com.google.devtools.build.lib.concurrent.Sharder.java Source code

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Here is the source code for com.google.devtools.build.lib.concurrent.Sharder.java

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// Copyright 2014 The Bazel Authors. All rights reserved.
//
// 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.google.devtools.build.lib.concurrent;

import com.google.common.base.Predicate;
import com.google.common.collect.Iterables;
import com.google.common.collect.Lists;
import com.google.devtools.build.lib.util.Preconditions;

import java.util.Collections;
import java.util.Iterator;
import java.util.List;

/**
 * A class to build shards (work queues) for a given task.
 *
 * <p>{@link #add}ed elements will be equally distributed among the shards.
 *
 * @param <T> the type of collection over which we're sharding
 */
public final class Sharder<T> implements Iterable<List<T>> {
    private final List<List<T>> shards;
    private int nextShard = 0;

    public Sharder(int maxNumShards, int expectedTotalSize) {
        Preconditions.checkArgument(maxNumShards > 0);
        Preconditions.checkArgument(expectedTotalSize >= 0);
        this.shards = immutableListOfLists(maxNumShards, expectedTotalSize / maxNumShards);
    }

    public void add(T item) {
        shards.get(nextShard).add(item);
        nextShard = (nextShard + 1) % shards.size();
    }

    /**
     * Returns an immutable list of mutable lists.
     *
     * @param numLists the number of top-level lists.
     * @param expectedSize the exepected size of each mutable list.
     * @return a list of lists.
     */
    private static <T> List<List<T>> immutableListOfLists(int numLists, int expectedSize) {
        List<List<T>> list = Lists.newArrayListWithCapacity(numLists);
        for (int i = 0; i < numLists; i++) {
            list.add(Lists.<T>newArrayListWithExpectedSize(expectedSize));
        }
        return Collections.unmodifiableList(list);
    }

    @Override
    public Iterator<List<T>> iterator() {
        return Iterables.filter(shards, new Predicate<List<T>>() {
            @Override
            public boolean apply(List<T> list) {
                return !list.isEmpty();
            }
        }).iterator();
    }
}