Add fold and reduce examples (#9188)

Co-authored-by: Yevgen Pikus <yevgen.pikus@eon.com>

core-kotlin-modules/core-kotlin-collections/src/test/kotlin/com/baeldung/foldvsreduce/FoldAndReduceTest.kt

@@ -0,0 +1,59 @@
+package com.baeldung.foldvsreduce
+
+import org.junit.Test
+import org.junit.jupiter.api.assertThrows
+import java.lang.RuntimeException
+import kotlin.test.assertEquals
+
+class FoldAndReduceTest {
+
+    @Test
+    fun testReduceLimitations() {
+        val numbers: List<Int> = listOf(1, 2, 3)
+        val sum: Number = numbers.reduce { acc, next -> acc + next }
+        assertEquals(6, sum)
+
+        val emptyList = listOf<Int>()
+        assertThrows<RuntimeException> { emptyList.reduce { acc, next -> acc + next } }
+
+        // doesn't compile
+        // val sum = numbers.reduce { acc, next -> acc.toLong() + next.toLong()}
+    }
+
+    @Test
+    fun testFold() {
+
+        val numbers: List<Int> = listOf(1, 2, 3)
+        val sum: Int = numbers.fold(0, { acc, next -> acc + next })
+        assertEquals(6, sum)
+
+        //change result type
+        val sumLong: Long = numbers.fold(0L, { acc, next -> acc + next.toLong() })
+        assertEquals(6L, sumLong)
+
+        val emptyList = listOf<Int>()
+        val emptySum = emptyList.fold(0, { acc, next -> acc + next })
+        assertEquals(0, emptySum)
+
+        //power of changing result type
+        val (even, odd) = numbers.fold(Pair(listOf<Int>(), listOf<Int>()), { acc, next ->
+            if (next % 2 == 0) Pair(acc.first + next, acc.second)
+            else Pair(acc.first, acc.second + next)
+        })
+
+        assertEquals(listOf(2), even)
+        assertEquals(listOf(1, 3), odd)
+    }
+
+    @Test
+    fun testVariationsOfFold() {
+        val numbers = listOf(1, 2, 3)
+        val reversed = numbers.foldRight(listOf<Int>(), { next, acc -> acc + next})
+        assertEquals(listOf(3,2,1), reversed)
+
+        val reversedIndexes = numbers.foldRightIndexed(listOf<Int>(), { i, _, acc -> acc + i })
+        assertEquals(listOf(2,1,0), reversedIndexes)
+    }
+
+
+}