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package com.sun.java_cup.internal.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.
Author: Frank Flannery See Also: - lr_parser
/** 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 com.sun.java_cup.internal.runtime.lr_parser
* @author Frank Flannery
*/
public class virtual_parse_stack {
/*-----------------------------------------------------------*/
/*--- Constructor(s) ----------------------------------------*/
/*-----------------------------------------------------------*/
Constructor to build a virtual stack out of a real stack. /** Constructor to build a virtual stack out of a real stack. */
public virtual_parse_stack(Stack<Symbol> 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.
/** 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<Symbol> 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.
/** 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.
/** 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<Integer> vstack;
/*-----------------------------------------------------------*/
/*--- General Methods ---------------------------------------*/
/*-----------------------------------------------------------*/
Transfer an element from the real to the virtual stack. This assumes
that the virtual stack is currently empty.
/** 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 = real_stack.get(real_stack.size()-1-real_next);
/* record the transfer */
real_next++;
/* put the state number from the Symbol onto the virtual stack */
vstack.push(stack_sym.parse_state);
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
Indicate whether the stack is empty. /** 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). /** 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 (vstack.peek());
}
/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
Pop the stack. /** 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. /** Push a state number onto the stack. */
public void push(int state_num)
{
vstack.push(state_num);
}
/*-----------------------------------------------------------*/
}