indexing
	description: "Objects that are able to iterate over two-way chains, on which they can perform repeated actions and tests according to a number of predefined control structures such as ``if%'%', ``until%'%' and others."
	names: iterators, iteration, two_way_chain_iterators, two_way_chain_iteration
	traversal: sequential
	exploration: forward, backward

class interface
	TWO_WAY_CHAIN_ITERATOR [G]

feature -- Access

	target: CHAIN [G]
	
feature -- Status report

	invariant_value: BOOLEAN
			-- Is the invariant satisfied?
			-- (Redefinitions of this feature will usually involve
			-- target; if so, make sure that the result is defined
			-- when target = Void.)
			-- (from ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	item_test (v: G): BOOLEAN
			-- Test to be applied to item v
			-- (default: false)
			-- (from ITERATOR)

	test: BOOLEAN
			-- Test to be applied to item at current position in target
			-- (default: value of item_test on item)
			-- (from ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void;
			not_off: not target.off
		ensure -- from ITERATOR
			not_off: not target.off
	
feature -- Status setting

	set (s: like target)
			-- Make s the new target of iterations.
			-- (from ITERATOR)
		require -- from ITERATOR
			s /= void
		ensure -- from ITERATOR
			target = s;
			target /= void
	
feature -- Cursor movement

	back
			-- Move cursor of target backward one position.
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	continue_for_back (n, k: INTEGER)
			-- Apply action to every k-th item,
			-- n times if possible.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			valid_repetition: n >= 0;
			valid_skip: k >= 1

	continue_for (n, k: INTEGER)
			-- Apply action to every k-th item,
			-- n times if possible.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			valid_repetition: n >= 0;
			valid_skip: k >= 1

	continue_search_back (b: BOOLEAN)
			-- Search the first item of target
			-- satisfying: test equals to b
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			found: not exhausted = (b = test)

	continue_search (b: BOOLEAN)
			-- Search the first item of target
			-- satisfying: test equals to b
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			found: not exhausted = (b = test)

	continue_until_back
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	continue_until
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	continue_while_back
			-- Apply action to every item of target up to
			-- and including first one not satisfying test
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require else -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	continue_while
			-- Apply action to every item of target up to
			-- and including first one not satisfying test
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		require else -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	do_all_back
			-- Apply action to every item of target.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			exhausted

	do_all
			-- Apply action to every item of target.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			exhausted

	do_for_back (i, n, k: INTEGER)
			-- Apply action to every k-th item,
			-- n times if possible, starting from i-th.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			valid_start: i >= 1;
			valid_repetition: n >= 0;
			valid_skip: k >= 1

	do_for (i, n, k: INTEGER)
			-- Apply action to every k-th item,
			-- n times if possible, starting from i-th.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			valid_start: i >= 1;
			valid_repetition: n >= 0;
			valid_skip: k >= 1

	do_if
			-- Apply action to every item of target
			-- satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	do_if_back
			-- Apply action to every item of target
			-- satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	do_until_back
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	do_until
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	do_while_back
			-- Apply action to every item of target up to
			-- and including first one not satisfying test.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	do_while
			-- Apply action to every item of target up to
			-- and including first one not satisfying test.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	exhausted: BOOLEAN
			-- Is target exhausted?
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	exists_back: BOOLEAN
			-- Does test return true for
			-- at least one item of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	exists: BOOLEAN
			-- Does test return true for
			-- at least one item of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	finish
			-- Move cursor of target to last position.
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	forall: BOOLEAN
			-- Does test return true for
			-- all items of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	forall_back: BOOLEAN
			-- Does test return true for
			-- all items of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void

	forth
			-- Move to next position of target.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	off: BOOLEAN
			-- Is position of target off?
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	search (b: BOOLEAN)
			-- Search the first item of target for which test
			-- has the same value as b (both true or both false).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	search_back (b: BOOLEAN)
			-- Search the first item of target for which test
			-- has the same value as b (both true or both false).
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	start
			-- Move to first position of target.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void

	until_continue
			-- Apply action to every item of target from current
			-- position, up to but excluding first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure -- from LINEAR_ITERATOR
			achieved: exhausted or else test;
			invariant_satisfied: invariant_value

	until_continue_back
			-- Apply action to every item of target from current
			-- position, up to but excluding first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			traversable_exists: target /= void;
			invariant_satisfied: invariant_value
		ensure -- from LINEAR_ITERATOR
			achieved: exhausted or else test;
			invariant_satisfied: invariant_value

	until_do
			-- Apply action to every item of target up to
			-- but excluding first one satisfying test.
			-- (Apply to full list if no item satisfies test.)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	until_do_back
			-- Apply action to every item of target up to
			-- but excluding first one satisfying test.
			-- (Apply to full list if no item satisfies test.)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test

	while_continue
			-- Apply action to every item of target up to
			-- but excluding first one not satisfying test.
			-- (from LINEAR_ITERATOR)
		ensure -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	while_continue_back
			-- Apply action to every item of target up to
			-- but excluding first one not satisfying test.
			-- (from LINEAR_ITERATOR)
		ensure -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	while_do
			-- Apply action to every item of target up to
			-- but excluding first one not satisfying test.
			-- (Apply to full list if all items satisfy test.)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test

	while_do_back
			-- Apply action to every item of target up to
			-- but excluding first one not satisfying test.
			-- (Apply to full list if all items satisfy test.)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test
	
feature -- Element change

	action
			-- Action to be applied to item at current position
			-- in target (default: item_action on that item).
			-- For iterators to work properly, redefined versions of
			-- this feature should not change the traversable's
			-- structure.
			-- (from ITERATOR)
		require -- from ITERATOR
			traversable_exists: target /= void;
			not_off: not target.off;
			invariant_satisfied: invariant_value
		ensure -- from ITERATOR
			not_off: not target.off;
			invariant_satisfied: invariant_value

	item_action (v: G)
			-- Action to be applied to item v
			-- (Default: do nothing.)
			-- (from ITERATOR)
	
invariant

		-- from GENERAL
	reflexive_equality: standard_is_equal (Current);
	reflexive_conformance: conforms_to (Current);

end -- class TWO_WAY_CHAIN_ITERATOR