Fix message loading

app/src/main/java/ru/risdeveau/pixeldragon/ui/layout/Room.kt

@@ -83,6 +83,7 @@ import de.connect2x.trixnity.client.store.TimelineEvent
 import de.connect2x.trixnity.client.store.eventId
 import de.connect2x.trixnity.core.model.RoomId
 import de.connect2x.trixnity.core.model.events.ClientEvent
+import de.connect2x.trixnity.core.model.events.m.FullyReadEventContent
 import de.connect2x.trixnity.core.model.events.m.room.AvatarEventContent
 import de.connect2x.trixnity.core.model.events.m.room.CanonicalAliasEventContent
 import de.connect2x.trixnity.core.model.events.m.room.HistoryVisibilityEventContent
@@ -101,6 +102,7 @@ import kotlinx.coroutines.flow.MutableStateFlow
 import kotlinx.coroutines.flow.combine
 import kotlinx.coroutines.flow.flatMapLatest
 import kotlinx.coroutines.flow.flowOf
+import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.launch
 import org.jsoup.Jsoup
 import org.jsoup.nodes.Element
@@ -117,6 +119,10 @@ import kotlin.math.roundToInt
 
 private const val WEBVIEW_HEIGHT_TITLE_PREFIX = "pd-height:"
 private const val MESSAGE_GROUP_WINDOW_MS = 5 * 60 * 1000L
+private const val TIMELINE_PAGE_SIZE = 50
+private const val TIMELINE_TOP_PREFETCH_THRESHOLD = 4
+private const val TIMELINE_BOTTOM_AUTOSCROLL_THRESHOLD = 3
+private const val MAX_INITIAL_UNREAD_SEARCH_LIMIT = 500
 private val TIMELINE_AVATAR_SIZE = 32.dp
 private val MESSAGE_FIELD_TOP_PADDING = 8.dp
 
@@ -182,9 +188,14 @@ private val headingMetrics = mapOf(
 @Composable
 fun Room(modifier: Modifier = Modifier, rid: String) {
     val roomId = remember(rid) { RoomId(rid) }
-    val limit = remember { MutableStateFlow(50) }
+    var requestedLimit by remember(roomId) { mutableStateOf(TIMELINE_PAGE_SIZE) }
+    val limit = remember(roomId) { MutableStateFlow(TIMELINE_PAGE_SIZE) }
 
-    val eventsFlow = remember(roomId) {
+    LaunchedEffect(requestedLimit) {
+        limit.value = requestedLimit
+    }
+
+    val eventsFlow = remember(roomId, client!!) {
         client!!.room
             .getLastTimelineEvents(roomId)
             .toFlowList(limit)
@@ -194,23 +205,105 @@ fun Room(modifier: Modifier = Modifier, rid: String) {
             }
     }
     val events by eventsFlow.collectAsState(initial = emptyList())
-    val currentUserId = remember { client!!.userId.toString() }
+
-    val timelineItems = remember(events, currentUserId) {
+    val readMarkerFlow = remember(roomId, client!!) {
-        buildTimelineItems(events = events, currentUserId = currentUserId)
+        client!!.room
+            .getAccountData(roomId, FullyReadEventContent::class)
+            .map { content -> ReadMarkerState.Ready(content?.eventId?.full) as ReadMarkerState }
+    }
+    val readMarkerState by readMarkerFlow.collectAsState(initial = ReadMarkerState.Loading)
+    val fullyReadEventId = (readMarkerState as? ReadMarkerState.Ready)?.eventId
+
+    val currentUserId = remember(client!!) { client!!.userId.toString() }
+    val timelineItems = remember(events, currentUserId, fullyReadEventId) {
+        buildTimelineItems(
+            events = events,
+            currentUserId = currentUserId,
+            fullyReadEventId = fullyReadEventId,
+        )
     }
     val latestTimelineKey = remember(timelineItems) {
         timelineItems.lastOrNull()?.latestKey()
     }
 
