kmath-coroutines/src/commonMain/kotlin/scientifik/kmath/CoroutinesExtra.kt

@@ -1,13 +1,32 @@
 package scientifik.kmath
 
 import kotlinx.coroutines.*
+import kotlinx.coroutines.channels.ReceiveChannel
 import kotlinx.coroutines.channels.produce
-import kotlinx.coroutines.flow.*
+import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.flow.FlowCollector
+import kotlinx.coroutines.flow.collect
+import kotlinx.coroutines.flow.map
 
 val Dispatchers.Math: CoroutineDispatcher get() = Dispatchers.Default
 
+/**
+ * An imitator of [Deferred] which holds a suspended function block and dispatcher
+ */
+class LazyDeferred<T>(val dispatcher: CoroutineDispatcher, val block: suspend CoroutineScope.() -> T) {
+    private var deferred: Deferred<T>? = null
+
+    fun CoroutineScope.start() {
+        if(deferred==null) {
+            deferred = async(dispatcher, block = block)
+        }
+    }
+
+    suspend fun await(): T = deferred?.await() ?: error("Coroutine not started")
+}
+
 @FlowPreview
-inline class AsyncFlow<T>(val deferredFlow: Flow<Deferred<T>>) : Flow<T> {
+inline class AsyncFlow<T>(val deferredFlow: Flow<LazyDeferred<T>>) : Flow<T> {
     override suspend fun collect(collector: FlowCollector<T>) {
         deferredFlow.collect {
             collector.emit((it.await()))
@@ -18,20 +37,20 @@ inline class AsyncFlow<T>(val deferredFlow: Flow<Deferred<T>>) : Flow<T> {
 @FlowPreview
 fun <T, R> Flow<T>.async(
     dispatcher: CoroutineDispatcher = Dispatchers.Default,
-    block: suspend (T) -> R
+    block: suspend CoroutineScope.(T) -> R
 ): AsyncFlow<R> {
     val flow = map {
-        coroutineScope {
-            async(dispatcher, start = CoroutineStart.LAZY) { block(it) }
-        }
+        LazyDeferred(dispatcher) { block(it) }
     }
     return AsyncFlow(flow)
 }
 
 @FlowPreview
 fun <T, R> AsyncFlow<T>.map(action: (T) -> R) = deferredFlow.map { input ->
-    coroutineScope {
-        async(start = CoroutineStart.LAZY) { action(input.await()) }
+    //TODO add actual composition
+    LazyDeferred(input.dispatcher) {
+        input.run { start() }
+        action(input.await())
     }
 }
 
@@ -43,7 +62,7 @@ suspend fun <T> AsyncFlow<T>.collect(concurrency: Int, collector: FlowCollector<
         //Starting up to N deferred coroutines ahead of time
         val channel = produce(capacity = concurrency) {
             deferredFlow.collect { value ->
-                value.start()
+                value.run { start() }
                 send(value)
             }
         }
@@ -71,3 +90,4 @@ suspend fun <T> AsyncFlow<T>.collect(concurrency: Int, action: suspend (value: T
         override suspend fun emit(value: T) = action(value)
     })
 }
+

kmath-coroutines/src/jvmTest/kotlin/scientifik/kmath/streaming/BufferFlowTest.kt

@@ -11,11 +11,10 @@ import scientifik.kmath.collect
 @FlowPreview
 class BufferFlowTest {
 
-
-    @Test
+    @Test(timeout = 2000)
     fun mapParallel() {
         runBlocking {
-            (1..20).asFlow().async(Dispatchers.IO) {
+            (1..20).asFlow().async(Dispatchers.Default) {
                 println("Started $it")
                 @Suppress("BlockingMethodInNonBlockingContext")
                 Thread.sleep(200)