------------------------------------------------------------------------------
--                                                                          --
--                         GNAT COMPILER COMPONENTS                         --
--                                                                          --
--                               N L I S T S                                --
--                                                                          --
--                                 B o d y                                  --
--                                                                          --
--          Copyright (C) 1992-2021, 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.  See the GNU General Public License --
-- for  more details.  You should have  received  a copy of the GNU General --
-- Public License  distributed with GNAT; see file COPYING3.  If not, go to --
-- http://www.gnu.org/licenses for a complete copy of the license.          --
--                                                                          --
-- GNAT was originally developed  by the GNAT team at  New York University. --
-- Extensive contributions were provided by Ada Core Technologies Inc.      --
--                                                                          --
------------------------------------------------------------------------------

--  WARNING: There is a C version of this package. Any changes to this source
--  file must be properly reflected in the corresponding C header a-nlists.h

with Alloc;
with Atree;       use Atree;
with Debug;       use Debug;
with Output;      use Output;
with Sinfo;       use Sinfo;
with Sinfo.Nodes; use Sinfo.Nodes;
with Table;

package body Nlists is
   Locked : Boolean := False;
   --  Compiling with assertions enabled, list contents modifications are
   --  permitted only when this switch is set to False; compiling without
   --  assertions this lock has no effect.

   ----------------------------------
   -- Implementation of Node Lists --
   ----------------------------------

   --  A node list is represented by a list header which contains
   --  three fields:

   type List_Header is record
      First : Node_Or_Entity_Id;
      --  Pointer to first node in list. Empty if list is empty

      Last : Node_Or_Entity_Id;
      --  Pointer to last node in list. Empty if list is empty

      Parent : Node_Id;
      --  Pointer to parent of list. Empty if list has no parent
   end record;

   --  The node lists are stored in a table indexed by List_Id values

   package Lists is new Table.Table (
     Table_Component_Type => List_Header,
     Table_Index_Type     => List_Id'Base,
     Table_Low_Bound      => First_List_Id,
     Table_Initial        => Alloc.Lists_Initial,
     Table_Increment      => Alloc.Lists_Increment,
     Table_Name           => "Lists");

   --  The nodes in the list all have the In_List flag set, and their Link
   --  fields (which otherwise point to the parent) contain the List_Id of
   --  the list header giving immediate access to the list containing the
   --  node, and its parent and first and last elements.

   --  Two auxiliary tables, indexed by Node_Id values and built in parallel
   --  with the main nodes table and always having the same size contain the
   --  list link values that allow locating the previous and next node in a
   --  list. The entries in these tables are valid only if the In_List flag
   --  is set in the corresponding node. Next_Node is Empty at the end of a
   --  list and Prev_Node is Empty at the start of a list.

   package Next_Node is new Table.Table (
      Table_Component_Type => Node_Or_Entity_Id,
      Table_Index_Type     => Node_Or_Entity_Id'Base,
      Table_Low_Bound      => First_Node_Id,
      Table_Initial        => Alloc.Node_Offsets_Initial,
      Table_Increment      => Alloc.Node_Offsets_Increment,
      Table_Name           => "Next_Node");

   package Prev_Node is new Table.Table (
      Table_Component_Type => Node_Or_Entity_Id,
      Table_Index_Type     => Node_Or_Entity_Id'Base,
      Table_Low_Bound      => First_Node_Id,
      Table_Initial        => Alloc.Node_Offsets_Initial,
      Table_Increment      => Alloc.Node_Offsets_Increment,
      Table_Name           => "Prev_Node");

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

   procedure Set_First (List : List_Id; To : Node_Or_Entity_Id);
   pragma Inline (Set_First);
   --  Sets First field of list header List to reference To

   procedure Set_Last (List : List_Id; To : Node_Or_Entity_Id);
   pragma Inline (Set_Last);
   --  Sets Last field of list header List to reference To

   procedure Set_List_Link (Node : Node_Or_Entity_Id; To : List_Id);
   pragma Inline (Set_List_Link);
   --  Sets list link of Node to list header To

   procedure Set_Next (Node : Node_Or_Entity_Id; To : Node_Or_Entity_Id);
   pragma Inline (Set_Next);
   --  Sets the Next_Node pointer for Node to reference To

   procedure Set_Prev (Node : Node_Or_Entity_Id; To : Node_Or_Entity_Id);
   pragma Inline (Set_Prev);
   --  Sets the Prev_Node pointer for Node to reference To

