code: /*
   * 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/publicdomain/zero/1.0/
   */
  
  // This is based on 1.9 version.
  
  package com.concurrent_ruby.ext.jsr166e.nounsafe;
  
  import java.util.concurrent.atomic.AtomicLong;
  import java.io.IOException;
  import java.io.Serializable;
  import java.io.ObjectInputStream;
  
  /**
   * One or more variables that together maintain an initially zero
   * {@code long} sum.  When updates (method {@link #add}) are contended
   * across threads, the set of variables may grow dynamically to reduce
   * contention. Method {@link #sum} (or, equivalently, {@link
   * #longValue}) returns the current total combined across the
   * variables maintaining the sum.
   *
   * <p>This class is usually preferable to {@link AtomicLong} when
   * multiple threads update a common sum that is used for purposes such
   * as collecting statistics, not for fine-grained synchronization
   * control.  Under low update contention, the two classes have similar
   * characteristics. But under high contention, expected throughput of
   * this class is significantly higher, at the expense of higher space
   * consumption.
   *
   * <p>This class extends {@link Number}, but does <em>not</em> define
   * methods such as {@code hashCode} and {@code compareTo} because
   * instances are expected to be mutated, and so are not useful as
   * collection keys.
   *
   * <p><em>jsr166e note: This class is targeted to be placed in
   * java.util.concurrent.atomic.</em>
   *
   * @since 1.8
   * @author Doug Lea
   */
  public class LongAdder extends Striped64 implements Serializable {
      private static final long serialVersionUID = 7249069246863182397L;
  
      /**
       * Version of plus for use in retryUpdate
       */
      final long fn(long v, long x) { return v + x; }
  
      /**
       * Creates a new adder with initial sum of zero.
       */
      public LongAdder() {
      }
  
      /**
       * Adds the given value.
       *
       * @param x the value to add
       */
      public void add(long x) {
          Cell[] as; long b, v; HashCode hc; Cell a; int n;
          if ((as = cells) != null || !casBase(b = base, b + x)) {
              boolean uncontended = true;
              int h = (hc = threadHashCode.get()).code;
              if (as == null || (n = as.length) < 1 ||
                      (a = as[(n - 1) & h]) == null ||
                      !(uncontended = a.cas(v = a.value, v + x)))
                  retryUpdate(x, hc, uncontended);
          }
      }
  
      /**
       * Equivalent to {@code add(1)}.
       */
      public void increment() {
          add(1L);
      }
  
      /**
       * Equivalent to {@code add(-1)}.
       */
      public void decrement() {
          add(-1L);
      }
  
      /**
       * Returns the current sum.  The returned value is <em>NOT</em> an
       * atomic snapshot: Invocation in the absence of concurrent
       * updates returns an accurate result, but concurrent updates that
       * occur while the sum is being calculated might not be
       * incorporated.
       *
       * @return the sum
       */
      public long sum() {
          long sum = base;
          Cell[] as = cells;
          if (as != null) {
              int n = as.length;
              for (int i = 0; i < n; ++i) {
                  Cell a = as[i];
                  if (a != null)
                      sum += a.value;
              }
          }
          return sum;
      }
  
      /**
       * Resets variables maintaining the sum to zero.  This method may
       * be a useful alternative to creating a new adder, but is only
       * effective if there are no concurrent updates.  Because this
       * method is intrinsically racy, it should only be used when it is
       * known that no threads are concurrently updating.
       */
      public void reset() {
          internalReset(0L);
      }
  
      /**
       * Equivalent in effect to {@link #sum} followed by {@link
       * #reset}. This method may apply for example during quiescent
       * points between multithreaded computations.  If there are
       * updates concurrent with this method, the returned value is
       * <em>not</em> guaranteed to be the final value occurring before
       * the reset.
       *
       * @return the sum
       */
      public long sumThenReset() {
          long sum = base;
          Cell[] as = cells;
          base = 0L;
          if (as != null) {
              int n = as.length;
              for (int i = 0; i < n; ++i) {
                  Cell a = as[i];
                  if (a != null) {
                      sum += a.value;
                      a.value = 0L;
                  }
              }
          }
          return sum;
      }
  
      /**
       * Returns the String representation of the {@link #sum}.
       * @return the String representation of the {@link #sum}
       */
      public String toString() {
          return Long.toString(sum());
      }
  
      /**
       * Equivalent to {@link #sum}.
       *
       * @return the sum
       */
      public long longValue() {
          return sum();
      }
  
      /**
       * Returns the {@link #sum} as an {@code int} after a narrowing
       * primitive conversion.
       */
      public int intValue() {
          return (int)sum();
      }
  
      /**
       * Returns the {@link #sum} as a {@code float}
       * after a widening primitive conversion.
       */
      public float floatValue() {
          return (float)sum();
      }
  
      /**
       * Returns the {@link #sum} as a {@code double} after a widening
       * primitive conversion.
       */
      public double doubleValue() {
          return (double)sum();
      }
  
      private void writeObject(java.io.ObjectOutputStream s)
              throws java.io.IOException {
          s.defaultWriteObject();
          s.writeLong(sum());
      }
  
      private void readObject(ObjectInputStream s)
              throws IOException, ClassNotFoundException {
          s.defaultReadObject();
          busy = 0;
          cells = null;
          base = s.readLong();
      }
  
  }
  