/*
* @(#)$Id: Expression.java 1627 2004-01-26 18:55:10Z kohsuke $
*
* Copyright 2001 Sun Microsystems, Inc. All Rights Reserved.
*
* This software is the proprietary information of Sun Microsystems, Inc.
* Use is subject to license terms.
*
*/
package com.sun.msv.grammar;
import com.sun.msv.grammar.util.RefExpRemover;
/**
* Primitive of the tree regular expression.
*
* most of the derived class is immutable (except ReferenceExp, ElementExp and OtherExp).
*
* <p>
* By making it immutable, it becomes possible to share subexpressions among expressions.
* This is very important for regular-expression-derivation based validation algorithm,
* as well as for smaller memory footprint.
* This sharing is automatically achieved by ExpressionPool.
*
* <p>
* ReferebceExp, ElementExp, and OtherExp are also placed in the pool,
* but these are not unified. Since they are not unified,
* application can derive classes from these expressions
* and mix them into AGM. This technique is heavily used to introduce schema language
* specific primitives into AGM. See various sub-packages of this package for examples.
*
* <p>
* The equals method must be implemented by the derived type. equals method will be
* used to unify the expressions. equals method can safely assume that its children
* are already unified (therefore == can be used to test the equality, rather than
* equals method).
*
* <p>
* To achieve unification, we overload the equals method so that
* <code>o1.equals(o2)</code> is true if o1 and o2 are identical.
* There, those two objects must return the same hash code. For this purpose,
* the hash code is calculated statically and cached internally.
*
*
* @author <a href="mailto:kohsuke.kawaguchi@eng.sun.com">Kohsuke KAWAGUCHI</a>
*/
public abstract class Expression implements java.io.Serializable {
/** cached value of epsilon reducibility.
*
* Epsilon reducibility can only be calculated after parsing the entire expression,
* because of forward reference to other pattern.
*/
private Boolean epsilonReducibility;
/** returns true if this expression accepts empty sequence.
*
* <p>
* If this method is called while creating Expressions, then this method
* may return approximated value. When this method is used while validation,
* this method is guaranteed to return the correct value.
*/
public boolean isEpsilonReducible() {
// epsilon reducibility is cached internally.
if (epsilonReducibility == null)
epsilonReducibility = calcEpsilonReducibility() ? Boolean.TRUE : Boolean.FALSE;
return epsilonReducibility.booleanValue();
}
/** computes epsilon reducibility */
protected abstract boolean calcEpsilonReducibility();
/**
* Cached value of the expression after ReferenceExps are removed.
* This value is computed on demand.
*/
private Expression expandedExp = null;
/**
* Gets the expression after removing all ReferenceExps, until child
* AttributeExp or ElementExp.
*/
public Expression getExpandedExp(ExpressionPool pool) {
if (expandedExp == null) {
// this part of the code may be called by the multiple threads
// even if that happens, there is no consistency problem
// because two thread will compute the same value.
expandedExp = this.visit(new RefExpRemover(pool, false));
}
return expandedExp;
}
/**
* Peels the occurence expressions from this expression.
* <p>
* In AGM, 'X?','X+' and 'X*' are represented by using
* other primitives. This method returns the 'X' part by
* removing occurence related expressions.
*/
public final Expression peelOccurence() {
// 'X?' is represented as 'choice(X,epsilon)'.
if (this instanceof ChoiceExp) {
ChoiceExp cexp = (ChoiceExp)this;
if (cexp.exp1 == Expression.epsilon)
return cexp.exp2.peelOccurence();
if (cexp.exp2 == Expression.epsilon)
return cexp.exp1.peelOccurence();
// note that epsilon may be in some branch deep under the tree.
// for example, when the expression is ((A|epsilon)|B)
// the above code won't be able to peel the epsilon in it.
// but this is OK, since this method still returns ChoiceExp,
// and the type of the expression is what matters.
}
// 'X+' is represented as 'oneOrMore(X)'
if (this instanceof OneOrMoreExp)
return ((OneOrMoreExp)this).exp.peelOccurence();
// 'X*' is represented as '(X+)?'
// therefore it is important to recursively process it.
// otherwise we've finished.
return this;
}
protected Expression(int hashCode) {
setHashCode(hashCode);
}
/**
* this constructor can be used for the ununified expressions.
* the only reason there are two parameters is to prevent unintentional
* use of the default constructor.
*/
protected Expression() {
this.cachedHashCode = System.identityHashCode(this);
}
/**
* this field can be used by Verifier implementation to speed up
* validation. Due to its nature, this field is not serialized.
*
* TODO: revisit this decision of not to serialize this field.
