Introduce filterIndexed

#KT-9502 Fixed

libraries/stdlib/test/collections/ArraysTest.kt

@@ -736,6 +736,15 @@ class ArraysTest {
         expect(listOf("b"), { arrayOf("a", "b").filter { it > "a" } })
     }
 
+    @test fun filterIndexed() {
+        expect(listOf(), { intArrayOf().filterIndexed { i, v -> i > v } })
+        expect(listOf(2, 5, 8), { intArrayOf(2, 4, 3, 5, 8).filterIndexed { index, value -> index % 2 == value % 2 } })
+        expect(listOf<Long>(2, 5, 8), { longArrayOf(2, 4, 3, 5, 8).filterIndexed { index, value -> index % 2 == (value % 2).toInt() } })
+        expect(listOf<Byte>(2, 5, 8), { byteArrayOf(2, 4, 3, 5, 8).filterIndexed { index, value -> index % 2 == (value % 2).toInt() } })
+        expect(listOf('9', 'e', 'a'), { charArrayOf('9', 'e', 'd', 'a').filterIndexed { index, c -> c == 'a' || index < 2 }})
+        expect(listOf("a", "c", "d"), { arrayOf("a", "b", "c", "d").filterIndexed { index, s -> s == "a" || index >= 2 } })
+    }
+
     @test fun filterNot() {
         expect(listOf(), { intArrayOf().filterNot { it > 2 } })
         expect(listOf(1), { intArrayOf(1).filterNot { it > 2 } })

libraries/stdlib/test/collections/IterableTests.kt

@@ -146,6 +146,11 @@ abstract class IterableTests<T : Iterable<String>>(val data: T, val empty: T) {
         assertEquals(listOf("foo"), foo)
     }
 
+    @Test fun filterIndexed() {
+        val result = data.filterIndexed { index, value -> value.first() == ('a' + index) }
+        assertEquals(listOf("bar"), result)
+    }
+
     @Test
     fun drop() {
         val foo = data.drop(1)

libraries/stdlib/test/collections/SequenceTest.kt

@@ -45,6 +45,10 @@ public class SequenceTest {
         }
     }
 
+    @test fun filterIndexed() {
+        assertEquals(listOf(1, 2, 5, 13, 34), fibonacci().filterIndexed { index, value -> index % 2 == 1 }.take(5).toList())
+    }
+
     @test fun filterNullable() {
         val data = sequenceOf(null, "foo", null, "bar")
         val filtered = data.filter { it == null || it == "foo" }

libraries/stdlib/test/text/StringTest.kt

@@ -746,6 +746,15 @@ class StringTest {
         assertContentEquals("abcd", arg1("a1b2c3d4").filterNot { it.isAsciiDigit() })
     }
 
+    @test fun filterIndexed() {
+        val data = "abedcf"
+        assertEquals("abdf", data.filterIndexed { index, c -> c == 'a' + index })
+    }
+
+    @test fun filterIndexedCharSequence() = withOneCharSequenceArg("abedcf") { data ->
+        assertContentEquals("abdf", data.filterIndexed { index, c -> c == 'a' + index })
+    }
+
     @test fun all() = withOneCharSequenceArg("AbCd") { data ->
         assertTrue {
             data.all { it.isAsciiLetter() }