Exercise: distinct
Implement a distinct function that returns a flow of distinct elements. It should keep all the previously emitted elements in memory and emit only those that were not emitted before.
fun <T> Flow<T>.distinct(): Flow<T> = TODO()
This problem can either be solved in the below playground or you can clone kotlin-exercises project and solve it locally. In the project, you can find code template for this exercise in coroutines/flow/Distinct.kt. You can find there starting code and unit tests.
Once you are done with the exercise, you can check your solution here.
Playground
import junit.framework.TestCase.assertEquals
import kotlinx.coroutines.delay
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.currentTime
import kotlinx.coroutines.test.runTest
import org.junit.Test
fun <T> Flow<T>.distinct(): Flow<T> = TODO()
class DistinctTest {
@Test
fun `should remove duplicates`() = runTest {
val flow = flowOf(1, 3, 1, 2, 3, 1, 2, 3)
val distinctFlow = flow.distinct()
assertEquals(listOf(1, 3, 2), distinctFlow.toList())
}
@Test
fun `should not introduce unnecessary delays`() = runTest {
val f = flowOf(1, 1, 3, 1, 2, 3, 1, 2, 3, 1)
val f1 = f
.onEach { delay(100) }
.distinct()
.map { currentTime to it }
.toList()
assertEquals(listOf(100L to 1, 300L to 3, 500L to 2), f1)
assertEquals(1000L, currentTime)
val f2 = f
.distinct()
.onEach { delay(100) }
.map { currentTime to it }
.toList()
assertEquals(listOf(1100L to 1, 1200L to 3, 1300L to 2), f2)
}
@Test
fun `should keep data flow-specific`() = runTest {
val f = flowOf(1, 1, 3, 1, 2, 3, 1, 2, 3, 1)
.distinct()
assertEquals(listOf(1, 3, 2), f.toList())
assertEquals(listOf(1, 3, 2), f.toList())
}
}
