stdlib: add flatten()

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

@@ -0,0 +1,34 @@
+package kotlin
+
+import java.util.ArrayList
+import kotlin.platform.platformName
+
+/**
+ * Returns a single list of all elements from all collections in original collection
+ */
+public fun <T> Iterable<Iterable<T>>.flatten(): List<T> {
+    val result = ArrayList<T>()
+    for (element in this) {
+        result.addAll(element)
+    }
+    return result
+}
+
+/**
+ * Returns a stream of all elements from all streams in original stream
+ */
+public fun <T> Stream<Stream<T>>.flatten(): Stream<T> {
+    return FlatteningStream(this, { it })
+}
+
+/**
+ * Returns a single list of all elements from all collections in original collection
+ */
+public fun <T> Array<Array<out T>>.flatten(): List<T> {
+    val result = ArrayList<T>(sumBy { it.size() })
+    for (element in this) {
+        result.addAll(element)
+    }
+    return result
+}
+

libraries/stdlib/test/collections/IterableTests.kt

@@ -164,6 +164,11 @@ abstract class IterableTests<T : Iterable<String>>(val data: T, val empty: T) {
         assertEquals(listOf(3, 3), lengths)
     }
 
+    Test
+    fun flatten() {
+        assertEquals(listOf(0, 1, 2, 3, 0, 1, 2, 3), data.map { 0..it.length() }.flatten())
+    }
+
     Test
     fun mapIndexed() {
         val shortened = data.mapIndexed {(index, value) -> value.substring(0..index) }

libraries/stdlib/test/collections/StreamTest.kt

@@ -213,6 +213,13 @@ public class StreamTest {
         assertEquals(listOf(0, 1, 3, 4), result.toList())
     }
 
+    test
+    fun flatten() {
+        val data = streamOf(1, 2).map { streamOf(0..it) }
+        assertEquals(listOf(0, 1, 0, 1, 2), data.flatten().toList())
+    }
+
+
     /*
     test fun pairIterator() {
         val pairStr = (fibonacci() zip fibonacci().map { i -> i*2 }).joinToString(limit = 10)