   --------------------------
   -- Allocate_List_Tables --
   --------------------------

   procedure Allocate_List_Tables (N : Node_Or_Entity_Id) is
      Old_Last : constant Node_Or_Entity_Id'Base := Next_Node.Last;

   begin
      pragma Assert (N >= Old_Last);
      Next_Node.Set_Last (N);
      Prev_Node.Set_Last (N);

      --  Make sure we have no uninitialized junk in any new entries added.

      for J in Old_Last + 1 .. N loop
         Next_Node.Table (J) := Empty;
         Prev_Node.Table (J) := Empty;
      end loop;
   end Allocate_List_Tables;

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

   procedure Append (Node : Node_Or_Entity_Id; To : List_Id) is
      L : constant Node_Or_Entity_Id := Last (To);

      procedure Append_Debug;
      pragma Inline (Append_Debug);
      --  Output debug information if Debug_Flag_N set

      ------------------
      -- Append_Debug --
      ------------------

      procedure Append_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Append node ");
            Write_Int (Int (Node));
            Write_Str (" to list ");
            Write_Int (Int (To));
            Write_Eol;
         end if;
      end Append_Debug;

   --  Start of processing for Append

   begin
      pragma Assert (not Is_List_Member (Node));

      if Node = Error then
         return;
      end if;

      pragma Debug (Append_Debug);

      if No (L) then
         Set_First (To, Node);
      else
         Set_Next (L, Node);
      end if;

      Set_Last (To, Node);

      Set_In_List (Node, True);

      Set_Next      (Node, Empty);
      Set_Prev      (Node, L);
      Set_List_Link (Node, To);
   end Append;

   -----------------
   -- Append_List --
   -----------------

   procedure Append_List (List : List_Id; To : List_Id) is
      procedure Append_List_Debug;
      pragma Inline (Append_List_Debug);
      --  Output debug information if Debug_Flag_N set

      -----------------------
      -- Append_List_Debug --
      -----------------------

      procedure Append_List_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Append list ");
            Write_Int (Int (List));
            Write_Str (" to list ");
            Write_Int (Int (To));
            Write_Eol;
         end if;
      end Append_List_Debug;

   --  Start of processing for Append_List

   begin
      if Is_Empty_List (List) then
         return;

      else
         declare
            L : constant Node_Or_Entity_Id := Last (To);
            F : constant Node_Or_Entity_Id := First (List);
            N : Node_Or_Entity_Id;

         begin
            pragma Debug (Append_List_Debug);

            N := F;
            loop
               Set_List_Link (N, To);
               Next (N);
               exit when No (N);
            end loop;

            if No (L) then
               Set_First (To, F);
            else
               Set_Next (L, F);
            end if;

            Set_Prev (F, L);
            Set_Last (To, Last (List));

            Set_First (List, Empty);
            Set_Last  (List, Empty);
         end;
      end if;
   end Append_List;

   --------------------
   -- Append_List_To --
   --------------------

   procedure Append_List_To (To : List_Id; List : List_Id) is
   begin
      Append_List (List, To);
   end Append_List_To;

   ----------------
   -- Append_New --
   ----------------

   procedure Append_New (Node : Node_Or_Entity_Id; To : in out List_Id) is
   begin
      if No (To) then
         To := New_List;
      end if;

      Append (Node, To);
   end Append_New;

   -------------------
   -- Append_New_To --
   -------------------

   procedure Append_New_To (To : in out List_Id; Node : Node_Or_Entity_Id) is
   begin
      Append_New (Node, To);
   end Append_New_To;

   ---------------
   -- Append_To --
   ---------------

   procedure Append_To (To : List_Id; Node : Node_Or_Entity_Id) is
   begin
      Append (Node, To);
   end Append_To;

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

   function First (List : List_Id) return Node_Or_Entity_Id is
   begin
      if List = No_List then
         return Empty;
      else
         pragma Assert (List <= Lists.Last);
         return Lists.Table (List).First;
      end if;
   end First;

   ----------------------
   -- First_Non_Pragma --
   ----------------------

   function First_Non_Pragma (List : List_Id) return Node_Or_Entity_Id is
      N : constant Node_Or_Entity_Id := First (List);
   begin
      if Nkind (N) /= N_Pragma
           and then
         Nkind (N) /= N_Null_Statement
      then
         return N;
      else
         return Next_Non_Pragma (N);
      end if;
   end First_Non_Pragma;

