package com.bigdata.bop;
import java.io.ObjectStreamException;
import java.util.UUID;
import com.bigdata.cache.ConcurrentWeakValueCache;
import com.bigdata.relation.rule.Rule;
/**
* A variable.
* <p>
* Note: This implementation provides reference testing for equality. The rest
* of the package <em>assumes</em> that it can use reference testing for
* equality when comparing variables.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
* @version $Id$
*
* @todo it is not entirely satisfactory to have variable names disappear from
* rules as they are specialized. It might be better to define a BoundVar
* (extends Var) whose id was the bound value and which reported true for
* both isVariable() and isConstant(). However this would mean that var(x) !=
* var(x,id) where the latter is bound to a constant. Or if {@link Var} is
* rule-local (or rule execution thread local) then it could define
* set(Object) and get():Object to access its binding. A rule that was
* specialized would then flag bound variables as immutable.
*
* @todo Variable canonicalization could be limited in scope to a rule or its
* derived rules (via specialization or for truth maintenance). This would
* make it easier to place constraints directly on the variable so that it
* can limit the types of binding that it will accept. Constraints on
* {@link IPredicate}s limit the matched tuples. Constraints on rules
* limit the binding patterns across the rule. (In fact, variables can be
* "named" by their index into the binding set for most purposes.)
*/
final public class Var<E> extends ImmutableBOp implements IVariable<E>,
Comparable<IVariable<E>> {
private static final long serialVersionUID = -7100443208125002485L;
private boolean anonymous = false;
public void setAnonymous(boolean anonymous) {
this.anonymous = anonymous;
}
@Override
public boolean isAnonymous() {
return anonymous;
}
final private String name;
@Override
final public boolean isVar() {
return true;
}
@Override
final public boolean isConstant() {
return false;
}
/**
* Private constructor - use {@link Rule#var(String)} to obtain an instance.
*
* @param name
*/
private Var(final String name) {
// super(new BOp[] {}, null/* annotations */);
super(BOp.NOARGS, BOp.NOANNS);
assert name != null;
this.name = name;
}
/**
* Clone is overridden to prevent variables from becoming aliased (this is
* part of the canonicalizing mapping). Because we override clone we do not
* need to provide the deep copy constructor (it is never invoked).
*/
@Override
final public Var<E> clone() {
return this;
}
/**
* @todo Why two versions of equals? This one is coming from
* IConstantOrVariable.
*/
@Override
public final boolean equals(final IVariableOrConstant<E> o) {
if (this == o)
return true;
if (o instanceof IVariable<?>) {
return name.equals(((IVariable<?>) o).getName());
}
return false;
}
@Override
public final boolean equals(final Object o) {
if (this == o)
return true;
if (o instanceof IVariable<?>) {
return name.equals(((IVariable<?>) o).getName());
}
return false;
}
@Override
public final int hashCode() {
return name.hashCode();
}
@Override
public String toString() {
return name;
}
@Override
public boolean isWildcard() {
return name.length() == 1 && name.charAt(0) == '*';
}
// public int compareTo(IVariableOrConstant arg0) {
//
// // order vars before ids
// if(arg0.isConstant()) return -1;
//
// return name.compareTo(((Var)arg0).name);
//
// }
@Override
public E get() {
throw new UnsupportedOperationException();
}
public E get(final IBindingSet bindingSet) {
if (bindingSet == null)
throw new IllegalArgumentException();
@SuppressWarnings("unchecked")
final IConstant<E> c = bindingSet.get(this);
return c == null ? null : c.get();
}
@Override
public String getName() {
return name;
}
/**
* Canonicalizing hash map with weak values for {@link Var}s.
*
* @see ConcurrentWeakValueCache
*/
static private final ConcurrentWeakValueCache<String, Var<?>> vars = new ConcurrentWeakValueCache<String, Var<?>>(
0// queue capacity (no hard reference queue).
);
/**
* Generate an anonymous random variable.
*/
static public Var<?> var() {
return Var.var(UUID.randomUUID().toString());
}
/**
* Singleton factory for {@link Var}s.
* <p>
* Note: While only a single instance of a variable object will be created
* for any given variable name, the "scope" of the variable is always
* constrained by the rule within which it is used. The purpose of the
* singleton factory is to let us test for the same variable using "=="
* (reference testing) and also to have a shorthand for variable creation.
*
* @param name
* The variable name.
*
* @return The singleton variable for that name.
*/
static public Var var(final String name) {
if (name == null)
throw new IllegalArgumentException();
if (name.length() == 0)
throw new IllegalArgumentException();
Var<?> var = vars.get(name);
if (var == null) {
final Var<?> tmp = vars.putIfAbsent(name, var = new Var(name));
if(tmp != null) {
// race condition - someone else inserted first.
var = tmp;
}
// synchronized (vars) {
//
// // Only one thread gets to create the variable for that name.
//
// var = new Var(name);
//
// vars.put(name, var);
//
// }
}
return var;
}
/**
* Orders variables alphabetically.
*/
public int compareTo(IVariable<E> o) {
return name.compareTo(o.getName());
}
// public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
//
// name = in.readUTF();
//
// }
//
// public void writeExternal(ObjectOutput out) throws IOException {
//
// out.writeUTF(name);
//
// }
/**
* Imposes the canonicalizing mapping during object de-serialization.
*/
private Object readResolve() throws ObjectStreamException {
return var(name);
}
}