Class DS_BILINKED

Class DS_BILINKED_LIST

note

description:

    "Lists implemented with bilinked cells"

library:    "Gobo Eiffel Structure Library"
author:     "Eric Bezault <ericb@gobosoft.com>"
copyright:  "Copyright (c) 1999-2001, Eric Bezault and others"
license:    "MIT License"

class interface

DS_BILINKED_LIST [G]

inherit

DS_LINKED_LIST [G]
    DS_LIST [G]
        DS_BILINEAR [G]
            DS_LINEAR [G]
                DS_TRAVERSABLE [G]
                    DS_CONTAINER [G]
                DS_SEARCHABLE [G]
                    DS_CONTAINER [G]
        DS_INDEXABLE [G]
            DS_SORTABLE [G]
                DS_CONTAINER [G]

create

make
        -- Create an empty list.
        -- Use `=' as comparison criterion.
        -- (From DS_LINKED_LIST.)
    ensure
        empty: is_empty
        before: before

make_equal
        -- Create an empty list.
        -- Use equal as comparison criterion.
        -- (From DS_LINKED_LIST.)
    ensure
        empty: is_empty
        before: before

make_from_linear (other: DS_LINEAR [G])
        -- Create a new list and fill it with items of other.
        -- Use `=' as comparison criterion.
        -- (From DS_LINKED_LIST.)
    require
        other_not_void: other /= Void
    ensure
        count_set: count = other.count
        before: before

make_default
        -- Create an empty list.
        -- Use `=' as comparison criterion.
        -- (From DS_CONTAINER.)
    ensure
        empty: is_empty
        before: before

feature -- Access

infix "@", item (i: INTEGER): G
        -- Item at index i
        -- (Performance: O(i).)
        -- (From DS_INDEXABLE.)
    require
        valid_index: 1 <= i and i <= count

item_for_iteration: G
        -- Item at internal cursor position
        -- (Performance: O(1).)
        -- (From DS_TRAVERSABLE.)
    require
        not_off: not off

first: G
        -- First item in list
        -- (Performance: O(1).)
        -- (From DS_LINEAR and DS_INDEXABLE.)
    require
        not_empty: not is_empty
    ensure
        has_first: has (Result)
        definition: Result = item (1)

last: G
        -- Last item in list
        -- (Performance: O(1).)
        -- (From DS_BILINEAR and DS_INDEXABLE.)
    require
        not_empty: not is_empty
    ensure
        has_last: has (Result)
        definition: Result = item (count)

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

index: INTEGER
        -- Index of current internal cursor position
        -- (Performance: O(count).)
        -- (From DS_LIST.)
    ensure
        valid_index: valid_index (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 list
        -- (Performance: O(1).)
        -- (From DS_CONTAINER.)

occurrences (v: G): INTEGER
        -- Number of times v appears in list
        -- (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 list 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 list 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?
        -- (From DS_BILINEAR.)
    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?
        -- (From DS_BILINEAR.)

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

valid_index (i: INTEGER): BOOLEAN
        -- Is i a valid index value?
        -- (From DS_LIST.)
    ensure
        definition: Result = (0 <= i and i <= (count + 1))

extendible (n: INTEGER): BOOLEAN
        -- May list be extended with n items?
        -- (From DS_INDEXABLE.)
    require
        positive_n: n >= 0
    ensure
        definition: Result = True

sorted (a_sorter: DS_SORTER [G]): BOOLEAN
        -- Is list sorted according to a_sorter's criterion?
        -- (From DS_SORTABLE.)
    require
        a_sorter_not_void: a_sorter /= 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 list equal to other?
        -- Do not take cursor positions nor
        -- equality_tester into account.
        -- (Performance: O(count).)
        -- (From GENERAL.)
    require
        other_not_void: other /= Void
    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 list.
        -- Move all cursors off (unless other = Current).
        -- (Performance: O(other.count).)
        -- (From GENERAL.)
    require
        other_not_void: other /= Void
        type_identity: same_type (other)
    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 -- Sort

sort (a_sorter: DS_SORTER [G])
        -- Sort list using a_sorter's algorithm.
        -- (From DS_SORTABLE.)
    require
        a_sorter_not_void: a_sorter /= Void
    ensure
        sorted: sorted (a_sorter)

feature -- Cursor movement

start
        -- Move internal cursor to first position.
        -- (Performance: O(1).)
        -- (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.
        -- (Performance: O(1).)
        -- (From DS_BILINEAR.)
    ensure
        empty_behavior: is_empty implies before
        not_empty_behavior: not is_empty implies is_last

