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package java_cup.runtime;
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import java.util.Stack;
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/** This class implements a temporary or "virtual" parse stack that
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* replaces the top portion of the actual parse stack (the part that
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* has been changed by some set of operations) while maintaining its
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* original contents. This data structure is used when the parse needs
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* to "parse ahead" to determine if a given error recovery attempt will
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* allow the parse to continue far enough to consider it successful. Once
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* success or failure of parse ahead is determined the system then
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* reverts to the original parse stack (which has not actually been
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* modified). Since parse ahead does not execute actions, only parse
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* state is maintained on the virtual stack, not full Symbol objects.
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* @see java_cup.runtime.lr_parser
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* @version last updated: 7/3/96
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* @author Frank Flannery
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public class virtual_parse_stack {
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/*-----------------------------------------------------------*/
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/*--- Constructor(s) ----------------------------------------*/
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/*-----------------------------------------------------------*/
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/** Constructor to build a virtual stack out of a real stack. */
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public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception
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if (shadowing_stack == null)
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"Internal parser error: attempt to create null virtual stack");
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/* set up our internals */
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real_stack = shadowing_stack;
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/* get one element onto the virtual portion of the stack */
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/*-----------------------------------------------------------*/
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/*--- (Access to) Instance Variables ------------------------*/
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/*-----------------------------------------------------------*/
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/** The real stack that we shadow. This is accessed when we move off
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* the bottom of the virtual portion of the stack, but is always left
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protected Stack real_stack;
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** Top of stack indicator for where we leave off in the real stack.
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* This is measured from top of stack, so 0 would indicate that no
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* elements have been "moved" from the real to virtual stack.
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protected int real_next;
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** The virtual top portion of the stack. This stack contains Integer
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* objects with state numbers. This stack shadows the top portion
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* of the real stack within the area that has been modified (via operations
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* on the virtual stack). When this portion of the stack becomes empty we
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* transfer elements from the underlying stack onto this stack.
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protected Stack vstack;
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/*-----------------------------------------------------------*/
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/*--- General Methods ---------------------------------------*/
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/*-----------------------------------------------------------*/
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/** Transfer an element from the real to the virtual stack. This assumes
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* that the virtual stack is currently empty.
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protected void get_from_real()
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/* don't transfer if the real stack is empty */
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if (real_next >= real_stack.size()) return;
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/* get a copy of the first Symbol we have not transfered */
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stack_sym = (Symbol)real_stack.elementAt(real_stack.size()-1-real_next);
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/* record the transfer */
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/* put the state number from the Symbol onto the virtual stack */
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vstack.push(new Integer(stack_sym.parse_state));
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** Indicate whether the stack is empty. */
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public boolean empty()
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/* if vstack is empty then we were unable to transfer onto it and
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the whole thing is empty. */
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return vstack.empty();
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** Return value on the top of the stack (without popping it). */
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public int top() throws java.lang.Exception
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"Internal parser error: top() called on empty virtual stack");
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return ((Integer)vstack.peek()).intValue();
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** Pop the stack. */
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public void pop() throws java.lang.Exception
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"Internal parser error: pop from empty virtual stack");
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/* if we are now empty transfer an element (if there is one) */
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/*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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/** Push a state number onto the stack. */
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public void push(int state_num)
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vstack.push(new Integer(state_num));
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/*-----------------------------------------------------------*/
added
removed
src/java_cup/runtime/virtual_parse_stack.java