     val listState = rememberLazyListState()
+    var didInitialScroll by remember(roomId) { mutableStateOf(false) }
+    var lastAutoScrollTimelineKey by remember(roomId) { mutableStateOf<String?>(null) }
+
+    val isNearBottom by remember(timelineItems, listState) {
+        derivedStateOf {
+            val totalItems = listState.layoutInfo.totalItemsCount
+            if (totalItems == 0) return@derivedStateOf true
+
+            val lastVisibleIndex = listState.layoutInfo.visibleItemsInfo.lastOrNull()?.index
+                ?: return@derivedStateOf false
+            lastVisibleIndex >= totalItems - 1 - TIMELINE_BOTTOM_AUTOSCROLL_THRESHOLD
+        }
+    }
+
+    val shouldLoadOlder by remember(listState, didInitialScroll) {
+        derivedStateOf {
+            didInitialScroll &&
+                    listState.layoutInfo.totalItemsCount > 0 &&
+                    listState.firstVisibleItemIndex <= TIMELINE_TOP_PREFETCH_THRESHOLD
+        }
+    }
 
     Column(modifier) {
         var message by remember { mutableStateOf("") }
 
-        LaunchedEffect(latestTimelineKey) {
+        LaunchedEffect(timelineItems, readMarkerState, events.size, requestedLimit) {
-            if (timelineItems.isNotEmpty()) {
+            if (didInitialScroll || timelineItems.isEmpty()) return@LaunchedEffect
-                listState.scrollToItem(timelineItems.lastIndex)
+            if (readMarkerState is ReadMarkerState.Loading) return@LaunchedEffect
+
+            val readMarkerEventId = (readMarkerState as ReadMarkerState.Ready).eventId
+            val unreadDividerIndex = timelineItems.indexOfFirst { it is UnreadDividerItem }
+            if (unreadDividerIndex >= 0) {
+                listState.scrollToItem(unreadDividerIndex)
+                didInitialScroll = true
+                lastAutoScrollTimelineKey = latestTimelineKey
+                return@LaunchedEffect
             }
+
+            val readMarkerIsLoaded = readMarkerEventId != null &&
+                    events.any { it.eventId.full == readMarkerEventId }
+            val shouldSearchOlderForUnreadMarker = readMarkerEventId != null &&
+                    !readMarkerIsLoaded &&
+                    events.size >= requestedLimit &&
+                    requestedLimit < MAX_INITIAL_UNREAD_SEARCH_LIMIT
+
+            if (shouldSearchOlderForUnreadMarker) {
+                requestedLimit = (requestedLimit + TIMELINE_PAGE_SIZE)
+                    .coerceAtMost(MAX_INITIAL_UNREAD_SEARCH_LIMIT)
+                return@LaunchedEffect
+            }
+
+            listState.scrollToItem(timelineItems.lastIndex)
+            didInitialScroll = true
+            lastAutoScrollTimelineKey = latestTimelineKey
+        }
+
+        LaunchedEffect(shouldLoadOlder, events.size, requestedLimit) {
+            if (!shouldLoadOlder) return@LaunchedEffect
+            if (events.size < requestedLimit) return@LaunchedEffect
+
+            requestedLimit += TIMELINE_PAGE_SIZE
+        }
+
+        LaunchedEffect(latestTimelineKey) {
+            val currentLatestKey = latestTimelineKey ?: return@LaunchedEffect
+            val previousLatestKey = lastAutoScrollTimelineKey
+
+            if (!didInitialScroll) {
+                lastAutoScrollTimelineKey = currentLatestKey
+                return@LaunchedEffect
+            }
+
+            if (previousLatestKey != null && previousLatestKey != currentLatestKey && isNearBottom) {
+                listState.animateScrollToItem(timelineItems.lastIndex)
+            }
+
+            lastAutoScrollTimelineKey = currentLatestKey
         }
 
         val density = LocalDensity.current
@@ -297,6 +390,11 @@ fun Room(modifier: Modifier = Modifier, rid: String) {
     }
 }
 
