note
	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.
	]"
	library: "Free implementation of ELKS library"
	legal: "See notice at end of class."
	status: "See notice at end of class."
	names: iterators, iteration, two_way_chain_iterators, two_way_chain_iteration
	traversal: sequential
	exploration: forward, backward
	date: "$Date: 2017-04-10 07:37:21 +0000 (Mon, 10 Apr 2017) $"
	revision: "$Revision: 100119 $"

class interface
	TWO_WAY_CHAIN_ITERATOR [G]

create 
	set (s: like target)
			-- Make s the new target of iterations.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			target_exists: s /= Void
		ensure -- from ITERATOR
				target = s
				target /= Void

feature -- Initialization

	set (s: like target)
			-- Make s the new target of iterations.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			target_exists: s /= Void
		ensure -- from ITERATOR
				target = s
				target /= Void
	
feature -- Access

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

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

	item: G
			-- The item at cursor position in target
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			not_off: not target.off

	item_tuple: TUPLE [G]
			-- Tuple containing a single element,
			-- the item at cursor position in target''
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			not_off: not target.off
		ensure -- from LINEAR_ITERATOR
			item_tuple_attached: Result /= Void

	target: CHAIN [G]
			-- The structure to which iteration features will apply.
	
feature -- Comparison

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

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

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

	is_equal (other: TWO_WAY_CHAIN_ITERATOR [G]): BOOLEAN
			-- Is other attached to an object considered
			-- equal to current object?
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		ensure -- from ANY
			symmetric: Result implies other ~ Current
			consistent: standard_is_equal (other) implies Result

	frozen standard_equal (a: detachable ANY; b: like arg #1): BOOLEAN
			-- Are a and b either both void or attached to
			-- field-by-field identical objects of the same type?
			-- Always uses default object comparison criterion.
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			definition: Result = (a = Void and b = Void) or else ((a /= Void and b /= Void) and then a.standard_is_equal (b))

	frozen standard_is_equal alias "≜" (other: TWO_WAY_CHAIN_ITERATOR [G]): BOOLEAN
			-- Is other attached to an object of the same type
			-- as current object, and field-by-field identical to it?
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		ensure -- from ANY
			same_type: Result implies same_type (other)
			symmetric: Result implies other.standard_is_equal (Current)
	
feature -- Status report

	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

	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)

	same_type (other: ANY): BOOLEAN
			-- Is type of current object identical to type of other?
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
		ensure -- from ANY
			definition: Result = (conforms_to (other) and other.conforms_to (Current))
	
feature -- Cursor movement

	back
			-- Move cursor of target backward one position.

	exhausted: BOOLEAN
			-- Is target exhausted?
			-- (from LINEAR_ITERATOR)

	finish
			-- Move cursor of target to last position.

	forth
			-- Move to next position of target.
			-- (from LINEAR_ITERATOR)

	off: BOOLEAN
			-- Is position of target off?
			-- (from LINEAR_ITERATOR)

	start
			-- Move to first position of target.
			-- (from LINEAR_ITERATOR)
	
feature -- Duplication

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

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

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

	frozen standard_copy (other: TWO_WAY_CHAIN_ITERATOR [G])
			-- Copy every field of other onto corresponding field
			-- of current object.
			-- (from ANY)
		require -- from ANY
			other_not_void: other /= Void
			type_identity: same_type (other)
		ensure -- from ANY
			is_standard_equal: standard_is_equal (other)

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

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

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

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

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

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

	continue_for_back (action: PROCEDURE [G]; n, k: INTEGER_32)
			-- Apply action to every k-th item,
			-- n times if possible.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			valid_repetition: n >= 0
			valid_skip: k >= 1

	continue_for (action: PROCEDURE [G]; n, k: INTEGER_32)
			-- Apply action to every k-th item,
			-- n times if possible.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			valid_repetition: n >= 0
			valid_skip: k >= 1

	continue_search (test: FUNCTION [G, BOOLEAN]; b: BOOLEAN)
			-- Search the first item of target
			-- satisfying: test equals to b
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		ensure then -- from LINEAR_ITERATOR
			found: not exhausted = (b = test.item ([target.item]))

