINKED_LIST
			empty_convention: empty implies after
	
feature -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
			-- (from SEQUENCE)
		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.
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from BAG
			one_more_occurrence: occurrences (v) = old (occurrences (v)) + 1

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

	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: like Current)
			-- Merge other into current structure before cursor
			-- position. Do not move cursor. Empty other.
		require -- from DYNAMIC_CHAIN
			extendible: extendible;
			not_off: not before;
			other_exists: other /= void
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + old other.count;
			new_index: index = old index + old other.count;
			other_is_empty: other.empty

	merge_right (other: like Current)
			-- Merge other into current structure after cursor
			-- position. Do not move cursor. Empty other.
		require -- from DYNAMIC_CHAIN
			extendible: extendible;
			not_off: not after;
			other_exists: other /= void
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count + old other.count;
			same_index: index = old index;
			other_is_empty: other.empty

	put (v: like item)
			-- Replace current item by v.
			-- (Synonym for replace)
			-- (from CHAIN)
		require -- from COLLECTION
			extendible: extendible
		ensure -- from COLLECTION
			item_inserted: has (v)
		ensure then -- from CHAIN
			same_count: count = old count

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

	put_i_th (v: like item; i: INTEGER)
			-- Put v at i-th position.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
		ensure then -- from INDEXABLE
			insertion_done: i_th (k) = v

	put_left (v: like item)
			-- Add v to the left of cursor position.
			-- Do not move cursor.
		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.
		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: like item)
			-- Replace current item by v.
			-- (from LINKED_LIST)
		require -- from ACTIVE
			writable: writable
		ensure -- from ACTIVE
			item_replaced: item = v
	
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

	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 -- DYNAMIC_CHAIN
			precursor: True
		require -- from COLLECTION
			prunable
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)
		ensure then -- from DYNAMIC_CHAIN
			is_exhausted: exhausted

	remove
			-- Remove current item.
			-- Move cursor to right neighbor
			-- (or after if no right neighbor).
		require -- from ACTIVE
			prunable: prunable;
			writable: writable
		ensure then -- from DYNAMIC_LIST
			after_when_empty: empty implies after

	remove_left
			-- Remove item to the left of cursor position.
			-- Do not move cursor.
		require -- from DYNAMIC_CHAIN
			left_exists: index > 1
		require else -- from DYNAMIC_LIST
			not_before: not before
		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.
		require -- from DYNAMIC_CHAIN
			right_exists: index < count
		ensure -- from DYNAMIC_CHAIN
			new_count: count = old count - 1;
			same_index: index = old index

	remove_sublist

	split (n: INTEGER)
			-- 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.
		require
			not_off: not off;
			valid_sublist: n >= 0

	wipe_out
			-- Remove all items.
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			wiped_out: empty
		ensure then -- from DYNAMIC_LIST
			is_before: before
	
feature -- Transformation

	swap (i: INTEGER)
			-- 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)
	
feature -- Duplication

	duplicate (n: INTEGER): like Current
			-- 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
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);
		-- from LINKED_LIST
	prunable: prunable;
		-- 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 empty) and (index = 1));
	islast_definition: islast = ((not empty) and (index = count));
	item_corresponds_to_index: (not off) implies (item = i_th (index));
		-- from ACTIVE
	writable_constraint: writable implies readable;
	empty_constraint: empty implies (not readable) and (not writable);
		-- from BILINEAR
	not_both: not (after and before);
	empty_property: empty implies (after or before);
	before_constraint: before implies off;
		-- from LINEAR
	after_constraint: after implies off;
		-- from TRAVERSABLE
	empty_constraint: empty implies off;
		-- from FINITE
	empty_definition: empty = (count = 0);
	non_negative_count: count >= 0;
		-- from DYNAMIC_CHAIN
	extendible: extendible;

end -- class TWO_WAY_LIST