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200 | -- Abstract :
--
-- A generic doubly linked list with definite elements, with
-- reference counting on cursors to detect dangling references.
--
-- WORKAROUND: there is a bug in GNAT Community 2020 (Eurocontrol
-- ticket V107-045) that causes reference counting to be inaccurate
-- in some cases, so we support turning off the reference counting.
--
-- Rationale for not implementing reference types and iterators:
-- Consider a typical reference type use:
--
-- declare
-- A : Element_Type renames List.First;
-- To_Delete : Cursor := List.First;
-- begin
-- Delete (To_Delete);
-- end;
--
-- The reference object exists only while evaluating the renames, so
-- it cannot assert any kind of lock on the element or list that
-- survives thru the call to Delete and is then released. We would
-- have to use something like:
--
-- declare
-- A_Ref : constant Reference_Type := List.First;
-- A : Element_Type renames Element (A_Ref);
-- To_Delete : Cursor := List.First;
-- begin
-- Delete (To_Delete);
-- end;
--
-- Where "Reference_Type" is opaque, and thus cannot be used for iterators.
--
-- Copyright (C) 2017 - 2022 Free Software Foundation, Inc.
--
-- This library is free software; you can redistribute it and/or
-- modify it under terms of the GNU General Public License as
-- published by the Free Software Foundation; either version 3, or (at
-- your option) any later version. This library is distributed in the
-- hope that it will be useful, but WITHOUT ANY WARRANTY; without even
-- the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
-- PURPOSE. See the GNU General Public License for more details. You
-- should have received a copy of the GNU General Public License
-- distributed with this program; see file COPYING. If not, write to
-- the Free Software Foundation, 59 Temple Place - Suite 330, Boston,
-- MA 02111-1307, USA.
--
-- As a special exception under Section 7 of GPL version 3, you are granted
-- additional permissions described in the GCC Runtime Library Exception,
-- version 3.1, as published by the Free Software Foundation.
pragma License (Modified_GPL);
with Ada.Containers;
with Ada.Finalization;
with Ada.Unchecked_Deallocation;
generic
type Element_Type is private;
package SAL.Gen_Definite_Doubly_Linked_Lists_Ref_Count is
type List is new Ada.Finalization.Controlled with private;
-- We cannot implement reference counting that detects reference
-- types (see discussion above), so no reference types, no iterators.
type List_Access_Constant is access constant List;
for List_Access_Constant'Storage_Size use 0;
type List_Access is access all List;
for List_Access'Storage_Size use 0;
Empty_List : constant List;
overriding procedure Adjust (Container : in out List);
-- Deep copy.
overriding procedure Finalize (Container : in out List);
-- Free all items in List.
procedure Enable_Ref_Count_Check (Container : in out List; Enable : in Boolean);
-- Enable or disable checks; default is enabled.
--
-- This is useful when there are bugs that you want to ignore.
procedure Check_Ref_Counts (Container : in out List);
-- Raises SAL.Invalid_Operation if any ref count is non-zero.
procedure Clear (Container : in out List) renames Finalize;
function Length (Container : in List) return Ada.Containers.Count_Type;
procedure Append (Container : in out List; Element : in Element_Type);
procedure Prepend (Container : in out List; Element : in Element_Type);
function To_List (Element : in Element_Type) return List;
type Cursor is tagged private;
-- Cursor is not simply 'private', to allow implementing reference
-- counting.
function Has_Element (Position : in Cursor) return Boolean;
No_Element : constant Cursor;
function Ref_Count (Position : in Cursor) return Integer;
-- For debugging, tests, preconditions.
function First (Container : in List'Class) return Cursor;
function Last (Container : in List'Class) return Cursor;
procedure Next (Position : in out Cursor)
with Pre => Has_Element (Position);
function Next (Position : in Cursor) return Cursor
with Pre => Has_Element (Position);
procedure Previous (Position : in out Cursor)
with Pre => Has_Element (Position);
function Previous (Position : in Cursor) return Cursor
with Pre => Has_Element (Position);
function Element (Position : in Cursor) return Element_Type
with Pre => Has_Element (Position);
procedure Delete (Container : in out List; Position : in out Cursor'Class)
with Pre => Has_Element (Position) and then Ref_Count (Position) = 1;
-- Raises SAL.Invalid_Operation if any other cursors, iterators, or
-- reference objects reference the same element as Position.
procedure Replace_Element
(Position : in Cursor'Class;
New_Element : in Element_Type)
with Pre => Has_Element (Position);
function Append (Container : in out List'Class; Element : in Element_Type) return Cursor;
procedure Insert
(Container : in out List;
Before : in Cursor'Class;
Element : in Element_Type);
function Insert
(Container : in out List'Class;
Before : in Cursor'Class;
Element : in Element_Type)
return Cursor;
-- If Before is No_Element, insert after Last.
----------
-- Iterator replacements, since we don't have Ada.Iterator_Interfaces
-- iterators.
function Contains
(Container : in List;
Item : in Cursor'Class)
return Boolean;
private
type Node_Type;
type Node_Access is access Node_Type;
type Node_Type is record
Element : aliased Element_Type;
Prev : Node_Access;
Next : Node_Access;
Ref_Count : Integer := 0;
-- Ref_Count does not include the internal pointers Prev, Next, Head,
-- Tail, since there are always two of those for each node.
end record;
procedure Free is new Ada.Unchecked_Deallocation (Node_Type, Node_Access);
type List is new Ada.Finalization.Controlled with record
Head : Node_Access := null;
Tail : Node_Access := null;
Count : Ada.Containers.Count_Type := 0;
Enable_Checks : Boolean := True;
end record;
Empty_List : constant List := (Ada.Finalization.Controlled with others => <>);
type Cursor is new Ada.Finalization.Controlled with record
Ptr : Node_Access;
end record;
overriding procedure Finalize (Object : in out Cursor);
-- Decrement external reference count of Object.Ptr.all.
overriding procedure Adjust (Object : in out Cursor);
-- Increment external reference count of Object.Ptr.all.
No_Element : constant Cursor := (Ada.Finalization.Controlled with Ptr => null);
function Ref_Count (Position : in Cursor) return Integer
is (Position.Ptr.Ref_Count);
end SAL.Gen_Definite_Doubly_Linked_Lists_Ref_Count;
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