[KTLN-773] Get Current and Previous Value in Kotlin Flow's collect (#1123)

* redone code

* code changes

* code  changes

Author: albertache1998

core-kotlin-modules/core-kotlin-advanced-4/src/test/kotlin/com/baeldung/firstAndPrevFlowValue/FirstAndPrevFlowValueUnitTest.kt

@@ -0,0 +1,39 @@
+package com.baeldung.firstAndPrevFlowValue
+
+import junit.framework.TestCase.assertEquals
+import kotlinx.coroutines.flow.*
+import kotlinx.coroutines.test.runTest
+import org.junit.jupiter.api.Test
+
+class FirstAndPrevFlowValueUnitTest {
+    @Test
+    fun `test manually tracking previous and current value`() = runTest {
+        val flow = flowOf(1, 2, 3, 4, 5)
+        var previousValue: Int? = null
+        val results = mutableListOf<Pair<Int?, Int>>()
+
+        flow.collect { currentValue ->
+            results.add(Pair(previousValue, currentValue))
+            previousValue = currentValue
+        }
+
+        assertEquals(listOf(Pair(null, 1), Pair(1, 2), Pair(2, 3), Pair(3, 4), Pair(4, 5)), results)
+    }
+
+    @Test
+    fun `test runningFold for previous and current value`() = runTest {
+        val flow = flowOf(1, 2, 3, 4, 5)
+        val initial: Pair<Int?, Int?> = null to null
+        val results = mutableListOf<Pair<Int?, Int?>>()
+
+        flow.runningFold(initial) { lastPair, next ->
+            lastPair.run {
+                val (_, last) = this
+                last to next
+            }
+        }
+            .collect { results.add(it) }
+
+        assertEquals(listOf(Pair(null, null), Pair(null, 1), Pair(1, 2), Pair(2, 3), Pair(3, 4), Pair(4, 5)), results)
+    }
+}