Extension random() to select random element from a collection

Fixes #KT-15539

libraries/stdlib/test/collections/ArraysTest.kt

@@ -11,6 +11,7 @@ import test.collections.behaviors.*
 import test.comparisons.STRING_CASE_INSENSITIVE_ORDER
 import kotlin.test.*
 import kotlin.comparisons.*
+import kotlin.random.Random
 
 fun <T> assertArrayNotSameButEquals(expected: Array<out T>, actual: Array<out T>, message: String = "") { assertTrue(expected !== actual && expected contentEquals actual, message) }
 fun assertArrayNotSameButEquals(expected: IntArray, actual: IntArray, message: String = "") {             assertTrue(expected !== actual && expected contentEquals actual, message) }
@@ -592,6 +593,31 @@ class ArraysTest {
         expect(2) { arrayOf(1, 2, 3).last { it % 2 == 0 } }
     }
 
+
+    @Test fun random() {
+        Array(100) { it }.let { array ->
+            val tosses = List(10) { array.random() }
+            assertTrue(tosses.distinct().size > 1, "Should be some distinct elements in $tosses")
+
+            val seed = Random.nextInt()
+            val random1 = Random(seed)
+            val random2 = Random(seed)
+
+            val tosses1 = List(10) { array.random(random1) }
+            val tosses2 = List(10) { array.random(random2) }
+
+            assertEquals(tosses1, tosses2)
+        }
+
+        arrayOf("x").let { singletonArray ->
+            val tosses = List(10) { singletonArray.random() }
+            assertEquals(singletonArray.toList(), tosses.distinct())
+        }
+
+        assertFailsWith<NoSuchElementException> { emptyArray<Any>().random() }
+    }
+
+
     @Test fun contains() {
         assertTrue(arrayOf("1", "2", "3", "4").contains("2"))
         assertTrue("3" in arrayOf("1", "2", "3", "4"))

libraries/stdlib/test/collections/CollectionTest.kt

@@ -10,6 +10,8 @@ import kotlin.test.*
 import test.collections.behaviors.*
 import test.comparisons.STRING_CASE_INSENSITIVE_ORDER
 import kotlin.comparisons.*
+import kotlin.math.sin
+import kotlin.random.Random
 
 class CollectionTest {
 
@@ -620,6 +622,31 @@ class CollectionTest {
         assertFails { arrayListOf<Int>().last() }
     }
 
+    @Test fun random() {
+        val list = List(100) { it }
+        val set = list.toSet()
+        listOf(list, set).forEach { collection: Collection<Int> ->
+            val tosses = List(10) { collection.random() }
+            assertTrue(tosses.distinct().size > 1, "Should be some distinct elements in $tosses")
+
+            val seed = Random.nextInt()
+            val random1 = Random(seed)
+            val random2 = Random(seed)
+
+            val tosses1 = List(10) { collection.random(random1) }
+            val tosses2 = List(10) { collection.random(random2) }
+
+            assertEquals(tosses1, tosses2)
+        }
+
+        listOf("x").let { singletonList ->
+            val tosses = List(10) { singletonList.random() }
+            assertEquals(singletonList, tosses.distinct())
+        }
+
+        assertFailsWith<NoSuchElementException> { emptyList<Any>().random() }
+    }
+
     @Test fun subscript() {
         val list = arrayListOf("foo", "bar")
         assertEquals("foo", list[0])

libraries/stdlib/test/text/StringTest.kt

@@ -10,6 +10,7 @@ import test.*
 import test.collections.behaviors.iteratorBehavior
 import test.collections.compare
 import kotlin.math.sign
+import kotlin.random.Random
 
 
 fun createString(content: String): CharSequence = content
@@ -936,6 +937,29 @@ class StringTest {
         assertNull(data.filterNot { it.isAsciiLetter() || it.isAsciiDigit() }.firstOrNull())
     }
 
+    @Test fun random() = withOneCharSequenceArg { data ->
+        data("abcdefg").let { charSeq ->
+            val tosses = List(10) { charSeq.random() }
+            assertTrue(tosses.distinct().size > 1, "Should be some distinct elements in $tosses")
+
+            val seed = Random.nextInt()
+            val random1 = Random(seed)
+            val random2 = Random(seed)
+
+            val tosses1 = List(10) { charSeq.random(random1) }
+            val tosses2 = List(10) { charSeq.random(random2) }
+
+            assertEquals(tosses1, tosses2)
+        }
+
+        data("x").let { singletonCharSeq ->
+            val tosses = List(10) { singletonCharSeq.random() }
+            assertEquals(singletonCharSeq.toList(), tosses.distinct())
+        }
+
+        assertFailsWith<NoSuchElementException> { data("").random() }
+    }
+
     @Test fun partition() {
         val data = "a1b2c3"
         val pair = data.partition { it.isAsciiDigit() }