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indexing
	description: "[
		Sequential, one-way linked lists that call an action
		sequence when an item is removed or added.
	]"
	status: "See notice at end of class"
	keywords: "event, action, linked, list"
	date: "$Date: 2001/11/16 20:34:10 $"
	revision: "$Revision: 1.1.1.1 $"

class 
	ACTIVE_LIST [G]

create 
	default_create

feature {NONE} -- Initialization

	default_create is
			-- Initialize
		do
			Precursor {INTERACTIVE_LIST}
			create add_actions
			create remove_actions
		end
	
feature -- Initialization

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

	add_actions: ACTION_SEQUENCE [TUPLE [G]]
			-- Actions performed when an item has just been added.

	cursor: CURSOR is
			-- Current cursor position
			-- (from LINKED_LIST)
		do
			create {LINKED_LIST_CURSOR [G]} Result.make (active, after, before)
		end

	first: like item is
			-- Item at first position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_empty
		do
			Result := first_element.item
		end

	has (v: like item): BOOLEAN is
			-- Does chain include v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from CHAIN)
		local
			pos: CURSOR
		do
			pos := cursor
			Result := sequential_has (v)
			go_to (pos)
		ensure -- from CONTAINER
			not_found_in_empty: Result implies not is_empty
		end

	i_th (i: INTEGER): like item is
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of @.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
		local
			pos: CURSOR
		do
			pos := cursor
			go_i_th (i)
			Result := item
			go_to (pos)
		end

	index: INTEGER is
			-- Index of current position
			-- (from LINKED_LIST)
		local
			p: LINKED_LIST_CURSOR [G]
		do
			if after then
				Result := count + 1
			elseif not before then
				p ?= cursor;
				check
					p /= void
				end;
				from
					start
					Result := 1
				until
					p.active = active
				loop
					forth
					Result := Result + 1
				end;
				go_to (p)
			end
		end

	index_of (v: like item; i: INTEGER): INTEGER is
			-- 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
		local
			pos: CURSOR
		do
			pos := cursor
			Result := sequential_index_of (v, i)
			go_to (pos)
		ensure -- from LINEAR
			non_negative_result: Result >= 0
		end

	item: G is
			-- Current item
			-- (from LINKED_LIST)
		require -- from TRAVERSABLE
			not_off: not off
		require -- from ACTIVE
			readable: readable
		do
			Result := active.item
		end

	last: like item is
			-- Item at last position
			-- (from LINKED_LIST)
		require -- from CHAIN
			not_empty: not is_empty
		do
			Result := last_element.item
		end

	sequential_occurrences (v: G): INTEGER is
			-- Number of times v appears.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		do
			from
				start
				search (v)
			until
				exhausted
			loop
				Result := Result + 1
				forth
				search (v)
			end
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
		end

	remove_actions: ACTION_SEQUENCE [TUPLE [G]]
			-- Actions performed when an item has just been removed.

	infix "@" (i: INTEGER): like item is
			-- Item at i-th position
			-- Was declared in CHAIN as synonym of i_th.
			-- (from CHAIN)
		require -- from TABLE
			valid_key: valid_index (k)
		local
			pos: CURSOR
		do
			pos := cursor
			go_i_th (i)
			Result := item
			go_to (pos)
		end
	
feature {NONE} -- Access

	sequential_has (v: like item): BOOLEAN is
			-- Does structure include an occurrence of v?
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- (from LINEAR)
		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

	sequential_index_of (v: like item; i: INTEGER): INTEGER is
			-- 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
		do
			if object_comparison and v /= void then
				from
					start
					pos := 1
				until
					off or (occur = i)
				loop
					if item /= void and then v.is_equal (item) then
						occur := occur + 1
					end
					forth
					pos := pos + 1
				end
			else
				from
					start
					pos := 1
				until
					off 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

	sequential_search (v: like item) is
			-- Move to first position (at or after current
			-- position) where item and v are equal.
			-- (Reference or object equality,
			-- based on object_comparison.)
			-- If no such position ensure that exhausted will be true.
			-- (from LINEAR)
		do
			if object_comparison and v /= void then
				from
				until
					exhausted or else (item /= void and then v.is_equal (item))
				loop
					forth
				end
			else
				from
				until
					exhausted or else v = item
				loop
					forth
				end
			end
		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
		end
	
feature -- Measurement

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

	index_set: INTEGER_INTERVAL is
			-- Range of acceptable indexes
			-- (from CHAIN)
		do
			create Result.make (1, count)
		ensure -- from INDEXABLE
			not_void: Result /= void
		ensure then -- from CHAIN
			count_definition: Result.count = count
		end

	occurrences (v: like item): INTEGER is
			-- 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)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
		end

	occurrences (v: like item): INTEGER is
			-- 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)
		ensure -- from BAG
			non_negative_occurrences: Result >= 0
		end
	
