2011-08-02 Yannick Moy <moy@adacore.com>

[thirdparty/gcc.git] / gcc / ada / a-cfdlli.adb

------------------------------------------------------------------------------
--                                                                          --
--                         GNAT LIBRARY COMPONENTS                          --
--                                                                          --
--                 ADA.CONTAINERS.FORMAL_DOUBLY_LINKED_LISTS                --
--                                                                          --
--                                 B o d y                                  --
--                                                                          --
--          Copyright (C) 2010, Free Software Foundation, Inc.              --
--                                                                          --
-- GNAT is free software;  you can  redistribute it  and/or modify it under --
-- terms of the  GNU General Public License as published  by the Free Soft- --
-- ware  Foundation;  either version 3,  or (at your option) any later ver- --
-- sion.  GNAT is distributed in the hope that it will be useful, but WITH- --
-- OUT ANY WARRANTY;  without even the  implied warranty of MERCHANTABILITY --
-- or FITNESS FOR A PARTICULAR PURPOSE.                                     --
--                                                                          --
-- 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.               --
--                                                                          --
-- You should have received a copy of the GNU General Public License and    --
-- a copy of the GCC Runtime Library Exception along with this program;     --
-- see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see    --
-- <http://www.gnu.org/licenses/>.                                          --
------------------------------------------------------------------------------

with System;  use type System.Address;

package body Ada.Containers.Formal_Doubly_Linked_Lists is

   -----------------------
   -- Local Subprograms --
   -----------------------

   procedure Allocate
     (Container : in out List;
      New_Item  : Element_Type;
      New_Node  : out Count_Type);

   procedure Allocate
     (Container : in out List;
      New_Node  : out Count_Type);

   function Copy
     (Source   : Plain_List;
      Capacity : Count_Type := 0) return PList_Access;

   function Find_Between
     (Container : Plain_List;
      Item      : Element_Type;
      From      : Count_Type;
      To        : Count_Type;
      Bg        : Count_Type) return Cursor;

   function Element_Unchecked
     (Container : List;
      Position  : Count_Type) return Element_Type;

   procedure Free
     (Container : in out Plain_List;
      X         : Count_Type);

   function Has_Element_Base
     (Container : Plain_List;
      Position  : Cursor) return Boolean;

   procedure Insert_Internal
     (Container : in out List;
      Before    : Count_Type;
      New_Node  : Count_Type);

   procedure Iterate_Between
     (Container : List;
      From      : Count_Type;
      To        : Count_Type;
      Process   :
        not null access procedure (Container : List; Position : Cursor));

   function Next_Unchecked
     (Container : List;
      Position  : Count_Type) return Count_Type;

   procedure Query_Element_Plain
     (Container : Plain_List; Position : Cursor;
      Process   : not null access procedure (Element : Element_Type));

   function Reverse_Find_Between
     (Container : Plain_List;
      Item      : Element_Type;
      From      : Count_Type;
      To        : Count_Type) return Cursor;

   procedure Reverse_Iterate_Between
     (Container : List;
      From      : Count_Type;
      To        : Count_Type;
      Process   :
        not null access procedure (Container : List; Position : Cursor));

   function Vet (L : List; Position : Cursor) return Boolean;

   procedure Write_Between
     (Stream : not null access Root_Stream_Type'Class;
      Item   : Plain_List;
      Length : Count_Type;
      From   : Count_Type;
      To     : Count_Type);

   ---------
   -- "=" --
   ---------

   function "=" (Left, Right : List) return Boolean is
      LI, RI : Count_Type;

   begin
      if Left'Address = Right'Address then
         return True;
      end if;

      if Left.Length /= Right.Length then
         return False;
      end if;

      LI := Left.First;
      RI := Right.First;
      while LI /= 0 loop
         if Element_Unchecked (Left, LI) /= Element_Unchecked (Right, LI) then
            return False;
         end if;

         LI := Next_Unchecked (Left, LI);
         RI := Next_Unchecked (Right, RI);
      end loop;

      return True;
   end "=";

   --------------
   -- Allocate --
   --------------

   procedure Allocate
     (Container : in out List;
      New_Item  : Element_Type;
      New_Node  : out Count_Type)
   is
      ContainerP : Plain_List renames Container.Plain.all;
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.all.Nodes;

      begin
         if ContainerP.Free >= 0 then
            New_Node := ContainerP.Free;
            N (New_Node).Element := New_Item;
            ContainerP.Free := N (New_Node).Next;

         else
            New_Node := abs ContainerP.Free;
            N (New_Node).Element := New_Item;
            ContainerP.Free := ContainerP.Free - 1;
         end if;
      end;
   end Allocate;

   procedure Allocate
     (Container : in out List;
      New_Node  : out Count_Type)
   is
      ContainerP : Plain_List renames Container.Plain.all;
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames ContainerP.Nodes;

      begin
         if ContainerP.Free >= 0 then
            New_Node := ContainerP.Free;
            ContainerP.Free := N (New_Node).Next;

         else
            New_Node := abs ContainerP.Free;
            ContainerP.Free := ContainerP.Free - 1;
         end if;
      end;
   end Allocate;

   ------------
   -- Append --
   ------------

   procedure Append
     (Container : in out List;
      New_Item  : Element_Type;
      Count     : Count_Type := 1)
   is
   begin
      Insert (Container, No_Element, New_Item, Count);
   end Append;

   ------------
   -- Assign --
   ------------

   procedure Assign (Target : in out List; Source : List) is
   begin
      if Target.K /= Plain or Source.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Source.Plain.Nodes;
         J : Count_Type;

      begin
         if Target'Address = Source'Address then
            return;
         end if;

         if Target.Capacity < Source.Length then
            raise Constraint_Error with  -- ???
              "Source length exceeds Target capacity";
         end if;

         Clear (Target);

         J := Source.First;
         while J /= 0 loop
            Append (Target, N (J).Element);
            J := N (J).Next;
         end loop;
      end;
   end Assign;

   -----------
   -- Clear --
   -----------

   procedure Clear (Container : in out List) is
   begin
      if Container.K /= Plain then
         raise Constraint_Error;
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;
         X : Count_Type;

      begin
         if Container.Length = 0 then
            pragma Assert (Container.First = 0);
            pragma Assert (Container.Last = 0);
            pragma Assert (Container.Plain.Busy = 0);
            pragma Assert (Container.Plain.Lock = 0);
            return;
         end if;

         pragma Assert (Container.First >= 1);
         pragma Assert (Container.Last >= 1);
         pragma Assert (N (Container.First).Prev = 0);
         pragma Assert (N (Container.Last).Next = 0);

