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Class java_cup.runtime.virtual_parse_stack

java.lang.Object
   |
   +----java_cup.runtime.virtual_parse_stack

public class virtual_parse_stack
extends 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

Variable Index

 o real_next
Top of stack indicator for where we leave off in the real stack.
 o real_stack
The real stack that we shadow.
 o vstack
The virtual top portion of the stack.

Constructor Index

 o virtual_parse_stack(Stack)
Constructor to build a virtual stack out of a real stack.

Method Index

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

Variables

 o real_stack
 protected 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.

 o 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.

 o vstack
 protected 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.

Constructors

 o virtual_parse_stack
 public virtual_parse_stack(Stack shadowing_stack) throws Exception
Constructor to build a virtual stack out of a real stack.

Methods

 o 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.

 o empty
 public boolean empty()
Indicate whether the stack is empty.

 o top
 public int top() throws Exception
Return value on the top of the stack (without popping it).

 o pop
 public void pop() throws Exception
Pop the stack.

 o push
 public void push(int state_num)
Push a state number onto the stack.


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