java_cup.runtime
Class virtual_parse_stack

java.lang.Object
  java_cup.runtime.virtual_parse_stack

public class virtual_parse_stack
extends java.lang.Object

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.

Version:

last updated: 7/3/96

Author:

Frank Flannery

See Also:

lr_parser

Field Summary

protected int	real_next Top of stack indicator for where we leave off in the real stack.
protected java.util.Stack	real_stack The real stack that we shadow.
protected java.util.Stack	vstack The virtual top portion of the stack.

Constructor Summary

virtual_parse_stack(java.util.Stack shadowing_stack)
Constructor to build a virtual stack out of a real stack.

Method Summary

boolean	empty() Indicate whether the stack is empty.
protected void	get_from_real() Transfer an element from the real to the virtual stack.
void	pop() Pop the stack.
void	push(int state_num) Push a state number onto the stack.
int	top() Return value on the top of the stack (without popping it).

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

real_stack

protected java.util.Stack real_stack

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.

real_next

protected int real_next

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.

vstack

protected java.util.Stack vstack

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.

Constructor Detail

virtual_parse_stack

public virtual_parse_stack(java.util.Stack shadowing_stack)
                    throws java.lang.Exception

Constructor to build a virtual stack out of a real stack.

Throws:

java.lang.Exception

Method Detail

get_from_real

protected void get_from_real()

Transfer an element from the real to the virtual stack. This assumes that the virtual stack is currently empty.

empty

public boolean empty()

Indicate whether the stack is empty.

top

public int top()
        throws java.lang.Exception

Return value on the top of the stack (without popping it).

Throws:

java.lang.Exception

pop

public void pop()
         throws java.lang.Exception

Pop the stack.

Throws:

java.lang.Exception

push

public void push(int state_num)

Push a state number onto the stack.