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         while Container.Length > 1 loop
            X := Container.First;

            Container.First := N (X).Next;
            N (Container.First).Prev := 0;

            Container.Length := Container.Length - 1;

            Free (Container.Plain.all, X);
         end loop;

         X := Container.First;

         Container.First := 0;
         Container.Last := 0;
         Container.Length := 0;

         Free (Container.Plain.all, X);
      end;
   end Clear;

   --------------
   -- Contains --
   --------------

   function Contains
     (Container : List;
      Item      : Element_Type) return Boolean
   is
   begin
      return Find (Container, Item) /= No_Element;
   end Contains;

   ----------
   -- Copy --
   ----------

   function Copy
     (Source   : Plain_List;
      Capacity : Count_Type := 0) return PList_Access
   is
      C : constant Count_Type := Count_Type'Max (Source.Capacity, Capacity);
      P : PList_Access;
      N : Count_Type := 1;
   begin
      P := new Plain_List (C);
      while N <= Source.Capacity loop
         P.Nodes (N).Prev := Source.Nodes (N).Prev;
         P.Nodes (N).Next := Source.Nodes (N).Next;
         P.Nodes (N).Element := Source.Nodes (N).Element;
         N := N + 1;
      end loop;
      P.Free := Source.Free;
      if P.Free >= 0 then
         N := Source.Capacity + 1;
         while N <= C loop
            Free (P.all, N);
            N := N + 1;
         end loop;
      end if;
      return P;
   end Copy;

   function Copy
     (Source   : List;
      Capacity : Count_Type := 0) return List
   is
      Cap : constant Count_Type := Count_Type'Max (Source.Capacity, Capacity);
   begin
      case Source.K is
         when Plain =>
            return (Capacity => Cap,
                    Length   => Source.Length,
                    Plain    => Copy (Source.Plain.all, Cap),
                    First    => Source.First,
                    Last     => Source.Last,
                    others   => <>);
         when Part =>
            declare
               Target : List (Capacity => Cap);
               C      : Cursor;
               P      : Cursor;
            begin
               Target := (Capacity => Cap,
                          Length   => Source.Part.LLength,
                          Plain    => Copy (Source.Plain.all, Cap),
                          First    => Source.Part.LFirst,
                          Last     => Source.Part.LLast,
                          others   => <>);
               C := (Node => Target.First);
               while C.Node /= Source.First loop
                  P := Next (Target, C);
                  Delete (Container => Target, Position => C);
                  C := P;
               end loop;
               if Source.Last /= 0 then
                  C := (Node => Source.Plain.all.Nodes (Source.Last).Next);
                  while C.Node /= 0 loop
                     P := Next (Target, C);
                     Delete (Container => Target, Position => C);
                     C := P;
                  end loop;
               end if;
               return Target;
            end;
      end case;
   end Copy;

   ------------
   -- Delete --
   ------------

   procedure Delete
     (Container : in out List;
      Position  : in out Cursor;
      Count     : Count_Type := 1)
   is
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;
         X : Count_Type;

      begin
         if not Has_Element (Container => Container,
                             Position  => Position) then
            raise Constraint_Error with
              "Position cursor has no element";
         end if;

         pragma Assert (Vet (Container, Position), "bad cursor in Delete");
         pragma Assert (Container.First >= 1);
         pragma Assert (Container.Last >= 1);
         pragma Assert (N (Container.First).Prev = 0);
         pragma Assert (N (Container.Last).Next = 0);

         if Position.Node = Container.First then
            Delete_First (Container, Count);
            Position := No_Element;
            return;
         end if;

         if Count = 0 then
            Position := No_Element;
            return;
         end if;

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         for Index in 1 .. Count loop
            pragma Assert (Container.Length >= 2);

            X := Position.Node;
            Container.Length := Container.Length - 1;

            if X = Container.Last then
               Position := No_Element;

               Container.Last := N (X).Prev;
               N (Container.Last).Next := 0;

               Free (Container.Plain.all, X);
               return;
            end if;

            Position.Node := N (X).Next;
            pragma Assert (N (Position.Node).Prev >= 0);

            N (N (X).Next).Prev := N (X).Prev;
            N (N (X).Prev).Next := N (X).Next;

            Free (Container.Plain.all, X);
         end loop;
         Position := No_Element;
      end;
   end Delete;

   ------------------
   -- Delete_First --
   ------------------

   procedure Delete_First
     (Container : in out List;
      Count     : Count_Type := 1)
   is
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;
         X : Count_Type;

      begin
         if Count >= Container.Length then
            Clear (Container);
            return;
         end if;

         if Count = 0 then
            return;
         end if;

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         for I in 1 .. Count loop
            X := Container.First;
            pragma Assert (N (N (X).Next).Prev = Container.First);

            Container.First := N (X).Next;
            N (Container.First).Prev := 0;

            Container.Length := Container.Length - 1;

            Free (Container.Plain.all, X);
         end loop;
      end;
   end Delete_First;

   -----------------
   -- Delete_Last --
   -----------------

   procedure Delete_Last
     (Container : in out List;
      Count     : Count_Type := 1)
   is
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;
         X : Count_Type;

      begin
         if Count >= Container.Length then
            Clear (Container);
            return;
         end if;

         if Count = 0 then
            return;
         end if;

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         for I in 1 .. Count loop
            X := Container.Last;
            pragma Assert (N (N (X).Prev).Next = Container.Last);

            Container.Last := N (X).Prev;
            N (Container.Last).Next := 0;

            Container.Length := Container.Length - 1;

            Free (Container.Plain.all, X);
         end loop;
      end;
   end Delete_Last;

   -------------
   -- Element --
   -------------

   function Element_Unchecked
     (Container : List;
      Position  : Count_Type) return Element_Type is
   begin
      case Container.K is
         when Plain =>
            return Container.Plain.Nodes (Position).Element;
         when others =>
            return Container.Plain.all.Nodes (Position).Element;
      end case;
   end Element_Unchecked;

   function Element
     (Container : List;
      Position  : Cursor) return Element_Type is
   begin
      if not Has_Element (Container => Container, Position  => Position) then
         raise Constraint_Error with
           "Position cursor has no element";
      end if;

      return Element_Unchecked (Container => Container,
                                Position  => Position.Node);
   end Element;

