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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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|
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package jsr166e; |
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|
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import java.io.IOException; |
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import java.io.ObjectInputStream; |
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import java.io.ObjectOutputStream; |
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import java.io.Serializable; |
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import java.util.concurrent.atomic.AtomicLong; |
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|
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/** |
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* One or more variables that together maintain an initially zero |
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* {@code long} sum. When updates (method {@link #add}) are contended |
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* across threads, the set of variables may grow dynamically to reduce |
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* contention. Method {@link #sum} (or, equivalently, {@link |
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* #longValue}) returns the current total combined across the |
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* variables maintaining the sum. |
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* |
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* <p>This class is usually preferable to {@link AtomicLong} when |
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* multiple threads update a common sum that is used for purposes such |
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* as collecting statistics, not for fine-grained synchronization |
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* control. Under low update contention, the two classes have similar |
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* characteristics. But under high contention, expected throughput of |
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* this class is significantly higher, at the expense of higher space |
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* consumption. |
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* |
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* <p>This class extends {@link Number}, but does <em>not</em> define |
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* methods such as {@code equals}, {@code hashCode} and {@code |
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* compareTo} because instances are expected to be mutated, and so are |
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* not useful as collection keys. |
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* |
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* <p><em>jsr166e note: This class is targeted to be placed in |
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* java.util.concurrent.atomic.</em> |
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* |
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* @since 1.8 |
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* @author Doug Lea |
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*/ |
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public class LongAdder extends Striped64 implements Serializable { |
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private static final long serialVersionUID = 7249069246863182397L; |
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|
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/** |
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* Version of plus for use in retryUpdate |
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*/ |
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final long fn(long v, long x) { return v + x; } |
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|
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/** |
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* Creates a new adder with initial sum of zero. |
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*/ |
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public LongAdder() { |
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} |
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|
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/** |
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* Adds the given value. |
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* |
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* @param x the value to add |
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*/ |
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public void add(long x) { |
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Cell[] as; long b, v; int[] hc; Cell a; int n; |
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if ((as = cells) != null || !casBase(b = base, b + x)) { |
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boolean uncontended = true; |
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if ((hc = threadHashCode.get()) == null || |
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as == null || (n = as.length) < 1 || |
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(a = as[(n - 1) & hc[0]]) == null || |
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!(uncontended = a.cas(v = a.value, v + x))) |
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retryUpdate(x, hc, uncontended); |
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} |
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} |
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|
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/** |
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* Equivalent to {@code add(1)}. |
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*/ |
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public void increment() { |
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add(1L); |
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} |
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|
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/** |
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* Equivalent to {@code add(-1)}. |
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*/ |
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public void decrement() { |
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add(-1L); |
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} |
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|
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/** |
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* Returns the current sum. The returned value is <em>NOT</em> an |
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* atomic snapshot; invocation in the absence of concurrent |
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* updates returns an accurate result, but concurrent updates that |
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* occur while the sum is being calculated might not be |
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* incorporated. |
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* |
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* @return the sum |
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*/ |
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public long sum() { |
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long sum = base; |
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Cell[] as = cells; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Cell a = as[i]; |
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if (a != null) |
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sum += a.value; |
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} |
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} |
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return sum; |
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} |
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|
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/** |
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* Resets variables maintaining the sum to zero. This method may |
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* be a useful alternative to creating a new adder, but is only |
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* effective if there are no concurrent updates. Because this |
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* method is intrinsically racy, it should only be used when it is |
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* known that no threads are concurrently updating. |
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*/ |
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public void reset() { |
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internalReset(0L); |
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} |
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|
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/** |
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* Equivalent in effect to {@link #sum} followed by {@link |
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* #reset}. This method may apply for example during quiescent |
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* points between multithreaded computations. If there are |
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* updates concurrent with this method, the returned value is |
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* <em>not</em> guaranteed to be the final value occurring before |
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* the reset. |
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* |
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* @return the sum |
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*/ |
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public long sumThenReset() { |
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long sum = base; |
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Cell[] as = cells; |
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base = 0L; |
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if (as != null) { |
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int n = as.length; |
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for (int i = 0; i < n; ++i) { |
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Cell a = as[i]; |
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if (a != null) { |
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sum += a.value; |
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a.value = 0L; |
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} |
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} |
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} |
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return sum; |
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} |
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|
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/** |
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* Returns the String representation of the {@link #sum}. |
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* @return the String representation of the {@link #sum} |
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*/ |
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public String toString() { |
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return Long.toString(sum()); |
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} |
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|
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/** |
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* Equivalent to {@link #sum}. |
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* |
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* @return the sum |
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*/ |
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public long longValue() { |
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return sum(); |
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} |
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|
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/** |
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* Returns the {@link #sum} as an {@code int} after a narrowing |
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* primitive conversion. |
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*/ |
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public int intValue() { |
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return (int)sum(); |
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} |
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|
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/** |
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* Returns the {@link #sum} as a {@code float} |
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* after a widening primitive conversion. |
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*/ |
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public float floatValue() { |
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return (float)sum(); |
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} |
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|
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/** |
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* Returns the {@link #sum} as a {@code double} after a widening |
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* primitive conversion. |
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*/ |
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public double doubleValue() { |
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return (double)sum(); |
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} |
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|
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private void writeObject(ObjectOutputStream s) throws IOException { |
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s.defaultWriteObject(); |
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s.writeLong(sum()); |
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} |
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|
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private void readObject(ObjectInputStream s) |
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throws IOException, ClassNotFoundException { |
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s.defaultReadObject(); |
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busy = 0; |
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cells = null; |
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base = s.readLong(); |
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} |
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|
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} |