   ----------------
   -- Initialize --
   ----------------

   procedure Initialize is
   begin
      Lists.Init;
      Next_Node.Init;
      Prev_Node.Init;

      --  Allocate Error_List list header

      Lists.Increment_Last;
      Set_Parent (Error_List, Empty);
      Set_First  (Error_List, Empty);
      Set_Last   (Error_List, Empty);
   end Initialize;

   ------------------
   -- In_Same_List --
   ------------------

   function In_Same_List (N1, N2 : Node_Or_Entity_Id) return Boolean is
   begin
      return List_Containing (N1) = List_Containing (N2);
   end In_Same_List;

   ------------------
   -- Insert_After --
   ------------------

   procedure Insert_After
     (After : Node_Or_Entity_Id;
      Node  : Node_Or_Entity_Id)
   is
      procedure Insert_After_Debug;
      pragma Inline (Insert_After_Debug);
      --  Output debug information if Debug_Flag_N set

      ------------------------
      -- Insert_After_Debug --
      ------------------------

      procedure Insert_After_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Insert node");
            Write_Int (Int (Node));
            Write_Str (" after node ");
            Write_Int (Int (After));
            Write_Eol;
         end if;
      end Insert_After_Debug;

   --  Start of processing for Insert_After

   begin
      pragma Assert
        (Is_List_Member (After) and then not Is_List_Member (Node));

      if Node = Error then
         return;
      end if;

      pragma Debug (Insert_After_Debug);

      declare
         Before : constant Node_Or_Entity_Id := Next (After);
         LC     : constant List_Id           := List_Containing (After);

      begin
         if Present (Before) then
            Set_Prev (Before, Node);
         else
            Set_Last (LC, Node);
         end if;

         Set_Next (After, Node);

         Set_In_List (Node, True);

         Set_Prev      (Node, After);
         Set_Next      (Node, Before);
         Set_List_Link (Node, LC);
      end;
   end Insert_After;

   -------------------
   -- Insert_Before --
   -------------------

   procedure Insert_Before
     (Before : Node_Or_Entity_Id;
      Node   : Node_Or_Entity_Id)
   is
      procedure Insert_Before_Debug;
      pragma Inline (Insert_Before_Debug);
      --  Output debug information if Debug_Flag_N set

      -------------------------
      -- Insert_Before_Debug --
      -------------------------

      procedure Insert_Before_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Insert node");
            Write_Int (Int (Node));
            Write_Str (" before node ");
            Write_Int (Int (Before));
            Write_Eol;
         end if;
      end Insert_Before_Debug;

   --  Start of processing for Insert_Before

   begin
      pragma Assert
        (Is_List_Member (Before) and then not Is_List_Member (Node));

      if Node = Error then
         return;
      end if;

      pragma Debug (Insert_Before_Debug);

      declare
         After : constant Node_Or_Entity_Id := Prev (Before);
         LC    : constant List_Id           := List_Containing (Before);

      begin
         if Present (After) then
            Set_Next (After, Node);
         else
            Set_First (LC, Node);
         end if;

         Set_Prev (Before, Node);

         Set_In_List (Node, True);

         Set_Prev      (Node, After);
         Set_Next      (Node, Before);
         Set_List_Link (Node, LC);
      end;
   end Insert_Before;

   -----------------------
   -- Insert_List_After --
   -----------------------

   procedure Insert_List_After (After : Node_Or_Entity_Id; List : List_Id) is

      procedure Insert_List_After_Debug;
      pragma Inline (Insert_List_After_Debug);
      --  Output debug information if Debug_Flag_N set

      -----------------------------
      -- Insert_List_After_Debug --
      -----------------------------

      procedure Insert_List_After_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Insert list ");
            Write_Int (Int (List));
            Write_Str (" after node ");
            Write_Int (Int (After));
            Write_Eol;
         end if;
      end Insert_List_After_Debug;

   --  Start of processing for Insert_List_After

   begin
      pragma Assert (Is_List_Member (After));

      if Is_Empty_List (List) then
         return;

      else
         declare
            Before : constant Node_Or_Entity_Id := Next (After);
            LC     : constant List_Id           := List_Containing (After);
            F      : constant Node_Or_Entity_Id := First (List);
            L      : constant Node_Or_Entity_Id := Last (List);
            N      : Node_Or_Entity_Id;

         begin
            pragma Debug (Insert_List_After_Debug);