   ----------
   -- Find --
   ----------

   function Find_Between
     (Container : Plain_List;
      Item      : Element_Type;
      From      : Count_Type;
      To        : Count_Type;
      Bg        : Count_Type) return Cursor
   is
      Nodes : Node_Array renames Container.Nodes;
      Node  : Count_Type := Bg;
   begin
      while Node /= From loop
         if Node = 0 or else Node = To then
            raise Constraint_Error with
              "Position cursor has no element";
         end if;
         Node := Nodes (Node).Next;
      end loop;
      while Node /= Nodes (To).Next loop
         if Nodes (Node).Element = Item then
            return (Node => Node);
         end if;
         Node := Nodes (Node).Next;
      end loop;

      return No_Element;
   end Find_Between;

   function Find
     (Container : List;
      Item      : Element_Type;
      Position  : Cursor := No_Element) return Cursor
   is
      From : Count_Type := Position.Node;
   begin
      if From = 0 and Container.Length = 0 then
         return No_Element;
      end if;
      if From = 0 then
         From := Container.First;
      end if;
      if Position.Node /= 0 and then
        not Has_Element_Base (Container.Plain.all, Position) then
         raise Constraint_Error with
           "Position cursor has no element";
      end if;
      return Find_Between (Container => Container.Plain.all,
                           Item      => Item,
                           From      => From,
                           To        => Container.Last,
                           Bg        => Container.First);
   end Find;

   -----------
   -- First --
   -----------

   function First (Container : List) return Cursor is
   begin
      if Container.First = 0 then
         return No_Element;
      end if;
      return (Node => Container.First);
   end First;

   -------------------
   -- First_Element --
   -------------------

   function First_Element (Container : List) return Element_Type is
      F : constant Count_Type := Container.First;
   begin
      if F = 0 then
         raise Constraint_Error with "list is empty";
      else
         return Element_Unchecked (Container, F);
      end if;
   end First_Element;

   ----------
   -- Free --
   ----------

   procedure Free
     (Container : in out Plain_List;
      X         : Count_Type)
   is
      pragma Assert (X > 0);
      pragma Assert (X <= Container.Capacity);

      N : Node_Array renames Container.Nodes;

   begin
      N (X).Prev := -1;  -- Node is deallocated (not on active list)

      if Container.Free >= 0 then
         N (X).Next := Container.Free;
         Container.Free := X;

      elsif X + 1 = abs Container.Free then
         N (X).Next := 0;  -- Not strictly necessary, but marginally safer
         Container.Free := Container.Free + 1;

      else
         Container.Free := abs Container.Free;

         if Container.Free > Container.Capacity then
            Container.Free := 0;

         else
            for I in Container.Free .. Container.Capacity - 1 loop
               N (I).Next := I + 1;
            end loop;

            N (Container.Capacity).Next := 0;
         end if;

         N (X).Next := Container.Free;
         Container.Free := X;
      end if;
   end Free;

   ---------------------
   -- Generic_Sorting --
   ---------------------

   package body Generic_Sorting is

      ---------------
      -- Is_Sorted --
      ---------------

      function Is_Sorted (Container : List) return Boolean is
         Nodes : Node_Array renames Container.Plain.all.Nodes;
         Node  : Count_Type := Container.First;
      begin
         for I in 2 .. Container.Length loop
            if Nodes (Nodes (Node).Next).Element < Nodes (Node).Element then
               return False;
            end if;

            Node := Nodes (Node).Next;
         end loop;

         return True;
      end Is_Sorted;

      -----------
      -- Merge --
      -----------

      procedure Merge
        (Target : in out List;
         Source : in out List)
      is
      begin
         if Target.K /= Plain or Source.K /= Plain then
            raise Program_Error with "cannot modify part of container";
         end if;

         declare
            LN : Node_Array renames Target.Plain.Nodes;
            RN : Node_Array renames Source.Plain.Nodes;
            LI : Cursor;
            RI : Cursor;

         begin
            if Target'Address = Source'Address then
               return;
            end if;

            if Target.Plain.Busy > 0 then
               raise Program_Error with
                 "attempt to tamper with cursors of Target (list is busy)";
            end if;

            if Source.Plain.Busy > 0 then
               raise Program_Error with
                 "attempt to tamper with cursors of Source (list is busy)";
            end if;

            LI := First (Target);
            RI := First (Source);
            while RI.Node /= 0 loop
               pragma Assert (RN (RI.Node).Next = 0
                 or else not (RN (RN (RI.Node).Next).Element <
                     RN (RI.Node).Element));

               if LI.Node = 0 then
                  Splice (Target, No_Element, Source);
                  return;
               end if;

               pragma Assert (LN (LI.Node).Next = 0
                 or else not (LN (LN (LI.Node).Next).Element <
                     LN (LI.Node).Element));

               if RN (RI.Node).Element < LN (LI.Node).Element then
                  declare
                     RJ : Cursor := RI;
                     pragma Warnings (Off, RJ);
                  begin
                     RI.Node := RN (RI.Node).Next;
                     Splice (Target, LI, Source, RJ);
                  end;

               else
                  LI.Node := LN (LI.Node).Next;
               end if;
            end loop;
         end;
      end Merge;

      ----------
      -- Sort --
      ----------

      procedure Sort (Container : in out List) is
      begin
         if Container.K /= Plain then
            raise Program_Error with "cannot modify part of container";
         end if;

         declare
            N : Node_Array renames Container.Plain.Nodes;

            procedure Partition (Pivot, Back : Count_Type);
            procedure Sort (Front, Back : Count_Type);

            ---------------
            -- Partition --
            ---------------

            procedure Partition (Pivot, Back : Count_Type) is
               Node : Count_Type := N (Pivot).Next;

            begin
               while Node /= Back loop
                  if N (Node).Element < N (Pivot).Element then
                     declare
                        Prev : constant Count_Type := N (Node).Prev;
                        Next : constant Count_Type := N (Node).Next;

                     begin
                        N (Prev).Next := Next;

                        if Next = 0 then
                           Container.Last := Prev;
                        else
                           N (Next).Prev := Prev;
                        end if;

                        N (Node).Next := Pivot;
                        N (Node).Prev := N (Pivot).Prev;

                        N (Pivot).Prev := Node;

                        if N (Node).Prev = 0 then
                           Container.First := Node;
                        else
                           N (N (Node).Prev).Next := Node;
                        end if;