forth
        -- Move internal cursor to next position.
        -- (Performance: O(1).)
        -- (From DS_LINEAR.)
    require
        not_after: not after

back
        -- Move internal cursor to previous position.
        -- (Performance: O(1).)
        -- (From DS_BILINEAR.)
    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.
        -- (From DS_BILINEAR.)
    require
        not_off: not off or before

go_after
        -- Move cursor to after position.
        -- (Performance: O(1).)
        -- (From DS_LINEAR.)
    ensure
        after: after

go_before
        -- Move cursor to before position.
        -- (Performance: O(1).)
        -- (From DS_BILINEAR.)
    ensure
        before: before

go_to (a_cursor: like new_cursor)
        -- Move internal cursor to a_cursor's position.
        -- (Performance: O(1).)
        -- (From DS_TRAVERSABLE.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
    ensure
        same_position: same_position (a_cursor)

go_i_th (i: INTEGER)
        -- Move internal cursor to i-th position.
        -- (Performance: O(min(i,count-i)).)
        -- (From DS_LIST.)
    require
        valid_index: valid_index (i)
    ensure
        moved: index = i

feature -- Element change

put_first (v: G)
        -- Add v to beginning of list.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    require
        extendible: extendible (1)
    ensure
        one_more: count = old count + 1
        inserted: first = v

put_last (v: G)
        -- Add v to end of list.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    require
        extendible: extendible (1)
    ensure
        one_more: count = old count + 1
        inserted: last = v

put_left (v: G)
        -- Add v to left of internal cursor position.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        extendible: extendible (1)
        not_before: not before
    ensure
        one_more: count = old count + 1

put_left_cursor (v: G; a_cursor: like new_cursor)
        -- Add v to left of a_cursor position.
        -- Do not move cursors.
        -- (Synonym of a_cursor.put_left (v).)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        extendible: extendible (1)
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_before: not a_cursor.before
    ensure
        one_more: count = old count + 1

put_right (v: G)
        -- Add v to right of internal cursor position.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        extendible: extendible (1)
        not_after: not after
    ensure
        one_more: count = old count + 1

put_right_cursor (v: G; a_cursor: like new_cursor)
        -- Add v to right of a_cursor position.
        -- Do not move cursors.
        -- (Synonym of a_cursor.put_right (v).)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        extendible: extendible (1)
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_after: not a_cursor.after
    ensure
        one_more: count = old count + 1

put (v: G; i: INTEGER)
        -- Add v at i-th position.
        -- Do not move cursors.
        -- (Performance: O(i).)
        -- (From DS_INDEXABLE.)
    require
        extendible: extendible (1)
        valid_index: 1 <= i and i <= (count + 1)
    ensure
        one_more: count = old count + 1
        inserted: item (i) = v

force_first (v: G)
        -- Add v to beginning of list.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    ensure
        one_more: count = old count + 1
        inserted: first = v

force_last (v: G)
        -- Add v to end of list.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    ensure
        one_more: count = old count + 1
        inserted: last = v

force_left (v: G)
        -- Add v to left of internal cursor position.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_before: not before
    ensure
        one_more: count = old count + 1

force_left_cursor (v: G; a_cursor: like new_cursor)
        -- Add v to left of a_cursor position.
        -- Do not move cursors.
        -- (Synonym of a_cursor.force_left (v).)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_before: not a_cursor.before
    ensure
        one_more: count = old count + 1

force_right (v: G)
        -- Add v to right of internal cursor position.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_after: not after
    ensure
        one_more: count = old count + 1

force_right_cursor (v: G; a_cursor: like new_cursor)
        -- Add v to right of a_cursor position.
        -- Do not move cursors.
        -- (Synonym of a_cursor.force_right (v).)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_after: not a_cursor.after
    ensure
        one_more: count = old count + 1

force (v: G; i: INTEGER)
        -- Add v at i-th position.
        -- Do not move cursors.
        -- (Performance: O(i).)
        -- (From DS_INDEXABLE.)
    require
        valid_index: 1 <= i and i <= (count + 1)
    ensure
        one_more: count = old count + 1
        inserted: item (i) = v

replace_at (v: G)
        -- Replace item at internal cursor position by v.
        -- Do not move cursors.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_off: not off
    ensure
        same_count: count = old count
        replaced: item_for_iteration = v

replace_at_cursor (v: G; a_cursor: like new_cursor)
        -- Replace item at a_cursor position by v.
        -- Do not move cursors.
        -- (Synonym of a_cursor.replace (v).)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_off: not a_cursor.off
    ensure
        same_count: count = old count
        replaced: a_cursor.item = v

