/*******************************************************************************
* Copyright 2015-2016 - CNRS (Centre National de Recherche Scientifique)
*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you 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.
*
*******************************************************************************/
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright 2, 2015nership. The ASF licenses this file
* to you 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 eu.project.ttc.metrics;
/**
* Longest Common Subsequence.<br />
* <pre>
* | ls(i-1,j-1) + 1, if a[i]=b[j]
* ls(i,j) = |
* | max(ls(i-1,j), ls(i,j-1)), else
* Boundary conditions: let think that ls[i][-1] = 0, ls[-1][j] = 0
* </pre>
*/
public class LongestCommonSubsequence extends AbstractEditDistance {
@Override
public int compute(String a, String b) {
this.a = a;
this.b = b;
this.set();
return this.get();
}
// strings a, b
private String a;
private String b;
// ls(i,j) - maximum length of common subsequence that end at a[i] and b[j].
private int ls[][];
// specifies which neighboring cell ls(i,j) it got its value
Direction direction[][];
private void set() {
// init with 0 by default
ls = new int[a.length() + 1][b.length() + 1];
// so this could be skipped
for (int i = 0; i <= a.length(); i++) {
ls[i][0] = 0;
}
for (int j = 0; j <= b.length(); j++) {
ls[0][j] = 0;
}
direction = new Direction[a.length() + 1][b.length() + 1];
for (int i = 1; i <= a.length(); i++) {
for (int j = 1; j <= b.length(); j++) {
if (a.charAt(i - 1) == b.charAt(j - 1)) {
ls[i][j] = ls[i - 1][j - 1] + 1;
direction[i][j] = Direction.DIAG;
} else if (ls[i - 1][j] >= ls[i][j - 1]) {
ls[i][j] = ls[i - 1][j];
direction[i][j] = Direction.UP;
} else {
ls[i][j] = ls[i][j - 1];
direction[i][j] = Direction.LEFT;
}
}
}
}
private int get() {
StringBuilder sb = new StringBuilder();
int i = a.length();
int j = b.length();
while (i > 0 && j > 0) {
if (direction[i][j] == Direction.DIAG) {
sb.append(a.charAt(i - 1));
i--;
j--;
} else if (direction[i][j] == Direction.UP)
i--;
else
j--;
}
return sb.reverse().toString().length();
}
private enum Direction {
LEFT, UP, DIAG;
@Override
public String toString() {
switch (this) {
case LEFT:
return "<";
case UP:
return "^";
case DIAG:
return "\\";
}
throw new IllegalStateException("wrong direction");
}
}
@Override
public double normalize(int distance, String source, String target) {
return (double) (2 * distance) / (double) (source.length() + target.length());
}
@Override
public boolean isFailFast() {
return false;
}
@Override
public void setFailThreshold(double threshold) {
// Nothing
}
}