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      1 /*
      2 [The "BSD licence"]
      3 Copyright (c) 2005-2008 Terence Parr
      4 All rights reserved.
      5 
      6 Redistribution and use in source and binary forms, with or without
      7 modification, are permitted provided that the following conditions
      8 are met:
      9 1. Redistributions of source code must retain the above copyright
     10 notice, this list of conditions and the following disclaimer.
     11 2. Redistributions in binary form must reproduce the above copyright
     12 notice, this list of conditions and the following disclaimer in the
     13 documentation and/or other materials provided with the distribution.
     14 3. The name of the author may not be used to endorse or promote products
     15 derived from this software without specific prior written permission.
     16 
     17 THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
     18 IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
     19 OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
     20 IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
     21 INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
     22 NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     23 DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     24 THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     25 (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
     26 THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     27 */
     28 package org.antlr.runtime.misc;
     29 
     30 import java.util.List;
     31 import java.util.ArrayList;
     32 
     33 /** A lookahead queue that knows how to mark/release locations
     34  *  in the buffer for backtracking purposes. Any markers force the FastQueue
     35  *  superclass to keep all tokens until no more markers; then can reset
     36  *  to avoid growing a huge buffer.
     37  */
     38 public abstract class LookaheadStream<T> extends FastQueue<T> {
     39     public static final int UNINITIALIZED_EOF_ELEMENT_INDEX = Integer.MAX_VALUE;
     40 
     41     /** Set to buffer index of eof when nextElement returns eof */
     42     protected int eofElementIndex = UNINITIALIZED_EOF_ELEMENT_INDEX;
     43 
     44     /** Returned by nextElement upon end of stream; we add to buffer also */
     45     public T eof = null;
     46 
     47     /** Track the last mark() call result value for use in rewind(). */
     48     protected int lastMarker;
     49 
     50     /** tracks how deep mark() calls are nested */
     51     protected int markDepth = 0;
     52 
     53     public LookaheadStream(T eof) {
     54         this.eof = eof;
     55     }
     56 
     57     public void reset() {
     58         eofElementIndex = UNINITIALIZED_EOF_ELEMENT_INDEX;
     59         super.reset();
     60     }
     61 
     62     /** Implement nextElement to supply a stream of elements to this
     63      *  lookahead buffer.  Return eof upon end of the stream we're pulling from.
     64      */
     65     public abstract T nextElement();
     66 
     67     /** Get and remove first element in queue; override FastQueue.remove() */
     68     public T remove() {
     69         T o = get(0);
     70         p++;
     71         // have we hit end of buffer and not backtracking?
     72         if ( p == data.size() && markDepth==0 ) {
     73             // if so, it's an opportunity to start filling at index 0 again
     74             clear(); // size goes to 0, but retains memory
     75         }
     76         return o;
     77     }
     78 
     79     /** Make sure we have at least one element to remove, even if EOF */
     80     public void consume() { sync(1); remove(); }
     81 
     82     /** Make sure we have 'need' elements from current position p. Last valid
     83      *  p index is data.size()-1.  p+need-1 is the data index 'need' elements
     84      *  ahead.  If we need 1 element, (p+1-1)==p must be < data.size().
     85      */
     86     public void sync(int need) {
     87         int n = (p+need-1) - data.size() + 1; // how many more elements we need?
     88         if ( n > 0 ) fill(n);                 // out of elements?
     89     }
     90 
     91     /** add n elements to buffer */
     92     public void fill(int n) {
     93         for (int i=1; i<=n; i++) {
     94             T o = nextElement();
     95             if ( o==eof ) {
     96                 data.add(eof);
     97                 eofElementIndex = data.size()-1;
     98             }
     99             else data.add(o);
    100         }
    101     }
    102 
    103     //public boolean hasNext() { return eofElementIndex!=UNINITIALIZED_EOF_ELEMENT_INDEX; }
    104 
    105     /** Size of entire stream is unknown; we only know buffer size from FastQueue */
    106     public int size() { throw new UnsupportedOperationException("streams are of unknown size"); }
    107 
    108     public Object LT(int k) {
    109 		if ( k==0 ) {
    110 			return null;
    111 		}
    112 		if ( k<0 ) {
    113 			return LB(-k);
    114 		}
    115 		//System.out.print("LT(p="+p+","+k+")=");
    116 		if ( (p+k-1) >= eofElementIndex ) { // move to super.LT
    117 			return eof;
    118 		}
    119         sync(k);
    120         return get(k-1);
    121 	}
    122 
    123 	/** Look backwards k nodes */
    124 	protected Object LB(int k) {
    125 		if ( k==0 ) {
    126 			return null;
    127 		}
    128 		if ( (p-k)<0 ) {
    129 			return null;
    130 		}
    131 		return get(-k);
    132 	}
    133 
    134     public Object getCurrentSymbol() { return LT(1); }
    135 
    136     public int index() { return p; }
    137 
    138 	public int mark() {
    139         markDepth++;
    140         lastMarker = index();
    141         return lastMarker;
    142 	}
    143 
    144 	public void release(int marker) {
    145 		// no resources to release
    146 	}
    147 
    148 	public void rewind(int marker) {
    149         markDepth--;
    150         seek(marker); // assume marker is top
    151         // release(marker); // waste of call; it does nothing in this class
    152     }
    153 
    154 	public void rewind() {
    155         seek(lastMarker); // rewind but do not release marker
    156     }
    157 
    158     /** Seek to a 0-indexed position within data buffer.  Can't handle
    159      *  case where you seek beyond end of existing buffer.  Normally used
    160      *  to seek backwards in the buffer. Does not force loading of nodes.
    161      */
    162     public void seek(int index) { p = index; }
    163 }