feat(rx): add filter operator (#172)

rx/observable-operator-filter_test.go

@@ -0,0 +1,56 @@
+package rx_test
+
+import (
+	"context"
+
+	"github.com/fortytw2/leaktest"
+	. "github.com/onsi/ginkgo/v2" //nolint:revive // ginkgo ok
+	"github.com/snivilised/lorax/rx"
+)
+
+func even(item rx.Item[int]) bool {
+	return item.V%2 == 0
+}
+
+var _ = Describe("Observable operator", func() {
+	Context("Filter", func() {
+		When("principle", func() {
+			It("🧪 should: return filtered items", func() {
+				// rxgo: Test_Observable_Filter
+				defer leaktest.Check(GinkgoT())()
+
+				ctx, cancel := context.WithCancel(context.Background())
+				defer cancel()
+
+				obs := testObservable[int](ctx, 1, 2, 3, 4).Filter(even)
+				rx.Assert(ctx, obs,
+					rx.HasItems[int]{
+						Expected: []int{2, 4},
+					},
+					rx.HasNoError[int]{},
+				)
+			})
+		})
+
+		Context("Parallel", func() {
+			When("foo", func() {
+				It("🧪 should: return filtered items", func() {
+					// rxgo: Test_Observable_Filter_Parallel
+					defer leaktest.Check(GinkgoT())()
+
+					ctx, cancel := context.WithCancel(context.Background())
+					defer cancel()
+					obs := testObservable[int](ctx, 1, 2, 3, 4).Filter(even,
+						rx.WithCPUPool[int](),
+					)
+					rx.Assert(ctx, obs,
+						rx.HasItemsNoOrder[int]{
+							Expected: []int{2, 4},
+						},
+						rx.HasNoError[int]{},
+					)
+				})
+			})
+		})
+	})
+})

rx/observable-operator.go

@@ -668,6 +668,44 @@ func (o *ObservableImpl[T]) Errors(opts ...Option[T]) []error {
 	}
 }
 
+// Filter emits only those items from an Observable that pass a predicate test.
+func (o *ObservableImpl[T]) Filter(apply Predicate[T], opts ...Option[T]) Observable[T] {
+	const (
+		forceSeq     = false
+		bypassGather = true
+	)
+
+	return observable(o.parent, o, func() operator[T] {
+		return &filterOperator[T]{apply: apply}
+	}, forceSeq, bypassGather, opts...)
+}
+
+type filterOperator[T any] struct {
+	apply Predicate[T]
+}
+
+func (op *filterOperator[T]) next(ctx context.Context, item Item[T],
+	dst chan<- Item[T], _ operatorOptions[T],
+) {
+	if op.apply(item) {
+		item.SendContext(ctx, dst)
+	}
+}
+
+func (op *filterOperator[T]) err(ctx context.Context, item Item[T],
+	dst chan<- Item[T], operatorOptions operatorOptions[T],
+) {
+	defaultErrorFuncOperator(ctx, item, dst, operatorOptions)
+}
+
+func (op *filterOperator[T]) end(_ context.Context, _ chan<- Item[T]) {
+}
+
+func (op *filterOperator[T]) gatherNext(_ context.Context, _ Item[T],
+	_ chan<- Item[T], _ operatorOptions[T],
+) {
+}
+
 // !!!
 
 // Max determines and emits the maximum-valued item emitted by an Observable according to a comparator.

rx/observable.go

@@ -26,6 +26,7 @@ type Observable[T any] interface {
 	ElementAt(index uint, opts ...Option[T]) Single[T]
 	Error(opts ...Option[T]) error
 	Errors(opts ...Option[T]) []error
+	Filter(apply Predicate[T], opts ...Option[T]) Observable[T]
 	Max(comparator Comparator[T], initLimit InitLimit[T], opts ...Option[T]) OptionalSingle[T]
 	Map(apply Func[T], opts ...Option[T]) Observable[T]
 	Min(comparator Comparator[T], initLimit InitLimit[T], opts ...Option[T]) OptionalSingle[T]