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+/*
+ * Copyright (C) 2011 The Guava Authors
+ *
+ * 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.common.util.concurrent;
+
+import com.google.common.annotations.Beta;
+import com.google.common.base.Functions;
+import com.google.common.base.Preconditions;
+import com.google.common.base.Supplier;
+import com.google.common.collect.Iterables;
+import com.google.common.collect.MapMaker;
+import com.google.common.math.IntMath;
+import com.google.common.primitives.Ints;
+
+import java.math.RoundingMode;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
+import java.util.concurrent.ConcurrentMap;
+import java.util.concurrent.Semaphore;
+import java.util.concurrent.locks.Lock;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
+
+/**
+ * A striped {@code Lock/Semaphore/ReadWriteLock}. This offers the underlying lock striping
+ * similar to that of {@code ConcurrentHashMap} in a reusable form, and extends it for
+ * semaphores and read-write locks. Conceptually, lock striping is the technique of dividing a lock
+ * into many <i>stripes</i>, increasing the granularity of a single lock and allowing independent
+ * operations to lock different stripes and proceed concurrently, instead of creating contention
+ * for a single lock.
+ *
+ * <p>The guarantee provided by this class is that equal keys lead to the same lock (or semaphore),
+ * i.e. {@code if (key1.equals(key2))} then {@code striped.get(key1) == striped.get(key2)}
+ * (assuming {@link Object#hashCode()} is correctly implemented for the keys). Note
+ * that if {@code key1} is <strong>not</strong> equal to {@code key2}, it is <strong>not</strong>
+ * guaranteed that {@code striped.get(key1) != striped.get(key2)}; the elements might nevertheless
+ * be mapped to the same lock. The lower the number of stripes, the higher the probability of this
+ * happening.
+ *
+ * <p>There are three flavors of this class: {@code Striped<Lock>}, {@code Striped<Semaphore>},
+ * and {@code Striped<ReadWriteLock>}. For each type, two implementations are offered:
+ * {@linkplain #lock(int) strong} and {@linkplain #lazyWeakLock(int) weak}
+ * {@code Striped<Lock>}, {@linkplain #semaphore(int, int) strong} and {@linkplain
+ * #lazyWeakSemaphore(int, int) weak} {@code Striped<Semaphore>}, and {@linkplain
+ * #readWriteLock(int) strong} and {@linkplain #lazyWeakReadWriteLock(int) weak}
+ * {@code Striped<ReadWriteLock>}. <i>Strong</i> means that all stripes (locks/semaphores) are
+ * initialized eagerly, and are not reclaimed unless {@code Striped} itself is reclaimable.
+ * <i>Weak</i> means that locks/semaphores are created lazily, and they are allowed to be reclaimed
+ * if nobody is holding on to them. This is useful, for example, if one wants to create a {@code
+ * Striped<Lock>} of many locks, but worries that in most cases only a small portion of these
+ * would be in use.
+ *
+ * <p>Prior to this class, one might be tempted to use {@code Map<K, Lock>}, where {@code K}
+ * represents the task. This maximizes concurrency by having each unique key mapped to a unique
+ * lock, but also maximizes memory footprint. On the other extreme, one could use a single lock
+ * for all tasks, which minimizes memory footprint but also minimizes concurrency. Instead of
+ * choosing either of these extremes, {@code Striped} allows the user to trade between required
+ * concurrency and memory footprint. For example, if a set of tasks are CPU-bound, one could easily
+ * create a very compact {@code Striped<Lock>} of {@code availableProcessors() * 4} stripes,
+ * instead of possibly thousands of locks which could be created in a {@code Map<K, Lock>}
+ * structure.
+ *
+ * @author Dimitris Andreou
+ * @since 13.0
+ */
+@Beta
+public abstract class Striped<L> {
+  private Striped() {}
+
+  /**
+   * Returns the stripe that corresponds to the passed key. It is always guaranteed that if
+   * {@code key1.equals(key2)}, then {@code get(key1) == get(key2)}.
+   *
+   * @param key an arbitrary, non-null key
+   * @return the stripe that the passed key corresponds to
+   */
+  public abstract L get(Object key);
+
+  /**
+   * Returns the stripe at the specified index. Valid indexes are 0, inclusively, to
+   * {@code size()}, exclusively.
+   *
+   * @param index the index of the stripe to return; must be in {@code [0...size())}
+   * @return the stripe at the specified index
+   */
+  public abstract L getAt(int index);
+
+  /**
+   * Returns the index to which the given key is mapped, so that getAt(indexFor(key)) == get(key).
+   */
+  abstract int indexFor(Object key);
+
+  /**
+   * Returns the total number of stripes in this instance.
+   */
+  public abstract int size();
+
+  /**
+   * Returns the stripes that correspond to the passed objects, in ascending (as per
+   * {@link #getAt(int)}) order. Thus, threads that use the stripes in the order returned
+   * by this method are guaranteed to not deadlock each other.
