1 files changed, 71 insertions, 0 deletions
diff --git a/src/main/java/com/google/devtools/build/lib/concurrent/Sharder.java b/src/main/java/com/google/devtools/build/lib/concurrent/Sharder.java
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index 0000000000..67a63e070c
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+++ b/src/main/java/com/google/devtools/build/lib/concurrent/Sharder.java
@@ -0,0 +1,71 @@
+// Copyright 2014 Google Inc. 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.Preconditions;
+import com.google.common.base.Predicate;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.Lists;
+
+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();
+  }
+}