KT-33761 Add reduceOrNull

libraries/stdlib/test/collections/ArraysTest.kt

@@ -1006,6 +1006,20 @@ class ArraysTest {
         }
     }
 
+    @Test fun reduceOrNull() {
+        expect(-4) { intArrayOf(1, 2, 3).reduceOrNull { a, b -> a - b } }
+        expect(-4.toLong()) { longArrayOf(1, 2, 3).reduceOrNull { a, b -> a - b } }
+        expect(-4F) { floatArrayOf(1F, 2F, 3F).reduceOrNull { a, b -> a - b } }
+        expect(-4.0) { doubleArrayOf(1.0, 2.0, 3.0).reduceOrNull { a, b -> a - b } }
+        expect('3') { charArrayOf('1', '3', '2').reduceOrNull { a, b -> if (a > b) a else b } }
+        expect(false) { booleanArrayOf(true, true, false).reduceOrNull { a, b -> a && b } }
+        expect(true) { booleanArrayOf(true, true).reduceOrNull { a, b -> a && b } }
+        expect(0.toByte()) { byteArrayOf(3, 2, 1).reduceOrNull { a, b -> (a - b).toByte() } }
+        expect(0.toShort()) { shortArrayOf(3, 2, 1).reduceOrNull { a, b -> (a - b).toShort() } }
+
+        expect(null, { intArrayOf().reduceOrNull { a, b -> a + b } })
+    }
+
     @Test fun reduceRight() {
         expect(2) { intArrayOf(1, 2, 3).reduceRight { a, b -> a - b } }
         expect(2.toLong()) { longArrayOf(1, 2, 3).reduceRight { a, b -> a - b } }

libraries/stdlib/test/collections/CollectionTest.kt

@@ -315,6 +315,15 @@ class CollectionTest {
         }
     }
 
+    @Test fun reduceOrNull() {
+        expect("1234") {
+            val list = listOf("1", "2", "3", "4")
+            list.reduceOrNull { a, b -> a + b }
+        }
+
+        expect(null, { arrayListOf<Int>().reduceOrNull { a, b -> a + b } })
+    }
+
     @Test fun reduceRight() {
         expect("1234") {
             val list = listOf("1", "2", "3", "4")

libraries/stdlib/test/collections/SequenceTest.kt

@@ -379,6 +379,10 @@ public class SequenceTest {
         assertEquals(expected_, b.toList())
     }
 
+    @Test fun reduceOrNullOnEmpty() {
+        expect(null, { sequenceOf<Int>().reduceOrNull { acc, i -> acc + i } })
+    }
+
     @Test fun minusElement() = testMinus(expected = listOf("foo", "bar")) { it - "bar" - "zoo" }
     @Test fun minusCollection() = testMinus { it - listOf("bar", "zoo") }
     @Test fun minusArray() = testMinus { it - arrayOf("bar", "zoo") }

libraries/stdlib/test/text/StringTest.kt

@@ -1281,6 +1281,13 @@ class StringTest {
         }
     }
 
+    @Test fun reduceOrNull() = withOneCharSequenceArg { arg1 ->
+        // get the smallest character(by char value)
+        assertEquals('a', arg1("bacfd").reduceOrNull { v, c -> if (v > c) c else v })
+
+        expect(null, { arg1("").reduceOrNull { _, _ -> '\n' } })
+    }
+
     @Test fun groupBy() = withOneCharSequenceArg("abAbaABcD") { data ->
         // group characters by their case
         val result = data.groupBy { it.isAsciiUpperCase() }