Multiplication is commutative

And document an idea.

Author: Stephan202

error-prone-contrib/src/main/java/tech/picnic/errorprone/refasterrules/RandomGeneratorRules.java

@@ -7,6 +7,10 @@
 import tech.picnic.errorprone.refaster.annotation.OnlineDocumentation;
 
 /** Refaster rules related to expressions dealing with {@link RandomGenerator} instances. */
+// XXX: Consider introducing an Error Prone bug pattern that identifies expressions of the form `c +
+// r.next{Int,Long,Double}(b)` for literals `c` and `b`, and replaces them with
+// `r.next{Int,Long,Double}(c, s)` where `s = c + b`. Similarly with the operands commuted, and
+// for the latter variant also for operator `-`.
 @OnlineDocumentation
 final class RandomGeneratorRules {
   private RandomGeneratorRules() {}
@@ -18,7 +22,7 @@ private RandomGeneratorRules() {}
   static final class RandomGeneratorNextDouble {
     @BeforeTemplate
     double before(RandomGenerator random, double bound) {
-      return random.nextDouble() * bound;
+      return Refaster.anyOf(random.nextDouble() * bound, bound * random.nextDouble());
     }
 
     @AfterTemplate

error-prone-contrib/src/test/resources/tech/picnic/errorprone/refasterrules/RandomGeneratorRulesTestInput.java

@@ -11,7 +11,7 @@ final class RandomGeneratorRulesTest implements RefasterRuleCollectionTestCase {
   ImmutableSet<Double> testRandomGeneratorNextDouble() {
     return ImmutableSet.of(
         new Random().nextDouble() * 1,
-        new SplittableRandom().nextDouble() * 2L,
+        2L * new SplittableRandom().nextDouble(),
         new SecureRandom().nextDouble() * 3.0);
   }