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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));
}
/*-----------------------------------------------------------*/
}
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