package org.apache.lucene.util.automaton;
/**
* 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.
*/
import java.util.Iterator;
import java.util.SortedSet;
import java.util.TreeSet;
/**
* Class to construct DFAs that match a word within some edit distance.
* <p>
* Implements the algorithm described in:
* Schulz and Mihov: Fast String Correction with Levenshtein Automata
* <p>
* @lucene.experimental
*/
public class LevenshteinAutomata {
/** @lucene.internal */
public static final int MAXIMUM_SUPPORTED_DISTANCE = 2;
/* input word */
final String input;
final int word[];
/* the automata alphabet. */
final int alphabet[];
/* the unicode ranges outside of alphabet */
final int rangeLower[];
final int rangeUpper[];
int numRanges = 0;
ParametricDescription descriptions[];
/**
* Create a new LevenshteinAutomata for some input String.
*/
public LevenshteinAutomata(String input) {
this.input = input;
int length = Character.codePointCount(input, 0, input.length());
word = new int[length];
for (int i = 0, j = 0, cp = 0; i < input.length(); i += Character.charCount(cp)) {
word[j++] = cp = input.codePointAt(i);
}
// calculate the alphabet
SortedSet<Integer> set = new TreeSet<Integer>();
for (int i = 0; i < word.length; i++)
set.add(word[i]);
alphabet = new int[set.size()];
Iterator<Integer> iterator = set.iterator();
for (int i = 0; i < alphabet.length; i++)
alphabet[i] = iterator.next();
rangeLower = new int[alphabet.length + 2];
rangeUpper = new int[alphabet.length + 2];
// calculate the unicode range intervals that exclude the alphabet
// these are the ranges for all unicode characters not in the alphabet
int lower = 0;
for (int i = 0; i < alphabet.length; i++) {
int higher = alphabet[i];
if (higher > lower) {
rangeLower[numRanges] = lower;
rangeUpper[numRanges] = higher - 1;
numRanges++;
}
lower = higher + 1;
}
/* add the final endpoint */
if (lower <= Character.MAX_CODE_POINT) {
rangeLower[numRanges] = lower;
rangeUpper[numRanges] = Character.MAX_CODE_POINT;
numRanges++;
}
descriptions = new ParametricDescription[] {
null, /* for n=0, we do not need to go through the trouble */
new Lev1ParametricDescription(word.length),
new Lev2ParametricDescription(word.length),
};
}
/**
* Compute a DFA that accepts all strings within an edit distance of <code>n</code>.
* <p>
* All automata have the following properties:
* <ul>
* <li>They are deterministic (DFA).
* <li>There are no transitions to dead states.
* <li>They are not minimal (some transitions could be combined).
* </ul>
* </p>
*/
public Automaton toAutomaton(int n) {
if (n == 0)
return BasicAutomata.makeString(input);
if (n >= descriptions.length)
return null;
final int range = 2*n+1;
ParametricDescription description = descriptions[n];
// the number of states is based on the length of the word and n
State states[] = new State[description.size()];
// create all states, and mark as accept states if appropriate
for (int i = 0; i < states.length; i++) {
states[i] = new State();
states[i].number = i;
states[i].setAccept(description.isAccept(i));
}
// create transitions from state to state
for (int k = 0; k < states.length; k++) {
final int xpos = description.getPosition(k);
if (xpos < 0)
continue;
final int end = xpos + Math.min(word.length - xpos, range);
for (int x = 0; x < alphabet.length; x++) {
final int ch = alphabet[x];
// get the characteristic vector at this position wrt ch
final int cvec = getVector(ch, xpos, end);
int dest = description.transition(k, xpos, cvec);
if (dest >= 0)
states[k].addTransition(new Transition(ch, states[dest]));
}
// add transitions for all other chars in unicode
// by definition, their characteristic vectors are always 0,
// because they do not exist in the input string.
int dest = description.transition(k, xpos, 0); // by definition
if (dest >= 0)
for (int r = 0; r < numRanges; r++)
states[k].addTransition(new Transition(rangeLower[r], rangeUpper[r], states[dest]));
// reduce the state: this doesn't appear to help anything
//states[k].reduce();
}
Automaton a = new Automaton(states[0]);
a.setDeterministic(true);
a.setNumberedStates(states);
// we need not trim transitions to dead states, as they are not created.
