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import java.util.*; |
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/** |
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* A Random number generator with the same properties as {@link |
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* A random number generator with the same properties as class {@link |
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* Random} but isolated to the current Thread. Like the global |
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* generator used by {@link java.lang.Math}, a ThreadLocalRandom is |
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* initialized with an internally generated seed that may not |
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* otherwise be modified. When applicable, use of ThreadLocalRandom |
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* rather than shared Random objects in concurrent programs will |
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* typically encounter less overhead and contention. The most common |
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* usage form is: <code>ThreadLocalRandom.current().nextX()</code>. |
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* generator used by the {@link java.lang.Math} class, a |
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* ThreadLocalRandom is initialized with an internally generated seed |
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* that may not otherwise be modified. When applicable, use of |
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* ThreadLocalRandom rather than shared Random objects in concurrent |
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* programs will typically encounter much less overhead and |
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* contention. ThreadLocalRandoms are particularly appropriate when |
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* multiple tasks (for example, each a {@link ForkJoinTask}), use |
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* random numbers in parallel in thread pools. |
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* |
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* <p>Usages of this class should typically be of the form: |
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* <code>ThreadLocalRandom.current().nextX(...)</code> (where |
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* <code>X</code> is <code>Int</code>, <code>Long</code>, etc). |
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* When all usages are of this form, it is never possible to |
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* accidently share ThreadLocalRandoms across multiple threads. |
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* |
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* <p>This class also provides additional commonly used bounded random |
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* generation methods. |
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* Initialization flag to permit the first and only allowed call |
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* to setSeed (inside Random constructor) to succeed. We can't |
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* allow others since it would cause setting seed in one part of a |
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* program to inintentionally impact other usages by the thread. |
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* program to unintentionally impact other usages by the thread. |
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*/ |
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boolean initialized; |
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// each other. |
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private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7; |
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/** |
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* The actual ThreadLocal |
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*/ |
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private static final ThreadLocal<ThreadLocalRandom> localRandom = |
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new ThreadLocal<ThreadLocalRandom>() { |
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protected ThreadLocalRandom initialValue() { |
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} |
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}; |
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ThreadLocalRandom() { // construct only in localRandom.initialValue |
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super(); // super constructor calls setSeed |
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/** |
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* Constructor called only by localRandom.initialValue. |
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* We rely on the fact that the superclass no-arg constructor |
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* invokes setSeed exactly once to initialize. |
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*/ |
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ThreadLocalRandom() { |
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super(); |
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} |
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/** |
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public long nextLong(long n) { |
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if (n <= 0) |
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throw new IllegalArgumentException("n must be positive"); |
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// Divide n by two until small enough for nextInt. On each |
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// iteration (at most 31 of them but usually much less), |
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// randomly choose both whether to include high bit in result |
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// (offset) and whether to continue with the lower vs upper |
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// half (which makes a difference only if odd). |
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long offset = 0; |
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while (n >= Integer.MAX_VALUE) { // randomly pick half range |
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int bits = next(2); // 2nd bit for odd vs even split |
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while (n >= Integer.MAX_VALUE) { |
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int bits = next(2); |
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long half = n >>> 1; |
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long nextn = ((bits & 2) == 0)? half : n - half; |
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if ((bits & 1) == 0) |
