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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 Keith Stevens
 *
 * 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 java.util.ArrayDeque;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Deque;


/**
 * A traversal class for iterating over a dependency tree of {@link Relation}s.
 * Given the tree and a starting index, the traverser will find all paths that
 * satisfy several different criteria: length of the path and accepted relations
 * in the path.  
 *
 * 
 *
 * Note that this class is NOT thread safe.
 */
public class DependencyIterator implements Iterator {

    /**
     * The maximum length of the returned paths.  The length is considedered to
     * not include the first term.
     */
    private final int maxPathLength;

    /**
     * The paths that have been expanded from the starting node but have not yet
     * been returned.
     */
    protected final Queue frontier;

    /**
     * The {@link DependencyRelationAcceptor} that validates each link before it is
     * traversed and returned as part of a {@link DependencyPath}.
     */
    private DependencyRelationAcceptor acceptor;

    /**
     * Creates a new {@link DependencyIterator} that will return all {@link
     * DependencyPath}s rooted at the term with index {@code startTerm}.  Each
     * link in the path will be valied with {@code acceptor} and weighted with
     * {@code weighter}.  Each path will have length 1 + {@code maxPathLength}.
     *
     * @param startNode the node that will start all the paths to be generated.
     * @param acceptor The {@link DependencyRelationAcceptor} that will validate
     *        each link the a path
     * @param maxPathLength the maximum length of any path returned
     * 
     * @throws IllegalArgumentException if {@code maxPathLength} is less than 1
     */
    public DependencyIterator(DependencyTreeNode startNode,
                              DependencyRelationAcceptor acceptor,
                              int maxPathLength) {
        if (maxPathLength < 1)
            throw new IllegalArgumentException(
                "Must specify a path length greater than or equal to 1");
        this.maxPathLength = maxPathLength;
        this.acceptor = acceptor;
        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.
            if (acceptor.accept(rel)) {
                frontier.offer(new SimpleDependencyPath(
                               rel, rel.headNode().equals(startNode)));
            }
        }
    }

    /**
     * Extends the path in its growth direction and adds to the frontier those
     * relations that are shorter than the maximum path length and that are
     * accepted by the {@code DependencyRelationAcceptor} used by this instance.
     */
    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) || !acceptor.accept(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");
    }
}