                        Node := Next;
                     end;

                  else
                     Node := N (Node).Next;
                  end if;
               end loop;
            end Partition;

            ----------
            -- Sort --
            ----------

            procedure Sort (Front, Back : Count_Type) is
               Pivot : Count_Type;

            begin
               if Front = 0 then
                  Pivot := Container.First;
               else
                  Pivot := N (Front).Next;
               end if;

               if Pivot /= Back then
                  Partition (Pivot, Back);
                  Sort (Front, Pivot);
                  Sort (Pivot, Back);
               end if;
            end Sort;

            --  Start of processing for Sort

         begin
            if Container.Length <= 1 then
               return;
            end if;

            pragma Assert (N (Container.First).Prev = 0);
            pragma Assert (N (Container.Last).Next = 0);

            if Container.Plain.Busy > 0 then
               raise Program_Error with
                 "attempt to tamper with elements (list is busy)";
            end if;

            Sort (Front => 0, Back => 0);

            pragma Assert (N (Container.First).Prev = 0);
            pragma Assert (N (Container.Last).Next = 0);
         end;
      end Sort;

   end Generic_Sorting;

   -----------------
   -- Has_Element --
   -----------------

   function Has_Element_Base (Container : Plain_List; Position : Cursor)
                              return Boolean
   is
   begin
      return Container.Nodes (Position.Node).Prev /= -1;
   end Has_Element_Base;

   function Has_Element (Container : List; Position : Cursor) return Boolean is
   begin
      if Position.Node = 0 then
         return False;
      end if;

      case Container.K is
         when Plain =>
            return Container.Plain.Nodes (Position.Node).Prev /= -1;
         when Part =>
            declare
               Current : Count_Type := Container.First;
            begin
               if Container.Plain.Nodes (Position.Node).Prev = -1 then
                  return False;
               end if;
               while Current /= 0 loop
                  if Current = Position.Node then
                     return True;
                  end if;
                  Current := Next_Unchecked (Container, Current);
               end loop;
               return False;
            end;
      end case;
   end Has_Element;

   ------------
   -- Insert --
   ------------

   procedure Insert
     (Container : in out List;
      Before    : Cursor;
      New_Item  : Element_Type;
      Position  : out Cursor;
      Count     : Count_Type := 1)
   is
      J : Count_Type;

   begin

      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      if Before.Node /= 0 then
         null;
         pragma Assert (Vet (Container, Before), "bad cursor in Insert");
      end if;

      if Count = 0 then
         Position := Before;
         return;
      end if;

      if Container.Length > Container.Capacity - Count then
         raise Constraint_Error with "new length exceeds capacity";
      end if;

      if Container.Plain.Busy > 0 then
         raise Program_Error with
           "attempt to tamper with elements (list is busy)";
      end if;

      Allocate (Container, New_Item, New_Node => J);
      Insert_Internal (Container, Before.Node, New_Node => J);
      Position := (Node => J);

      for Index in 2 .. Count loop
         Allocate (Container, New_Item, New_Node => J);
         Insert_Internal (Container, Before.Node, New_Node => J);
      end loop;
   end Insert;

   procedure Insert
     (Container : in out List;
      Before    : Cursor;
      New_Item  : Element_Type;
      Count     : Count_Type := 1)
   is
      Position : Cursor;

   begin
      Insert (Container, Before, New_Item, Position, Count);
   end Insert;

   procedure Insert
     (Container : in out List;
      Before    : Cursor;
      Position  : out Cursor;
      Count     : Count_Type := 1)
   is
      J : Count_Type;

   begin

      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      if Before.Node /= 0 then
         null;
         pragma Assert (Vet (Container, Before), "bad cursor in Insert");
      end if;

      if Count = 0 then
         Position := Before;
         return;
      end if;

      if Container.Length > Container.Capacity - Count then
         raise Constraint_Error with "new length exceeds capacity";
      end if;

      if Container.Plain.Busy > 0 then
         raise Program_Error with
           "attempt to tamper with elements (list is busy)";
      end if;

      Allocate (Container, New_Node => J);
      Insert_Internal (Container, Before.Node, New_Node => J);
      Position := (Node => J);

      for Index in 2 .. Count loop
         Allocate (Container, New_Node => J);
         Insert_Internal (Container, Before.Node, New_Node => J);
      end loop;
   end Insert;

   ---------------------
   -- Insert_Internal --
   ---------------------

   procedure Insert_Internal
     (Container : in out List;
      Before    : Count_Type;
      New_Node  : Count_Type)
   is
   begin
      if Container.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;

      begin
         if Container.Length = 0 then
            pragma Assert (Before = 0);
            pragma Assert (Container.First = 0);
            pragma Assert (Container.Last = 0);

            Container.First := New_Node;
            Container.Last := New_Node;

            N (Container.First).Prev := 0;
            N (Container.Last).Next := 0;

         elsif Before = 0 then
            pragma Assert (N (Container.Last).Next = 0);

            N (Container.Last).Next := New_Node;
            N (New_Node).Prev := Container.Last;

            Container.Last := New_Node;
            N (Container.Last).Next := 0;

         elsif Before = Container.First then
            pragma Assert (N (Container.First).Prev = 0);

            N (Container.First).Prev := New_Node;
            N (New_Node).Next := Container.First;

            Container.First := New_Node;
            N (Container.First).Prev := 0;

         else
            pragma Assert (N (Container.First).Prev = 0);
            pragma Assert (N (Container.Last).Next = 0);

            N (New_Node).Next := Before;
            N (New_Node).Prev := N (Before).Prev;

            N (N (Before).Prev).Next := New_Node;
            N (Before).Prev := New_Node;
         end if;

         Container.Length := Container.Length + 1;
      end;
   end Insert_Internal;

   --------------
   -- Is_Empty --
   --------------

   function Is_Empty (Container : List) return Boolean is
   begin
      return Length (Container) = 0;
   end Is_Empty;

   -------------
   -- Iterate --
   -------------

   procedure Iterate_Between
     (Container : List;
      From      : Count_Type;
      To        : Count_Type;
      Process   :
        not null access procedure (Container : List; Position : Cursor))
   is
      C : Plain_List renames Container.Plain.all;
      N : Node_Array renames C.Nodes;
      B : Natural renames C.Busy;

