/*
* dk.brics.automaton
*
* Copyright (c) 2001-2009 Anders Moeller
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. The name of the author may not be used to endorse or promote products
* derived from this software without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
* IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
* OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
* IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
* INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
* NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
* DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
* THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
* (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
* THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
*/
package org.apache.lucene.util.automaton;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Set;
import org.apache.lucene.util.BytesRef;
/**
* Special automata operations.
*
* @lucene.experimental
*/
final public class SpecialOperations {
private SpecialOperations() {}
/**
* Finds the largest entry whose value is less than or equal to c, or 0 if
* there is no such entry.
*/
static int findIndex(int c, int[] points) {
int a = 0;
int b = points.length;
while (b - a > 1) {
int d = (a + b) >>> 1;
if (points[d] > c) b = d;
else if (points[d] < c) a = d;
else return d;
}
return a;
}
/**
* Returns true if the language of this automaton is finite.
*/
public static boolean isFinite(Automaton a) {
if (a.isSingleton()) return true;
return isFinite(a.initial, new HashSet<State>());
}
/**
* Checks whether there is a loop containing s. (This is sufficient since
* there are never transitions to dead states.)
*/
// TODO: not great that this is recursive... in theory a
// large automata could exceed java's stack
private static boolean isFinite(State s, HashSet<State> path) {
path.add(s);
for (Transition t : s.getTransitions())
if (path.contains(t.to) || !isFinite(t.to, path)) return false;
path.remove(s);
return true;
}
/**
* Returns the longest string that is a prefix of all accepted strings and
* visits each state at most once.
*
* @return common prefix
*/
public static String getCommonPrefix(Automaton a) {
if (a.isSingleton()) return a.singleton;
StringBuilder b = new StringBuilder();
HashSet<State> visited = new HashSet<State>();
State s = a.initial;
boolean done;
do {
done = true;
visited.add(s);
if (!s.accept && s.numTransitions() == 1) {
Transition t = s.getTransitions().iterator().next();
if (t.min == t.max && !visited.contains(t.to)) {
b.appendCodePoint(t.min);
s = t.to;
done = false;
}
}
} while (!done);
return b.toString();
}
// TODO: this currently requites a determinized machine,
// but it need not -- we can speed it up by walking the
// NFA instead. it'd still be fail fast.
public static BytesRef getCommonPrefixBytesRef(Automaton a) {
if (a.isSingleton()) return new BytesRef(a.singleton);
BytesRef ref = new BytesRef(10);
HashSet<State> visited = new HashSet<State>();
State s = a.initial;
boolean done;
do {
done = true;
visited.add(s);
if (!s.accept && s.numTransitions() == 1) {
Transition t = s.getTransitions().iterator().next();
if (t.min == t.max && !visited.contains(t.to)) {
ref.grow(++ref.length);
ref.bytes[ref.length - 1] = (byte)t.min;
s = t.to;
done = false;
}
}
} while (!done);
return ref;
}
/**
* Returns the longest string that is a suffix of all accepted strings and
* visits each state at most once.
*
* @return common suffix
*/
public static String getCommonSuffix(Automaton a) {
if (a.isSingleton()) // if singleton, the suffix is the string itself.
return a.singleton;
// reverse the language of the automaton, then reverse its common prefix.
Automaton r = a.clone();
reverse(r);
r.determinize();
return new StringBuilder(SpecialOperations.getCommonPrefix(r)).reverse().toString();
}
public static BytesRef getCommonSuffixBytesRef(Automaton a) {
if (a.isSingleton()) // if singleton, the suffix is the string itself.
return new BytesRef(a.singleton);
// reverse the language of the automaton, then reverse its common prefix.
Automaton r = a.clone();
reverse(r);
r.determinize();
BytesRef ref = SpecialOperations.getCommonPrefixBytesRef(r);
reverseBytes(ref);
return ref;
}
private static void reverseBytes(BytesRef ref) {
if (ref.length <= 1) return;
int num = ref.length >> 1;
for (int i = ref.offset; i < ( ref.offset + num ); i++) {
byte b = ref.bytes[i];
ref.bytes[i] = ref.bytes[ref.offset * 2 + ref.length - i - 1];
ref.bytes[ref.offset * 2 + ref.length - i - 1] = b;
}
}
/**
* Reverses the language of the given (non-singleton) automaton while returning
* the set of new initial states.
*/
private static Set<State> reverse(Automaton a) {
a.expandSingleton();
// reverse all edges
HashMap<State, HashSet<Transition>> m = new HashMap<State, HashSet<Transition>>();
State[] states = a.getNumberedStates();
Set<State> accept = new HashSet<State>();
for (State s : states)
if (s.isAccept())
accept.add(s);
for (State r : states) {
m.put(r, new HashSet<Transition>());
r.accept = false;
}
for (State r : states)
for (Transition t : r.getTransitions())
m.get(t.to).add(new Transition(t.min, t.max, r));
for (State r : states) {
Set<Transition> tr = m.get(r);
r.setTransitions(tr.toArray(new Transition[tr.size()]));
}
// make new initial+final states
a.initial.accept = true;
a.initial = new State();
for (State r : accept)
a.initial.addEpsilon(r); // ensures that all initial states are reachable
a.deterministic = false;
a.clearNumberedStates();
return accept;
}
}