code: /*
   * #%L
   * Simmetrics Core
   * %%
   * Copyright (C) 2014 - 2016 Simmetrics Authors
   * %%
   * 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.
   * #L%
   */
  
  package org.simmetrics.metrics;
  
  import static com.google.common.base.Preconditions.checkArgument;
  import static com.google.common.base.Strings.commonPrefix;
  import static java.lang.Math.min;
  
  import org.simmetrics.StringDistance;
  import org.simmetrics.StringMetric;
  
  /**
   * Calculates the Jaro-Winkler distance (similarity) over two strings.
   * <p>
   * <code>
   * similarity(a,b) = jaro-winkler(a,b)
   * <br>
   * distance(a,b) = 1 - similarity(a,b)
   * </code>
   * <p>
   * Can be configured with a prefix adjustment scale, max prefix length and boost
   * threshold.
   * <p>
   * This class is immutable and thread-safe.
   * 
   * @see <a
   *      href="http://en.wikipedia.org/wiki/Jaro%E2%80%93Winkler_distance">Wikipedia
   *      - Jaro-Winkler distance</a>
   * 
   * @see Jaro
   * 
   * 
   */
  public final class JaroWinkler implements StringMetric, StringDistance {
  
  	private final Jaro jaro = new Jaro();
  
  	private static final float PREFIX_SCALE = 0.1f;
  	private static final float WINKLER_BOOST_THRESHOLD = 0.7f;
  
  	private static final float BOOST_THRESHOLD = 0.0f;
  	private static final int MAX_PREFIX_LENGTH = 4;
  
  	private final float boostThreshold;
  	private final float prefixScale;
  	private final int maxPrefixLength;
  
  	/**
  	 * Constructs a new JaroWinkler metric.
  	 */
  	public JaroWinkler() {
  		this(BOOST_THRESHOLD, PREFIX_SCALE, MAX_PREFIX_LENGTH);
  	}
  
  	/**
  	 * Constructs a new JaroWinkler metric with Winklers boost threshold of 0.7.
  	 * 
  	 * @return a new JaroWinkler metric
  	 */
  	public static JaroWinkler createWithBoostThreshold() {
  		return new JaroWinkler(WINKLER_BOOST_THRESHOLD, PREFIX_SCALE,
  				MAX_PREFIX_LENGTH);
  	}
  
  	/**
  	 * Constructs a new JaroWinkler metric.
  	 * 
  	 * 
  	 * @param boostThreshold
  	 *            minimum jaro score for which the score is boosted
  	 * @param prefixScale
  	 *            scale at which a common prefix adds a bonus
  	 * @param maxPrefixLength
  	 *            cutoff at which a longer common prefix does not improve the
  	 *            score
  	 */
  	public JaroWinkler(float boostThreshold, float prefixScale,
  			int maxPrefixLength) {
  		checkArgument(boostThreshold >= 0);
  		checkArgument(0 <= prefixScale && prefixScale <= 1);
  		checkArgument(maxPrefixLength >= 0);
  
  		this.boostThreshold = boostThreshold;
  		this.prefixScale = prefixScale;
  		this.maxPrefixLength = maxPrefixLength;
  	}
  
  	@Override
  	public float distance(String a, String b) {
  		return 1.0f - compare(a, b);
  	}
  	
  	@Override
  	public float compare(final String a, final String b) {
  		final float jaroScore = jaro.compare(a, b);
  
  		if (jaroScore < boostThreshold) {
  			return jaroScore;
  		}
  
  		int prefixLength = min(commonPrefix(a, b).length(), maxPrefixLength);
  
  		return jaroScore + (prefixLength * prefixScale * (1.0f - jaroScore));
  	}
  
  	@Override
  	public String toString() {
  		return "JaroWinkler [boostThreshold=" + boostThreshold
  				+ ", prefixScale=" + prefixScale + ", maxPrefixLength="
  				+ maxPrefixLength + "]";
  	}
  
  }
  