fix(notifications): throttle dictionary progress reposts

A fast download reposted the ongoing notification several times per
second. Publish stage changes immediately and rate-limit intermediate
updates to the latest snapshot.

app/src/main/java/com/example/research/common/progress/DictionaryProgressPresenter.kt

@@ -1,37 +1,86 @@
 package com.example.research.common.progress
 
+import android.os.SystemClock
 import kotlinx.coroutines.CoroutineDispatcher
 import kotlinx.coroutines.CoroutineScope
 import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.Job
+import kotlinx.coroutines.delay
 import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.launch
+import kotlin.time.Duration.Companion.milliseconds
 
 class DictionaryProgressPresenter(
-    scope: CoroutineScope,
+    private val scope: CoroutineScope,
     private val progressSnapshot: StateFlow<DictionaryProgressModel.Snapshot?>,
     private val notifications: NotificationPort,
-    mainDispatcher: CoroutineDispatcher = Dispatchers.Main,
+    private val mainDispatcher: CoroutineDispatcher = Dispatchers.Main,
 ) : PipelineTerminalUi {
+
+    private companion object {
+        val MIN_NOTIFICATION_INTERVAL = 500.milliseconds
+    }
+
     @Volatile
     private var terminalActive = false
 
+    private var lastShownTitleRes: Int = 0
+    private var lastEmitMs: Long = 0L
+    private var pendingSnapshot: DictionaryProgressModel.Snapshot? = null
+    private var flushJob: Job? = null
+
     init {
         scope.launch(mainDispatcher) {
             progressSnapshot.collect { snapshot ->
                 when {
                     snapshot == null -> {
+                        resetThrottle()
                         if (!terminalActive && notifications.hasActiveProgressNotification()) {
                             notifications.cancelProgressNotification()
                         }
                     }
-                    !terminalActive -> notifications.showProgress(snapshot.percent, snapshot.titleRes)
+                    !terminalActive -> emitThrottled(snapshot)
                 }
             }
         }
     }
 
+    private fun emitThrottled(snapshot: DictionaryProgressModel.Snapshot) {
+        val now = SystemClock.elapsedRealtime()
+        val stageChanged = snapshot.titleRes != lastShownTitleRes
+        if (stageChanged || (now - lastEmitMs).milliseconds >= MIN_NOTIFICATION_INTERVAL) {
+            flushJob?.cancel()
+            pendingSnapshot = null
+            publish(snapshot, now)
+        } else {
+            pendingSnapshot = snapshot
+            if (flushJob?.isActive != true) {
+                flushJob = scope.launch(mainDispatcher) {
+                    delay(MIN_NOTIFICATION_INTERVAL - (now - lastEmitMs).milliseconds)
+                    pendingSnapshot?.let { publish(it, SystemClock.elapsedRealtime()) }
+                    pendingSnapshot = null
+                }
+            }
+        }
+    }
+
+    private fun publish(snapshot: DictionaryProgressModel.Snapshot, now: Long) {
+        if (terminalActive) return
+        lastShownTitleRes = snapshot.titleRes
+        lastEmitMs = now
+        notifications.showProgress(snapshot.percent, snapshot.titleRes)
+    }
+
+    private fun resetThrottle() {
+        flushJob?.cancel()
+        pendingSnapshot = null
+        lastShownTitleRes = 0
+        lastEmitMs = 0L
+    }
+
     fun beginFlow() {
         terminalActive = false
+        resetThrottle()
         notifications.cancelTerminalNotification()
     }
 
@@ -52,6 +101,7 @@ class DictionaryProgressPresenter(
 
     override fun cancelProgress() {
         terminalActive = false
+        resetThrottle()
         notifications.cancelProgressNotification()
     }
 }