1 2 package java_cup.runtime; 3 4 import java.util.Stack; 5 6 /** This class implements a temporary or "virtual" parse stack that 7 * replaces the top portion of the actual parse stack (the part that 8 * has been changed by some set of operations) while maintaining its 9 * original contents. This data structure is used when the parse needs 10 * to "parse ahead" to determine if a given error recovery attempt will 11 * allow the parse to continue far enough to consider it successful. Once 12 * success or failure of parse ahead is determined the system then 13 * reverts to the original parse stack (which has not actually been 14 * modified). Since parse ahead does not execute actions, only parse 15 * state is maintained on the virtual stack, not full symbol objects. 16 * 17 * @see java_cup.runtime.lr_parser 18 * @version last updated: 11/25/95 19 * @author Scott Hudson 20 */ 21 22 public class virtual_parse_stack { 23 /*-----------------------------------------------------------*/ 24 /*--- Constructor(s) ----------------------------------------*/ 25 /*-----------------------------------------------------------*/ 26 27 /** Constructor to build a virtual stack out of a real stack. */ virtual_parse_stack(Stack shadowing_stack)28 public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception 29 { 30 /* sanity check */ 31 if (shadowing_stack == null) 32 throw new Exception( 33 "Internal parser error: attempt to create null virtual stack"); 34 35 /* set up our internals */ 36 real_stack = shadowing_stack; 37 vstack = new Stack(); 38 real_next = 0; 39 40 /* get one element onto the virtual portion of the stack */ 41 get_from_real(); 42 } 43 44 /*-----------------------------------------------------------*/ 45 /*--- (Access to) Instance Variables ------------------------*/ 46 /*-----------------------------------------------------------*/ 47 48 /** The real stack that we shadow. This is accessed when we move off 49 * the bottom of the virtual portion of the stack, but is always left 50 * unmodified. 51 */ 52 protected Stack real_stack; 53 54 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 55 56 /** Top of stack indicator for where we leave off in the real stack. 57 * This is measured from top of stack, so 0 would indicate that no 58 * elements have been "moved" from the real to virtual stack. 59 */ 60 protected int real_next; 61 62 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 63 64 /** The virtual top portion of the stack. This stack contains Integer 65 * objects with state numbers. This stack shadows the top portion 66 * of the real stack within the area that has been modified (via operations 67 * on the virtual stack). When this portion of the stack becomes empty we 68 * transfer elements from the underlying stack onto this stack. 69 */ 70 protected Stack vstack; 71 72 /*-----------------------------------------------------------*/ 73 /*--- General Methods ---------------------------------------*/ 74 /*-----------------------------------------------------------*/ 75 76 /** Transfer an element from the real to the virtual stack. This assumes 77 * that the virtual stack is currently empty. 78 */ get_from_real()79 protected void get_from_real() 80 { 81 symbol stack_sym; 82 83 /* don't transfer if the real stack is empty */ 84 if (real_next >= real_stack.size()) return; 85 86 /* get a copy of the first symbol we have not transfered */ 87 stack_sym = (symbol)real_stack.elementAt(real_stack.size()-1-real_next); 88 89 /* record the transfer */ 90 real_next++; 91 92 /* put the state number from the symbol onto the virtual stack */ 93 vstack.push(new Integer(stack_sym.parse_state)); 94 } 95 96 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 97 98 /** Indicate whether the stack is empty. */ empty()99 public boolean empty() 100 { 101 /* if vstack is empty then we were unable to transfer onto it and 102 the whole thing is empty. */ 103 return vstack.empty(); 104 } 105 106 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 107 108 /** Return value on the top of the stack (without popping it). */ top()109 public int top() throws java.lang.Exception 110 { 111 if (vstack.empty()) 112 throw new Exception( 113 "Internal parser error: top() called on empty virtual stack"); 114 115 return ((Integer)vstack.peek()).intValue(); 116 } 117 118 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 119 120 /** Pop the stack. */ pop()121 public void pop() throws java.lang.Exception 122 { 123 if (vstack.empty()) 124 throw new Exception( 125 "Internal parser error: pop from empty virtual stack"); 126 127 /* pop it */ 128 vstack.pop(); 129 130 /* if we are now empty transfer an element (if there is one) */ 131 if (vstack.empty()) 132 get_from_real(); 133 } 134 135 /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/ 136 137 /** Push a state number onto the stack. */ push(int state_num)138 public void push(int state_num) 139 { 140 vstack.push(new Integer(state_num)); 141 } 142 143 /*-----------------------------------------------------------*/ 144 145 }; 146