            N := F;
            loop
               Set_List_Link (N, LC);
               exit when N = L;
               Next (N);
            end loop;

            if Present (Before) then
               Set_Prev (Before, L);
            else
               Set_Last (LC, L);
            end if;

            Set_Next (After, F);
            Set_Prev (F, After);
            Set_Next (L, Before);

            Set_First (List, Empty);
            Set_Last  (List, Empty);
         end;
      end if;
   end Insert_List_After;

   ------------------------
   -- Insert_List_Before --
   ------------------------

   procedure Insert_List_Before (Before : Node_Or_Entity_Id; List : List_Id) is

      procedure Insert_List_Before_Debug;
      pragma Inline (Insert_List_Before_Debug);
      --  Output debug information if Debug_Flag_N set

      ------------------------------
      -- Insert_List_Before_Debug --
      ------------------------------

      procedure Insert_List_Before_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Insert list ");
            Write_Int (Int (List));
            Write_Str (" before node ");
            Write_Int (Int (Before));
            Write_Eol;
         end if;
      end Insert_List_Before_Debug;

   --  Start of processing for Insert_List_Before

   begin
      pragma Assert (Is_List_Member (Before));

      if Is_Empty_List (List) then
         return;

      else
         declare
            After : constant Node_Or_Entity_Id := Prev (Before);
            LC    : constant List_Id           := List_Containing (Before);
            F     : constant Node_Or_Entity_Id := First (List);
            L     : constant Node_Or_Entity_Id := Last (List);
            N     : Node_Or_Entity_Id;

         begin
            pragma Debug (Insert_List_Before_Debug);

            N := F;
            loop
               Set_List_Link (N, LC);
               exit when N = L;
               Next (N);
            end loop;

            if Present (After) then
               Set_Next (After, F);
            else
               Set_First (LC, F);
            end if;

            Set_Prev (Before, L);
            Set_Prev (F, After);
            Set_Next (L, Before);

            Set_First (List, Empty);
            Set_Last  (List, Empty);
         end;
      end if;
   end Insert_List_Before;

   -------------------
   -- Is_Empty_List --
   -------------------

   function Is_Empty_List (List : List_Id) return Boolean is
   begin
      return First (List) = Empty;
   end Is_Empty_List;

   --------------------
   -- Is_List_Member --
   --------------------

   function Is_List_Member (Node : Node_Or_Entity_Id) return Boolean is
   begin
      return In_List (Node);
   end Is_List_Member;

   -----------------------
   -- Is_Non_Empty_List --
   -----------------------

   function Is_Non_Empty_List (List : List_Id) return Boolean is
   begin
      return First (List) /= Empty;
   end Is_Non_Empty_List;

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

   function Last (List : List_Id) return Node_Or_Entity_Id is
   begin
      pragma Assert (List <= Lists.Last);
      return Lists.Table (List).Last;
   end Last;

   ------------------
   -- Last_List_Id --
   ------------------

   function Last_List_Id return List_Id is
   begin
      return Lists.Last;
   end Last_List_Id;

   ---------------------
   -- Last_Non_Pragma --
   ---------------------

   function Last_Non_Pragma (List : List_Id) return Node_Or_Entity_Id is
      N : constant Node_Or_Entity_Id := Last (List);
   begin
      if Nkind (N) /= N_Pragma then
         return N;
      else
         return Prev_Non_Pragma (N);
      end if;
   end Last_Non_Pragma;

   ---------------------
   -- List_Containing --
   ---------------------

   function List_Containing (Node : Node_Or_Entity_Id) return List_Id is
   begin
      pragma Assert (Is_List_Member (Node));
      return List_Id (Link (Node));
   end List_Containing;

   -----------------
   -- List_Length --
   -----------------

   function List_Length (List : List_Id) return Nat is
      Result : Nat;
      Node   : Node_Or_Entity_Id;

   begin
      Result := 0;
      Node := First (List);
      while Present (Node) loop
         Result := Result + 1;
         Next (Node);
      end loop;

      return Result;
   end List_Length;

   -------------------
   -- Lists_Address --
   -------------------

   function Lists_Address return System.Address is
   begin
      return Lists.Table (First_List_Id)'Address;
   end Lists_Address;

