8280550: SplittableRandom#nextDouble(double,double) can return result >= bound

Reviewed-by: jlaskey, psandoz

src/java.base/share/classes/jdk/internal/util/random/RandomSupport.java

@@ -645,7 +645,7 @@ public class RandomSupport {
         if (origin < bound) {
             r = r * (bound - origin) + origin;
             if (r >= bound)  // may need to correct a rounding problem
-                r = Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
+                r = Math.nextAfter(r, origin);
         }
         return r;
     }
@@ -677,7 +677,7 @@ public class RandomSupport {
         double r = rng.nextDouble();
         r = r * bound;
         if (r >= bound)  // may need to correct a rounding problem
-            r = Double.longBitsToDouble(Double.doubleToLongBits(bound) - 1);
+            r = Math.nextDown(r);
         return r;
     }
 

test/jdk/java/util/Random/RandomNextDoubleBoundary.java

@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2022, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @summary Verify nextDouble stays within range
+ * @bug 8280550
+ */
+
+import java.util.SplittableRandom;
+
+public class RandomNextDoubleBoundary {
+    public static void main(String... args) {
+        // Both bounds are negative
+        double lowerBound = -1.0000000000000002;
+        double upperBound = -1.0;
+        var sr = new SplittableRandom(42L);
+        var r = sr.nextDouble(lowerBound, upperBound);
+
+        if (r >= upperBound) {
+            System.err.println("r = " + r + "\t" + Double.toHexString(r));
+            System.err.println("ub = " + upperBound + "\t" + Double.toHexString(upperBound));
+            throw new RuntimeException("Greater than upper bound");
+        }
+
+        if (r < lowerBound) {
+            System.err.println("r = " + r + "\t" + Double.toHexString(r));
+            System.err.println("lb = " + lowerBound + "\t" + Double.toHexString(lowerBound));
+            throw new RuntimeException("Less than lower bound");
+        }
+    }
+}