+   *
+   * <p>It should be noted that using a {@code Striped<L>} with relatively few stripes, and
+   * {@code bulkGet(keys)} with a relative large number of keys can cause an excessive number
+   * of shared stripes (much like the birthday paradox, where much fewer than anticipated birthdays
+   * are needed for a pair of them to match). Please consider carefully the implications of the
+   * number of stripes, the intended concurrency level, and the typical number of keys used in a
+   * {@code bulkGet(keys)} operation. See <a href="http://www.mathpages.com/home/kmath199.htm">Balls
+   * in Bins model</a> for mathematical formulas that can be used to estimate the probability of
+   * collisions.
+   *
+   * @param keys arbitrary non-null keys
+   * @return the stripes corresponding to the objects (one per each object, derived by delegating
+   *         to {@link #get(Object)}; may contain duplicates), in an increasing index order.
+   */
+  public Iterable<L> bulkGet(Iterable<?> keys) {
+    // Initially using the array to store the keys, then reusing it to store the respective L's
+    final Object[] array = Iterables.toArray(keys, Object.class);
+    int[] stripes = new int[array.length];
+    for (int i = 0; i < array.length; i++) {
+      stripes[i] = indexFor(array[i]);
+    }
+    Arrays.sort(stripes);
+    for (int i = 0; i < array.length; i++) {
+      array[i] = getAt(stripes[i]);
+    }
+    /*
+     * Note that the returned Iterable holds references to the returned stripes, to avoid
+     * error-prone code like:
+     *
+     * Striped<Lock> stripedLock = Striped.lazyWeakXXX(...)'
+     * Iterable<Lock> locks = stripedLock.bulkGet(keys);
+     * for (Lock lock : locks) {
+     *   lock.lock();
+     * }
+     * operation();
+     * for (Lock lock : locks) {
+     *   lock.unlock();
+     * }
+     *
+     * If we only held the int[] stripes, translating it on the fly to L's, the original locks
+     * might be garbage collected after locking them, ending up in a huge mess.
+     */
+    @SuppressWarnings("unchecked") // we carefully replaced all keys with their respective L's
+    List<L> asList = (List<L>) Arrays.asList(array);
+    return Collections.unmodifiableList(asList);
+  }
+
+  // Static factories
+
+  /**
+   * Creates a {@code Striped<Lock>} with eagerly initialized, strongly referenced locks, with the
+   * specified fairness. Every lock is reentrant.
+   *
+   * @param stripes the minimum number of stripes (locks) required
+   * @return a new {@code Striped<Lock>}
+   */
+  public static Striped<Lock> lock(int stripes) {
+    return new CompactStriped<Lock>(stripes, new Supplier<Lock>() {
+      public Lock get() {
+        return new PaddedLock();
+      }
+    });
+  }
+
+  /**
+   * Creates a {@code Striped<Lock>} with lazily initialized, weakly referenced locks, with the
+   * specified fairness. Every lock is reentrant.
+   *
+   * @param stripes the minimum number of stripes (locks) required
+   * @return a new {@code Striped<Lock>}
+   */
+  public static Striped<Lock> lazyWeakLock(int stripes) {
+    return new LazyStriped<Lock>(stripes, new Supplier<Lock>() {
+      public Lock get() {
+        return new ReentrantLock(false);
+      }
+    });
+  }
+
+  /**
+   * Creates a {@code Striped<Semaphore>} with eagerly initialized, strongly referenced semaphores,
+   * with the specified number of permits and fairness.
+   *
+   * @param stripes the minimum number of stripes (semaphores) required
+   * @param permits the number of permits in each semaphore
+   * @return a new {@code Striped<Semaphore>}
+   */
+  public static Striped<Semaphore> semaphore(int stripes, final int permits) {
+    return new CompactStriped<Semaphore>(stripes, new Supplier<Semaphore>() {
+      public Semaphore get() {
+        return new PaddedSemaphore(permits);
+      }
+    });
+  }
+
+  /**
+   * Creates a {@code Striped<Semaphore>} with lazily initialized, weakly referenced semaphores,
+   * with the specified number of permits and fairness.
+   *
+   * @param stripes the minimum number of stripes (semaphores) required
+   * @param permits the number of permits in each semaphore
+   * @return a new {@code Striped<Semaphore>}
+   */
+  public static Striped<Semaphore> lazyWeakSemaphore(int stripes, final int permits) {
+    return new LazyStriped<Semaphore>(stripes, new Supplier<Semaphore>() {
+      public Semaphore get() {
+        return new Semaphore(permits, false);
+      }
+    });
+  }
+
+  /**
+   * Creates a {@code Striped<ReadWriteLock>} with eagerly initialized, strongly referenced
+   * read-write locks, with the specified fairness. Every lock is reentrant.
+   *
+   * @param stripes the minimum number of stripes (locks) required
+   * @return a new {@code Striped<ReadWriteLock>}
+   */
+  public static Striped<ReadWriteLock> readWriteLock(int stripes) {
+    return new CompactStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER);
+  }
+
+  /**
+   * Creates a {@code Striped<ReadWriteLock>} with lazily initialized, weakly referenced
+   * read-write locks, with the specified fairness. Every lock is reentrant.
