Fix Random.nextInt with bound that returned negative values

backend.native/tests/build.gradle

@@ -499,8 +499,13 @@ task runtime_random(type: RunKonanTest) {
     source = "runtime/basic/random.kt"
 }
 
+task runtime_random_bound(type: RunKonanTest) {
+    disabled = (project.testTarget == 'wasm32') // Uses exceptions.
+    source = "runtime/basic/random_bound.kt"
+}
+
 task runtime_worker_random(type: RunKonanTest) {
-    disabled = (project.testTarget == 'wasm32')
+    disabled = (project.testTarget == 'wasm32') // Uses workers.
     source = "runtime/basic/worker_random.kt"
 }
 

backend.native/tests/runtime/basic/random.kt

@@ -52,11 +52,20 @@ fun testDiffInt() = testDifference { Random.nextInt() }
 fun testDiffLong() = testDifference { Random.nextLong() }
 
 @Test
-fun testBoundInt() {
-    testReproducibility(1000L, { Random.nextInt(1000) })
+fun testNextInt() {
+    testReproducibility(getTimeMillis(), { Random.nextInt(1000) })
+    testReproducibility(1000L, { Random.nextInt(1024) })
+}
 
-    // test bounds
-    val bound = 5000
+@Test
+fun testBoundsNextInt() {
+    boundTest(5000)
+    boundTest(32)
+    boundTest(2)
+    boundTest(Int.MAX_VALUE)
+}
+
+private fun boundTest(bound: Int) {
     val a = Array<Int>(100, { Random.nextInt(bound) })
     a.forEach {
         assertTrue(it >= 0, "Should be: $it >= 0")

backend.native/tests/runtime/basic/random_bound.kt

@@ -0,0 +1,11 @@
+package runtime.basic.random_bound
+
+import kotlin.random.*
+import kotlin.test.*
+
+@Test
+fun testBoundsNextInt() {
+    assertFailsWith<IllegalArgumentException>("Should fail on bound 0", { Random.nextInt(0) })
+    assertFailsWith<IllegalArgumentException>("Should fail on bound -100", { Random.nextInt(-100) })
+    assertFailsWith<IllegalArgumentException>("Should fail on bound ${Int.MIN_VALUE}", { Random.nextInt(Int.MIN_VALUE) })
+}

runtime/src/main/kotlin/kotlin/random/Random.kt

@@ -15,9 +15,7 @@
  */
 package kotlin.random
 
-import konan.internal.*
 import konan.worker.AtomicLong
-import kotlin.random.Random.Companion.MULTIPLIER
 import kotlin.system.getTimeNanos
 
 abstract class Random {
@@ -61,12 +59,12 @@ abstract class Random {
             if (bound <= 0) throw IllegalArgumentException("Incorrect bound: $bound")
 
             if (bound and (bound - 1) == 0) {
-                return ((bound * nextInt().toLong()) shr 31).toInt();
+                return ((bound * (nextInt() ushr 1).toLong()) ushr 31).toInt();
             }
 
             var m: Int
             do {
-                var r = nextInt()
+                var r = nextInt() ushr 1
                 m = r % bound
             } while (r - m + (bound - 1) < 0)
             return m