replace (v: G; i: INTEGER)
        -- Replace item at i-th position by v.
        -- Do not move cursors.
        -- (Performance: O(i).)
        -- (From DS_INDEXABLE.)
    require
        valid_index: 1 <= i and i <= count
    ensure
        same_count: count = old count
        replaced: item (i) = v

swap (i, j: INTEGER)
        -- Exchange items at indexes i and j.
        -- Do not move cursors.
        -- (Performance: O(max(i,j)).)
        -- (From DS_INDEXABLE.)
    require
        valid_i: 1 <= i and i <= count
        valid_j: 1 <= j and j <= count
    ensure
        same_count: count = old count
        new_i: item (i) = old item (j)
        new_j: item (j) = old item (i)

extend_first (other: DS_LINEAR [G])
        -- Add items of other to beginning of list.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (first = other.first)

extend_last (other: DS_LINEAR [G])
        -- Add items of other to end of list.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old count + 1) = other.first)

extend_left (other: DS_LINEAR [G])
        -- Add items of other to left of internal cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
        not_before: not before
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old index) = other.first)

extend_left_cursor (other: DS_LINEAR [G]; a_cursor: like new_cursor)
        -- Add items of other to left of a_cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Synonym of a_cursor.extend_left (other).)
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_before: not a_cursor.before
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old (a_cursor.index)) = other.first)

extend_right (other: DS_LINEAR [G])
        -- Add items of other to right of internal cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
        not_after: not after
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (index + 1) = other.first)

extend_right_cursor (other: DS_LINEAR [G]; a_cursor: like new_cursor)
        -- Add items of other to right of a_cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Synonym of a_cursor.extend_right (other).)
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_after: not a_cursor.after
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (a_cursor.index + 1) = other.first)

extend (other: DS_LINEAR [G]; i: INTEGER)
        -- Add items of other at i-th position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(i+other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
        extendible: extendible (other.count)
        valid_index: 1 <= i and i <= (count + 1)
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (i) = other.first)

append_first (other: DS_LINEAR [G])
        -- Add items of other to beginning of list.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (first = other.first)

append_last (other: DS_LINEAR [G])
        -- Add items of other to end of list.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old count + 1) = other.first)

append_left (other: DS_LINEAR [G])
        -- Add items of other to left of internal cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        not_before: not before
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old index) = other.first)

append_left_cursor (other: DS_LINEAR [G]; a_cursor: like new_cursor)
        -- Add items of other to left of a_cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Synonym of a_cursor.append_left (other).)
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_before: not a_cursor.before
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (old (a_cursor.index)) = other.first)

append_right (other: DS_LINEAR [G])
        -- Add items of other to right of internal cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        not_after: not after
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (index + 1) = other.first)

append_right_cursor (other: DS_LINEAR [G]; a_cursor: like new_cursor)
        -- Add items of other to right of a_cursor position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Synonym of a_cursor.append_right (other).)
        -- (Performance: O(other.count).)
        -- (From DS_LIST.)
    require
        other_not_void: other /= Void
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_after: not a_cursor.after
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (a_cursor.index + 1) = other.first)

append (other: DS_LINEAR [G]; i: INTEGER)
        -- Add items of other at i-th position.
        -- Keep items of other in the same order.
        -- Do not move cursors.
        -- (Performance: O(i+other.count).)
        -- (From DS_INDEXABLE.)
    require
        other_not_void: other /= Void
        valid_index: 1 <= i and i <= (count + 1)
    ensure
        new_count: count = old count + other.count
        same_order: (not other.is_empty) implies (item (i) = other.first)

feature -- Removal

remove (i: INTEGER)
        -- Remove item at i-th position.
        -- Move any cursors at this position forth.
        -- (Performance: O(i).)
        -- (From DS_INDEXABLE.)
    require
        not_empty: not is_empty
        valid_index: 1 <= i and i <= count
    ensure
        one_less: count = old count - 1

remove_at
        -- Remove item at internal cursor position.
        -- Move any cursors at this position forth.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_off: not off
    ensure
        one_less: count = old count - 1

remove_at_cursor (a_cursor: like new_cursor)
        -- Remove item at a_cursor position.
        -- Move any cursors at this position forth.
        -- (Synonym of a_cursor.remove.)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_off: not a_cursor.off
    ensure
        one_less: count = old count - 1

remove_first
        -- Remove first item from list.
        -- Move any cursors at this position forth.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    require
        not_empty: not is_empty
    ensure
        one_less: count = old count - 1

