Common shuffle/shuffled with the specified random source

KT-17261, KT-9010

libraries/stdlib/js/src/kotlin/collections.kt

@@ -6,7 +6,7 @@
 package kotlin.collections
 
 import kotlin.comparisons.naturalOrder
-import kotlin.math.floor
+import kotlin.random.Random
 
 /** Returns the array if it's not `null`, or an empty array otherwise. */
 @kotlin.internal.InlineOnly
@@ -87,16 +87,7 @@ public actual fun <T> MutableList<T>.fill(value: T): Unit {
  * See: https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#The_modern_algorithm
  */
 @SinceKotlin("1.2")
-public actual fun <T> MutableList<T>.shuffle(): Unit {
-    for (i in lastIndex downTo 1) {
-        val j = rand(i + 1)
-        val copy = this[i]
-        this[i] = this[j]
-        this[j] = copy
-    }
-}
-
-private fun rand(upperBound: Int) = floor(kotlin.js.Math.random() * upperBound).toInt()
+public actual fun <T> MutableList<T>.shuffle(): Unit = shuffle(Random)
 
 /**
  * Returns a new list with the elements of this list randomly shuffled.

libraries/stdlib/src/kotlin/collections/MutableCollections.kt

@@ -8,6 +8,8 @@
 
 package kotlin.collections
 
+import kotlin.random.Random
+
 /**
  * Removes a single instance of the specified element from this
  * collection, if it is present.
@@ -259,3 +261,26 @@ private fun MutableCollection<*>.retainNothing(): Boolean {
     clear()
     return result
 }
+
+/**
+ * Randomly shuffles elements in this mutable list using the specified [random] instance as the source of randomness.
+ *
+ * See: https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle#The_modern_algorithm
+ */
+@SinceKotlin("1.3")
+public fun <T> MutableList<T>.shuffle(random: Random): Unit {
+    for (i in lastIndex downTo 1) {
+        val j = random.nextInt(i + 1)
+        val copy = this[i]
+        this[i] = this[j]
+        this[j] = copy
+    }
+}
+
+/**
+ * Returns a new list with the elements of this list randomly shuffled
+ * using the specified [random] instance as the source of randomness.
+ */
+@SinceKotlin("1.3")
+public fun <T> Iterable<T>.shuffled(random: Random): List<T> = toMutableList().apply { shuffle(random) }
+

libraries/stdlib/test/collections/MutableCollectionsTest.kt

@@ -5,6 +5,7 @@
 
 package test.collections
 
+import kotlin.random.Random
 import kotlin.test.*
 
 
@@ -114,4 +115,16 @@ class MutableCollectionTest {
         assertEquals(list.size, shuffled.distinct().size)
     }
 
+    @Test fun shuffledPredictably() {
+        val list = List(10) { it }
+        val shuffled1 = list.shuffled(Random(1))
+        val shuffled11 = list.shuffled(Random(1))
+
+        assertEquals(shuffled1, shuffled11)
+        assertEquals("[1, 4, 0, 6, 2, 8, 9, 7, 3, 5]", shuffled1.toString())
+
+        val shuffled2 = list.shuffled(Random(42))
+        assertEquals("[5, 0, 4, 9, 2, 8, 1, 7, 6, 3]", shuffled2.toString())
+    }
+
 }