feat: only emit latest statistics within time window using SSE

This will reduce browser activity and updates statistics only once in
500ms with the latest statistics update event within this time window
and event type.

If no updates are available, no SSE will be sent.

src/main/kotlin/dev/pcvolkmer/oncoanalytics/monitor/web/EventStreamController.kt

@@ -7,6 +7,8 @@ import org.springframework.http.codec.ServerSentEvent
 import org.springframework.web.bind.annotation.GetMapping
 import org.springframework.web.bind.annotation.RestController
 import reactor.core.publisher.Flux
+import kotlin.time.Duration.Companion.milliseconds
+import kotlin.time.toJavaDuration
 
 @RestController
 class EventStreamController(
@@ -15,9 +17,13 @@ class EventStreamController(
 
     @GetMapping(path = ["/events"], produces = [MediaType.TEXT_EVENT_STREAM_VALUE])
     fun eventStream(): Flux<ServerSentEvent<Statistics>> {
-        return statisticsEventProducer.asFlux().map {
+        return statisticsEventProducer.asFlux()
-            ServerSentEvent.builder(it).event(it.name).build()
+            .window(500.milliseconds.toJavaDuration())
-        }.doOnComplete { println("X") }
+            .flatMap { statistics -> statistics.groupBy { it.name } }
+            .flatMap { group -> group.last() }
+            .map {
+                ServerSentEvent.builder(it).event(it.name).build()
+            }
     }
 
 }

src/test/kotlin/dev/pcvolkmer/oncoanalytics/monitor/web/EventStreamControllerTest.kt

@@ -2,6 +2,7 @@ package dev.pcvolkmer.oncoanalytics.monitor.web
 
 import dev.pcvolkmer.oncoanalytics.monitor.StatisticsSink
 import dev.pcvolkmer.oncoanalytics.monitor.conditions.Statistics
+import dev.pcvolkmer.oncoanalytics.monitor.conditions.StatisticsEntry
 import org.junit.jupiter.api.BeforeEach
 import org.junit.jupiter.api.Test
 import org.junit.jupiter.api.extension.ExtendWith
@@ -10,6 +11,7 @@ import org.springframework.http.MediaType
 import org.springframework.test.context.junit.jupiter.SpringExtension
 import org.springframework.test.web.reactive.server.WebTestClient
 import org.springframework.test.web.reactive.server.returnResult
+import reactor.core.publisher.Flux
 import reactor.core.publisher.Sinks
 import reactor.test.StepVerifier
 import kotlin.time.Duration.Companion.seconds
@@ -46,4 +48,35 @@ class EventStreamControllerTest {
             .verify()
     }
 
+    @Test
+    fun shouldSendOnlyLatestEventsWithinTimeWindow() {
+        val latestValue = 10
+
+        Flux.fromIterable(0..latestValue).subscribe { value ->
+            this.statisticsEventProducer.emitNext(
+                Statistics(
+                    "test",
+                    listOf(StatisticsEntry("test", value))
+                )
+            ) { _, _ -> false }
+        }
+
+        val result = webClient
+            .get().uri("/events")
+            .accept(MediaType.TEXT_EVENT_STREAM)
+            .exchange()
+            .expectStatus().isOk
+            .returnResult<Statistics>()
+
+        StepVerifier.create(result.responseBody)
+            .expectNext(
+                Statistics(
+                    "test",
+                    listOf(StatisticsEntry("test", latestValue))
+                )
+            )
+            .expectTimeout(3.seconds.toJavaDuration())
+            .verify()
+    }
+
 }