#include "lisp.h"

/* stack routines for SIMULISP */

/* stack is implemented a linked list of pages each containing OBJPAGESIZE-1
 * objects.  The first entry in each page points to the last stack page, if
 * any.
 */

page *o_stack;			/* pointer to current stack page */
int stack_index;		/* index of top of stack */

/* initialize stack */
i_stack()
  {	o_stack = NULL;
	stack_index = 0;
}

/* push object onto stack.  returns its argument. */
object push(obj)
  object obj;
  {	register page *current = o_stack;

	/* if we need new stack page, get one */
	if (stack_index==OBJPAGESIZE-1 || current==NULL) {
	  o_stack = al_page(0);
	  (*o_stack)[0] = (object)current;
	  current = o_stack;
	  stack_index = 0;
	}

	/* there is room in current page, push object and return */
	(*current)[++stack_index] = obj;
	return(obj);
}

/* pop n objects off the stack.  returns former top of stack. */
object pop(n)
  register int n;
  {	register page *current = o_stack;
	object tos;

	tos = (current==NULL)? NULL : (*current)[stack_index];

	/* pop off pages til we get to the one we want */
	while (n >= stack_index) {
	  if (current == NULL) {
	    error("; overpopped stack\n\n");
	    return(tos);
	  }
	  o_stack = (page *)((*current)[0]);
	  n -= stack_index;
	  fr_page(current);
	  current = o_stack;
	  stack_index = OBJPAGESIZE-1;
	}

	/* pop what we need to off this page, then we're done */
	stack_index -= n;
	return(tos);
}

/* return nth object from top of stack. */
object nth_stack(n)
  register int n;
  {	register page *current = o_stack;
	register int current_index = stack_index;

	/* run through pages til we get to the one we want */
	while (n >= current_index) {
	  if (current==NULL) {
	    error(";looking too far into stack\n\n");
	    return(NULL);
	  }
	  n -= current_index;
	  current = (page *)((*current)[0]);
	  current_index = OBJPAGESIZE-1;
	}

	/* what we need is in this page */
	return((*current)[current_index - n]);
}

/* replace nth object from top of stack with given object, returns what was there */
object nth_exch(n,obj)
  register int n;
  object obj;
  {	register page *current = o_stack;
	register int current_index = stack_index;
	object old;

	/* run through pages til we get to the one we want */
	while (n >= current_index) {
	  if (current==NULL) {
	    error(";looking too far into stack\n\n");
	    return(NULL);
	  }
	  current = (page *)((*current)[0]);
	  n -= current_index;
	  current_index = OBJPAGESIZE-1;
	}

	/* what we need is in this page */
	old = (*current)[current_index - n];
	(*current)[current_index - n] = obj;
	return(old);
}

/* mark stack for garbage collection */
m_stack()
  {	register page *current = o_stack;
	register int current_index = stack_index;

	/* run through pages til we get to bottom of things */
	while (current != NULL) {
	  while (current_index) m_obj((*current)[current_index--]);
	  current = (page *)((*current)[0]);
	  current_index = OBJPAGESIZE-1;
	}
}