BilouDecoder.java

/*
 * Copyright 2016
 * Ubiquitous Knowledge Processing (UKP) Lab and FG Language Technology
 * 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.api.io;

import java.util.List;

import org.apache.uima.cas.CAS;
import org.apache.uima.cas.Feature;
import org.apache.uima.cas.Type;
import org.apache.uima.cas.text.AnnotationFS;

import de.tudarmstadt.ukp.dkpro.core.api.resources.MappingProvider;

/**
 * Creates annotations from BILOU encoded data.
 */
public class BilouDecoder
{
    private CAS cas;
    private Feature chunkValue;

    private MappingProvider mappingProvider;

    private boolean internTags = true;
    
    private String openChunk;
    private int start;
    private int end;
    

    public BilouDecoder(CAS aCas, Feature aValueFeature, MappingProvider aMappingProvider)
    {
        super();
        cas = aCas;
        chunkValue = aValueFeature;
        mappingProvider = aMappingProvider;
    }

    public void setInternTags(boolean aInternTags)
    {
        internTags = aInternTags;
    }
    
    public void decode(List<? extends AnnotationFS> aTokens, String[] aTags)
    {
        int i = 0;
        for (AnnotationFS token : aTokens) {
            String tag = aTags[i];
            
            // Check if the BILOU encoding is present and fail if not
            if (
                !(tag.length() == 1 && tag.charAt(0) == 'O') &&
                !(tag.length() >= 2 && tag.charAt(1) == '-')
            ) {
                throw new IllegalStateException("Tag is not BILOU-encoded: [" + tag + "]");
            }
            
            switch (tag.charAt(0)) {
            case 'B':
                start = token.getBegin();
                openChunk = tag.substring(2);
                break;
            case 'I':
                // Nothing to do because we wait for the L
                break;
            case 'L':
                // End of previous chunk
                end = token.getEnd();
                chunkComplete();
                break;
            case 'O':
                // Nothing to do, no annotation here
                break;
            case 'U':
                start = token.getBegin();
                end = token.getEnd();
                openChunk = tag.substring(2);
                chunkComplete();
                break;
            default:
                throw new IllegalStateException("Tag is not BILOU-encoded: [" + tag + "]");
            }
            
            i++;
        }
        
        // End of processing signal
        chunkComplete();
    }
    
    private void chunkComplete()
    {
        if (openChunk != null) {
            Type chunkType = mappingProvider.getTagType(openChunk);
            AnnotationFS chunk = cas.createAnnotation(chunkType, start, end);
            chunk.setStringValue(chunkValue, internTags ? openChunk.intern() :
                openChunk);
            cas.addFsToIndexes(chunk);
            openChunk = null;
        }
    }
}