      Node : Count_Type := From;

   begin
      B := B + 1;

      begin
         while Node /= N (To).Next loop
            pragma Assert (N (Node).Prev >= 0);
            Process (Container, Position => (Node => Node));
            Node := N (Node).Next;
         end loop;
      exception
         when others =>
            B := B - 1;
            raise;
      end;

      B := B - 1;
   end Iterate_Between;

   procedure Iterate
     (Container : List;
      Process   :
        not null access procedure (Container : List; Position : Cursor))
   is
   begin
      if Container.Length = 0 then
         return;
      end if;
      Iterate_Between (Container, Container.First, Container.Last, Process);
   end Iterate;

   ----------
   -- Last --
   ----------

   function Last (Container : List) return Cursor is
   begin
      if Container.Last = 0 then
         return No_Element;
      end if;
      return (Node => Container.Last);
   end Last;

   ------------------
   -- Last_Element --
   ------------------

   function Last_Element (Container : List) return Element_Type is
      L : constant Count_Type := Container.Last;
   begin
      if L = 0 then
         raise Constraint_Error with "list is empty";
      else
         return Element_Unchecked (Container, L);
      end if;
   end Last_Element;

   ----------
   -- Left --
   ----------

   function Left (Container : List; Position : Cursor) return List is
      L   : Count_Type := 0;
      C   : Count_Type := Container.First;
      LLe : Count_Type;
      LF  : Count_Type;
      LLa : Count_Type;
   begin
      case Container.K is
         when Plain =>
            LLe := Container.Length;
            LF := Container.First;
            LLa := Container.Last;
         when Part =>
            LLe := Container.Part.LLength;
            LF := Container.Part.LFirst;
            LLa := Container.Part.LLast;
      end case;
      if Position.Node = 0 then
         return (Capacity => Container.Capacity,
                 K        => Part,
                 Length   => Container.Length,
                 First    => Container.First,
                 Last     => Container.Last,
                 Plain    => Container.Plain,
                 Part     => (LLength => LLe, LFirst => LF, LLast => LLa));
      else
         while C /= Position.Node loop
            if C = Container.Last or C = 0 then
               raise Constraint_Error with
                 "Position cursor has no element";
            end if;
            C := Next_Unchecked (Container, C);
            L := L + 1;
         end loop;
         if L = 0 then
            return (Capacity => Container.Capacity,
                    K        => Part,
                    Length   => 0,
                    First    => 0,
                    Last     => 0,
                    Plain    => Container.Plain,
                    Part     => (LLength => LLe, LFirst => LF, LLast => LLa));
         else
            return (Capacity => Container.Capacity,
                    K        => Part,
                    Length   => L,
                    First    => Container.First,
                    Last     => Container.Plain.Nodes (C).Prev,
                    Plain    => Container.Plain,
                    Part     => (LLength => LLe, LFirst => LF, LLast => LLa));
         end if;
      end if;
   end Left;

   ------------
   -- Length --
   ------------

   function Length (Container : List) return Count_Type is
   begin
      return Container.Length;
   end Length;

   ----------
   -- Move --
   ----------

   procedure Move
     (Target : in out List;
      Source : in out List)
   is
   begin
      if Target.K /= Plain or Source.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare

         N : Node_Array renames Source.Plain.Nodes;
         X : Count_Type;

      begin
         if Target'Address = Source'Address then
            return;
         end if;

         if Target.Capacity < Source.Length then
            raise Constraint_Error with  -- ???
              "Source length exceeds Target capacity";
         end if;

         if Source.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with cursors of Source (list is busy)";
         end if;

         Clear (Target);

         while Source.Length > 0 loop
            X := Source.First;
            Append (Target, N (X).Element);  -- optimize away???

            Source.First := N (X).Next;
            N (Source.First).Prev := 0;

            Source.Length := Source.Length - 1;
            Free (Source.Plain.all, X);
         end loop;
      end;
   end Move;

   ----------
   -- Next --
   ----------

   procedure Next (Container : List; Position : in out Cursor) is
   begin
      Position := Next (Container, Position);
   end Next;

   function Next (Container : List; Position : Cursor) return Cursor is
   begin
      if Position.Node = 0 then
         return No_Element;
      end if;
      if not Has_Element (Container, Position) then
         raise Program_Error with "Position cursor has no element";
      end if;
      return (Node => Next_Unchecked (Container, Position.Node));
   end Next;

   function Next_Unchecked (Container : List; Position : Count_Type)
                            return Count_Type
   is
   begin
      case Container.K is
         when Plain =>
            return Container.Plain.Nodes (Position).Next;
         when Part =>
            if Position = Container.Last then
               return 0;
            else
               return Container.Plain.Nodes (Position).Next;
            end if;
      end case;
   end Next_Unchecked;

   -------------
   -- Prepend --
   -------------

   procedure Prepend
     (Container : in out List;
      New_Item  : Element_Type;
      Count     : Count_Type := 1)
   is
   begin
      Insert (Container, First (Container), New_Item, Count);
   end Prepend;

   --------------
   -- Previous --
   --------------

   procedure Previous (Container : List; Position : in out Cursor) is
   begin
      Position := Previous (Container, Position);
   end Previous;

   function Previous (Container : List; Position : Cursor) return Cursor is
   begin
      if Position.Node = 0 then
         return No_Element;
      end if;

      if not Has_Element (Container, Position) then
         raise Program_Error with "Position cursor has no element";
      end if;

      case Container.K is
         when Plain =>
            return (Node => Container.Plain.Nodes (Position.Node).Prev);
         when Part =>
            if Container.First = Position.Node then
               return No_Element;
            else
               return (Node => Container.Plain.Nodes (Position.Node).Prev);
            end if;
      end case;
   end Previous;

   -------------------
   -- Query_Element --
   -------------------

   procedure Query_Element_Plain
     (Container : Plain_List; Position : Cursor;
      Process   : not null access procedure (Element : Element_Type))
   is
      C : Plain_List renames Container'Unrestricted_Access.all;
      B : Natural renames C.Busy;
      L : Natural renames C.Lock;

   begin
      B := B + 1;
      L := L + 1;

      declare
         N : Node_Type renames C.Nodes (Position.Node);
      begin
         Process (N.Element);
      exception
         when others =>
            L := L - 1;
            B := B - 1;
            raise;
      end;

