StringsComparator.java
/*
* 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.
*/
package org.apache.commons.text.diff;
/**
* <p>
* It is guaranteed that the comparisons will always be done as
* <code>o1.equals(o2)</code> where <code>o1</code> belongs to the first
* sequence and <code>o2</code> belongs to the second sequence. This can
* be important if subclassing is used for some elements in the first
* sequence and the <code>equals</code> method is specialized.
* </p>
* <p>
* Comparison can be seen from two points of view: either as giving the smallest
* modification allowing to transform the first sequence into the second one, or
* as giving the longest sequence which is a subsequence of both initial
* sequences. The <code>equals</code> method is used to compare objects, so any
* object can be put into sequences. Modifications include deleting, inserting
* or keeping one object, starting from the beginning of the first sequence.
* </p>
* <p>
* This class implements the comparison algorithm, which is the very efficient
* algorithm from Eugene W. Myers
* <a href="http://www.cis.upenn.edu/~bcpierce/courses/dd/papers/diff.ps">
* An O(ND) Difference Algorithm and Its Variations</a>. This algorithm produces
* the shortest possible {@link EditScript edit script} containing all the
* {@link EditCommand commands} needed to transform the first sequence into
* the second one.
*
* <p>
* This code has been adapted from Apache Commons Collections 4.0.
* </p>
*
* @see EditScript
* @see EditCommand
* @see CommandVisitor
* @since 1.0
*/
public class StringsComparator {
/**
* First character sequence.
*/
private final String left;
/**
* Second character sequence.
*/
private final String right;
/**
* Temporary array.
*/
private final int[] vDown;
/**
* Temporary array.
*/
private final int[] vUp;
/**
* Simple constructor.
* <p>
* Creates a new instance of StringsComparator.
* </p>
* <p>
* It is <em>guaranteed</em> that the comparisons will always be done as
* <code>o1.equals(o2)</code> where <code>o1</code> belongs to the first
* sequence and <code>o2</code> belongs to the second sequence. This can be
* important if subclassing is used for some elements in the first sequence
* and the <code>equals</code> method is specialized.
* </p>
*
* @param left first character sequence to be compared
* @param right second character sequence to be compared
*/
public StringsComparator(final String left, final String right) {
this.left = left;
this.right = right;
final int size = left.length() + right.length() + 2;
vDown = new int[size];
vUp = new int[size];
}
/**
* Get the {@link EditScript} object.
* <p>
* It is guaranteed that the objects embedded in the {@link InsertCommand
* insert commands} come from the second sequence and that the objects
* embedded in either the {@link DeleteCommand delete commands} or
* {@link KeepCommand keep commands} come from the first sequence. This can
* be important if subclassing is used for some elements in the first
* sequence and the <code>equals</code> method is specialized.
* </p>
*
* @return the edit script resulting from the comparison of the two
* sequences
*/
public EditScript<Character> getScript() {
final EditScript<Character> script = new EditScript<>();
buildScript(0, left.length(), 0, right.length(), script);
return script;
}
/**
* Build an edit script.
*
* @param start1 the begin of the first sequence to be compared
* @param end1 the end of the first sequence to be compared
* @param start2 the begin of the second sequence to be compared
* @param end2 the end of the second sequence to be compared
* @param script the edited script
*/
private void buildScript(final int start1, final int end1, final int start2, final int end2,
final EditScript<Character> script) {
final Snake middle = getMiddleSnake(start1, end1, start2, end2);
if (middle == null
|| middle.getStart() == end1 && middle.getDiag() == end1 - end2
|| middle.getEnd() == start1 && middle.getDiag() == start1 - start2) {
int i = start1;
int j = start2;
while (i < end1 || j < end2) {
if (i < end1 && j < end2 && left.charAt(i) == right.charAt(j)) {
script.append(new KeepCommand<>(left.charAt(i)));
++i;
++j;
} else {
if (end1 - start1 > end2 - start2) {
script.append(new DeleteCommand<>(left.charAt(i)));
++i;
} else {
script.append(new InsertCommand<>(right.charAt(j)));
++j;
}
}
}
} else {
buildScript(start1, middle.getStart(),
start2, middle.getStart() - middle.getDiag(),
script);
for (int i = middle.getStart(); i < middle.getEnd(); ++i) {
script.append(new KeepCommand<>(left.charAt(i)));
}
buildScript(middle.getEnd(), end1,
middle.getEnd() - middle.getDiag(), end2,
script);
}
}
/**
* Get the middle snake corresponding to two subsequences of the
* main sequences.
