FlowOps: sample operator (#254)

Author: corem

core/src/main/scala/ox/flow/FlowOps.scala

@@ -73,6 +73,19 @@ class FlowOps[+T]:
   def filter(f: T => Boolean): Flow[T] = Flow.usingEmitInline: emit =>
     last.run(FlowEmit.fromInline(t => if f(t) then emit.apply(t)))
 
+  /** Emits only every nth element emitted by this flow.
+    *
+    * @param n
+    *   The interval between two emitted elements.
+    */
+  def sample(n: Int): Flow[T] = Flow.usingEmitInline: emit =>
+    var sampleCounter = 0
+    last.run(
+      FlowEmit.fromInline: t =>
+        sampleCounter += 1
+        if n != 0 && sampleCounter % n == 0 then emit(t)
+    )
+
   /** Applies the given mapping function `f` to each element emitted by this flow, for which the function is defined, and emits the result.
     * If `f` is not defined at an element, the element will be skipped.
     *
@@ -82,6 +95,23 @@ class FlowOps[+T]:
   def collect[U](f: PartialFunction[T, U]): Flow[U] = Flow.usingEmitInline: emit =>
     last.run(FlowEmit.fromInline(t => if f.isDefinedAt(t) then emit.apply(f(t))))
 
+  /** Transforms the elements of the flow by applying an accumulation function to each element, producing a new value at each step. The
+    * resulting flow contains the accumulated values at each point in the original flow.
+    *
+    * @param initial
+    *   The initial value to start the accumulation.
+    * @param f
+    *   The accumulation function that is applied to each element of the flow.
+    */
+  def scan[V](initial: V)(f: (V, T) => V): Flow[V] = Flow.usingEmitInline: emit =>
+    emit(initial)
+    var accumulator = initial
+    last.run(
+      FlowEmit.fromInline: t =>
+        accumulator = f(accumulator, t)
+        emit(accumulator)
+    )
+
   /** Applies the given effectful function `f` to each element emitted by this flow. The returned flow emits the elements unchanged. If `f`
     * throws an exceptions, the flow fails and propagates the exception.
     */
@@ -441,6 +471,17 @@ class FlowOps[+T]:
                 emit((t, otherDefault)); true
             )
 
+  /** Combines each element from this and the index of the element (starting at 0).
+    */
+  def zipWithIndex: Flow[(T, Long)] = Flow.usingEmitInline: emit =>
+    var index = 0L
+    last.run(
+      FlowEmit.fromInline: t =>
+        val zipped = (t, index)
+        index += 1
+        emit(zipped)
+    )
+
   /** Emits a given number of elements (determined byc `segmentSize`) from this flow to the returned flow, then emits the same number of
     * elements from the `other` flow and repeats. The order of elements in both flows is preserved.
     *

core/src/test/scala/ox/flow/FlowOpsFilterTest.scala

@@ -0,0 +1,30 @@
+package ox.flow
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class FlowOpsFilterTest extends AnyFlatSpec with Matchers:
+  behavior of "filter"
+
+  it should "not filter anything from the empty flow" in:
+    val c = Flow.empty[Int]
+    val s = c.filter(_ % 2 == 0)
+    s.runToList() shouldBe List.empty
+
+  it should "filter out everything if no element meets 'f'" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val s = c.filter(_ => false)
+    s.runToList() shouldBe List.empty
+
+  it should "not filter anything if all the elements meet 'f'" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val s = c.filter(_ => true)
+    s.runToList() shouldBe (1 to 10)
+
+  it should "filter out elements that don't meet 'f'" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val s = c.filter(_ % 2 == 0)
+    s.runToList() shouldBe (2 to 10 by 2)
+
+end FlowOpsFilterTest

core/src/test/scala/ox/flow/FlowOpsSampleTest.scala

@@ -0,0 +1,32 @@
+package ox.flow
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class FlowOpsSampleTest extends AnyFlatSpec with Matchers:
+  behavior of "sample"
+
+  it should "not sample anything from an empty flow" in:
+    val c = Flow.empty[Int]
+    val s = c.sample(5)
+    s.runToList() shouldBe List.empty
+
+  it should "not sample anything when 'n == 0'" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val s = c.sample(0)
+    s.runToList() shouldBe List.empty
+
+  it should "sample every element of the flow when 'n == 1'" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val n = 1
+    val s = c.sample(n)
+    s.runToList() shouldBe (n to 10 by n)
+
+  it should "sample every nth element of the flow" in:
+    val c = Flow.fromValues(1 to 10: _*)
+    val n = 3
+    val s = c.sample(n)
+    s.runToList() shouldBe (n to 10 by n)
+
+end FlowOpsSampleTest

core/src/test/scala/ox/flow/FlowOpsScanTest.scala

@@ -0,0 +1,30 @@
+package ox.flow
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class FlowOpsScanTest extends AnyFlatSpec with Matchers:
+  behavior of "scan"
+
+  it should "scan the empty flow" in:
+    val flow: Flow[Int] = Flow.empty
+    val scannedFlow = flow.scan(0)((acc, el) => acc + el)
+    scannedFlow.runToList() shouldBe List(0)
+
+  it should "scan a flow of summed Int" in:
+    val flow = Flow.fromValues(1 to 10: _*)
+    val scannedFlow = flow.scan(0)((acc, el) => acc + el)
+    scannedFlow.runToList() shouldBe List(0, 1, 3, 6, 10, 15, 21, 28, 36, 45, 55)
+
+  it should "scan a flow of multiplied Int" in:
+    val flow = Flow.fromValues(1 to 10: _*)
+    val scannedFlow = flow.scan(1)((acc, el) => acc * el)
+    scannedFlow.runToList() shouldBe List(1, 1, 2, 6, 24, 120, 720, 5040, 40320, 362880, 3628800)
+
+  it should "scan a flow of concatenated String" in:
+    val flow = Flow.fromValues("f", "l", "o", "w")
+    val scannedFlow = flow.scan("my")((acc, el) => acc + el)
+    scannedFlow.runToList() shouldBe List("my", "myf", "myfl", "myflo", "myflow")
+
+end FlowOpsScanTest

core/src/test/scala/ox/flow/FlowOpsZipWithIndexTest.scala

@@ -0,0 +1,20 @@
+package ox.flow
+
+import org.scalatest.concurrent.Eventually
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+
+class FlowOpsZipWithIndexTest extends AnyFlatSpec with Matchers with Eventually:
+  behavior of "zipWithIndex"
+
+  it should "not zip anything from an empty flow" in:
+    val c = Flow.empty[Int]
+    val s = c.zipWithIndex
+    s.runToList() shouldBe List.empty
+
+  it should "zip flow with index" in:
+    val c = Flow.fromValues(1 to 5: _*)
+    val s = c.zipWithIndex
+    s.runToList() shouldBe List((1, 0), (2, 1), (3, 2), (4, 3), (5, 4))
+
+end FlowOpsZipWithIndexTest