      L := L - 1;
      B := B - 1;
   end Query_Element_Plain;

   procedure Query_Element
     (Container : List; Position : Cursor;
      Process   : not null access procedure (Element : Element_Type))
   is
   begin
      if not Has_Element (Container, Position) then
         raise Constraint_Error with
           "Position cursor has no element";
      end if;
      Query_Element_Plain (Container.Plain.all, Position, Process);
   end Query_Element;

   ----------
   -- Read --
   ----------

   procedure Read
     (Stream : not null access Root_Stream_Type'Class;
      Item   : out List)
   is
      N : Count_Type'Base;

   begin
      Clear (Item);

      Count_Type'Base'Read (Stream, N);

      if N < 0 then
         raise Program_Error with "bad list length";
      end if;

      if N = 0 then
         return;
      end if;

      if N > Item.Capacity then
         raise Constraint_Error with "length exceeds capacity";
      end if;

      for J in 1 .. N loop
         Item.Append (Element_Type'Input (Stream));  -- ???
      end loop;
   end Read;

   procedure Read
     (Stream : not null access Root_Stream_Type'Class;
      Item   : out Cursor)
   is
   begin
      raise Program_Error with "attempt to stream list cursor";
   end Read;

   ---------------------
   -- Replace_Element --
   ---------------------

   procedure Replace_Element
     (Container : in out List;
      Position  : Cursor;
      New_Item  : Element_Type)
   is
   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      if not Has_Element (Container, Position) then
         raise Constraint_Error with "Position cursor has no element";
      end if;

      if Container.Plain.Lock > 0 then
         raise Program_Error with
           "attempt to tamper with cursors (list is locked)";
      end if;

      pragma Assert (Vet (Container, Position),
                     "bad cursor in Replace_Element");

      declare
         N : Node_Array renames Container.Plain.Nodes;
      begin
         N (Position.Node).Element := New_Item;
      end;
   end Replace_Element;

   ----------------------
   -- Reverse_Elements --
   ----------------------

   procedure Reverse_Elements (Container : in out List) is
   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;
         I : Count_Type := Container.First;
         J : Count_Type := Container.Last;

         procedure Swap (L, R : Count_Type);

         ----------
         -- Swap --
         ----------

         procedure Swap (L, R : Count_Type) is
            LN : constant Count_Type := N (L).Next;
            LP : constant Count_Type := N (L).Prev;

            RN : constant Count_Type := N (R).Next;
            RP : constant Count_Type := N (R).Prev;

         begin
            if LP /= 0 then
               N (LP).Next := R;
            end if;

            if RN /= 0 then
               N (RN).Prev := L;
            end if;

            N (L).Next := RN;
            N (R).Prev := LP;

            if LN = R then
               pragma Assert (RP = L);

               N (L).Prev := R;
               N (R).Next := L;

            else
               N (L).Prev := RP;
               N (RP).Next := L;

               N (R).Next := LN;
               N (LN).Prev := R;
            end if;
         end Swap;

         --  Start of processing for Reverse_Elements

      begin
         if Container.Length <= 1 then
            return;
         end if;

         pragma Assert (N (Container.First).Prev = 0);
         pragma Assert (N (Container.Last).Next = 0);

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         Container.First := J;
         Container.Last := I;
         loop
            Swap (L => I, R => J);

            J := N (J).Next;
            exit when I = J;

            I := N (I).Prev;
            exit when I = J;

            Swap (L => J, R => I);

            I := N (I).Next;
            exit when I = J;

            J := N (J).Prev;
            exit when I = J;
         end loop;

         pragma Assert (N (Container.First).Prev = 0);
         pragma Assert (N (Container.Last).Next = 0);
      end;
   end Reverse_Elements;

   ------------------
   -- Reverse_Find --
   ------------------

   function Reverse_Find_Between
     (Container : Plain_List;
      Item      : Element_Type;
      From      : Count_Type;
      To        : Count_Type) return Cursor
   is
      Nodes : Node_Array renames Container.Nodes;
      Node  : Count_Type := To;
   begin
      while Node /= Nodes (From).Prev loop
         if Nodes (Node).Element = Item then
            return (Node => Node);
         end if;
         Node := Nodes (Node).Prev;
      end loop;

      return No_Element;
   end Reverse_Find_Between;

   function Reverse_Find
     (Container : List;
      Item      : Element_Type;
      Position  : Cursor := No_Element) return Cursor
   is
      CFirst : Count_Type := Position.Node;
   begin
      if CFirst = 0 then
         CFirst := Container.First;
      end if;

      if Container.Length = 0 then
         return No_Element;
      end if;
      return Reverse_Find_Between (Container => Container.Plain.all,
                                   Item      => Item,
                                   From      => CFirst,
                                   To        => Container.Last);
   end Reverse_Find;

   ---------------------
   -- Reverse_Iterate --
   ---------------------

   procedure Reverse_Iterate_Between
     (Container : List;
      From      : Count_Type;
      To        : Count_Type;
      Process   :
        not null access procedure (Container : List; Position : Cursor))
   is
      C : Plain_List renames Container.Plain.all;
      N : Node_Array renames C.Nodes;
      B : Natural renames C.Busy;

      Node : Count_Type := To;

   begin
      B := B + 1;

      begin
         while Node /= N (From).Prev loop
            pragma Assert (N (Node).Prev >= 0);
            Process (Container, Position => (Node => Node));
            Node := N (Node).Prev;
         end loop;

      exception
         when others =>
            B := B - 1;
            raise;
      end;

      B := B - 1;
   end Reverse_Iterate_Between;

   procedure Reverse_Iterate
     (Container : List;
      Process   :
        not null access procedure (Container : List; Position : Cursor))
   is
   begin
      if Container.Length = 0 then
         return;
      end if;
      Reverse_Iterate_Between
        (Container, Container.First, Container.Last, Process);
   end Reverse_Iterate;

   -----------
   -- Right --
   -----------

   function Right (Container : List; Position : Cursor) return List is
      L   : Count_Type := 0;
      C   : Count_Type := Container.First;
      LLe : Count_Type;
      LF  : Count_Type;
      LLa : Count_Type;
   begin
      case Container.K is
         when Plain =>
            LLe := Container.Length;
            LF := Container.First;
            LLa := Container.Last;
         when Part =>
            LLe := Container.Part.LLength;
            LF := Container.Part.LFirst;
            LLa := Container.Part.LLast;
      end case;
      if Position.Node = 0 then
         return (Capacity => Container.Capacity,
                 K        => Part,
                 Length   => 0,
                 First    => 0,
                 Last     => 0,
                 Plain    => Container.Plain,
                 Part     => (LLength => LLe, LFirst => LF, LLast => LLa));
      else
         while C /= Position.Node loop
            if C = Container.Last or C = 0 then
               raise Constraint_Error with
                 "Position cursor has no element";
            end if;
            C := Next_Unchecked (Container, C);
            L := L + 1;
         end loop;
         return (Capacity => Container.Capacity,
                 K        => Part,
                 Length   => Container.Length - L,
                 First    => Position.Node,
                 Last     => Container.Last,
                 Plain    => Container.Plain,
                 Part     => (LLength => LLe, LFirst => LF, LLast => LLa));
      end if;
   end Right;

