1/* 2 [The "BSD license"] 3 Copyright (c) 2005-2009 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 */ 28package org.antlr.runtime.misc; 29 30import org.antlr.runtime.Token; 31 32import java.util.NoSuchElementException; 33 34/** A lookahead queue that knows how to mark/release locations 35 * in the buffer for backtracking purposes. Any markers force the FastQueue 36 * superclass to keep all tokens until no more markers; then can reset 37 * to avoid growing a huge buffer. 38 */ 39public abstract class LookaheadStream<T> extends FastQueue<T> { 40 public static final int UNINITIALIZED_EOF_ELEMENT_INDEX = Integer.MAX_VALUE; 41 42 /** Absolute token index. It's the index of the symbol about to be 43 * read via LT(1). Goes from 0 to numtokens. 44 */ 45 protected int currentElementIndex = 0; 46 47 protected T prevElement; 48 49 /** Track object returned by nextElement upon end of stream; 50 * Return it later when they ask for LT passed end of input. 51 */ 52 public T eof = null; 53 54 /** Track the last mark() call result value for use in rewind(). */ 55 protected int lastMarker; 56 57 /** tracks how deep mark() calls are nested */ 58 protected int markDepth = 0; 59 60 public void reset() { 61 super.reset(); 62 currentElementIndex = 0; 63 p = 0; 64 prevElement=null; 65 } 66 67 /** Implement nextElement to supply a stream of elements to this 68 * lookahead buffer. Return eof upon end of the stream we're pulling from. 69 */ 70 public abstract T nextElement(); 71 72 public abstract boolean isEOF(T o); 73 74 /** Get and remove first element in queue; override FastQueue.remove(); 75 * it's the same, just checks for backtracking. 76 */ 77 public T remove() { 78 T o = elementAt(0); 79 p++; 80 // have we hit end of buffer and not backtracking? 81 if ( p == data.size() && markDepth==0 ) { 82 // if so, it's an opportunity to start filling at index 0 again 83 clear(); // size goes to 0, but retains memory 84 } 85 return o; 86 } 87 88 /** Make sure we have at least one element to remove, even if EOF */ 89 public void consume() { 90 syncAhead(1); 91 prevElement = remove(); 92 currentElementIndex++; 93 } 94 95 /** Make sure we have 'need' elements from current position p. Last valid 96 * p index is data.size()-1. p+need-1 is the data index 'need' elements 97 * ahead. If we need 1 element, (p+1-1)==p must be < data.size(). 98 */ 99 protected void syncAhead(int need) { 100 int n = (p+need-1) - data.size() + 1; // how many more elements we need? 101 if ( n > 0 ) fill(n); // out of elements? 102 } 103 104 /** add n elements to buffer */ 105 public void fill(int n) { 106 for (int i=1; i<=n; i++) { 107 T o = nextElement(); 108 if ( isEOF(o) ) eof = o; 109 data.add(o); 110 } 111 } 112 113 /** Size of entire stream is unknown; we only know buffer size from FastQueue */ 114 public int size() { throw new UnsupportedOperationException("streams are of unknown size"); } 115 116 public T LT(int k) { 117 if ( k==0 ) { 118 return null; 119 } 120 if ( k<0 ) return LB(-k); 121 //System.out.print("LT(p="+p+","+k+")="); 122 syncAhead(k); 123 if ( (p+k-1) > data.size() ) return eof; 124 return elementAt(k-1); 125 } 126 127 public int index() { return currentElementIndex; } 128 129 public int mark() { 130 markDepth++; 131 lastMarker = p; // track where we are in buffer not absolute token index 132 return lastMarker; 133 } 134 135 public void release(int marker) { 136 // no resources to release 137 } 138 139 public void rewind(int marker) { 140 markDepth--; 141 seek(marker); // assume marker is top 142 // release(marker); // waste of call; it does nothing in this class 143 } 144 145 public void rewind() { 146 seek(lastMarker); // rewind but do not release marker 147 } 148 149 /** Seek to a 0-indexed position within data buffer. Can't handle 150 * case where you seek beyond end of existing buffer. Normally used 151 * to seek backwards in the buffer. Does not force loading of nodes. 152 * Doesn't see to absolute position in input stream since this stream 153 * is unbuffered. Seeks only into our moving window of elements. 154 */ 155 public void seek(int index) { p = index; } 156 157 protected T LB(int k) { 158 if ( k==1 ) return prevElement; 159 throw new NoSuchElementException("can't look backwards more than one token in this stream"); 160 } 161}