Provide stable sorting when JS engine sorting doesn't look stable

#KT-12473

libraries/stdlib/test/collections/ArraysTest.kt

@@ -1,5 +1,5 @@
 /*
- * Copyright 2010-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
+ * Copyright 2010-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license
  * that can be found in the license/LICENSE.txt file.
  */
 
@@ -1412,6 +1412,21 @@ class ArraysTest {
         }
     }
 
+    @Test fun sortStable() {
+        val keyRange = 'A'..'D'
+        for (size in listOf(10, 100, 2000)) {
+            val array = Array(size) { index -> Sortable(keyRange.random(), index) }
+
+            array.sortedArray().assertStableSorted()
+            array.sortedArrayDescending().assertStableSorted(descending = true)
+
+            array.sort()
+            array.assertStableSorted()
+            array.sortDescending()
+            array.assertStableSorted(descending = true)
+        }
+    }
+
     @Test fun sortByInPlace() {
         val data = arrayOf("aa" to 20, "ab" to 3, "aa" to 3)
         data.sortBy { it.second }
@@ -1431,6 +1446,22 @@ class ArraysTest {
         assertEquals(listOf(1, 2, 0), indices.sortedBy { values[it] })
     }
 
+    @Test fun sortByStable() {
+        val keyRange = 'A'..'D'
+        for (size in listOf(10, 100, 2000)) {
+            val array = Array(size) { index -> Sortable(keyRange.random(), index) }
+
+            array.sortedBy { it.key }.iterator().assertStableSorted()
+            array.sortedByDescending { it.key }.iterator().assertStableSorted(descending = true)
+
+            array.sortBy { it.key }
+            array.assertStableSorted()
+
+            array.sortByDescending { it.key }
+            array.assertStableSorted(descending = true)
+        }
+    }
+
     @Test fun sortedNullableBy() {
         fun String.nullIfEmpty() = if (this.isEmpty()) null else this
         arrayOf(null, "").let {
@@ -1457,6 +1488,20 @@ class ArraysTest {
     }
 }
 
+private data class Sortable<K : Comparable<K>>(val key: K, val index: Int) : Comparable<Sortable<K>> {
+    override fun compareTo(other: Sortable<K>): Int = this.key.compareTo(other.key)
+}
+
+private fun <K : Comparable<K>> Array<out Sortable<K>>.assertStableSorted(descending: Boolean = false) =
+    iterator().assertStableSorted(descending = descending)
+
+private fun <K : Comparable<K>> Iterator<Sortable<K>>.assertStableSorted(descending: Boolean = false) {
+    assertSorted { a, b ->
+        val relation = a.key.compareTo(b.key)
+        (if (descending) relation > 0 else relation < 0) || relation == 0 && a.index < b.index
+    }
+}
+
 private class ArraySortedChecker<A, T>(val array: A, val comparator: Comparator<in T>) {
     public fun <R> checkSorted(sorted: A.() -> R, sortedDescending: A.() -> R, iterator: R.() -> Iterator<T>) {
         array.sorted().iterator().assertSorted { a, b -> comparator.compare(a, b) <= 0 }