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The S-Space Package is a collection of algorithms for building Semantic Spaces as well as a highly-scalable library for designing new distributional semantics algorithms. Distributional algorithms process text corpora and represent the semantic for words as high dimensional feature vectors. This package also includes matrices, vectors, and numerous clustering algorithms. These approaches are known by many names, such as word spaces, semantic spaces, or distributed semantics and rest upon the Distributional Hypothesis: words that appear in similar contexts have similar meanings.

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/*
 * Copyright 2010 David Jurgens
 *
 * This file is part of the S-Space package and is covered under the terms and
 * conditions therein.
 *
 * The S-Space package is free software: you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 as published
 * by the Free Software Foundation and distributed hereunder to you.
 *
 * THIS SOFTWARE IS PROVIDED "AS IS" AND NO REPRESENTATIONS OR WARRANTIES,
 * EXPRESS OR IMPLIED ARE MADE.  BY WAY OF EXAMPLE, BUT NOT LIMITATION, WE MAKE
 * NO REPRESENTATIONS OR WARRANTIES OF MERCHANT- ABILITY OR FITNESS FOR ANY
 * PARTICULAR PURPOSE OR THAT THE USE OF THE LICENSED SOFTWARE OR DOCUMENTATION
 * WILL NOT INFRINGE ANY THIRD PARTY PATENTS, COPYRIGHTS, TRADEMARKS OR OTHER
 * RIGHTS.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program. If not, see .
 */

package edu.ucla.sspace.dependency;

import edu.ucla.sspace.util.Pair;

import edu.ucla.sspace.util.SortedMultiMap;
import edu.ucla.sspace.util.TreeMultiMap;

import java.util.ArrayDeque;
import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Deque;


/**
 * A class for performing a breadth-first iteration of the links in a dependency
 * tree starting at any arbitrary node.  The paths returned will always begin at
 * the specified start node.
 *
 * 
 *
 * This class is not thread safe.
 */
public class BreadthFirstPathIterator implements Iterator {
    
    /**
     * The paths that have been expanded from the starting node but have not yet
     * been returned.
     */
    protected final Queue frontier;

    /**
     * The maximum length of any path returned by the iterator.
     */
    private final int maxPathLength;

    /**
     * Creates a new iterator over all the paths starting at the provided index.
     *
     * @param startNode the node that will start all the paths to be generated.
     */
    public BreadthFirstPathIterator(DependencyTreeNode startNode) {
        this(startNode, Integer.MAX_VALUE);
    }

    /**
     * Creates a new iterator over all the paths starting at the provided index
     * that will only return paths up to the specified maximum length.
     *
     * @param startNode the node that will start all the paths to be generated.
     * @param maxPathLength the maximum path length to return
     *
     * @throws IllegalArgumentException if {@maxPathLength} is < 1.
     */
    public BreadthFirstPathIterator(DependencyTreeNode startNode, 
                                    int maxPathLength) {
        if (maxPathLength < 1)
            throw new IllegalArgumentException(
                "Must specify a path length greater than or equal to 1");
        this.maxPathLength = maxPathLength;
        frontier = new ArrayDeque();

        // Base-case: find all the paths of length 1
        for (DependencyRelation rel : startNode.neighbors()) {
            // Orient the path depending on whether the root was the head of the
            // relationship or not.  This ensures that the root is always the
            // first node in the path and any expansion will continue away from
            // the root.
            frontier.offer(new SimpleDependencyPath(
                               rel, rel.headNode().equals(startNode)));
        }
    }

    /**
     * Expands the breadth-first frontier by adding all the new paths one link
     * away to the end of {@code frontier}.
     */
    /* package-private */ void advance(SimpleDependencyPath path) {
        if (path.length() >= maxPathLength)
            return;

        // Get the last node and last relation to decide how to expand.
        DependencyRelation lastRelation = path.lastRelation();
        DependencyTreeNode last = path.last(); 
        
        // Expand all of the possible relations from the last node, creating a
        // new path for each, except if the relation is the one that generated
        // this path.
        for (DependencyRelation rel : last.neighbors()) {
            // Skip re-adding the current relation
            if (lastRelation.equals(rel))
                continue;
            SimpleDependencyPath extended = path.extend(rel);
            frontier.offer(extended);
        }
    }

    /**
     * Returns {@code true} if there are still paths to return for the tree.
     */
    public boolean hasNext() {
        return !frontier.isEmpty();
    }

    /**
     * Returns the next {@code DependencyPath} in the tree whose length is equal
     * or greater than the previously returned path.
     */
    public DependencyPath next() {
        SimpleDependencyPath p = frontier.remove();
        // Expand the frontier 1 link starting from the current path
        advance(p);
        return p;
    }

    /**
     * Throws an {@code UnsupportedOperationException} if called
     */
    public void remove() {
        throw new UnsupportedOperationException("Removal is not possible");
    }
}