virtual_parse_stack.java

tiger run time supporting files

package java_cup.runtime;

import java.util.Stack;

/**
 * This class implements a temporary or "virtual" parse stack that replaces the
 * top portion of the actual parse stack (the part that has been changed by some
 * set of operations) while maintaining its original contents. This data
 * structure is used when the parse needs to "parse ahead" to determine if a
 * given error recovery attempt will allow the parse to continue far enough to
 * consider it successful. Once success or failure of parse ahead is determined
 * the system then reverts to the original parse stack (which has not actually
 * been modified). Since parse ahead does not execute actions, only parse state
 * is maintained on the virtual stack, not full Symbol objects.
 * 
 * @see java_cup.runtime.lr_parser
 * @version last updated: 7/3/96
 * @author Frank Flannery
 */

public class virtual_parse_stack {
	/*-----------------------------------------------------------*/
	/*--- Constructor(s) ----------------------------------------*/
	/*-----------------------------------------------------------*/

	/** Constructor to build a virtual stack out of a real stack. */
	public virtual_parse_stack(Stack shadowing_stack)
			throws java.lang.Exception {
		/* sanity check */
		if (shadowing_stack == null)
			throw new Exception(
					"Internal parser error: attempt to create null virtual stack");

		/* set up our internals */
		real_stack = shadowing_stack;
		vstack = new Stack();
		real_next = 0;

		/* get one element onto the virtual portion of the stack */
		get_from_real();
	}

	/*-----------------------------------------------------------*/
	/*--- (Access to) Instance Variables ------------------------*/
	/*-----------------------------------------------------------*/

	/**
	 * The real stack that we shadow. This is accessed when we move off the
	 * bottom of the virtual portion of the stack, but is always left
	 * unmodified.
	 */
	protected Stack real_stack;

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/**
	 * Top of stack indicator for where we leave off in the real stack. This is
	 * measured from top of stack, so 0 would indicate that no elements have
	 * been "moved" from the real to virtual stack.
	 */
	protected int real_next;

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/**
	 * The virtual top portion of the stack. This stack contains Integer objects
	 * with state numbers. This stack shadows the top portion of the real stack
	 * within the area that has been modified (via operations on the virtual
	 * stack). When this portion of the stack becomes empty we transfer elements
	 * from the underlying stack onto this stack.
	 */
	protected Stack vstack;

	/*-----------------------------------------------------------*/
	/*--- General Methods ---------------------------------------*/
	/*-----------------------------------------------------------*/

	/**
	 * Transfer an element from the real to the virtual stack. This assumes that
	 * the virtual stack is currently empty.
	 */
	protected void get_from_real() {
		Symbol stack_sym;

		/* don't transfer if the real stack is empty */
		if (real_next >= real_stack.size())
			return;

		/* get a copy of the first Symbol we have not transfered */
		stack_sym = (Symbol) real_stack.elementAt(real_stack.size() - 1
				- real_next);

		/* record the transfer */
		real_next++;

		/* put the state number from the Symbol onto the virtual stack */
		vstack.push(new Integer(stack_sym.parse_state));
	}

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/** Indicate whether the stack is empty. */
	public boolean empty() {
		/*
		 * if vstack is empty then we were unable to transfer onto it and the
		 * whole thing is empty.
		 */
		return vstack.empty();
	}

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/** Return value on the top of the stack (without popping it). */
	public int top() throws java.lang.Exception {
		if (vstack.empty())
			throw new Exception(
					"Internal parser error: top() called on empty virtual stack");

		return ((Integer) vstack.peek()).intValue();
	}

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/** Pop the stack. */
	public void pop() throws java.lang.Exception {
		if (vstack.empty())
			throw new Exception(
					"Internal parser error: pop from empty virtual stack");

		/* pop it */
		vstack.pop();

		/* if we are now empty transfer an element (if there is one) */
		if (vstack.empty())
			get_from_real();
	}

	/* . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . */

	/** Push a state number onto the stack. */
	public void push(int state_num) {
		vstack.push(new Integer(state_num));
	}

	/*-----------------------------------------------------------*/

}