From 8c0c91deb7bb6795b00da55207f60dadeda5dbf2 Mon Sep 17 00:00:00 2001
From: Ilia Mashkov <hikkikomori7@gmail.com>
Date: Fri, 16 Jan 2026 17:48:33 +0300
Subject: [PATCH] feat(createVirtualizer): enhance logic with binary search and
 requestAnimationFrame

---
 .../createVirtualizer.svelte.ts               | 77 +++++++++++--------
 1 file changed, 44 insertions(+), 33 deletions(-)

diff --git a/src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts b/src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts
index 3482292..ea96535 100644
--- a/src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts
+++ b/src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts
@@ -79,27 +79,23 @@ export interface VirtualizerOptions {
  * </div>
  * ```
  */
-export function createVirtualizer(optionsGetter: () => VirtualizerOptions) {
-    // Reactive State
+export function createVirtualizer<T>(optionsGetter: () => VirtualizerOptions & { data: T[] }) {
     let scrollOffset = $state(0);
     let containerHeight = $state(0);
     let measuredSizes = $state<Record<number, number>>({});
-
-    // Non-reactive ref for DOM manipulation (avoiding unnecessary state tracking)
     let elementRef: HTMLElement | null = null;
 
-    // Reactive Options
+    // By wrapping the getter in $derived, we track everything inside it
     const options = $derived(optionsGetter());
 
-    // Optimized Memoization (The Cache Layer)
-    // Only recalculates when item count or measured sizes change.
+    // This derivation now tracks: count, measuredSizes, AND the data array itself
     const offsets = $derived.by(() => {
         const count = options.count;
-        const result = Array.from<number>({ length: count });
+        const result = new Float64Array(count);
         let accumulated = 0;
-
         for (let i = 0; i < count; i++) {
             result[i] = accumulated;
+            // Accessing measuredSizes here creates the subscription
             accumulated += measuredSizes[i] ?? options.estimateSize(i);
         }
         return result;
@@ -112,24 +108,30 @@ export function createVirtualizer(optionsGetter: () => VirtualizerOptions) {
             : 0,
     );
 
-    // Visible Range Calculation
-    // Svelte tracks dependencies automatically here.
     const items = $derived.by((): VirtualItem[] => {
-        const count = options.count;
-        if (count === 0 || containerHeight === 0) return [];
+        // We MUST read options.data here so Svelte knows to re-run
+        // this derivation when the items array is replaced!
+        const { count, data } = options;
+        if (count === 0 || containerHeight === 0 || !data) return [];
 
         const overscan = options.overscan ?? 5;
-        const viewportStart = scrollOffset;
-        const viewportEnd = scrollOffset + containerHeight;
 
-        // Find Start (Linear Scan)
+        // Binary search for efficiency
+        let low = 0;
+        let high = count - 1;
         let startIdx = 0;
-        while (startIdx < count && offsets[startIdx + 1] < viewportStart) {
-            startIdx++;
+        while (low <= high) {
+            const mid = Math.floor((low + high) / 2);
+            if (offsets[mid] <= scrollOffset) {
+                startIdx = mid;
+                low = mid + 1;
+            } else {
+                high = mid - 1;
+            }
         }
 
-        // Find End
         let endIdx = startIdx;
+        const viewportEnd = scrollOffset + containerHeight;
         while (endIdx < count && offsets[endIdx] < viewportEnd) {
             endIdx++;
         }
@@ -139,18 +141,16 @@ export function createVirtualizer(optionsGetter: () => VirtualizerOptions) {
 
         const result: VirtualItem[] = [];
         for (let i = start; i < end; i++) {
-            const size = measuredSizes[i] ?? options.estimateSize(i);
             result.push({
                 index: i,
                 start: offsets[i],
-                size,
-                end: offsets[i] + size,
+                size: measuredSizes[i] ?? options.estimateSize(i),
+                end: offsets[i] + (measuredSizes[i] ?? options.estimateSize(i)),
                 key: options.getItemKey?.(i) ?? i,
             });
         }
         return result;
     });
-
     // Svelte Actions (The DOM Interface)
 
     /**
@@ -185,6 +185,8 @@ export function createVirtualizer(optionsGetter: () => VirtualizerOptions) {
         };
     }
 
+    let measurementBuffer: Record<number, number> = {};
+    let frameId: number | null = null;
     /**
      * Svelte action to measure individual item elements for dynamic height support.
      *
@@ -195,23 +197,32 @@ export function createVirtualizer(optionsGetter: () => VirtualizerOptions) {
      * @returns Object with destroy method for cleanup
      */
     function measureElement(node: HTMLElement) {
-        // Use a ResizeObserver on individual items for dynamic height support
         const resizeObserver = new ResizeObserver(([entry]) => {
-            if (entry) {
-                const index = parseInt(node.dataset.index || '', 10);
-                const height = entry.borderBoxSize[0]?.blockSize ?? node.offsetHeight;
+            if (!entry) return;
+            const index = parseInt(node.dataset.index || '', 10);
+            const height = entry.borderBoxSize[0]?.blockSize ?? node.offsetHeight;
 
-                // Only update if height actually changed to prevent loops
-                if (!isNaN(index) && measuredSizes[index] !== height) {
-                    measuredSizes[index] = height;
+            if (!isNaN(index) && measuredSizes[index] !== height) {
+                // 1. Stuff the measurement into a temporary buffer
+                measurementBuffer[index] = height;
+
+                // 2. Schedule a single update for the next animation frame
+                if (frameId === null) {
+                    frameId = requestAnimationFrame(() => {
+                        // 3. Update the state once for all collected measurements
+                        // We use spread to trigger a single fine-grained update
+                        measuredSizes = { ...measuredSizes, ...measurementBuffer };
+
+                        // 4. Reset the buffer
+                        measurementBuffer = {};
+                        frameId = null;
+                    });
                 }
             }
         });
 
         resizeObserver.observe(node);
-        return {
-            destroy: () => resizeObserver.disconnect(),
-        };
+        return { destroy: () => resizeObserver.disconnect() };
     }
 
     // Programmatic Scroll