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authorupstream source tree <ports@midipix.org>2015-03-15 20:14:05 -0400
committerupstream source tree <ports@midipix.org>2015-03-15 20:14:05 -0400
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tree976dc5ab7fddf506dadce60ae936f43f58787092 /libjava/classpath/java/util/Random.java
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+/* Random.java -- a pseudo-random number generator
+ Copyright (C) 1998, 1999, 2000, 2001, 2002 Free Software Foundation, Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING. If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library. Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module. An independent module is a module which is not derived from
+or based on this library. If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so. If you do not wish to do so, delete this
+exception statement from your version. */
+
+
+package java.util;
+
+import java.io.Serializable;
+
+/**
+ * This class generates pseudorandom numbers. It uses the same
+ * algorithm as the original JDK-class, so that your programs behave
+ * exactly the same way, if started with the same seed.
+ *
+ * The algorithm is described in <em>The Art of Computer Programming,
+ * Volume 2</em> by Donald Knuth in Section 3.2.1. It is a 48-bit seed,
+ * linear congruential formula.
+ *
+ * If two instances of this class are created with the same seed and
+ * the same calls to these classes are made, they behave exactly the
+ * same way. This should be even true for foreign implementations
+ * (like this), so every port must use the same algorithm as described
+ * here.
+ *
+ * If you want to implement your own pseudorandom algorithm, you
+ * should extend this class and overload the <code>next()</code> and
+ * <code>setSeed(long)</code> method. In that case the above
+ * paragraph doesn't apply to you.
+ *
+ * This class shouldn't be used for security sensitive purposes (like
+ * generating passwords or encryption keys. See <code>SecureRandom</code>
+ * in package <code>java.security</code> for this purpose.
+ *
+ * For simple random doubles between 0.0 and 1.0, you may consider using
+ * Math.random instead.
+ *
+ * @see java.security.SecureRandom
+ * @see Math#random()
+ * @author Jochen Hoenicke
+ * @author Eric Blake (ebb9@email.byu.edu)
+ * @status updated to 1.4
+ */
+public class Random implements Serializable
+{
+ /**
+ * True if the next nextGaussian is available. This is used by
+ * nextGaussian, which generates two gaussian numbers by one call,
+ * and returns the second on the second call.
+ *
+ * @serial whether nextNextGaussian is available
+ * @see #nextGaussian()
+ * @see #nextNextGaussian
+ */
+ private boolean haveNextNextGaussian;
+
+ /**
+ * The next nextGaussian, when available. This is used by nextGaussian,
+ * which generates two gaussian numbers by one call, and returns the
+ * second on the second call.
+ *
+ * @serial the second gaussian of a pair
+ * @see #nextGaussian()
+ * @see #haveNextNextGaussian
+ */
+ private double nextNextGaussian;
+
+ /**
+ * The seed. This is the number set by setSeed and which is used
+ * in next.
+ *
+ * @serial the internal state of this generator
+ * @see #next(int)
+ */
+ private long seed;
+
+ /**
+ * Compatible with JDK 1.0+.
+ */
+ private static final long serialVersionUID = 3905348978240129619L;
+
+ /**
+ * Creates a new pseudorandom number generator. The seed is initialized
+ * to the current time, as if by
+ * <code>setSeed(System.currentTimeMillis());</code>.
+ *
+ * @see System#currentTimeMillis()
+ */
+ public Random()
+ {
+ this(System.currentTimeMillis());
+ }
+
+ /**
+ * Creates a new pseudorandom number generator, starting with the
+ * specified seed, using <code>setSeed(seed);</code>.
+ *
+ * @param seed the initial seed
+ */
+ public Random(long seed)
+ {
+ setSeed(seed);
+ }
+
+ /**
+ * Sets the seed for this pseudorandom number generator. As described
+ * above, two instances of the same random class, starting with the
+ * same seed, should produce the same results, if the same methods
+ * are called. The implementation for java.util.Random is:
+ *
+<pre>public synchronized void setSeed(long seed)
+{
+ this.seed = (seed ^ 0x5DEECE66DL) & ((1L &lt;&lt; 48) - 1);
+ haveNextNextGaussian = false;
+}</pre>
+ *
+ * @param seed the new seed
+ */
+ public synchronized void setSeed(long seed)
+ {
+ this.seed = (seed ^ 0x5DEECE66DL) & ((1L << 48) - 1);
+ haveNextNextGaussian = false;
+ }
+
+ /**
+ * Generates the next pseudorandom number. This returns
+ * an int value whose <code>bits</code> low order bits are
+ * independent chosen random bits (0 and 1 are equally likely).