* <p>
* The snake is found using the MYERS Algorithm (this algorithms has
* also been implemented in the GNU diff program). This algorithm is
* explained in Eugene Myers article:
* <a href="http://www.cs.arizona.edu/people/gene/PAPERS/diff.ps">
* An O(ND) Difference Algorithm and Its Variations</a>.
* </p>
*
* @param start1 the begin of the first sequence to be compared
* @param end1 the end of the first sequence to be compared
* @param start2 the begin of the second sequence to be compared
* @param end2 the end of the second sequence to be compared
* @return the middle snake
*/
private Snake getMiddleSnake(final int start1, final int end1, final int start2, final int end2) {
// Myers Algorithm
// Initialisations
final int m = end1 - start1;
final int n = end2 - start2;
if (m == 0 || n == 0) {
return null;
}
final int delta = m - n;
final int sum = n + m;
final int offset = (sum % 2 == 0 ? sum : sum + 1) / 2;
vDown[1 + offset] = start1;
vUp[1 + offset] = end1 + 1;
for (int d = 0; d <= offset; ++d) {
// Down
for (int k = -d; k <= d; k += 2) {
// First step
final int i = k + offset;
if (k == -d || k != d && vDown[i - 1] < vDown[i + 1]) {
vDown[i] = vDown[i + 1];
} else {
vDown[i] = vDown[i - 1] + 1;
}
int x = vDown[i];
int y = x - start1 + start2 - k;
while (x < end1 && y < end2 && left.charAt(x) == right.charAt(y)) {
vDown[i] = ++x;
++y;
}
// Second step
if (delta % 2 != 0 && delta - d <= k && k <= delta + d) {
if (vUp[i - delta] <= vDown[i]) { // NOPMD
return buildSnake(vUp[i - delta], k + start1 - start2, end1, end2);
}
}
}
// Up
for (int k = delta - d; k <= delta + d; k += 2) {
// First step
final int i = k + offset - delta;
if (k == delta - d
|| k != delta + d && vUp[i + 1] <= vUp[i - 1]) {
vUp[i] = vUp[i + 1] - 1;
} else {
vUp[i] = vUp[i - 1];
}
int x = vUp[i] - 1;
int y = x - start1 + start2 - k;
while (x >= start1 && y >= start2
&& left.charAt(x) == right.charAt(y)) {
vUp[i] = x--;
y--;
}
// Second step
if (delta % 2 == 0 && -d <= k && k <= d) {
if (vUp[i] <= vDown[i + delta]) { // NOPMD
return buildSnake(vUp[i], k + start1 - start2, end1, end2);
}
}
}
}
// this should not happen
throw new RuntimeException("Internal Error");
}
/**
* Build a snake.
*
* @param start the value of the start of the snake
* @param diag the value of the diagonal of the snake
* @param end1 the value of the end of the first sequence to be compared
* @param end2 the value of the end of the second sequence to be compared
* @return the snake built
*/
private Snake buildSnake(final int start, final int diag, final int end1, final int end2) {
int end = start;
while (end - diag < end2
&& end < end1
&& left.charAt(end) == right.charAt(end - diag)) {
++end;
}
return new Snake(start, end, diag);
}
/**
* This class is a simple placeholder to hold the end part of a path
* under construction in a {@link StringsComparator StringsComparator}.
*/
private static class Snake {
/** Start index. */
private final int start;
/** End index. */
private final int end;
/** Diagonal number. */
private final int diag;
/**
* Simple constructor. Creates a new instance of Snake with specified indices.
*
* @param start start index of the snake
* @param end end index of the snake
* @param diag diagonal number
*/
public Snake(final int start, final int end, final int diag) {
this.start = start;
this.end = end;
this.diag = diag;
}
/**
* Get the start index of the snake.
*
* @return start index of the snake
*/
public int getStart() {
return start;
}
/**
* Get the end index of the snake.
*
* @return end index of the snake
*/
public int getEnd() {
return end;
}
/**
* Get the diagonal number of the snake.
*
* @return diagonal number of the snake
*/
public int getDiag() {
return diag;
}
}
}