   ------------
   -- Splice --
   ------------

   procedure Splice
     (Target : in out List;
      Before : Cursor;
      Source : in out List)
   is
   begin
      if Target.K /= Plain or Source.K /= Plain then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         SN : Node_Array renames Source.Plain.Nodes;

      begin
         if Before.Node /= 0 then
            null;
            pragma Assert (Vet (Target, Before), "bad cursor in Splice");
         end if;

         if Target'Address = Source'Address
           or else Source.Length = 0
         then
            return;
         end if;

         pragma Assert (SN (Source.First).Prev = 0);
         pragma Assert (SN (Source.Last).Next = 0);

         if Target.Length > Count_Type'Base'Last - Source.Length then
            raise Constraint_Error with "new length exceeds maximum";
         end if;

         if Target.Length + Source.Length > Target.Capacity then
            raise Constraint_Error;
         end if;

         if Target.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with cursors of Target (list is busy)";
         end if;

         if Source.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with cursors of Source (list is busy)";
         end if;

         loop
            Insert (Target, Before, SN (Source.Last).Element);
            Delete_Last (Source);
            exit when Is_Empty (Source);
         end loop;
      end;
   end Splice;

   procedure Splice
     (Target   : in out List;
      Before   : Cursor;
      Source   : in out List;
      Position : in out Cursor)
   is
      Target_Position : Cursor;

   begin
      if Target.K /= Plain or Source.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      if Target'Address = Source'Address then
         Splice (Target, Before, Position);
         return;
      end if;

      if Position.Node = 0 then
         raise Constraint_Error with "Position cursor has no element";
      end if;

      pragma Assert (Vet (Source, Position), "bad Position cursor in Splice");

      if Target.Length >= Target.Capacity then
         raise Constraint_Error;
      end if;

      if Target.Plain.Busy > 0 then
         raise Program_Error with
           "attempt to tamper with cursors of Target (list is busy)";
      end if;

      if Source.Plain.Busy > 0 then
         raise Program_Error with
           "attempt to tamper with cursors of Source (list is busy)";
      end if;

      Insert
        (Container => Target,
         Before    => Before,
         New_Item  => Source.Plain.Nodes (Position.Node).Element,
         Position  => Target_Position);

      Delete (Source, Position);
      Position := Target_Position;
   end Splice;

   procedure Splice
     (Container : in out List;
      Before    : Cursor;
      Position  : Cursor)
   is
   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames Container.Plain.Nodes;

      begin
         if Before.Node /= 0 then
            null;
            pragma Assert (Vet (Container, Before),
                           "bad Before cursor in Splice");
         end if;

         if Position.Node = 0 then
            raise Constraint_Error with "Position cursor has no element";
         end if;

         pragma Assert (Vet (Container, Position),
                        "bad Position cursor in Splice");

         if Position.Node = Before.Node
           or else N (Position.Node).Next = Before.Node
         then
            return;
         end if;

         pragma Assert (Container.Length >= 2);

         if Container.Plain.Busy > 0 then
            raise Program_Error with
              "attempt to tamper with elements (list is busy)";
         end if;

         if Before.Node = 0 then
            pragma Assert (Position.Node /= Container.Last);

            if Position.Node = Container.First then
               Container.First := N (Position.Node).Next;
               N (Container.First).Prev := 0;

            else
               N (N (Position.Node).Prev).Next := N (Position.Node).Next;
               N (N (Position.Node).Next).Prev := N (Position.Node).Prev;
            end if;

            N (Container.Last).Next := Position.Node;
            N (Position.Node).Prev := Container.Last;

            Container.Last := Position.Node;
            N (Container.Last).Next := 0;

            return;
         end if;

         if Before.Node = Container.First then
            pragma Assert (Position.Node /= Container.First);

            if Position.Node = Container.Last then
               Container.Last := N (Position.Node).Prev;
               N (Container.Last).Next := 0;

            else
               N (N (Position.Node).Prev).Next := N (Position.Node).Next;
               N (N (Position.Node).Next).Prev := N (Position.Node).Prev;
            end if;

            N (Container.First).Prev := Position.Node;
            N (Position.Node).Next := Container.First;

            Container.First := Position.Node;
            N (Container.First).Prev := 0;

            return;
         end if;

         if Position.Node = Container.First then
            Container.First := N (Position.Node).Next;
            N (Container.First).Prev := 0;

         elsif Position.Node = Container.Last then
            Container.Last := N (Position.Node).Prev;
            N (Container.Last).Next := 0;

         else
            N (N (Position.Node).Prev).Next := N (Position.Node).Next;
            N (N (Position.Node).Next).Prev := N (Position.Node).Prev;
         end if;

         N (N (Before.Node).Prev).Next := Position.Node;
         N (Position.Node).Prev := N (Before.Node).Prev;

         N (Before.Node).Prev := Position.Node;
         N (Position.Node).Next := Before.Node;

         pragma Assert (N (Container.First).Prev = 0);
         pragma Assert (N (Container.Last).Next = 0);
      end;
   end Splice;

   ------------------
   -- Strict_Equal --
   ------------------

   function Strict_Equal (Left, Right : List) return Boolean is
      CL : Count_Type := Left.First;
      CR : Count_Type := Right.First;
   begin
      while CL /= 0 or CR /= 0 loop
         if CL /= CR or else
           Element_Unchecked (Left, CL) /= Element_Unchecked (Right, CL) then
            return False;
         end if;
         CL := Next_Unchecked (Left, CL);
         CR := Next_Unchecked (Right, CR);
      end loop;
      return True;
   end Strict_Equal;