remove_last
        -- Remove last item from list.
        -- Move any cursors at this position forth.
        -- (Performance: O(1).)
        -- (From DS_INDEXABLE.)
    require
        not_empty: not is_empty
    ensure
        one_less: count = old count - 1

remove_left
        -- Remove item to left of internal cursor position.
        -- Move any cursors at this position forth.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_empty: not is_empty
        not_before: not before
        not_first: not is_first
    ensure
        one_less: count = old count - 1

remove_left_cursor (a_cursor: like new_cursor)
        -- Remove item to left of a_cursor position.
        -- Move any cursors at this position forth.
        -- (Synonym of a_cursor.remove_left.)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_empty: not is_empty
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_before: not a_cursor.before
        not_first: not a_cursor.is_first
    ensure
        one_less: count = old count - 1

remove_right
        -- Remove item to right of internal cursor position.
        -- Move any cursors at this position forth.
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_empty: not is_empty
        not_after: not after
        not_last: not is_last
    ensure
        one_less: count = old count - 1

remove_right_cursor (a_cursor: like new_cursor)
        -- Remove item to right of a_cursor position.
        -- Move any cursors at this position forth.
        -- (Synonym of a_cursor.remove_right.)
        -- (Performance: O(1).)
        -- (From DS_LIST.)
    require
        not_empty: not is_empty
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        not_after: not a_cursor.after
        not_last: not a_cursor.is_last
    ensure
        one_less: count = old count - 1

prune_first (n: INTEGER)
        -- Remove n first items from list.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_INDEXABLE.)
    require
        valid_n: 0 <= n and n <= count
    ensure
        new_count: count = old count - n

prune_last (n: INTEGER)
        -- Remove n last items from list.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_INDEXABLE.)
    require
        valid_n: 0 <= n and n <= count
    ensure
        new_count: count = old count - n

prune_left (n: INTEGER)
        -- Remove n items to left of internal cursor position.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_LIST.)
    require
        valid_n: 0 <= n and n < index
    ensure
        new_count: count = old count - n

prune_left_cursor (n: INTEGER; a_cursor: like new_cursor)
        -- Remove n items to left of a_cursor position.
        -- Move all cursors off.
        -- (Synonym of a_cursor.prune_left (n).)
       	-- (Performance: O(n).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        valid_n: 0 <= n and n < a_cursor.index
    ensure
        new_count: count = old count - n

prune_right (n: INTEGER)
        -- Remove n items to right of internal cursor position.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_LIST.)
    require
        valid_n: 0 <= n and n <= (count - index)
    ensure
        new_count: count = old count - n

prune_right_cursor (n: INTEGER; a_cursor: like new_cursor)
        -- Remove n items to right of a_cursor position.
        -- Move all cursors off.
        -- (Synonym of a_cursor.prune_right (n).)
        -- (Performance: O(n).)
        -- (From DS_LIST.)
    require
        cursor_not_void: a_cursor /= Void
        valid_cursor: valid_cursor (a_cursor)
        valid_n: 0 <= n and n <= (count - a_cursor.index)
    ensure
        new_count: count = old count - n

prune (n: INTEGER; i: INTEGER)
        -- Remove n items at and after i-th position.
        -- Move all cursors off.
        -- (Performance: O(i+n).)
        -- (From DS_INDEXABLE.)
    require
        valid_index: 1 <= i and i <= count
        valid_n: 0 <= n and n <= (count - i + 1)
    ensure
        new_count: count = old count - n

keep_first (n: INTEGER)
        -- Keep n first items in list.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_INDEXABLE.)
    require
        valid_n: 0 <= n and n <= count
    ensure
        new_count: count = n

keep_last (n: INTEGER)
        -- Keep n last items in list.
        -- Move all cursors off.
        -- (Performance: O(n).)
        -- (From DS_INDEXABLE.)
    require
        valid_n: 0 <= n and n <= count
    ensure
        new_count: count = n

delete (v: G)
        -- Remove all occurrences of v.
        -- (Use equality_tester's comparison criterion
        -- if not void, use `=' criterion otherwise.)
        -- Move all cursors off.
        -- (Performance: O(count).)
        -- (From DS_LIST.)
    ensure
        deleted: not has (v)
        new_count: count = old (count - occurrences (v))

wipe_out
        -- Remove all items from list.
        -- Move all cursors off.
        -- (Performance: O(1).)
        -- (From DS_CONTAINER.)
    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
    -- (From DS_BILINEAR.)

end -- class DS_BILINKED_LIST