indexing
	description: "Finite sequences: structures where existing items are arrangedand accessed sequentially, and new ones can be added at the end."
	names: sequence
	access: cursor, membership
	contents: generic

deferred class interface
	SEQUENCE [G]

feature -- Access

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

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

	index_of (v: like item; i: INTEGER): INTEGER
			-- 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
		ensure -- from LINEAR
			non_negative_result: Result >= 0

	item: G
			-- Item at current position
			-- (from TRAVERSABLE)
		require -- from ACTIVE
			readable: readable
		require -- from TRAVERSABLE
			not_off: not off
		require -- from TRAVERSABLE
			not_off: not off

	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

	off: BOOLEAN
			-- Is there no current item?
			-- (from BILINEAR)
	
feature -- Measurement

	count: INTEGER
			-- Number of items
			-- (from FINITE)
	
feature -- Status report

	after: BOOLEAN
			-- Is there no valid position to the right of current one?
			-- (from LINEAR)

	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
			-- May new items be added?
			-- (from COLLECTION)

	full: BOOLEAN
			-- Is structure filled to capacity?
			-- (from BOX)

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

	prunable: BOOLEAN
			-- May items be removed?
			-- (from COLLECTION)

	readable: BOOLEAN
			-- Is there a current item that may be read?

	writable: BOOLEAN
			-- Is there a current item that may be modified?
	
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 to previous position.
			-- (from BILINEAR)
		require -- from BILINEAR
			not_before: not before
		ensure then -- from BILINEAR
			moved_back: index = old index - 1

	before: BOOLEAN
			-- Is there no valid position to the left of current one?
			-- (from BILINEAR)

	finish
			-- Move to last position.
			-- (from LINEAR)

	forth
			-- Move to next position; if no next position,
			-- ensure that exhausted will be true.
			-- (from LINEAR)
		require -- from LINEAR
			not_after: not after
		require -- from LINEAR
			not_after: not 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 to first position if any.
			-- (from TRAVERSABLE)
	
feature -- Element change

	append (s: SEQUENCE [G])
			-- Append a copy of s.
		require
			argument_not_void: s /= void
		ensure
			new_count: count >= old count

	extend (v: G)
			-- Add a new occurrence of v.
			-- (from BAG)
		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.
		require
			extendible: extendible
		ensure then
			new_count: count = old count + 1;
			item_inserted: has (v)

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

	replace (v: G)
			-- Replace current item by v.
			-- (from ACTIVE)
		require -- from ACTIVE
			writable: writable
		ensure -- from ACTIVE
			item_replaced: item = v
	
feature -- Removal

	prune (v: like item)
			-- Remove the first occurrence of v if any.
			-- If no such occurrence go off.
		require -- from COLLECTION
			prunable: prunable

	prune_all (v: like item)
			-- Remove all occurrences of v; go off.
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			no_more_occurrences: not has (v)

	remove
			-- Remove current item.
			-- (from ACTIVE)
		require -- from ACTIVE
			prunable: prunable;
			writable: writable

	wipe_out
			-- Remove all items.
			-- (from COLLECTION)
		require -- from COLLECTION
			prunable
		ensure -- from COLLECTION
			wiped_out: empty
	
feature -- Conversion

	linear_representation: LINEAR [G]
			-- Representation as a linear structure
			-- (from LINEAR)
	
invariant

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

end -- class SEQUENCE