/*
    * 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/
    */
   
   /*
    * Source:
    * http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/LongAdder.java?revision=1.17
   */
  
  package com.google.common.cache;
  
  import com.google.common.annotations.GwtCompatible;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * 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 equals}, {@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
   */
  @GwtCompatible(emulated = true)
  final class LongAdder extends Striped64 implements Serializable, LongAddable {
      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; int[] hc; Cell a; int n;
          if ((as = cells) != null || !casBase(b = base, b + x)) {
              boolean uncontended = true;
              if ((hc = threadHashCode.get()) == null ||
                  as == null || (n = as.length) < 1 ||
                  (a = as[(n - 1) & hc[0]]) == 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(ObjectOutputStream s) throws IOException {
         s.defaultWriteObject();
         s.writeLong(sum());
     }
 
     private void readObject(ObjectInputStream s)
             throws IOException, ClassNotFoundException {
         s.defaultReadObject();
         busy = 0;
         cells = null;
         base = s.readLong();
     }
 
 }