	continue_search_back (test: FUNCTION [G, BOOLEAN]; b: BOOLEAN)
			-- Search the first item of target
			-- satisfying: test equals to b
			-- (from the current position of target).
			-- (from LINEAR_ITERATOR)
		ensure then -- from LINEAR_ITERATOR
			found: not exhausted = (b = test.item ([target.item]))

	continue_until_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	continue_until (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	continue_while (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])

	continue_while_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])

	do_all (action: PROCEDURE [G])
			-- Apply action to every item of target.
			-- (from ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void

	do_all_back (action: PROCEDURE [G])
			-- Apply action to every item of target.
			-- (from ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void

	do_for_back (action: PROCEDURE [G]; i, n, k: INTEGER_32)
			-- Apply action to every k-th item,
			-- n times if possible, starting from i-th.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			valid_start: i >= 1
			valid_repetition: n >= 0
			valid_skip: k >= 1

	do_for (action: PROCEDURE [G]; i, n, k: INTEGER_32)
			-- Apply action to every k-th item,
			-- n times if possible, starting from i-th.
			-- (from LINEAR_ITERATOR)
		require -- from LINEAR_ITERATOR
			valid_start: i >= 1
			valid_repetition: n >= 0
			valid_skip: k >= 1

	do_if (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target satisfying test.
			-- (from ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void

	do_if_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target satisfying test.
			-- (from ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void

	do_until_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	do_until (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target up to
			-- and including first one satisfying test.
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	do_while (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target up to
			-- and including first one not satisfying test.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])

	do_while_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- Apply action to every item of target up to
			-- and including first one not satisfying test.
			-- (from the start of target)
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])

	for_all (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Does test return true for
			-- all items of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			test_exists: test /= Void

	for_all_back (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Does test return true for
			-- all items of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			test_exists: test /= Void

	search (test: FUNCTION [G, BOOLEAN]; 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)

	search_back (test: FUNCTION [G, BOOLEAN]; 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)

	there_exists (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Does test return true for
			-- at least one item of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			test_exists: test /= Void

	there_exists_back (test: FUNCTION [G, BOOLEAN]): BOOLEAN
			-- Does test return true for
			-- at least one item of target?
			-- (from LINEAR_ITERATOR)
		require -- from ITERATOR
			test_exists: test /= Void

	until_continue (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure -- from LINEAR_ITERATOR
			achieved: exhausted or else test.item ([target.item])
			invariant_satisfied: invariant_value

	until_continue_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			invariant_satisfied: invariant_value
		ensure -- from LINEAR_ITERATOR
			achieved: exhausted or else test.item ([target.item])
			invariant_satisfied: invariant_value

	until_do_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	until_do (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			achieved: not exhausted implies test.item ([target.item])

	while_continue (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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.item ([target.item])

	while_continue_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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.item ([target.item])

	while_do_back (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])

	while_do (action: PROCEDURE [G]; test: FUNCTION [G, BOOLEAN])
			-- 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
			action_exists: action /= Void
			test_exists: test /= Void
		ensure then -- from LINEAR_ITERATOR
			finished: not exhausted implies not test.item ([target.item])
	
feature -- Output

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

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

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

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

	Operating_environment: OPERATING_ENVIRONMENT
			-- Objects available from the operating system
			-- (from ANY)
		ensure -- from ANY
			instance_free: class
			operating_environment_not_void: Result /= Void
	
invariant
		-- from LINEAR_ITERATOR
	target_exists: target /= Void

		-- from ITERATOR
	traversable_exists: target /= Void

		-- from ANY
	reflexive_equality: standard_is_equal (Current)
	reflexive_conformance: conforms_to (Current)

note
	copyright: "Copyright (c) 1984-2012, Eiffel Software and others"
	license: "Eiffel Forum License v2 (see http://www.eiffel.com/licensing/forum.txt)"
	source: "[
		Eiffel Software
		5949 Hollister Ave., Goleta, CA 93117 USA
		Telephone 805-685-1006, Fax 805-685-6869
		Website http://www.eiffel.com
		Customer support http://support.eiffel.com
	]"

end -- class TWO_WAY_CHAIN_ITERATOR

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