Merge pull request #1119 from sirmax/gen-pick-combinations

Fix Gen.pick producing only a subset of possible combinations

Author: satorg

core/jvm/src/test/scala/org/scalacheck/GenSpecification.scala

@@ -291,6 +291,24 @@ object GenSpecification extends Properties("Gen") with GenSpecificationVersionSp
     }
   }
 
+  property("pick produces enough distinct combinations") = {
+    val (n, nElements, nCombinations) = (3, 5, 10)
+    // The number of samples to produce with Gen.pick, that is enough to assert
+    // that every possible combination appears at least once. It depends on
+    // nSamples, but there is no specific math behind it. Rather, it's just an
+    // empirically chosen value, that has yielded no failures over thousands of
+    // test cycles.
+    val nSamples = 200
+
+    val genPick = pick(n, 1 to nElements)
+      // not interested in different permutations, only in distinct combinations
+      .map(_.sorted)
+
+    forAllNoShrink(listOfN(nSamples, genPick).map(_.distinct)) { distinctCombinations =>
+      distinctCombinations.size == nCombinations
+    }
+  }
+
   property("numChar") = forAll(numChar)(_.isDigit)
 
   property("calendar") = forAll(calendar) { cal =>

core/shared/src/main/scala/org/scalacheck/Gen.scala

@@ -1092,7 +1092,7 @@ object Gen extends GenArities with GenVersionSpecific {
           buf += t
         } else {
           val (x, s) = seed.long
-          val i = (x & Long.MaxValue % count).toInt
+          val i = ((x & Long.MaxValue) % count).toInt
           if (i < n) buf(i) = t
           seed = s
         }