LeftToRightSplitterAlgorithm.java

/*
 * Copyright 2010
 * Ubiquitous Knowledge Processing (UKP) Lab
 * Technische Universität Darmstadt
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 **/

package de.tudarmstadt.ukp.dkpro.core.decompounding.splitter;

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

import de.tudarmstadt.ukp.dkpro.core.decompounding.dictionary.Dictionary;
import de.tudarmstadt.ukp.dkpro.core.decompounding.dictionary.LinkingMorphemes;
import de.tudarmstadt.ukp.dkpro.core.decompounding.trie.ValueNode;

/**
 * Implements a simple left to right split algorithm.
 * 
 * Goes from left to right to the word. If a word is found the right side is evaluation from left to
 * right. At the end we have a right balanced tree. All leaves are the smallest word fractions.
 * 
 * The leaves will combined to all possible splits.
 * 
 */
public class LeftToRightSplitterAlgorithm
    implements SplitterAlgorithm
{
    private Dictionary dict;
    private LinkingMorphemes morphemes;
    private int maxTreeDepth = Integer.MAX_VALUE;
    private int minWordLength = 1;
    private int minRestLength = 2;

    /**
     * Empty constructor.
     * 
     * Before you use this class set the dictionary and the linking morphemes with the setter
     * methods
     */
    public LeftToRightSplitterAlgorithm()
    {
        // Nothing to do
    }

    /**
     * Create a instance of the algorithm
     * 
     * @param aDict
     *            A dictionary with all words
     * @param aMorphemes
     *            A LinkingMorphemes class
     */
    public LeftToRightSplitterAlgorithm(Dictionary aDict, LinkingMorphemes aMorphemes)
    {
        setDictionary(aDict);
        setLinkingMorphemes(aMorphemes);
    }

    @Override
    public DecompoundingTree split(String aWord)
    {
        aWord = aWord.toLowerCase();

        DecompoundingTree t = new DecompoundingTree(aWord);
        t.getRoot().getValue().getSplits().get(0).setSplitAgain(true);

        ltrSplit(t.getRoot(), 0);

        return t;
    }

    /**
     * The basic split algorithm. Moves the word from left to right and checks for valid words.
     * 
     * @param aParent
     *            The parent node
     * @param aDepth
     *            the current depth.
     */
    protected void ltrSplit(ValueNode<DecompoundedWord> aParent, int aDepth)
    {
        if (aDepth > maxTreeDepth) {
            return;
        }

        for (int i = 0; i < aParent.getValue().getSplits().size(); i++) {
            Fragment element = aParent.getValue().getSplits().get(i);

            if (element.shouldSplitAgain()) {
                List<DecompoundedWord> results = makeSplit(element.getWord());

                for (DecompoundedWord result : results) {
                    DecompoundedWord copy = aParent.getValue().createCopy();
                    if (result.getSplits().size() > 1) {
                        result.getSplits().get(1).setSplitAgain(true);
                        copy.replaceSplitElement(i, result);
                        ValueNode<DecompoundedWord> child = new ValueNode<DecompoundedWord>(copy);
                        aParent.addChild(child);
                        ltrSplit(child, aDepth + 1);
                    }
                    else if (result.getSplits().size() == 1 && !result.equals(aParent.getValue())) {
                        copy.replaceSplitElement(i, result);
                        ValueNode<DecompoundedWord> child = new ValueNode<DecompoundedWord>(copy);
                        aParent.addChild(child);
                    }
                }
            }
        }
    }

    /**
     * Splits a word in two word.
     * 
     * @param aWord
     *            a word.
     * @return the splits.
     */
    protected List<DecompoundedWord> makeSplit(String aWord)
    {
        List<DecompoundedWord> result = new ArrayList<DecompoundedWord>();

        for (int i = 0; i < aWord.length(); i++) {
            String leftWord = aWord.substring(0, i + 1);
            String rightWord = aWord.substring(i + 1);

            boolean leftGood = dict.contains(leftWord) && leftWord.length() >= minWordLength;
            boolean rightGood = rightWord.length() > minRestLength || rightWord.length() == 0;

            if (leftGood && rightGood) {
                // createFromString removes the trailing + if rightWord is empty.
                DecompoundedWord split = DecompoundedWord.createFromString(leftWord + "+"
                        + rightWord);
                split.setSplitPos(i);
                result.add(split);
            }

            // Check if left word contains linking morphemes
            for (String morpheme : morphemes.getAll()) {
                try {
                    String leftWithoutMorpheme = leftWord.substring(0,
                            leftWord.length() - morpheme.length());
                    if (leftWord.endsWith(morpheme) && dict.contains(leftWithoutMorpheme)
                            && rightGood) {
                        DecompoundedWord split = DecompoundedWord
                                .createFromString(leftWithoutMorpheme + "(" + morpheme + ")+"
                                        + rightWord);
                        split.setSplitPos(i);
                        result.add(split);
                    }
                }
                catch (StringIndexOutOfBoundsException e) {
                    continue;
                }
            }
        }

        return result;
    }

    @Override
    public void setDictionary(Dictionary aDict)
    {
        dict = aDict;
    }

    public Dictionary getDictionary()
    {
        return dict;
    }

    @Override
    public void setLinkingMorphemes(LinkingMorphemes aLinkingMorphemes)
    {
        morphemes = aLinkingMorphemes;
    }

    public LinkingMorphemes getMorphemes()
    {
        return morphemes;
    }

    @Override
    public void setMaximalTreeDepth(int aDepth)
    {
        maxTreeDepth = aDepth;
    }

    public int getMaxTreeDepth()
    {
        return maxTreeDepth;
    }

    public void setMinWordLength(int aMinWordLength)
    {
        minWordLength = aMinWordLength;
    }

    public int getMinWordLength()
    {
        return minWordLength;
    }

    public void setMinRestLength(int aMinRestLength)
    {
        minRestLength = aMinRestLength;
    }

    public int getMinRestLength()
    {
        return minRestLength;
    }
}