   ----------
   -- Lock --
   ----------

   procedure Lock is
   begin
      Lists.Release;
      Lists.Locked := True;
      Prev_Node.Release;
      Prev_Node.Locked := True;
      Next_Node.Release;
      Next_Node.Locked := True;
   end Lock;

   ----------------
   -- Lock_Lists --
   ----------------

   procedure Lock_Lists is
   begin
      pragma Assert (not Locked);
      Locked := True;
   end Lock_Lists;

   -------------------
   -- New_Copy_List --
   -------------------

   function New_Copy_List (List : List_Id) return List_Id is
      NL : List_Id;
      E  : Node_Or_Entity_Id;

   begin
      if List = No_List then
         return No_List;

      else
         NL := New_List;
         E := First (List);

         while Present (E) loop
            Append (New_Copy (E), NL);
            Next (E);
         end loop;

         return NL;
      end if;
   end New_Copy_List;

   ----------------------------
   -- New_Copy_List_Original --
   ----------------------------

   function New_Copy_List_Original (List : List_Id) return List_Id is
      NL : List_Id;
      E  : Node_Or_Entity_Id;

   begin
      if List = No_List then
         return No_List;

      else
         NL := New_List;

         E := First (List);
         while Present (E) loop
            if Comes_From_Source (E) then
               Append (New_Copy (E), NL);
            end if;

            Next (E);
         end loop;

         return NL;
      end if;
   end New_Copy_List_Original;

   --------------
   -- New_List --
   --------------

   function New_List return List_Id is

      procedure New_List_Debug;
      pragma Inline (New_List_Debug);
      --  Output debugging information if Debug_Flag_N is set

      --------------------
      -- New_List_Debug --
      --------------------

      procedure New_List_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Allocate new list, returned ID = ");
            Write_Int (Int (Lists.Last));
            Write_Eol;
         end if;
      end New_List_Debug;

   --  Start of processing for New_List

   begin
      Lists.Increment_Last;

      declare
         List : constant List_Id := Lists.Last;

      begin
         Set_Parent (List, Empty);
         Set_First  (List, Empty);
         Set_Last   (List, Empty);

         pragma Debug (New_List_Debug);
         return (List);
      end;
   end New_List;

   --  Since the one argument case is common, we optimize to build the right
   --  list directly, rather than first building an empty list and then doing
   --  the insertion, which results in some unnecessary work.

   function New_List (Node : Node_Or_Entity_Id) return List_Id is

      procedure New_List_Debug;
      pragma Inline (New_List_Debug);
      --  Output debugging information if Debug_Flag_N is set

      --------------------
      -- New_List_Debug --
      --------------------

      procedure New_List_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Allocate new list, returned ID = ");
            Write_Int (Int (Lists.Last));
            Write_Eol;
         end if;
      end New_List_Debug;

   --  Start of processing for New_List

   begin
      if Node = Error then
         return New_List;

      else
         pragma Assert (not Is_List_Member (Node));

         Lists.Increment_Last;

         declare
            List : constant List_Id := Lists.Last;

         begin
            Set_Parent (List, Empty);
            Set_First  (List, Node);
            Set_Last   (List, Node);

            Set_In_List (Node, True);
            Set_List_Link (Node, List);
            Set_Prev (Node, Empty);
            Set_Next (Node, Empty);
            pragma Debug (New_List_Debug);
            return List;
         end;
      end if;
   end New_List;

   function New_List
     (Node1 : Node_Or_Entity_Id;
      Node2 : Node_Or_Entity_Id) return List_Id
   is
      L : constant List_Id := New_List (Node1);
   begin
      Append (Node2, L);
      return L;
   end New_List;

   function New_List
     (Node1 : Node_Or_Entity_Id;
      Node2 : Node_Or_Entity_Id;
      Node3 : Node_Or_Entity_Id) return List_Id
   is
      L : constant List_Id := New_List (Node1);
   begin
      Append (Node2, L);
      Append (Node3, L);
      return L;
   end New_List;

   function New_List
     (Node1 : Node_Or_Entity_Id;
      Node2 : Node_Or_Entity_Id;
      Node3 : Node_Or_Entity_Id;
      Node4 : Node_Or_Entity_Id) return List_Id
   is
      L : constant List_Id := New_List (Node1);
   begin
      Append (Node2, L);
      Append (Node3, L);
      Append (Node4, L);
      return L;
   end New_List;