   ----------
   -- Swap --
   ----------

   procedure Swap
     (Container : in out List;
      I, J      : Cursor)
   is
   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      if I.Node = 0 then
         raise Constraint_Error with "I cursor has no element";
      end if;

      if J.Node = 0 then
         raise Constraint_Error with "J cursor has no element";
      end if;

      if I.Node = J.Node then
         return;
      end if;

      if Container.Plain.Lock > 0 then
         raise Program_Error with
           "attempt to tamper with cursors (list is locked)";
      end if;

      pragma Assert (Vet (Container, I), "bad I cursor in Swap");
      pragma Assert (Vet (Container, J), "bad J cursor in Swap");

      declare
         NN : Node_Array renames Container.Plain.Nodes;
         NI : Node_Type renames NN (I.Node);
         NJ : Node_Type renames NN (J.Node);

         EI_Copy : constant Element_Type := NI.Element;

      begin
         NI.Element := NJ.Element;
         NJ.Element := EI_Copy;
      end;
   end Swap;

   ----------------
   -- Swap_Links --
   ----------------

   procedure Swap_Links
     (Container : in out List;
      I, J      : Cursor)
   is
      I_Next, J_Next : Cursor;

   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      if I.Node = 0 then
         raise Constraint_Error with "I cursor has no element";
      end if;

      if J.Node = 0 then
         raise Constraint_Error with "J cursor has no element";
      end if;

      if I.Node = J.Node then
         return;
      end if;

      if Container.Plain.Busy > 0 then
         raise Program_Error with
           "attempt to tamper with elements (list is busy)";
      end if;

      pragma Assert (Vet (Container, I), "bad I cursor in Swap_Links");
      pragma Assert (Vet (Container, J), "bad J cursor in Swap_Links");

      I_Next := Next (Container, I);

      if I_Next = J then
         Splice (Container, Before => I, Position => J);

      else
         J_Next := Next (Container, J);

         if J_Next = I then
            Splice (Container, Before => J, Position => I);

         else
            pragma Assert (Container.Length >= 3);
            Splice (Container, Before => I_Next, Position => J);
            Splice (Container, Before => J_Next, Position => I);
         end if;
      end if;
   end Swap_Links;

   --------------------
   -- Update_Element --
   --------------------

   procedure Update_Element
     (Container : in out List;
      Position  : Cursor;
      Process   : not null access procedure (Element : in out Element_Type))
   is
   begin
      if Container.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      if Position.Node = 0 then
         raise Constraint_Error with "Position cursor has no element";
      end if;

      pragma Assert (Vet (Container, Position),
                     "bad cursor in Update_Element");

      declare
         B : Natural renames Container.Plain.Busy;
         L : Natural renames Container.Plain.Lock;

      begin
         B := B + 1;
         L := L + 1;

         declare
            N : Node_Type renames Container.Plain.Nodes (Position.Node);
         begin
            Process (N.Element);
         exception
            when others =>
               L := L - 1;
               B := B - 1;
               raise;
         end;

         L := L - 1;
         B := B - 1;
      end;
   end Update_Element;

   ---------
   -- Vet --
   ---------

   function Vet (L : List; Position : Cursor) return Boolean is
   begin
      if L.K /= Plain  then
         raise Program_Error with "cannot modify part of container";
      end if;

      declare
         N : Node_Array renames L.Plain.Nodes;

      begin
         if L.Length = 0 then
            return False;
         end if;

         if L.First = 0 then
            return False;
         end if;

         if L.Last = 0 then
            return False;
         end if;

         if Position.Node > L.Capacity then
            return False;
         end if;

         if N (Position.Node).Prev < 0
           or else N (Position.Node).Prev > L.Capacity
         then
            return False;
         end if;

         if N (Position.Node).Next > L.Capacity then
            return False;
         end if;

         if N (L.First).Prev /= 0 then
            return False;
         end if;

         if N (L.Last).Next /= 0 then
            return False;
         end if;

         if N (Position.Node).Prev = 0
           and then Position.Node /= L.First
         then
            return False;
         end if;

         if N (Position.Node).Next = 0
           and then Position.Node /= L.Last
         then
            return False;
         end if;

         if L.Length = 1 then
            return L.First = L.Last;
         end if;

         if L.First = L.Last then
            return False;
         end if;

         if N (L.First).Next = 0 then
            return False;
         end if;

         if N (L.Last).Prev = 0 then
            return False;
         end if;

         if N (N (L.First).Next).Prev /= L.First then
            return False;
         end if;

         if N (N (L.Last).Prev).Next /= L.Last then
            return False;
         end if;

         if L.Length = 2 then
            if N (L.First).Next /= L.Last then
               return False;
            end if;

            if N (L.Last).Prev /= L.First then
               return False;
            end if;

            return True;
         end if;

         if N (L.First).Next = L.Last then
            return False;
         end if;

         if N (L.Last).Prev = L.First then
            return False;
         end if;

         if Position.Node = L.First then
            return True;
         end if;

         if Position.Node = L.Last then
            return True;
         end if;

         if N (Position.Node).Next = 0 then
            return False;
         end if;

         if N (Position.Node).Prev = 0 then
            return False;
         end if;

         if N (N (Position.Node).Next).Prev /= Position.Node then
            return False;
         end if;

         if N (N (Position.Node).Prev).Next /= Position.Node then
            return False;
         end if;

         if L.Length = 3 then
            if N (L.First).Next /= Position.Node then
               return False;
            end if;

            if N (L.Last).Prev /= Position.Node then
               return False;
            end if;
         end if;

         return True;
      end;
   end Vet;

   -----------
   -- Write --
   -----------

   procedure Write_Between
     (Stream : not null access Root_Stream_Type'Class;
      Item   : Plain_List;
      Length : Count_Type;
      From   : Count_Type;
      To     : Count_Type) is

      N    : Node_Array renames Item.Nodes;
      Node : Count_Type;

   begin
      Count_Type'Base'Write (Stream, Length);

      Node := From;
      while Node /= N (To).Next loop
         Element_Type'Write (Stream, N (Node).Element);
         Node := N (Node).Next;
      end loop;
   end Write_Between;

   procedure Write
     (Stream : not null access Root_Stream_Type'Class;
      Item   : List)
   is
   begin
      Write_Between
        (Stream, Item.Plain.all, Item.Length, Item.First, Item.Last);
   end Write;

   procedure Write
     (Stream : not null access Root_Stream_Type'Class;
      Item   : Cursor)
   is
   begin
      raise Program_Error with "attempt to stream list cursor";
   end Write;

end Ada.Containers.Formal_Doubly_Linked_Lists;