refactor(web): replace per-asset viewport proximity with day-tier active indices

Binary search on asset positions replaces per-ViewerAsset $derived
proximity tracking. Reactive churn during scroll reduces from O(N)
per-asset deriveds to O(log N) per-day binary search.

Change-Id: Ib4bdaec5df4801d1347f41bbabd607956a6a6964

web/src/lib/components/timeline/AssetLayout.svelte

@@ -12,6 +12,8 @@
 
   type Props = {
     viewerAssets: ViewerAsset[];
+    firstActiveIndex: number;
+    lastActiveIndex: number;
     width: number;
     height: number;
     manager: VirtualScrollManager;
@@ -26,18 +28,27 @@
     customThumbnailLayout?: Snippet<[asset: TimelineAsset]>;
   };
 
-  const { viewerAssets, width, height, manager, thumbnail, customThumbnailLayout }: Props = $props();
+  const {
+    viewerAssets,
+    firstActiveIndex,
+    lastActiveIndex,
+    width,
+    height,
+    manager,
+    thumbnail,
+    customThumbnailLayout,
+  }: Props = $props();
 
   const transitionDuration = $derived(manager.suspendTransitions && !$isUploading ? 0 : 150);
   const scaleDuration = $derived(transitionDuration === 0 ? 0 : transitionDuration + 100);
-
-  const firstInOrNearViewport = $derived(viewerAssets.findIndex((a) => a.isInOrNearViewport));
-  const lastInOrNearViewport = $derived(viewerAssets.findLastIndex((a) => a.isInOrNearViewport));
+  const visibleViewerAssets = $derived(
+    firstActiveIndex === -1 ? [] : viewerAssets.slice(firstActiveIndex, lastActiveIndex + 1),
+  );
 </script>
 
 <!-- Image grid -->
 <div data-image-grid class="relative overflow-clip" style:height={height + 'px'} style:width={width + 'px'}>
-  {#each viewerAssets.slice(firstInOrNearViewport, lastInOrNearViewport + 1) as viewerAsset (viewerAsset.id)}
+  {#each visibleViewerAssets as viewerAsset (viewerAsset.id)}
     {@const position = viewerAsset.position!}
     {@const asset = viewerAsset.asset!}
 

web/src/lib/components/timeline/Month.svelte

@@ -101,6 +101,8 @@
     <AssetLayout
       {manager}
       viewerAssets={timelineDay.viewerAssets}
+      firstActiveIndex={timelineDay.firstActiveIndex}
+      lastActiveIndex={timelineDay.lastActiveIndex}
       height={timelineDay.height}
       width={timelineDay.width}
       {customThumbnailLayout}

web/src/lib/managers/timeline-manager/internal/intersection-support.svelte.ts

@@ -53,17 +53,3 @@ export function updateTimelineMonthViewportProximity(timelineManager: TimelineMa
     timelineManager.clearDeferredLayout(month);
   }
 }
-
-export function calculateViewerAssetViewportProximity(
-  timelineManager: TimelineManager,
-  positionTop: number,
-  positionHeight: number,
-) {
-  const headerHeight = timelineManager.headerHeight;
-  return calculateViewportProximity(
-    positionTop,
-    positionTop + positionHeight,
-    timelineManager.visibleWindow.top - headerHeight,
-    timelineManager.visibleWindow.bottom + headerHeight,
-  );
-}

web/src/lib/managers/timeline-manager/timeline-day.svelte.ts

@@ -1,12 +1,31 @@
 import { AssetOrder, AssetOrderBy } from '@immich/sdk';
 import { SvelteSet } from 'svelte/reactivity';
-import type { CommonLayoutOptions } from '$lib/utils/layout-utils';
+import type { CommonLayoutOptions, CommonPosition } from '$lib/utils/layout-utils';
 import { getJustifiedLayoutFromAssets } from '$lib/utils/layout-utils';
 import { getOrderingDate, plainDateTimeCompare } from '$lib/utils/timeline-util';
+import { TUNABLES } from '$lib/utils/tunables';
 import type { TimelineMonth } from './timeline-month.svelte';
 import type { Direction, MoveAsset, TimelineAsset } from './types';
 import { ViewerAsset } from './viewer-asset.svelte';
 
+const {
+  TIMELINE: { INTERSECTION_EXPAND_TOP, INTERSECTION_EXPAND_BOTTOM },
+} = TUNABLES;
+
+function lowerBound(assets: ViewerAsset[], target: number, key: (pos: CommonPosition) => number): number {
+  let lo = 0;
+  let hi = assets.length;
+  while (lo < hi) {
+    const mid = (lo + hi) >>> 1;
+    if (key(assets[mid].position!) < target) {
+      lo = mid + 1;
+    } else {
+      hi = mid;
+    }
+  }
+  return lo;
+}
+
 export class TimelineDay {
   readonly timelineMonth: TimelineMonth;
   readonly index: number;
@@ -18,12 +37,16 @@ export class TimelineDay {
   height = $state(0);
   width = $state(0);
 