+ * The implementation for java.util.Random is:
+ *
+<pre>protected synchronized int next(int bits)
+{
+ seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L &lt;&lt; 48) - 1);
+ return (int) (seed &gt;&gt;&gt; (48 - bits));
+}</pre>
+ *
+ * @param bits the number of random bits to generate, in the range 1..32
+ * @return the next pseudorandom value
+ * @since 1.1
+ */
+ protected synchronized int next(int bits)
+ {
+ seed = (seed * 0x5DEECE66DL + 0xBL) & ((1L << 48) - 1);
+ return (int) (seed >>> (48 - bits));
+ }
+
+ /**
+ * Fills an array of bytes with random numbers. All possible values
+ * are (approximately) equally likely.
+ * The JDK documentation gives no implementation, but it seems to be:
+ *
+<pre>public void nextBytes(byte[] bytes)
+{
+ for (int i = 0; i &lt; bytes.length; i += 4)
+ {
+ int random = next(32);
+ for (int j = 0; i + j &lt; bytes.length && j &lt; 4; j++)
+ {
+ bytes[i+j] = (byte) (random & 0xff)
+ random &gt;&gt;= 8;
+ }
+ }
+}</pre>
+ *
+ * @param bytes the byte array that should be filled
+ * @throws NullPointerException if bytes is null
+ * @since 1.1
+ */
+ public void nextBytes(byte[] bytes)
+ {
+ int random;
+ // Do a little bit unrolling of the above algorithm.
+ int max = bytes.length & ~0x3;
+ for (int i = 0; i < max; i += 4)
+ {
+ random = next(32);
+ bytes[i] = (byte) random;
+ bytes[i + 1] = (byte) (random >> 8);
+ bytes[i + 2] = (byte) (random >> 16);
+ bytes[i + 3] = (byte) (random >> 24);
+ }
+ if (max < bytes.length)
+ {
+ random = next(32);
+ for (int j = max; j < bytes.length; j++)
+ {
+ bytes[j] = (byte) random;
+ random >>= 8;
+ }
+ }
+ }
+
+ /**
+ * Generates the next pseudorandom number. This returns
+ * an int value whose 32 bits are independent chosen random bits
+ * (0 and 1 are equally likely). The implementation for
+ * java.util.Random is:
+ *
+<pre>public int nextInt()
+{
+ return next(32);
+}</pre>
+ *
+ * @return the next pseudorandom value
+ */
+ public int nextInt()
+ {
+ return next(32);
+ }
+
+ /**
+ * Generates the next pseudorandom number. This returns
+ * a value between 0(inclusive) and <code>n</code>(exclusive), and
+ * each value has the same likelihodd (1/<code>n</code>).
+ * (0 and 1 are equally likely). The implementation for
+ * java.util.Random is:
+ *
+<pre>
+public int nextInt(int n)
+{
+ if (n &lt;= 0)
+ throw new IllegalArgumentException("n must be positive");
+
+ if ((n & -n) == n) // i.e., n is a power of 2
+ return (int)((n * (long) next(31)) &gt;&gt; 31);
+
+ int bits, val;
+ do
+ {
+ bits = next(31);
+ val = bits % n;
+ }
+ while(bits - val + (n-1) &lt; 0);
+
+ return val;
+}</pre>
+ *
+ * <p>This algorithm would return every value with exactly the same
+ * probability, if the next()-method would be a perfect random number
+ * generator.
+ *
+ * The loop at the bottom only accepts a value, if the random
+ * number was between 0 and the highest number less then 1<<31,
+ * which is divisible by n. The probability for this is high for small
+ * n, and the worst case is 1/2 (for n=(1<<30)+1).
+ *
+ * The special treatment for n = power of 2, selects the high bits of
+ * the random number (the loop at the bottom would select the low order
+ * bits). This is done, because the low order bits of linear congruential
+ * number generators (like the one used in this class) are known to be
+ * ``less random'' than the high order bits.
