new fast rng for 64 bit numbers

Author: TilmanNeumann

src/main/java/de/tilman_neumann/jml/random/SplitMix64.java

@@ -0,0 +1,49 @@
+/*
+ * java-math-library is a Java library focused on number theory, but not necessarily limited to it. It is based on the PSIQS 4.0 factoring project.
+ * Copyright (C) 2018-2025 Tilman Neumann - tilman.neumann@web.de
+ *
+ * This program 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 3 of the License, or (at your option) any later version.
+ *
+ * This program 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 this program;
+ * if not, see <http://www.gnu.org/licenses/>.
+ */
+package de.tilman_neumann.jml.random;
+
+/**
+ * 64 bit random number generator adapted from https://rosettacode.org/wiki/Pseudo-random_numbers/Splitmix64.
+ * 
+ * Very fast.
+ * 
+ * @author Tilman Neumann
+ */
+final class SplitMix64 {
+	
+	private static final long constant1 = 0x9e3779b97f4a7c15L;
+	private static final long constant2 = 0xbf58476d1ce4e5b9L;
+	private static final long constant3 = 0x94d049bb133111ebL;
+    
+	private long state;
+
+	public SplitMix64() {
+		state = 90832436910930047L; // just some semiprime, arbitrary choice
+	}
+	
+	public SplitMix64(long aSeed) {
+		state = aSeed;
+	}
+	
+	public void seed(long aNumber) {
+		state = aNumber;
+	}
+    
+	public long nextLong() {
+	    long z = (state += constant1);
+	    z = (z ^ (z>>>30)) * constant2;
+	    z = (z ^ (z>>>27)) * constant3;
+	    return z ^ (z>>>31);
+	}
+}

src/test/java/de/tilman_neumann/jml/random/RngPerformanceTest.java

@@ -52,6 +52,7 @@ public class RngPerformanceTest {
 	private static final LehmerRng64 lehmer64 = new LehmerRng64();
 	private static final LehmerRng64MH lehmer64MH = new LehmerRng64MH();
 	private static final Xorshf64 xorshf64 = new Xorshf64();
+	private static final SplitMix64 splitMix64 = new SplitMix64();
 
 	/**
 	 * Test.
@@ -174,6 +175,10 @@ public static void main(String[] args) {
 			xorshf64.nextLong();
 		}
 		LOG.debug("Xorshf64.nextLong() took " + timer.capture() + " ms");
+		for (int i=0; i<NCOUNT; i++) {
+			splitMix64.nextLong();
+		}
+		LOG.debug("SplitMix64.nextLong() took " + timer.capture() + " ms");
 
 		// test nextLong(long upper)
 		for (int i=0; i<NCOUNT; i++) {

src/test/java/de/tilman_neumann/jml/random/SplitMix64Test.java

@@ -0,0 +1,68 @@
+/*
+ * java-math-library is a Java library focused on number theory, but not necessarily limited to it. It is based on the PSIQS 4.0 factoring project.
+ * Copyright (C) 2018-2025 Tilman Neumann - tilman.neumann@web.de
+ *
+ * This program 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 3 of the License, or (at your option) any later version.
+ *
+ * This program 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 this program;
+ * if not, see <http://www.gnu.org/licenses/>.
+ */
+package de.tilman_neumann.jml.random;
+
+import static org.junit.Assert.assertEquals;
+import static org.junit.Assert.assertTrue;
+
+import java.util.Arrays;
+
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+import de.tilman_neumann.util.ConfigUtil;
+
+public final class SplitMix64Test {
+	private static final Logger LOG = LogManager.getLogger(SplitMix64Test.class);
+
+	private static final int NCOUNT = 1000000;
+
+	private static final SplitMix64 rng = new SplitMix64();
+	
+	@BeforeClass
+	public static void setup() {
+		ConfigUtil.initProject();
+	}
+
+	@Test
+	public void testRosettaExamples() {
+		SplitMix64 random = new SplitMix64(1234567);
+		assertEquals(6457827717110365317L, random.nextLong());
+		assertEquals(3203168211198807973L, random.nextLong());
+		assertEquals(-8629252141511181193L, random.nextLong()); // 9817491932198370423 - 2^64
+		assertEquals(4593380528125082431L, random.nextLong());
+		assertEquals(-2037821214251327795L, random.nextLong()); // 16408922859458223821 - 2^64
+    }
+	
+	@Test
+	public void testNextLongNoArgs() {
+		long[] firstElements = new long[100];
+		long min = Long.MAX_VALUE, max = Long.MIN_VALUE;
+		boolean generatesEven = false, generatesOdd = false;
+		for (int i=0; i<NCOUNT; i++) {
+			long n = rng.nextLong();
+			if (i<100) firstElements[i] = n;
+			if (n<min) min = n;
+			if (n>max) max = n;
+			if ((n & 1) == 1) generatesOdd = true; else generatesEven = true;
+		}
+		LOG.debug("First 100 elements: " + Arrays.toString(firstElements));
+		LOG.debug(NCOUNT + " numbers from " + rng.getClass().getSimpleName() + ".nextLong() gave min = " + min + ", max = " + max);
+		assertTrue(min < 0);
+		assertTrue(generatesEven);
+		assertTrue(generatesOdd);
+	}
+}