+  // Assets in or near the viewport; active assets should be added to the DOM.
+  firstActiveIndex = $state(-1);
+  lastActiveIndex = $state(-1);
+  isInOrNearViewport = $derived(this.firstActiveIndex !== -1);
+
   #top: number = $state(0);
   #start: number = $state(0);
   #row = $state(0);
   #col = $state(0);
   #deferredLayout = false;
-  #lastInOrNearViewport = -1;
 
   constructor(timelineMonth: TimelineMonth, index: number, day: number, groupTitle: string, orderBy: AssetOrderBy) {
     this.index = index;
@@ -149,18 +172,32 @@ export class TimelineDay {
     for (let i = 0; i < this.viewerAssets.length; i++) {
       this.viewerAssets[i].position = geometry.getPosition(i);
     }
+    this.updateAssetBoundaries();
+  }
+
+  updateAssetBoundaries() {
+    const manager = this.timelineMonth.timelineManager;
+    const visibleWindow = manager.visibleWindow;
+    if (this.viewerAssets.length === 0 || !this.viewerAssets[0].position) {
+      this.firstActiveIndex = -1;
+      this.lastActiveIndex = -1;
+      return;
+    }
+
+    const dayOffset = this.absoluteTimelineDayTop;
+    const headerHeight = manager.headerHeight;
+    const expandedTop = visibleWindow.top - headerHeight - INTERSECTION_EXPAND_TOP - dayOffset;
+    const expandedBottom = visibleWindow.bottom + headerHeight + INTERSECTION_EXPAND_BOTTOM - dayOffset;
+
+    const first = lowerBound(this.viewerAssets, expandedTop, (p) => p.top + p.height);
+    const last = lowerBound(this.viewerAssets, expandedBottom, (p) => p.top) - 1;
+
+    const hasActive = last >= first && first < this.viewerAssets.length;
+    this.firstActiveIndex = hasActive ? first : -1;
+    this.lastActiveIndex = hasActive ? last : -1;
   }
 
   get absoluteTimelineDayTop() {
     return this.timelineMonth.top + this.#top;
   }
-
-  get isInOrNearViewport() {
-    if (this.#lastInOrNearViewport !== -1 && this.viewerAssets[this.#lastInOrNearViewport].isInOrNearViewport) {
-      return true;
-    }
-
-    this.#lastInOrNearViewport = this.viewerAssets.findIndex((viewAsset) => viewAsset.isInOrNearViewport);
-    return this.#lastInOrNearViewport !== -1;
-  }
 }

web/src/lib/managers/timeline-manager/timeline-manager.svelte.ts

@@ -214,6 +214,11 @@ export class TimelineManager extends VirtualScrollManager {
 
     for (const month of this.months) {
       updateTimelineMonthViewportProximity(this, month);
+      if (month.isInOrNearViewport && month.isLoaded) {
+        for (const day of month.timelineDays) {
+          day.updateAssetBoundaries();
+        }
+      }
     }
 
     const month = this.months.find((month) => month.isInViewport);

web/src/lib/managers/timeline-manager/timeline-month.svelte.ts

@@ -254,7 +254,7 @@ export class TimelineMonth {
       addContext.newTimelineDays.add(timelineDay);
     }
 
-    const viewerAsset = new ViewerAsset(timelineDay, timelineAsset);
+    const viewerAsset = new ViewerAsset(timelineAsset);
     timelineDay.viewerAssets.push(viewerAsset);
     addContext.changedTimelineDays.add(timelineDay);
   }

web/src/lib/managers/timeline-manager/viewer-asset.svelte.ts

@@ -1,36 +1,12 @@
 import type { CommonPosition } from '$lib/utils/layout-utils';
-import {
-  ViewportProximity,
-  calculateViewerAssetViewportProximity,
-  isInOrNearViewport,
-} from './internal/intersection-support.svelte';
-import type { TimelineDay } from './timeline-day.svelte';
 import type { TimelineAsset } from './types';
 
 export class ViewerAsset {
-  readonly #group: TimelineDay;
-
-  #viewportProximity = $derived.by(() => {
-    if (!this.position) {
-      return ViewportProximity.FarFromViewport;
-    }
-
-    const store = this.#group.timelineMonth.timelineManager;
-    const positionTop = this.#group.absoluteTimelineDayTop + this.position.top;
-
-    return calculateViewerAssetViewportProximity(store, positionTop, this.position.height);
-  });
-
-  get isInOrNearViewport() {
-    return isInOrNearViewport(this.#viewportProximity);
-  }
-
   position: CommonPosition | undefined = $state.raw();
   asset: TimelineAsset = $state() as TimelineAsset;
   id: string = $derived(this.asset.id);
 
-  constructor(group: TimelineDay, asset: TimelineAsset) {
-    this.#group = group;
+  constructor(asset: TimelineAsset) {
     this.asset = asset;
   }
 }