com.uber.hoodie.func.LazyIterableIterator Maven / Gradle / Ivy

Go to download
Show more of this group Show more artifacts with this name
Show all versions of hoodie-client Show documentation
There is a newer version: 0.4.7
/*
 * Copyright (c) 2016 Uber Technologies, Inc. ([email protected])
 *
 * 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 com.uber.hoodie.func;

import java.util.Iterator;

/**
 * (NOTE: Adapted from Apache SystemML) This class is a generic base class for lazy, single pass
 * inputItr classes in order to simplify the implementation of lazy iterators for mapPartitions use
 * cases. Note [SPARK-3369], which gives the reasons for backwards compatibility with regard to the
 * iterable API despite Spark's single pass nature.
 *
 * Provide a way to obtain a inputItr of type O  (output), out of an inputItr of type I (input)
 *
 * Things to remember: - Assumes Spark calls hasNext() to check for elements, before calling next()
 * to obtain them - Assumes hasNext() gets called atleast once. - Concrete Implementation is
 * responsible for calling inputIterator.next() and doing the processing in computeNext()
 */
public abstract class LazyIterableIterator implements Iterable, Iterator {
    protected Iterator inputItr = null;
    private boolean consumed = false;
    private boolean startCalled = false;
    private boolean endCalled = false;

    public LazyIterableIterator(Iterator in) {
        inputItr = in;
    }

    /**
     * Called once, before any elements are processed
     */
    protected abstract void start();

    /**
     * Block computation to be overwritten by sub classes.
     */
    protected abstract O computeNext();


    /**
     * Called once, after all elements are processed.
     */
    protected abstract void end();


    //////////////////
    // iterable implementation

    private void invokeStartIfNeeded() {
        if (!startCalled) {
            startCalled = true;
            try {
                start();
            } catch (Exception e) {
                throw new RuntimeException("Error in start()");
            }
        }
    }

    private void invokeEndIfNeeded() {
        // make the calls out to begin() & end()
        if (!endCalled) {
            endCalled = true;
            // if we are out of elements, and end has not been called yet
            try {
                end();
            } catch (Exception e) {
                throw new RuntimeException("Error in end()");
            }
        }
    }

    @Override
    public Iterator iterator() {
        //check for consumed inputItr
        if (consumed)
            throw new RuntimeException("Invalid repeated inputItr consumption.");

        //hand out self as inputItr exactly once (note: do not hand out the input
        //inputItr since it is consumed by the self inputItr implementation)
        consumed = true;
        return this;
    }

    //////////////////
    // inputItr implementation

    @Override
    public boolean hasNext() {
        boolean ret = inputItr.hasNext();
        // make sure, there is exactly one call to start()
        invokeStartIfNeeded();
        if (!ret) {
            // if we are out of elements, and end has not been called yet
            invokeEndIfNeeded();
        }

        return ret;
    }

    @Override
    public O next() {
        try {
            return computeNext();
        } catch (Exception ex) {
            throw new RuntimeException(ex);
        }
    }

    @Override
    public void remove() {
        throw new RuntimeException("Unsupported remove operation.");
    }
}