+private sealed interface ReadMarkerState {
+    data object Loading : ReadMarkerState
+    data class Ready(val eventId: String?) : ReadMarkerState
+}
+
 private sealed interface TimelineUiItem {
     val key: String
 }
@@ -306,6 +404,10 @@ private data class DateDividerItem(
     val label: String,
 ) : TimelineUiItem
 
+private data class UnreadDividerItem(
+    override val key: String,
+) : TimelineUiItem
+
 private data class MessageGroupItem(
     override val key: String,
     val senderId: String?,
@@ -338,6 +440,7 @@ private data class SystemTimelineItem(
 private fun buildTimelineItems(
     events: List<TimelineEvent>,
     currentUserId: String,
+    fullyReadEventId: String?,
 ): List<TimelineUiItem> {
     if (events.isEmpty()) return emptyList()
 
@@ -346,6 +449,8 @@ private fun buildTimelineItems(
 
     var previousDayStartMs: Long? = null
     var lastMessageGroupIndex: Int? = null
+    var insertUnreadDividerBeforeNextMessage = false
+    var unreadDividerInserted = false
 
     for (event in chronologicalEvents) {
         val content = event.content?.getOrNull()
@@ -361,6 +466,17 @@ private fun buildTimelineItems(
             lastMessageGroupIndex = null
         }
 
+        if (
+            content is RoomMessageEventContent &&
+            insertUnreadDividerBeforeNextMessage &&
+            !unreadDividerInserted
+        ) {
+            items += UnreadDividerItem(key = "unread:$fullyReadEventId")
+            unreadDividerInserted = true
+            insertUnreadDividerBeforeNextMessage = false
+            lastMessageGroupIndex = null
+        }
+
         when {
             content == null -> {
                 items += SystemTimelineItem(
@@ -429,6 +545,11 @@ private fun buildTimelineItems(
                 lastMessageGroupIndex = null
             }
         }
+
+        if (!unreadDividerInserted && fullyReadEventId != null && event.eventId.full == fullyReadEventId) {
+            insertUnreadDividerBeforeNextMessage = true
+            lastMessageGroupIndex = null
+        }
     }
 
     return items
@@ -441,6 +562,7 @@ private fun TimelineItem(
 ) {
     when (item) {
         is DateDividerItem -> DateDivider(item)
+        is UnreadDividerItem -> UnreadDivider()
         is MessageGroupItem -> MessageGroupRow(
             group = item,
             hideInlineAvatar = pinnedAvatarGroupKey == item.key,
@@ -472,6 +594,36 @@ private fun DateDivider(item: DateDividerItem) {
     }
 }
 
+@Composable
+private fun UnreadDivider() {
+    Row(
+        modifier = Modifier
+            .fillMaxWidth()
+            .padding(horizontal = 12.dp, vertical = 8.dp),
+        verticalAlignment = Alignment.CenterVertically,
+    ) {
+        Box(
+            modifier = Modifier
+                .height(1.dp)
+                .weight(1f)
+                .background(MaterialTheme.colorScheme.primary.copy(alpha = 0.45f)),
+        )
+        Text(
+            text = "Unread messages",
+            modifier = Modifier.padding(horizontal = 8.dp),
+            style = MaterialTheme.typography.labelSmall,
+            color = MaterialTheme.colorScheme.primary,
+            fontWeight = FontWeight.SemiBold,
+        )
+        Box(
+            modifier = Modifier
+                .height(1.dp)
+                .weight(1f)
+                .background(MaterialTheme.colorScheme.primary.copy(alpha = 0.45f)),
+        )
+    }
+}
+
 @Composable
 private fun SystemEventRow(item: SystemTimelineItem) {
     Text(