   function New_List
     (Node1 : Node_Or_Entity_Id;
      Node2 : Node_Or_Entity_Id;
      Node3 : Node_Or_Entity_Id;
      Node4 : Node_Or_Entity_Id;
      Node5 : Node_Or_Entity_Id) return List_Id
   is
      L : constant List_Id := New_List (Node1);
   begin
      Append (Node2, L);
      Append (Node3, L);
      Append (Node4, L);
      Append (Node5, L);
      return L;
   end New_List;

   function New_List
     (Node1 : Node_Or_Entity_Id;
      Node2 : Node_Or_Entity_Id;
      Node3 : Node_Or_Entity_Id;
      Node4 : Node_Or_Entity_Id;
      Node5 : Node_Or_Entity_Id;
      Node6 : Node_Or_Entity_Id) return List_Id
   is
      L : constant List_Id := New_List (Node1);
   begin
      Append (Node2, L);
      Append (Node3, L);
      Append (Node4, L);
      Append (Node5, L);
      Append (Node6, L);
      return L;
   end New_List;

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

   function Next (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id is
   begin
      pragma Assert (Is_List_Member (Node));
      return Next_Node.Table (Node);
   end Next;

   procedure Next (Node : in out Node_Or_Entity_Id) is
   begin
      Node := Next (Node);
   end Next;

   -----------------------
   -- Next_Node_Address --
   -----------------------

   function Next_Node_Address return System.Address is
   begin
      return Next_Node.Table (First_Node_Id)'Address;
   end Next_Node_Address;

   ---------------------
   -- Next_Non_Pragma --
   ---------------------

   function Next_Non_Pragma
     (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id
   is
      N : Node_Or_Entity_Id;

   begin
      N := Node;
      loop
         Next (N);
         exit when Nkind (N) not in N_Pragma | N_Null_Statement;
      end loop;

      return N;
   end Next_Non_Pragma;

   procedure Next_Non_Pragma (Node : in out Node_Or_Entity_Id) is
   begin
      Node := Next_Non_Pragma (Node);
   end Next_Non_Pragma;

   --------
   -- No --
   --------

   function No (List : List_Id) return Boolean is
   begin
      return List = No_List;
   end No;

   ---------------
   -- Num_Lists --
   ---------------

   function Num_Lists return Nat is
   begin
      return Int (Lists.Last) - Int (Lists.First) + 1;
   end Num_Lists;

   ------------
   -- Parent --
   ------------

   function Parent (List : List_Id) return Node_Or_Entity_Id is
   begin
      pragma Assert (Present (List));
      pragma Assert (List <= Lists.Last);
      return Lists.Table (List).Parent;
   end Parent;

   ----------
   -- Pick --
   ----------

   function Pick (List : List_Id; Index : Pos) return Node_Or_Entity_Id is
      Elmt : Node_Or_Entity_Id;

   begin
      Elmt := First (List);
      for J in 1 .. Index - 1 loop
         Next (Elmt);
      end loop;

      return Elmt;
   end Pick;

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

   procedure Prepend (Node : Node_Or_Entity_Id; To : List_Id) is
      F : constant Node_Or_Entity_Id := First (To);

      procedure Prepend_Debug;
      pragma Inline (Prepend_Debug);
      --  Output debug information if Debug_Flag_N set

      -------------------
      -- Prepend_Debug --
      -------------------

      procedure Prepend_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Prepend node ");
            Write_Int (Int (Node));
            Write_Str (" to list ");
            Write_Int (Int (To));
            Write_Eol;
         end if;
      end Prepend_Debug;

   --  Start of processing for Prepend_Debug

   begin
      pragma Assert (not Is_List_Member (Node));

      if Node = Error then
         return;
      end if;

      pragma Debug (Prepend_Debug);

      if No (F) then
         Set_Last (To, Node);
      else
         Set_Prev (F, Node);
      end if;

      Set_First (To, Node);

      Set_In_List (Node, True);

      Set_Next      (Node, F);
      Set_Prev      (Node, Empty);
      Set_List_Link (Node, To);
   end Prepend;

   ------------------
   -- Prepend_List --
   ------------------

   procedure Prepend_List (List : List_Id; To : List_Id) is

      procedure Prepend_List_Debug;
      pragma Inline (Prepend_List_Debug);
      --  Output debug information if Debug_Flag_N set

