org.simmetrics.metrics.LongestCommonSubstring Maven / Gradle / Ivy
/*
* #%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 java.lang.Math.max;
import org.simmetrics.StringDistance;
import org.simmetrics.StringMetric;
/**
* Applies the longest common substring algorithm to calculate the similarity
* and distance between two strings.
*
*
* similarity(a,b) = ∣lcs(a,b)∣ / max{∣a∣, ∣b∣}
*
* distance(a,b) = ∣a∣ + ∣b∣ - 2 * ∣lcs(a,b)∣
*
*
* This class is immutable and thread-safe.
*
* @see Wikipedia
* - Longest common substring problem
*/
public final class LongestCommonSubstring implements StringMetric,
StringDistance {
@Override
public float compare(String a, String b) {
if (a.isEmpty() && b.isEmpty()) {
return 1.0f;
}
if (a.isEmpty() || b.isEmpty()) {
return 0.0f;
}
return lcs(a, b) / (float) max(a.length(), b.length());
}
@Override
public float distance(String a, String b) {
if (a.isEmpty() && b.isEmpty()) {
return 0.0f;
}
if (a.isEmpty()) {
return b.length();
}
if (b.isEmpty()) {
return a.length();
}
return a.length() + b.length() - 2 * lcs(a, b);
}
private static int lcs(String a, String b) {
final int m = a.length();
final int n = b.length();
int[] v0 = new int[n];
int[] v1 = new int[n];
int z = 0;
for (int i = 0; i < m; i++) {
for (int j = 0; j < n; j++) {
if (a.charAt(i) == b.charAt(j)) {
if (i == 0 || j == 0) {
v1[j] = 1;
} else {
v1[j] = v0[j - 1] + 1;
}
if (v1[j] > z) {
z = v1[j];
}
} else {
v1[j] = 0;
}
}
final int[] swap = v0; v0 = v1; v1 = swap;
}
return z;
}
@Override
public String toString() {
return "LongestCommonSubstring";
}
}