feature -- Comparison

	is_equal (other: like Current): BOOLEAN is
			-- Does other contain the same elements?
			-- (from LIST)
		require -- from ANY
			other_not_void: other /= void
		local
			c1, c2: CURSOR
		do
			if Current = other then
				Result := True
			else
				Result := (is_empty = other.is_empty) and (object_comparison = other.object_comparison)
				if Result and not is_empty then
					c1 ?= cursor
					c2 ?= other.cursor
					check
						cursors_exist: c1 /= void and c2 /= void
					end
					from
						start
						other.start
					until
						after or not Result
					loop
						if object_comparison then
							Result := equal (item, other.item)
						else
							Result := (item = other.item)
						end
						forth
						other.forth
					end
					go_to (c1)
					other.go_to (c2)
				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.is_equal (Current)
			consistent: standard_is_equal (other) implies Result
		ensure then -- from LIST
			indices_unchanged: index = old index and other.index = old other.index
			true_implies_same_size: Result implies count = other.count
		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 is
			-- May object_comparison be changed?
			-- (Answer: yes by default.)
			-- (from CONTAINER)
		do
			Result := True
		end

	empty: BOOLEAN is
		obsolete "ELKS 2000: Use `is_empty' instead"
			-- Is there no element?
			-- (from CONTAINER)
		do
			Result := is_empty
		end

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

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

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

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

	is_inserted (v: G): BOOLEAN is
			-- Has v been inserted at the end by the most recent put or
			-- extend?
			-- (from LINKED_LIST)
		do
			check
				put_constraint: (v /= last_element.item) implies not off
			end
			Result := (v = last_element.item) or else (v = item)
		end

	isfirst: BOOLEAN is
			-- Is cursor at first position?
			-- (from LINKED_LIST)
		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
			-- Is cursor at last position?
			-- (from LINKED_LIST)
		do
			Result := not after and not before and (active /= void) and then (active.right = void)
		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
			-- Is there no current item?
			-- (from LINKED_LIST)
		do
			Result := after or before
		end

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

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

	valid_cursor (p: CURSOR): BOOLEAN is
			-- Can the cursor be moved to position p?
			-- (from LINKED_LIST)
		local
			ll_c: LINKED_LIST_CURSOR [G]
			temp, sought: like first_element
		do
			ll_c ?= p
			if ll_c /= void 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): BOOLEAN is
			-- 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): BOOLEAN is
			-- Is i within allowable bounds?
			-- (from CHAIN)
		do
			Result := (i >= 1) and (i <= count)
		ensure then -- from INDEXABLE
			only_if_in_index_set: Result implies ((i >= index_set.lower) and (i <= index_set.upper))
		ensure then -- from CHAIN
			valid_index_definition: Result = ((i >= 1) and (i <= count))
		end

	writable: BOOLEAN is
			-- Is there a current item that may be modified?
			-- (from SEQUENCE)
		do
			Result := not off
		end
	
feature -- Status setting

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

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

	back is
			-- Move to previous item.
			-- (from LINKED_LIST)
		require -- from BILINEAR
			not_before: not before
		do
			if is_empty then
				before := True
				after := False
			elseif after then
				after := False
			elseif isfirst then
				before := True
			else
				active := previous
			end
		end

	finish is
			-- Move cursor to last position.
			-- (Go before if empty)
			-- (from LINKED_LIST)
		local
			p: like first_element
		do
			if not is_empty then
				from
					p := active
				until
					p.right = void
				loop
					p := p.right
				end
				active := p
				after := False
				before := False
			else
				before := True
				after := False
			end
		ensure then -- from CHAIN
			at_last: not is_empty implies islast
		ensure then -- from LINKED_LIST
			empty_convention: is_empty implies before
		end

	forth is
			-- Move cursor to next position.
			-- (from LINKED_LIST)
		require -- from LINEAR
			not_after: not after
		local
			old_active: like first_element
		do
			if before then
				before := False
				if is_empty then
					after := True
				end
			else
				old_active := active
				active := active.right
				if active = void then
					active := old_active
					after := True
				end
			end
		ensure then -- from LIST
			moved_forth: index = old index + 1
		end

	go_i_th (i: INTEGER) is
			-- 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) is
			-- Move cursor to position p.
			-- (from LINKED_LIST)
		require -- from CURSOR_STRUCTURE
			cursor_position_valid: valid_cursor (p)
		local
			ll_c: LINKED_LIST_CURSOR [G]
		do
			ll_c ?= p
			check
				ll_c /= void
			end
			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
		end

	move (i: INTEGER) is
			-- Move cursor i positions. The cursor
			-- may end up off if the offset is too big.
			-- (from LINKED_LIST)
		local
			counter, new_index: INTEGER
			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) is
			-- 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)
		do
			if before and not is_empty then
				forth
			end
			sequential_search (v)
		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
		end

	start is
			-- Move cursor to first position.
			-- (from