Move empty day separator filtering to a timeline post-processor (#6866)

* Move empty day separator filtering to a timeline post-processor

* Split `FilterPublicMembershipChangesPostProcessor` from `RoomBeginningPostProcessor`

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/timeline/RustTimeline.kt

@@ -38,6 +38,8 @@ import io.element.android.libraries.matrix.impl.room.location.into
 import io.element.android.libraries.matrix.impl.timeline.item.event.EventTimelineItemMapper
 import io.element.android.libraries.matrix.impl.timeline.item.event.TimelineEventContentMapper
 import io.element.android.libraries.matrix.impl.timeline.item.virtual.VirtualTimelineItemMapper
+import io.element.android.libraries.matrix.impl.timeline.postprocessor.FilterEmptyDayPostProcessor
+import io.element.android.libraries.matrix.impl.timeline.postprocessor.FilterPublicMembershipChangesPostProcessor
 import io.element.android.libraries.matrix.impl.timeline.postprocessor.LastForwardIndicatorsPostProcessor
 import io.element.android.libraries.matrix.impl.timeline.postprocessor.LoadingIndicatorsPostProcessor
 import io.element.android.libraries.matrix.impl.timeline.postprocessor.RoomBeginningPostProcessor
@@ -84,7 +86,7 @@ private const val PAGINATION_SIZE = 50
 class RustTimeline(
     private val inner: InnerTimeline,
     override val mode: Timeline.Mode,
-    private val systemClock: SystemClock,
+    systemClock: SystemClock,
     private val joinedRoom: JoinedRoom,
     private val coroutineScope: CoroutineScope,
     private val dispatcher: CoroutineDispatcher,
@@ -123,6 +125,8 @@ class RustTimeline(
     private val loadingIndicatorsPostProcessor = LoadingIndicatorsPostProcessor(systemClock)
     private val lastForwardIndicatorsPostProcessor = LastForwardIndicatorsPostProcessor(mode)
     private val typingNotificationPostProcessor = TypingNotificationPostProcessor(mode)
+    private val publicMembershipChangesPostProcessor = FilterPublicMembershipChangesPostProcessor()
+    private val emptyDayPostProcessor = FilterEmptyDayPostProcessor()
 
     private data class RoomTimelineInfo(
         val roomCreators: ImmutableList<UserId>,
@@ -245,11 +249,21 @@ class RustTimeline(
                         items = items,
                         isDm = isDm,
                         roomCreator = roomCreators.firstOrNull(),
-                        joinRule = joinRule,
-                        isEncrypted = isEncrypted,
                         hasMoreToLoadBackwards = backwardPaginationStatus.hasMoreToLoad,
                     )
                 }
+                // This should be the first post processor after room beginning.
+                .let { items ->
+                    publicMembershipChangesPostProcessor.process(
+                        items = items,
+                        joinRule = joinRule,
+                        isEncrypted = isEncrypted,
+                    )
+                }
+                // After removing public membership changes, we might end up with empty days, so we need to filter them out.
+                .let { items ->
+                    emptyDayPostProcessor.process(items)
+                }
                 .let { items ->
                     loadingIndicatorsPostProcessor.process(
                         items = items,

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/timeline/postprocessor/FilterEmptyDayPostProcessor.kt

@@ -0,0 +1,38 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.matrix.impl.timeline.postprocessor
+
+import io.element.android.libraries.matrix.api.timeline.MatrixTimelineItem
+import io.element.android.libraries.matrix.api.timeline.item.virtual.VirtualTimelineItem
+
+/**
+ * Post-processor to filter out day separators for days that don't contain any events.
+ */
+class FilterEmptyDayPostProcessor {
+    /**
+     * Filters out day separators from [items] for days that don't contain any events.
+     */
+    fun process(items: List<MatrixTimelineItem>): List<MatrixTimelineItem> = buildList {
+        // The timeline is ordered by descending timestamp, so events that happened during a day appear before the day separator for that day.
+        // We can use this to determine if a day separator should be kept or not.
+        var hasEvent = false
+        for (item in items) {
+            if (item is MatrixTimelineItem.Event) {
+                hasEvent = true
+                add(item)
+            } else if (item is MatrixTimelineItem.Virtual && item.virtual is VirtualTimelineItem.DayDivider) {
+                if (hasEvent) {
+                    add(item)
+                    hasEvent = false
+                }
+            } else {
+                add(item)
+            }
+        }
+    }
+}

