--------------------------------------------------------------------
--                        a3.ads

-- This file is the specification file for the types
--    and functions used in the environment simulation
--    of Assignment 3, CS 152, Spring 2000.

--------------------------------------------------------------------

with TEXT_IO;    use TEXT_IO;

package a3 is


-- These are the record types and access types
--   used by the test program.  Note that the
--   implementation of the "binding" type must
--   be public in order to allow the test program
--   to convert integer literals to this type.
--   Also "frame_ptr" must be identified as an
--   access type so that the test program can
--   verify that NULL is a permissible value
--   for that type.

type binding is new integer;
type environment_ptr is private;
type frame is private;
type frame_ptr is access frame;


-- These are the procedures and functions used
-- in the test program


-- These two functions substitute for constructors, giving
--    a little more control over the initialization, etc.
-- The first function creates an empty environment.
-- The second creates a frame (with no bindings), given its
--    static link.

function new_environment return environment_ptr;
function new_frame(fr:frame_ptr) return frame_ptr;


-- These three functions modify frames or environments
-- The first adds a given binding to a given frame
-- The second adds (i.e., pushes) a given frame to a given environment.
-- The third removes a frame from (the front of) an
--    environment.

procedure add_binding(f:frame_ptr; bb:binding);
procedure add_frame(e:environment_ptr; fr:frame_ptr);
procedure delete_frame(e:environment_ptr);


-- This function returns the (value of) the binding found
--   in environment e, at position j in the frame found
--   at i links down the chain of static links from the
--   top frame.

procedure lookup_and_print(e:environment_ptr; i,j:integer);


private


--  An environment consists of a display (an array of frame pointers),
--    a display size (how many positions of the array are used)
--    and a pointer to a stack of frames of type "environment_node_ptr"
--  An environment_node is a linked stack of frame pointers, with a
--    header node.  Note that the static pointers are kept in the
--    frame nodes, while the dynamic pointers are kept in the
--    environment nodes.

MAX_DISPLAY_SIZE:constant integer:=10;

type display is array(0..MAX_DISPLAY_SIZE-1) of frame_ptr;

type environment_node;
type environment_node_ptr is access environment_node;


type environment is record
  d:display:=(others=>null);
  display_size:integer:=0;
  frames:environment_node_ptr;
end record;


type environment_node is record
  frame:frame_ptr;
  next:environment_node_ptr;
end record;


type environment_ptr is access environment;




-- A frame consists of a static link and a linked list of bindings.

-- The binding_node type represents a node in a linked list of bindings.
-- It has a link field "next", so these nodes know that they are in
--   a linked list.

type binding_node;
type binding_node_ptr is access binding_node;
type binding_node is record
   data:binding;
   next:binding_node_ptr;
end record;

-- Here is the definition of a frame

type frame is record
   static_link:frame_ptr;
   bindings: binding_node_ptr;
end record;

end a3;