Class DS_BILINEAR |

    note

>     description:

>         "Data structures that can be traversed forward and backward"

>     library:    "Gobo Eiffel Structure Library"
>     copyright:  "Copyright (c) 1999-2001, Eric Bezault and others"
>     license:    "MIT License"

    deferred class interface

>     DS_BILINEAR [G]

    inherit

>     DS_LINEAR [G]
>         DS_TRAVERSABLE [G]
>             DS_CONTAINER [G]
>         DS_SEARCHABLE [G]
>             DS_CONTAINER [G]

    feature {NONE} -- Initialization

>     make_default
>             -- Create an empty container.
>             -- (From DS_CONTAINER.)
>         deferred
>         ensure
>             empty: is_empty

    feature -- Access

>     item_for_iteration: G
>             -- Item at internal cursor position
>             -- (From DS_TRAVERSABLE.)
>         require
>             not_off: not off

>     first: G
>             -- First item in container
>             -- (From DS_LINEAR.)
>         require
>             not_empty: not is_empty
>         deferred
>         ensure
>             has_first: has (Result)

>     last: G
>             -- Last item in container
>         require
>             not_empty: not is_empty
>         deferred
>         ensure
>             has_last: has (Result)

>     new_cursor: DS_BILINEAR_CURSOR [G]
>             -- New external cursor for traversal
>             -- (From DS_TRAVERSABLE.)
>         deferred
>         ensure
>             cursor_not_void: Result /= Void
>             valid_cursor: valid_cursor (Result)

>     equality_tester: DS_EQUALITY_TESTER [G]
>             -- Equality tester;
>             -- A void equality tester means that `='
>             -- will be used as comparison criterion.
>             -- (From DS_SEARCHABLE.)

    feature -- Measurement

>     count: INTEGER
>             -- Number of items in container
>             -- (From DS_CONTAINER.)
>         deferred

>     occurrences (v: G): INTEGER
>             -- Number of times v appears in container
>             -- (Use equality_tester's comparison criterion
>             -- if not void, use `=' criterion otherwise.)
>             -- (From DS_SEARCHABLE.)
>         ensure
>             positive: Result >= 0
>             has: has (v) implies Result >= 1

    feature -- Status report

>     has (v: G): BOOLEAN
>             -- Does container include v?
>             -- (Use equality_tester's comparison criterion
>             -- if not void, use `=' criterion otherwise.)
>             -- (From DS_SEARCHABLE.)
>         ensure
>             not_empty: Result implies not is_empty

>     is_empty: BOOLEAN
>             -- Is container empty?
>             -- (From DS_CONTAINER.)

>     is_first: BOOLEAN
>             -- Is internal cursor on first item?
>             -- (From DS_LINEAR.)
>         ensure
>             not_empty: Result implies not is_empty
>             not_off: Result implies not off
>             definition: Result implies (item_for_iteration = first)

>     is_last: BOOLEAN
>             -- Is internal cursor on last item?
>         ensure
>             not_empty: Result implies not is_empty
>             not_off: Result implies not off
>             definition: Result implies (item_for_iteration = last)

>     after: BOOLEAN
>             -- Is there no valid position to right of internal cursor?
>             -- (From DS_LINEAR.)

>     before: BOOLEAN
>             -- Is there no valid position to left of internal cursor?

>     off: BOOLEAN
>             -- Is there no item at internal cursor position?
>             -- (From DS_TRAVERSABLE.)

>     same_position (a_cursor: like new_cursor): BOOLEAN
>             -- Is internal cursor at same position as a_cursor?
>             -- (From DS_TRAVERSABLE.)
>         require
>             a_cursor_not_void: a_cursor /= Void

>     valid_cursor (a_cursor: DS_CURSOR [G]): BOOLEAN
>             -- Is a_cursor a valid cursor?
>             -- (From DS_TRAVERSABLE.)
>         require
>             a_cursor_not_void: a_cursor /= Void

>     same_items (v, u: G): BOOLEAN
>             -- Are v and u considered equal?
>             -- (Use equality_tester's comparison criterion
>             -- if not void, use `=' criterion otherwise.)
>             -- (From DS_SEARCHABLE.)