//a.restoreInvariant();
return a;
}
/**
* Get the characteristic vector <code>X(x, V)</code>
* where V is <code>substring(pos, end)</code>
*/
int getVector(int x, int pos, int end) {
int vector = 0;
for (int i = pos; i < end; i++) {
vector <<= 1;
if (word[i] == x)
vector |= 1;
}
return vector;
}
/**
* A ParametricDescription describes the structure of a Levenshtein DFA for some degree n.
* <p>
* There are four components of a parametric description, all parameterized on the length
* of the word <code>w</code>:
* <ol>
* <li>The number of states: {@link #size()}
* <li>The set of final states: {@link #isAccept(int)}
* <li>The transition function: {@link #transition(int, int, int)}
* <li>Minimal boundary function: {@link #getPosition(int)}
* </ol>
*/
static abstract class ParametricDescription {
protected final int w;
protected final int n;
private final int[] minErrors;
ParametricDescription(int w, int n, int[] minErrors) {
this.w = w;
this.n = n;
this.minErrors = minErrors;
}
/**
* Return the number of states needed to compute a Levenshtein DFA
*/
int size() {
return minErrors.length * (w+1);
};
/**
* Returns true if the <code>state</code> in any Levenshtein DFA is an accept state (final state).
*/
boolean isAccept(int absState) {
// decode absState -> state, offset
int state = absState/(w+1);
int offset = absState%(w+1);
assert offset >= 0;
return w - offset + minErrors[state] <= n;
}
/**
* Returns the position in the input word for a given <code>state</code>.
* This is the minimal boundary for the state.
*/
int getPosition(int absState) {
return absState % (w+1);
}
/**
* Returns the state number for a transition from the given <code>state</code>,
* assuming <code>position</code> and characteristic vector <code>vector</code>
*/
abstract int transition(int state, int position, int vector);
private final static long[] MASKS = new long[] {0x1,0x3,0x7,0xf,
0x1f,0x3f,0x7f,0xff,
0x1ff,0x3ff,0x7ff,0xfff,
0x1fff,0x3fff,0x7fff,0xffff,
0x1ffff,0x3ffff,0x7ffff,0xfffff,
0x1fffff,0x3fffff,0x7fffff,0xffffff,
0x1ffffff,0x3ffffff,0x7ffffff,0xfffffff,
0x1fffffff,0x3fffffff,0x7fffffffL,0xffffffffL,
0x1ffffffffL,0x3ffffffffL,0x7ffffffffL,0xfffffffffL,
0x1fffffffffL,0x3fffffffffL,0x7fffffffffL,0xffffffffffL,
0x1ffffffffffL,0x3ffffffffffL,0x7ffffffffffL,0xfffffffffffL,
0x1fffffffffffL,0x3fffffffffffL,0x7fffffffffffL,0xffffffffffffL,
0x1ffffffffffffL,0x3ffffffffffffL,0x7ffffffffffffL,0xfffffffffffffL,
0x1fffffffffffffL,0x3fffffffffffffL,0x7fffffffffffffL,0xffffffffffffffL,
0x1ffffffffffffffL,0x3ffffffffffffffL,0x7ffffffffffffffL,0xfffffffffffffffL,
0x1fffffffffffffffL,0x3fffffffffffffffL,0x7fffffffffffffffL};
protected int unpack(long[] data, int index, int bitsPerValue) {
final long bitLoc = bitsPerValue * index;
final int dataLoc = (int) (bitLoc >> 6);
final int bitStart = (int) (bitLoc & 63);
//System.out.println("index=" + index + " dataLoc=" + dataLoc + " bitStart=" + bitStart + " bitsPerV=" + bitsPerValue);
if (bitStart + bitsPerValue <= 64) {
// not split
return (int) ((data[dataLoc] >> bitStart) & MASKS[bitsPerValue-1]);
} else {
// split
final int part = 64-bitStart;
return (int) (((data[dataLoc] >> bitStart) & MASKS[part-1]) +
((data[1+dataLoc] & MASKS[bitsPerValue-part-1]) << part));
}
}
}
}