      ------------------------
      -- Prepend_List_Debug --
      ------------------------

      procedure Prepend_List_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Prepend list ");
            Write_Int (Int (List));
            Write_Str (" to list ");
            Write_Int (Int (To));
            Write_Eol;
         end if;
      end Prepend_List_Debug;

   --  Start of processing for Prepend_List

   begin
      if Is_Empty_List (List) then
         return;

      else
         declare
            F : constant Node_Or_Entity_Id := First (To);
            L : constant Node_Or_Entity_Id := Last (List);
            N : Node_Or_Entity_Id;

         begin
            pragma Debug (Prepend_List_Debug);

            N := L;
            loop
               Set_List_Link (N, To);
               N := Prev (N);
               exit when No (N);
            end loop;

            if No (F) then
               Set_Last (To, L);
            else
               Set_Next (L, F);
            end if;

            Set_Prev (F, L);
            Set_First (To, First (List));

            Set_First (List, Empty);
            Set_Last  (List, Empty);
         end;
      end if;
   end Prepend_List;

   ---------------------
   -- Prepend_List_To --
   ---------------------

   procedure Prepend_List_To (To : List_Id; List : List_Id) is
   begin
      Prepend_List (List, To);
   end Prepend_List_To;

   -----------------
   -- Prepend_New --
   -----------------

   procedure Prepend_New (Node : Node_Or_Entity_Id; To : in out List_Id) is
   begin
      if No (To) then
         To := New_List;
      end if;

      Prepend (Node, To);
   end Prepend_New;

   --------------------
   -- Prepend_New_To --
   --------------------

   procedure Prepend_New_To (To : in out List_Id; Node : Node_Or_Entity_Id) is
   begin
      Prepend_New (Node, To);
   end Prepend_New_To;

   ----------------
   -- Prepend_To --
   ----------------

   procedure Prepend_To (To : List_Id; Node : Node_Or_Entity_Id) is
   begin
      Prepend (Node, To);
   end Prepend_To;

   -------------
   -- Present --
   -------------

   function Present (List : List_Id) return Boolean is
   begin
      return List /= No_List;
   end Present;

   ----------
   -- Prev --
   ----------

   function Prev (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id is
   begin
      pragma Assert (Is_List_Member (Node));
      return Prev_Node.Table (Node);
   end Prev;

   procedure Prev (Node : in out Node_Or_Entity_Id) is
   begin
      Node := Prev (Node);
   end Prev;

   -----------------------
   -- Prev_Node_Address --
   -----------------------

   function Prev_Node_Address return System.Address is
   begin
      return Prev_Node.Table (First_Node_Id)'Address;
   end Prev_Node_Address;

   ---------------------
   -- Prev_Non_Pragma --
   ---------------------

   function Prev_Non_Pragma
     (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id
   is
      N : Node_Or_Entity_Id;

   begin
      N := Node;
      loop
         N := Prev (N);
         exit when Nkind (N) /= N_Pragma;
      end loop;

      return N;
   end Prev_Non_Pragma;

   procedure Prev_Non_Pragma (Node : in out Node_Or_Entity_Id) is
   begin
      Node := Prev_Non_Pragma (Node);
   end Prev_Non_Pragma;

   ------------
   -- Remove --
   ------------

   procedure Remove (Node : Node_Or_Entity_Id) is
      Lst : constant List_Id           := List_Containing (Node);
      Prv : constant Node_Or_Entity_Id := Prev (Node);
      Nxt : constant Node_Or_Entity_Id := Next (Node);

      procedure Remove_Debug;
      pragma Inline (Remove_Debug);
      --  Output debug information if Debug_Flag_N set

      ------------------
      -- Remove_Debug --
      ------------------

      procedure Remove_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Remove node ");
            Write_Int (Int (Node));
            Write_Eol;
         end if;
      end Remove_Debug;

   --  Start of processing for Remove

   begin
      pragma Debug (Remove_Debug);

      if No (Prv) then
         Set_First (Lst, Nxt);
      else
         Set_Next (Prv, Nxt);
      end if;

      if No (Nxt) then
         Set_Last (Lst, Prv);
      else
         Set_Prev (Nxt, Prv);
      end if;

      Set_In_List (Node, False);
      Set_Parent (Node, Empty);
   end Remove;

