xref: /external/antlr/antlr-3.4/runtime/ObjC/Framework/FastQueue.h

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

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modification, are permitted provided that the following conditions
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documentation and/or other materials provided with the distribution.
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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.
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*/
package org.antlr.runtime.misc;

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

/** A queue that can dequeue and get(i) in O(1) and grow arbitrarily large.
 *  A linked list is fast at dequeue but slow at get(i).  An array is
 *  the reverse.  This is O(1) for both operations.
 *
 *  List grows until you dequeue last element at end of buffer. Then
 *  it resets to start filling at 0 again.  If adds/removes are balanced, the
 *  buffer will not grow too large.
 *
 *  No iterator stuff as that's not how we'll use it.
 */
public class FastQueue<T> {
    /** dynamically-sized buffer of elements */
    protected List<T> data = new ArrayList<T>();
    /** index of next element to fill */
    protected int p = 0;

    public void reset() { p = 0; data.clear(); }

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

    public void add(T o) { data.add(o); }

    public int size() { return data.size() - p; }

    public T head() { return get(0); }

    /** Return element i elements ahead of current element.  i==0 gets
     *  current element.  This is not an absolute index into the data list
     *  since p defines the start of the real list.
     */
    public T get(int i) {
        if ( p+i >= data.size() ) {
            throw new NoSuchElementException("queue index "+(p+i)+" > size "+data.size());
        }
        return data.get(p+i);
    }

    public void clear() { p = 0; data.clear(); }

    /** Return string of current buffer contents; non-destructive */
    public String toString() {
        StringBuffer buf = new StringBuffer();
        int n = size();
        for (int i=0; i<n; i++) {
            buf.append(get(i));
            if ( (i+1)<n ) buf.append(" ");
        }
        return buf.toString();
    }
}