--
-- This is the body of the linked integer stack package.
--

-- This is needed for the memory free operation.
with Ada.Unchecked_Deallocation;
use Ada;

package body IntStackPkg is
   -- Here's the deallocation function for nodes.
   procedure Free is new Unchecked_Deallocation(StackNode, NodePtr);

   -- Push onto the stack.
   procedure Push(S: in out IntStack; I: Integer) is
      begin
         S := new StackNode'(I, NodePtr(S));
      end Push;

   -- Remove from the stack.
   procedure Pop(S: in out IntStack; I: out Integer) is
      OldTop: NodePtr;
      begin
         if S /= null then
            -- Unlink the top node.
            OldTop := NodePtr(S);
            S := IntStack(S.Next);

            -- Get the data out of it.
            I := OldTop.Data;

            -- Say bye, little node.
            Free(OldTop);
         end if;
      end Pop;

   -- Find out what's on the top.
   function Top(S: IntStack) return Integer is
   begin
      if S = null then
         return 0;
      else
         return S.Data;
      end if;
   end Top;

   -- So, are we empty yet?
   function IsEmpty(S: IntStack) return Boolean is
      begin
         return S = null;
      end IsEmpty;

   -- Remove all memory from the stack.
   procedure Clear(S: in out IntStack) is
      Zombie,                   -- Ptr to node about to be deleted.
        Scan: NodePtr;         -- List scanner.
   begin
      Scan := NodePtr(S);
      while Scan /= null loop
         -- Advance the pointer and delete the old node.
         Zombie := Scan;
         Scan := Scan.Next;
         Free(Zombie);
      end loop;
      
      S := null;
   end Clear;

end IntStackPkg;