   -----------------
   -- Remove_Head --
   -----------------

   function Remove_Head (List : List_Id) return Node_Or_Entity_Id is
      Frst : constant Node_Or_Entity_Id := First (List);

      procedure Remove_Head_Debug;
      pragma Inline (Remove_Head_Debug);
      --  Output debug information if Debug_Flag_N set

      -----------------------
      -- Remove_Head_Debug --
      -----------------------

      procedure Remove_Head_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Remove head of list ");
            Write_Int (Int (List));
            Write_Eol;
         end if;
      end Remove_Head_Debug;

   --  Start of processing for Remove_Head

   begin
      pragma Debug (Remove_Head_Debug);

      if Frst = Empty then
         return Empty;

      else
         declare
            Nxt : constant Node_Or_Entity_Id := Next (Frst);

         begin
            Set_First (List, Nxt);

            if No (Nxt) then
               Set_Last (List, Empty);
            else
               Set_Prev (Nxt, Empty);
            end if;

            Set_In_List (Frst, False);
            Set_Parent (Frst, Empty);
            return Frst;
         end;
      end if;
   end Remove_Head;

   -----------------
   -- Remove_Next --
   -----------------

   function Remove_Next
     (Node : Node_Or_Entity_Id) return Node_Or_Entity_Id
   is
      Nxt : constant Node_Or_Entity_Id := Next (Node);

      procedure Remove_Next_Debug;
      pragma Inline (Remove_Next_Debug);
      --  Output debug information if Debug_Flag_N set

      -----------------------
      -- Remove_Next_Debug --
      -----------------------

      procedure Remove_Next_Debug is
      begin
         if Debug_Flag_N then
            Write_Str ("Remove next node after ");
            Write_Int (Int (Node));
            Write_Eol;
         end if;
      end Remove_Next_Debug;

   --  Start of processing for Remove_Next

   begin
      if Present (Nxt) then
         declare
            Nxt2 : constant Node_Or_Entity_Id := Next (Nxt);
            LC   : constant List_Id           := List_Containing (Node);

         begin
            pragma Debug (Remove_Next_Debug);
            Set_Next (Node, Nxt2);

            if No (Nxt2) then
               Set_Last (LC, Node);
            else
               Set_Prev (Nxt2, Node);
            end if;

            Set_In_List (Nxt, False);
            Set_Parent (Nxt, Empty);
         end;
      end if;

      return Nxt;
   end Remove_Next;

   ---------------
   -- Set_First --
   ---------------

   procedure Set_First (List : List_Id; To : Node_Or_Entity_Id) is
   begin
      pragma Assert (not Locked);
      Lists.Table (List).First := To;
   end Set_First;

   --------------
   -- Set_Last --
   --------------

   procedure Set_Last (List : List_Id; To : Node_Or_Entity_Id) is
   begin
      pragma Assert (not Locked);
      Lists.Table (List).Last := To;
   end Set_Last;

   -------------------
   -- Set_List_Link --
   -------------------

   procedure Set_List_Link (Node : Node_Or_Entity_Id; To : List_Id) is
   begin
      pragma Assert (not Locked);
      Set_Link (Node, Union_Id (To));
   end Set_List_Link;

   --------------
   -- Set_Next --
   --------------

   procedure Set_Next (Node : Node_Or_Entity_Id; To : Node_Or_Entity_Id) is
   begin
      pragma Assert (not Locked);
      Next_Node.Table (Node) := To;
   end Set_Next;

   ----------------
   -- Set_Parent --
   ----------------

   procedure Set_Parent (List : List_Id; Node : Node_Or_Entity_Id) is
   begin
      pragma Assert (not Locked);
      pragma Assert (List <= Lists.Last);
      Lists.Table (List).Parent := Node;
   end Set_Parent;

   --------------
   -- Set_Prev --
   --------------

   procedure Set_Prev (Node : Node_Or_Entity_Id; To : Node_Or_Entity_Id) is
   begin
      pragma Assert (not Locked);
      Prev_Node.Table (Node) := To;
   end Set_Prev;

   ------------
   -- Unlock --
   ------------

   procedure Unlock is
   begin
      Lists.Locked := False;
      Prev_Node.Locked := False;
      Next_Node.Locked := False;
   end Unlock;

   ------------------
   -- Unlock_Lists --
   ------------------

   procedure Unlock_Lists is
   begin
      pragma Assert (Locked);
      Locked := False;
   end Unlock_Lists;

end Nlists;