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/timeline/postprocessor/FilterPublicMembershipChangesPostProcessor.kt

@@ -0,0 +1,43 @@
+/*
+ * Copyright (c) 2026 Element Creations Ltd.
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only OR LicenseRef-Element-Commercial.
+ * Please see LICENSE files in the repository root for full details.
+ */
+
+package io.element.android.libraries.matrix.impl.timeline.postprocessor
+
+import io.element.android.libraries.matrix.api.room.join.JoinRule
+import io.element.android.libraries.matrix.api.timeline.MatrixTimelineItem
+import io.element.android.libraries.matrix.api.timeline.item.event.MembershipChange
+import io.element.android.libraries.matrix.api.timeline.item.event.ProfileChangeContent
+import io.element.android.libraries.matrix.api.timeline.item.event.RoomMembershipContent
+
+/**
+ * Post-processor to filter out public membership changes for non-encrypted, publicly joinable rooms.
+ */
+class FilterPublicMembershipChangesPostProcessor {
+    /**
+     * Filters out public membership changes from [items] if the room is publicly joinable and not encrypted.
+     */
+    fun process(
+        items: List<MatrixTimelineItem>,
+        joinRule: JoinRule?,
+        isEncrypted: Boolean?,
+    ): List<MatrixTimelineItem> {
+        return if (joinRule !is JoinRule.Invite && isEncrypted == false) {
+            filterMembershipEvents(items)
+        } else {
+            items
+        }
+    }
+
+    private fun filterMembershipEvents(items: List<MatrixTimelineItem>): List<MatrixTimelineItem> = items.filter { item ->
+        val eventContent = (item as? MatrixTimelineItem.Event)?.event?.content ?: return@filter true
+        when (eventContent) {
+            is RoomMembershipContent -> eventContent.change != null && eventContent.change !in listOf(MembershipChange.JOINED, MembershipChange.LEFT)
+            is ProfileChangeContent -> false
+            else -> true
+        }
+    }
+}

libraries/matrix/impl/src/main/kotlin/io/element/android/libraries/matrix/impl/timeline/postprocessor/RoomBeginningPostProcessor.kt

@@ -9,12 +9,10 @@
 package io.element.android.libraries.matrix.impl.timeline.postprocessor
 
 import io.element.android.libraries.matrix.api.core.UserId
-import io.element.android.libraries.matrix.api.room.join.JoinRule
 import io.element.android.libraries.matrix.api.timeline.MatrixTimelineItem
 import io.element.android.libraries.matrix.api.timeline.Timeline
 import io.element.android.libraries.matrix.api.timeline.item.event.MembershipChange
 import io.element.android.libraries.matrix.api.timeline.item.event.OtherState
-import io.element.android.libraries.matrix.api.timeline.item.event.ProfileChangeContent
 import io.element.android.libraries.matrix.api.timeline.item.event.RoomMembershipContent
 import io.element.android.libraries.matrix.api.timeline.item.event.StateContent
 
@@ -27,31 +25,17 @@ class RoomBeginningPostProcessor(private val mode: Timeline.Mode) {
         items: List<MatrixTimelineItem>,
         isDm: Boolean,
         roomCreator: UserId?,
-        joinRule: JoinRule?,
-        isEncrypted: Boolean?,
         hasMoreToLoadBackwards: Boolean,
     ): List<MatrixTimelineItem> {
         return when {
             items.isEmpty() -> items
             mode == Timeline.Mode.PinnedEvents -> items
-            joinRule !is JoinRule.Invite && isEncrypted == false -> filterRoomMemberEvents(items)
             isDm -> processForDM(items, roomCreator)
             hasMoreToLoadBackwards -> items
             else -> processForRoom(items)
         }
     }
 
-    private fun filterRoomMemberEvents(items: List<MatrixTimelineItem>): List<MatrixTimelineItem> {
-        return items.filter { item ->
-            val eventContent = (item as? MatrixTimelineItem.Event)?.event?.content
-            when (eventContent) {
-                is RoomMembershipContent -> eventContent.change !in listOf(MembershipChange.JOINED, MembershipChange.LEFT)
-                is ProfileChangeContent -> false
-                else -> true
-            }
-        }
-    }
-
     private fun processForRoom(items: List<MatrixTimelineItem>): List<MatrixTimelineItem> {
         // No changes needed, timeline start item is already added by the SDK
         return items