>     same_equality_tester (other: DS_SEARCHABLE [G]): BOOLEAN
>             -- Does container use the same comparison
>             -- criterion as other?
>             -- (From DS_SEARCHABLE.)
>         require
>             other_not_void: other /= Void

>     equality_tester_settable (a_tester: like equality_tester): BOOLEAN
>             -- Can set_equality_tester be called with a_tester
>             -- as argument in current state of container?
>             -- (Default answer: True.)
>             -- (From DS_SEARCHABLE.)

    feature -- Comparison

>     is_equal (other: like Current): BOOLEAN
>             -- Is current container equal to other?
>             -- (From GENERAL.)
>         require
>             other_not_void: other /= Void
>         deferred
>         ensure
>             consistent: standard_is_equal (other) implies Result
>             same_type: Result implies same_type (other)
>             symmetric: Result implies other.is_equal (Current)
>             same_count: Result implies count = other.count

    feature -- Duplication

>     copy (other: like Current)
>             -- Copy other to current container.
>             -- (From GENERAL.)
>         require
>             other_not_void: other /= Void
>             type_identity: same_type (other)
>         deferred
>         ensure
>             is_equal: is_equal (other)

    feature -- Setting

>     set_equality_tester (a_tester: like equality_tester)
>             -- Set equality_tester to a_tester.
>             -- A void equality tester means that `='
>             -- will be used as comparison criterion.
>             -- (From DS_SEARCHABLE.)
>         require
>             equality_tester_settable: equality_tester_settable (a_tester)
>         ensure
>             equality_tester_set: equality_tester = a_tester

    feature -- Cursor movement

>     start
>             -- Move internal cursor to first position.
>             -- (From DS_LINEAR.)
>         ensure
>             empty_behavior: is_empty implies after
>             not_empty_behavior: not is_empty implies is_first

>     finish
>             -- Move internal cursor to last position.
>         ensure
>             empty_behavior: is_empty implies before
>             not_empty_behavior: not is_empty implies is_last

>     forth
>             -- Move internal cursor to next position.
>             -- (From DS_LINEAR.)
>         require
>             not_after: not after

>     back
>             -- Move internal cursor to previous position.
>         require
>             not_before: not before

>     search_forth (v: G)
>             -- Move internal cursor to first position at or after current
>             -- position where item_for_iteration and v are equal.
>             -- (Use equality_tester's comparison criterion
>             -- if not void, use `=' criterion otherwise.)
>             -- Move after if not found.
>             -- (From DS_LINEAR.)
>         require
>             not_off: not off or after

>     search_back (v: G)
>             -- Move internal cursor to first position at or before current
>             -- position where item_for_iteration and v are equal.
>             -- (Use equality_tester's comparison criterion
>             -- if not void, use `=' criterion otherwise.)
>             -- Move before if not found.
>         require
>             not_off: not off or before

>     go_after
>             -- Move cursor to after position.
>             -- (From DS_LINEAR.)
>         ensure
>             after: after

>     go_before
>             -- Move cursor to before position.
>         ensure
>             before: before

>     go_to (a_cursor: like new_cursor)
>             -- Move internal cursor to a_cursor's position.
>             -- (From DS_TRAVERSABLE.)
>         require
>             cursor_not_void: a_cursor /= Void
>             valid_cursor: valid_cursor (a_cursor)
>         ensure
>             same_position: same_position (a_cursor)

    feature -- Removal

>     wipe_out
>             -- Remove all items from container.
>             -- (From DS_CONTAINER.)
>         deferred
>         ensure
>             wiped_out: is_empty

    invariant

>     positive_count: count >= 0
>     empty_definition: is_empty = (count = 0)
>         -- (From DS_CONTAINER.)

>     empty_constraint: is_empty implies off
>         -- (From DS_TRAVERSABLE.)

>     after_constraint: after implies off
>         -- (From DS_LINEAR.)

>     not_both: not (after and before)
>     before_constraint: before implies off

    end -- class DS_BILINEAR