ObjC/Framework/LookaheadStream.m

/*
[The "BSD licence"]
Copyright (c) 2005-2008 Terence Parr
All rights reserved.

Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions
are met:
1. Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
2. Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
3. The name of the author may not be used to endorse or promote products
derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
(INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
package org.antlr.runtime.misc;

import java.util.List;
import java.util.ArrayList;

/** A lookahead queue that knows how to mark/release locations
 *  in the buffer for backtracking purposes. Any markers force the FastQueue
 *  superclass to keep all tokens until no more markers; then can reset
 *  to avoid growing a huge buffer.
 */
public abstract class LookaheadStream<T> extends FastQueue<T> {
    public static final int UNINITIALIZED_EOF_ELEMENT_INDEX = Integer.MAX_VALUE;

    /** Set to buffer index of eof when nextElement returns eof */
    protected int eofElementIndex = UNINITIALIZED_EOF_ELEMENT_INDEX;

    /** Returned by nextElement upon end of stream; we add to buffer also */
    public T eof = null;

    /** Track the last mark() call result value for use in rewind(). */
    protected int lastMarker;

    /** tracks how deep mark() calls are nested */
    protected int markDepth = 0;

    public LookaheadStream(T eof) {
        this.eof = eof;
    }

    public void reset() {
        eofElementIndex = UNINITIALIZED_EOF_ELEMENT_INDEX;
        super.reset();
    }

    /** Implement nextElement to supply a stream of elements to this
     *  lookahead buffer.  Return eof upon end of the stream we're pulling from.
     */
    public abstract T nextElement();

    /** Get and remove first element in queue; override FastQueue.remove() */
    public T remove() {
        T o = get(0);
        p++;
        // have we hit end of buffer and not backtracking?
        if ( p == data.size() && markDepth==0 ) {
            // if so, it's an opportunity to start filling at index 0 again
            clear(); // size goes to 0, but retains memory
        }
        return o;
    }

    /** Make sure we have at least one element to remove, even if EOF */
    public void consume() { sync(1); remove(); }

    /** Make sure we have 'need' elements from current position p. Last valid
     *  p index is data.size()-1.  p+need-1 is the data index 'need' elements
     *  ahead.  If we need 1 element, (p+1-1)==p must be < data.size().
     */
    public void sync(int need) {
        int n = (p+need-1) - data.size() + 1; // how many more elements we need?
        if ( n > 0 ) fill(n);                 // out of elements?
    }

    /** add n elements to buffer */
    public void fill(int n) {
        for (int i=1; i<=n; i++) {
            T o = nextElement();
            if ( o==eof ) {
                data.add(eof);
                eofElementIndex = data.size()-1;
            }
            else data.add(o);
        }
    }

    //public boolean hasNext() { return eofElementIndex!=UNINITIALIZED_EOF_ELEMENT_INDEX; }

    /** Size of entire stream is unknown; we only know buffer size from FastQueue */
    public int size() { throw new UnsupportedOperationException("streams are of unknown size"); }

    public Object LT(int k) {
		if ( k==0 ) {
			return null;
		}
		if ( k<0 ) {
			return LB(-k);
		}
		//System.out.print("LT(p="+p+","+k+")=");
		if ( (p+k-1) >= eofElementIndex ) { // move to super.LT
			return eof;
		}
        sync(k);
        return get(k-1);
	}

	/** Look backwards k nodes */
	protected Object LB(int k) {
		if ( k==0 ) {
			return null;
		}
		if ( (p-k)<0 ) {
			return null;
		}
		return get(-k);
	}

    public Object getCurrentSymbol() { return LT(1); }

    public int index() { return p; }

	public int mark() {
        markDepth++;
        lastMarker = index();
        return lastMarker;
	}

	public void release(int marker) {
		// no resources to release
	}

	public void rewind(int marker) {
        markDepth--;
        seek(marker); // assume marker is top
        // release(marker); // waste of call; it does nothing in this class
    }

	public void rewind() {
        seek(lastMarker); // rewind but do not release marker
    }

    /** Seek to a 0-indexed position within data buffer.  Can't handle
     *  case where you seek beyond end of existing buffer.  Normally used
     *  to seek backwards in the buffer. Does not force loading of nodes.
     */
    public void seek(int index) { p = index; }
}