+ *
+ * @param n the upper bound
+ * @throws IllegalArgumentException if the given upper bound is negative
+ * @return the next pseudorandom value
+ * @since 1.2
+ */
+ public int nextInt(int n)
+ {
+ if (n <= 0)
+ throw new IllegalArgumentException("n must be positive");
+ if ((n & -n) == n) // i.e., n is a power of 2
+ return (int) ((n * (long) next(31)) >> 31);
+ int bits, val;
+ do
+ {
+ bits = next(31);
+ val = bits % n;
+ }
+ while (bits - val + (n - 1) < 0);
+ return val;
+ }
+
+ /**
+ * Generates the next pseudorandom long number. All bits of this
+ * long are independently chosen and 0 and 1 have equal likelihood.
+ * The implementation for java.util.Random is:
+ *
+<pre>public long nextLong()
+{
+ return ((long) next(32) &lt;&lt; 32) + next(32);
+}</pre>
+ *
+ * @return the next pseudorandom value
+ */
+ public long nextLong()
+ {
+ return ((long) next(32) << 32) + next(32);
+ }
+
+ /**
+ * Generates the next pseudorandom boolean. True and false have
+ * the same probability. The implementation is:
+ *
+<pre>public boolean nextBoolean()
+{
+ return next(1) != 0;
+}</pre>
+ *
+ * @return the next pseudorandom boolean
+ * @since 1.2
+ */
+ public boolean nextBoolean()
+ {
+ return next(1) != 0;
+ }
+
+ /**
+ * Generates the next pseudorandom float uniformly distributed
+ * between 0.0f (inclusive) and 1.0f (exclusive). The
+ * implementation is as follows.
+ *
+<pre>public float nextFloat()
+{
+ return next(24) / ((float)(1 &lt;&lt; 24));
+}</pre>
+ *
+ * @return the next pseudorandom float
+ */
+ public float nextFloat()
+ {
+ return next(24) / (float) (1 << 24);
+ }
+
+ /**
+ * Generates the next pseudorandom double uniformly distributed
+ * between 0.0 (inclusive) and 1.0 (exclusive). The
+ * implementation is as follows.
+ *
+<pre>public double nextDouble()
+{
+ return (((long) next(26) &lt;&lt; 27) + next(27)) / (double)(1L &lt;&lt; 53);
+}</pre>
+ *
+ * @return the next pseudorandom double
+ */
+ public double nextDouble()
+ {
+ return (((long) next(26) << 27) + next(27)) / (double) (1L << 53);
+ }
+
+ /**
+ * Generates the next pseudorandom, Gaussian (normally) distributed
+ * double value, with mean 0.0 and standard deviation 1.0.
+ * The algorithm is as follows.
+ *
+<pre>public synchronized double nextGaussian()
+{
+ if (haveNextNextGaussian)
+ {
+ haveNextNextGaussian = false;
+ return nextNextGaussian;
+ }
+ else
+ {
+ double v1, v2, s;
+ do
+ {
+ v1 = 2 * nextDouble() - 1; // between -1.0 and 1.0
+ v2 = 2 * nextDouble() - 1; // between -1.0 and 1.0
+ s = v1 * v1 + v2 * v2;
+ }
+ while (s >= 1);
+
+ double norm = Math.sqrt(-2 * Math.log(s) / s);
+ nextNextGaussian = v2 * norm;
+ haveNextNextGaussian = true;
+ return v1 * norm;
+ }
+}</pre>
+ *
+ * <p>This is described in section 3.4.1 of <em>The Art of Computer
+ * Programming, Volume 2</em> by Donald Knuth.
+ *
+ * @return the next pseudorandom Gaussian distributed double
+ */
+ public synchronized double nextGaussian()
+ {
+ if (haveNextNextGaussian)
+ {
+ haveNextNextGaussian = false;
+ return nextNextGaussian;
+ }
+ double v1, v2, s;
+ do
+ {
+ v1 = 2 * nextDouble() - 1; // Between -1.0 and 1.0.
+ v2 = 2 * nextDouble() - 1; // Between -1.0 and 1.0.
+ s = v1 * v1 + v2 * v2;
+ }
+ while (s >= 1);
+ double norm = Math.sqrt(-2 * Math.log(s) / s);
+ nextNextGaussian = v2 * norm;
+ haveNextNextGaussian = true;
+ return v1 * norm;
+ }
+}