+   *
+   * @param stripes the minimum number of stripes (locks) required
+   * @return a new {@code Striped<ReadWriteLock>}
+   */
+  public static Striped<ReadWriteLock> lazyWeakReadWriteLock(int stripes) {
+    return new LazyStriped<ReadWriteLock>(stripes, READ_WRITE_LOCK_SUPPLIER);
+  }
+
+  // ReentrantReadWriteLock is large enough to make padding probably unnecessary
+  private static final Supplier<ReadWriteLock> READ_WRITE_LOCK_SUPPLIER =
+      new Supplier<ReadWriteLock>() {
+    public ReadWriteLock get() {
+      return new ReentrantReadWriteLock();
+    }
+  };
+
+  private abstract static class PowerOfTwoStriped<L> extends Striped<L> {
+    /** Capacity (power of two) minus one, for fast mod evaluation */
+    final int mask;
+
+    PowerOfTwoStriped(int stripes) {
+      Preconditions.checkArgument(stripes > 0, "Stripes must be positive");
+      this.mask = stripes > Ints.MAX_POWER_OF_TWO ? ALL_SET : ceilToPowerOfTwo(stripes) - 1;
+    }
+
+    @Override final int indexFor(Object key) {
+      int hash = smear(key.hashCode());
+      return hash & mask;
+    }
+
+    @Override public final L get(Object key) {
+      return getAt(indexFor(key));
+    }
+  }
+
+  /**
+   * Implementation of Striped where 2^k stripes are represented as an array of the same length,
+   * eagerly initialized.
+   */
+  private static class CompactStriped<L> extends PowerOfTwoStriped<L> {
+    /** Size is a power of two. */
+    private final Object[] array;
+
+    private CompactStriped(int stripes, Supplier<L> supplier) {
+      super(stripes);
+      Preconditions.checkArgument(stripes <= Ints.MAX_POWER_OF_TWO, "Stripes must be <= 2^30)");
+
+      this.array = new Object[mask + 1];
+      for (int i = 0; i < array.length; i++) {
+        array[i] = supplier.get();
+      }
+    }
+
+    @SuppressWarnings("unchecked") // we only put L's in the array
+    @Override public L getAt(int index) {
+      return (L) array[index];
+    }
+
+    @Override public int size() {
+      return array.length;
+    }
+  }
+
+  /**
+   * Implementation of Striped where up to 2^k stripes can be represented, using a Cache
+   * where the key domain is [0..2^k). To map a user key into a stripe, we take a k-bit slice of the
+   * user key's (smeared) hashCode(). The stripes are lazily initialized and are weakly referenced.
+   */
+  private static class LazyStriped<L> extends PowerOfTwoStriped<L> {
+    final ConcurrentMap<Integer, L> cache;
+    final int size;
+
+    LazyStriped(int stripes, Supplier<L> supplier) {
+      super(stripes);
+      this.size = (mask == ALL_SET) ? Integer.MAX_VALUE : mask + 1;
+      this.cache = new MapMaker().weakValues().makeComputingMap(Functions.forSupplier(supplier));
+    }
+
+    @Override public L getAt(int index) {
+      Preconditions.checkElementIndex(index, size());
+      return cache.get(index);
+    }
+
+    @Override public int size() {
+      return size;
+    }
+  }
+
+  /**
+   * A bit mask were all bits are set.
+   */
+  private static final int ALL_SET = ~0;
+
+  private static int ceilToPowerOfTwo(int x) {
+    return 1 << IntMath.log2(x, RoundingMode.CEILING);
+  }
+
+  /*
+   * This method was written by Doug Lea with assistance from members of JCP
+   * JSR-166 Expert Group and released to the public domain, as explained at
+   * http://creativecommons.org/licenses/publicdomain
+   *
+   * As of 2010/06/11, this method is identical to the (package private) hash
+   * method in OpenJDK 7's java.util.HashMap class.
+   */
+  // Copied from java/com/google/common/collect/Hashing.java
+  private static int smear(int hashCode) {
+    hashCode ^= (hashCode >>> 20) ^ (hashCode >>> 12);
+    return hashCode ^ (hashCode >>> 7) ^ (hashCode >>> 4);
+  }
+
+  private static class PaddedLock extends ReentrantLock {
+    /*
+     * Padding from 40 into 64 bytes, same size as cache line. Might be beneficial to add
+     * a fourth long here, to minimize chance of interference between consecutive locks,
+     * but I couldn't observe any benefit from that.
+     */
+    @SuppressWarnings("unused")
+    long q1, q2, q3;
+
+    PaddedLock() {
+      super(false);
+    }
+  }
+
+  private static class PaddedSemaphore extends Semaphore {
+    // See PaddedReentrantLock comment
+    @SuppressWarnings("unused")
+    long q1, q2, q3;
+
+    PaddedSemaphore(int permits) {
+      super(permits, false);
+    }
+  }
+}