*/
public transient Object verifierTag = null;
public abstract Object visit(ExpressionVisitor visitor);
public abstract Expression visit(ExpressionVisitorExpression visitor);
public abstract boolean visit(ExpressionVisitorBoolean visitor);
public abstract void visit(ExpressionVisitorVoid visitor);
// if you don't need RELAX capability at all, cut these lines
public Object visit(com.sun.msv.grammar.relax.RELAXExpressionVisitor visitor) {
return visit((ExpressionVisitor)visitor);
}
public Expression visit(com.sun.msv.grammar.relax.RELAXExpressionVisitorExpression visitor) {
return visit((ExpressionVisitorExpression)visitor);
}
public boolean visit(com.sun.msv.grammar.relax.RELAXExpressionVisitorBoolean visitor) {
return visit((ExpressionVisitorBoolean)visitor);
}
public void visit(com.sun.msv.grammar.relax.RELAXExpressionVisitorVoid visitor) {
visit((ExpressionVisitorVoid)visitor);
}
// until here
/**
* Hash code of this object.
*
* <p>
* To memorize every sub expression, hash code is frequently used.
* And computation of the hash code requires full-traversal of
* the expression. Therefore, hash code is computed when the object
* is constructed, and kept cached thereafter.
*
* <p>
* This field is essentially final, but because of the serialization
* support, we cannot declare it as such.
*/
private transient int cachedHashCode;
public final int hashCode() {
return cachedHashCode;
}
private final void setHashCode(int hashCode) {
this.cachedHashCode = hashCode^getClass().hashCode();
}
/**
* Computes the hashCode again.
* <p>
* This method and the parameter to the constructor has to be
* the same. This method is used when the object is being read
* from the stream.
*/
protected abstract int calcHashCode();
public abstract boolean equals(Object o);
static protected int hashCode(Object o1, Object o2, int hashKey) {
// TODO: more efficient hashing algorithm
return o1.hashCode() + o2.hashCode() + hashKey;
}
static protected int hashCode(Object o, int hashKey) {
// TODO: more efficient hashing algorithm
return o.hashCode() + hashKey;
}
private static class EpsilonExpression extends Expression {
EpsilonExpression() {
}
protected final int calcHashCode() {
return System.identityHashCode(this);
}
public Object visit(ExpressionVisitor visitor) {
return visitor.onEpsilon();
}
public Expression visit(ExpressionVisitorExpression visitor) {
return visitor.onEpsilon();
}
public boolean visit(ExpressionVisitorBoolean visitor) {
return visitor.onEpsilon();
}
public void visit(ExpressionVisitorVoid visitor) {
visitor.onEpsilon();
}
protected boolean calcEpsilonReducibility() {
return true;
}
public boolean equals(Object o) {
return this == o;
} // this class is used as singleton.
// serialization support
private static final long serialVersionUID = 1;
protected Object readResolve() {
return Expression.epsilon;
}
};
/**
* Special expression object that represents epsilon (ε).
* This expression matches to "empty".
* Epsilon can be thought as an empty sequence.
*/
public static final Expression epsilon = new EpsilonExpression();
private static class NullSetExpression extends Expression {
NullSetExpression() {
}
protected final int calcHashCode() {
return System.identityHashCode(this);
}
public Object visit(ExpressionVisitor visitor) {
return visitor.onNullSet();
}
public Expression visit(ExpressionVisitorExpression visitor) {
return visitor.onNullSet();
}
public boolean visit(ExpressionVisitorBoolean visitor) {
return visitor.onNullSet();
}
public void visit(ExpressionVisitorVoid visitor) {
visitor.onNullSet();
}
protected boolean calcEpsilonReducibility() {
return false;
}
public boolean equals(Object o) {
return this == o;
} // this class is used as singleton.
// serialization support
private static final long serialVersionUID = 1;
protected Object readResolve() {
return Expression.nullSet;
}
};
/**
* special expression object that represents the empty set (Φ).
* This expression doesn't match to anything.
* NullSet can be thought as an empty choice.
*/
public static final Expression nullSet = new NullSetExpression();
private static class AnyStringExpression extends Expression {
AnyStringExpression() {
}
protected final int calcHashCode() {
return System.identityHashCode(this);
}
public Object visit(ExpressionVisitor visitor) {
return visitor.onAnyString();
}
public Expression visit(ExpressionVisitorExpression visitor) {
return visitor.onAnyString();
}
public boolean visit(ExpressionVisitorBoolean visitor) {
return visitor.onAnyString();
}
public void visit(ExpressionVisitorVoid visitor) {
visitor.onAnyString();
}
// anyString is consider to be epsilon reducible.
// In other words, one can always ignore anyString.
//
// Instead, anyString will remain in the expression even after
// consuming some StringToken.
// That is, residual of anyString by StringToken is not the epsilon but an anyString.
protected boolean calcEpsilonReducibility() {
return true;
}
public boolean equals(Object o) {
return this == o;
} // this class is used as singleton.
// serialization support
private static final long serialVersionUID = 1;
protected Object readResolve() {
return Expression.anyString;
}
};
/**
* special expression object that represents "any string".
* It is close to xsd:string datatype, but they have different semantics
* in several things.
*
* <p>
* This object is used as <anyString/> pattern of TREX and
* <text/> pattern of RELAX NG.
*/
public static final Expression anyString = new AnyStringExpression();
protected Object readResolve() {
setHashCode(calcHashCode());
return this;
}
private static final long serialVersionUID = -569561418606215601L;
}