[25584] | 1 |
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| 2 | package java_cup.runtime;
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| 3 |
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| 4 | import java.util.Stack;
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| 5 |
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| 6 | /** This class implements a temporary or "virtual" parse stack that
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| 7 | * replaces the top portion of the actual parse stack (the part that
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| 8 | * has been changed by some set of operations) while maintaining its
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| 9 | * original contents. This data structure is used when the parse needs
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| 10 | * to "parse ahead" to determine if a given error recovery attempt will
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| 11 | * allow the parse to continue far enough to consider it successful. Once
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| 12 | * success or failure of parse ahead is determined the system then
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| 13 | * reverts to the original parse stack (which has not actually been
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| 14 | * modified). Since parse ahead does not execute actions, only parse
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| 15 | * state is maintained on the virtual stack, not full Symbol objects.
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| 16 | *
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| 17 | * @see java_cup.runtime.lr_parser
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| 18 | * @version last updated: 7/3/96
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| 19 | * @author Frank Flannery
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| 20 | */
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| 21 |
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| 22 | public class virtual_parse_stack {
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| 23 | /*-----------------------------------------------------------*/
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| 24 | /*--- Constructor(s) ----------------------------------------*/
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| 25 | /*-----------------------------------------------------------*/
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| 26 |
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| 27 | /** Constructor to build a virtual stack out of a real stack. */
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| 28 | public virtual_parse_stack(Stack shadowing_stack) throws java.lang.Exception
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| 29 | {
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| 30 | /* sanity check */
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| 31 | if (shadowing_stack == null)
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| 32 | throw new Exception(
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| 33 | "Internal parser error: attempt to create null virtual stack");
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| 34 |
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| 35 | /* set up our internals */
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| 36 | real_stack = shadowing_stack;
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| 37 | vstack = new Stack();
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| 38 | real_next = 0;
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| 39 |
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| 40 | /* get one element onto the virtual portion of the stack */
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| 41 | get_from_real();
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| 42 | }
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| 43 |
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| 44 | /*-----------------------------------------------------------*/
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| 45 | /*--- (Access to) Instance Variables ------------------------*/
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| 46 | /*-----------------------------------------------------------*/
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| 47 |
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| 48 | /** The real stack that we shadow. This is accessed when we move off
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| 49 | * the bottom of the virtual portion of the stack, but is always left
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| 50 | * unmodified.
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| 51 | */
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| 52 | protected Stack real_stack;
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| 53 |
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| 54 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 55 |
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| 56 | /** Top of stack indicator for where we leave off in the real stack.
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| 57 | * This is measured from top of stack, so 0 would indicate that no
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| 58 | * elements have been "moved" from the real to virtual stack.
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| 59 | */
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| 60 | protected int real_next;
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| 61 |
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| 62 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 63 |
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| 64 | /** The virtual top portion of the stack. This stack contains Integer
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| 65 | * objects with state numbers. This stack shadows the top portion
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| 66 | * of the real stack within the area that has been modified (via operations
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| 67 | * on the virtual stack). When this portion of the stack becomes empty we
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| 68 | * transfer elements from the underlying stack onto this stack.
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| 69 | */
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| 70 | protected Stack vstack;
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| 71 |
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| 72 | /*-----------------------------------------------------------*/
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| 73 | /*--- General Methods ---------------------------------------*/
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| 74 | /*-----------------------------------------------------------*/
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| 75 |
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| 76 | /** Transfer an element from the real to the virtual stack. This assumes
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| 77 | * that the virtual stack is currently empty.
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| 78 | */
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| 79 | protected void get_from_real()
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| 80 | {
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| 81 | Symbol stack_sym;
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| 82 |
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| 83 | /* don't transfer if the real stack is empty */
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| 84 | if (real_next >= real_stack.size()) return;
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| 85 |
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| 86 | /* get a copy of the first Symbol we have not transfered */
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| 87 | stack_sym = (Symbol)real_stack.elementAt(real_stack.size()-1-real_next);
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| 88 |
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| 89 | /* record the transfer */
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| 90 | real_next++;
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| 91 |
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| 92 | /* put the state number from the Symbol onto the virtual stack */
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| 93 | vstack.push(new Integer(stack_sym.parse_state));
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| 94 | }
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| 95 |
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| 96 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 97 |
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| 98 | /** Indicate whether the stack is empty. */
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| 99 | public boolean empty()
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| 100 | {
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| 101 | /* if vstack is empty then we were unable to transfer onto it and
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| 102 | the whole thing is empty. */
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| 103 | return vstack.empty();
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| 104 | }
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| 105 |
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| 106 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 107 |
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| 108 | /** Return value on the top of the stack (without popping it). */
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| 109 | public int top() throws java.lang.Exception
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| 110 | {
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| 111 | if (vstack.empty())
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| 112 | throw new Exception(
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| 113 | "Internal parser error: top() called on empty virtual stack");
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| 114 |
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| 115 | return ((Integer)vstack.peek()).intValue();
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| 116 | }
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| 117 |
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| 118 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 119 |
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| 120 | /** Pop the stack. */
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| 121 | public void pop() throws java.lang.Exception
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| 122 | {
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| 123 | if (vstack.empty())
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| 124 | throw new Exception(
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| 125 | "Internal parser error: pop from empty virtual stack");
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| 126 |
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| 127 | /* pop it */
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| 128 | vstack.pop();
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| 129 |
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| 130 | /* if we are now empty transfer an element (if there is one) */
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| 131 | if (vstack.empty())
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| 132 | get_from_real();
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| 133 | }
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| 134 |
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| 135 | /*. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .*/
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| 136 |
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| 137 | /** Push a state number onto the stack. */
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| 138 | public void push(int state_num)
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| 139 | {
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| 140 | vstack.push(new Integer(state_num));
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| 141 | }
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| 142 |
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| 143 | /*-----------------------------------------------------------*/
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| 144 |
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| 145 | }
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