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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