pigpen.cascading.SingleIterationSeq Maven / Gradle / Ivy
/*
*
* Copyright 2015 Netflix, Inc.
*
* Licensed 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 pigpen.cascading;
import java.io.IOException;
import java.io.NotSerializableException;
import java.util.Iterator;
import clojure.lang.ASeq;
import clojure.lang.IPersistentMap;
import clojure.lang.ISeq;
/**
* This sequence behaves like a normal iterator-seq up to a certain size.
* If the max size is exceeded, it will only allow one iteration. The objective
* is to become immune to memory problems resulting from holding onto the head for
* large collections but still support operations such as count, max, etc.
*/
public class SingleIterationSeq extends ASeq {
private static int MAX_SIZE = 1000;
private final Iterator iter;
private final State state;
private static class State {
volatile Object val;
volatile Object rest;
volatile int index;
}
public static SingleIterationSeq create(Iterator iter) {
return create(iter, 0);
}
private static SingleIterationSeq create(Iterator iter, int index) {
if (iter.hasNext()) {
return new SingleIterationSeq(iter, index);
}
return null;
}
private SingleIterationSeq(Iterator iter, int index) {
this.iter = iter;
state = new State();
this.state.val = state;
this.state.rest = state;
this.state.index = index;
}
private SingleIterationSeq(IPersistentMap meta, Iterator iter, State state) {
super(meta);
this.iter = iter;
this.state = state;
}
public Object first() {
if (state.val == state) {
synchronized (state) {
if (state.val == state) {
state.val = iter.next();
}
}
}
return state.val;
}
public ISeq next() {
if (state.rest == state) {
synchronized (state) {
if (state.rest == state) {
first();
if (state.index < MAX_SIZE) {
state.rest = create(iter, state.index + 1);
return (ISeq)state.rest;
} else {
state.rest = iter;
return create(iter, state.index + 1);
}
}
}
}
if (state.rest == iter) {
throw new UnsupportedOperationException("This Seq can only be traversed once.");
}
return (ISeq)state.rest;
}
public SingleIterationSeq withMeta(IPersistentMap meta) {
return new SingleIterationSeq(meta, iter, state);
}
private void writeObject(java.io.ObjectOutputStream out) throws IOException {
throw new NotSerializableException(getClass().getName());
}
}