Virtualizer Core Engine

/**
 * Core virtualization engine for managing virtual scrolling
 * @template TScrollElement - Type of the scroll container (Element or Window)
 * @template TItemElement - Type of the virtualized item elements
 */
export class Virtualizer<TScrollElement extends Element | Window, TItemElement extends Element> {
  constructor(opts: VirtualizerOptions<TScrollElement, TItemElement>);
  
  // Configuration
  setOptions(opts: VirtualizerOptions<TScrollElement, TItemElement>): void;
  
  // Virtual item management
  getVirtualItems(): Array<VirtualItem>;
  getVirtualIndexes(): Array<number>;
  calculateRange(): { startIndex: number; endIndex: number } | null;
  
  // Measurement and sizing
  measureElement(node: TItemElement | null | undefined): void;
  resizeItem(index: number, size: number): void;
  getTotalSize(): number;
  measure(): void;
  
  // Scroll control
  scrollToIndex(index: number, options?: ScrollToIndexOptions): void;
  scrollToOffset(toOffset: number, options?: ScrollToOffsetOptions): void;
  scrollBy(delta: number, options?: ScrollToOffsetOptions): void;
  
  // Position calculations
  getOffsetForIndex(index: number, align?: ScrollAlignment): [number, ScrollAlignment] | undefined;
  getOffsetForAlignment(toOffset: number, align: ScrollAlignment, itemSize?: number): number;
  getVirtualItemForOffset(offset: number): VirtualItem | undefined;
  
  // Element utilities
  indexFromElement(node: TItemElement): number;
}

interface VirtualizerState {
  /** The current scroll element being observed */
  scrollElement: TScrollElement | null;
  /** Reference to the target window object */
  targetWindow: (Window & typeof globalThis) | null;
  /** Whether the virtualizer is currently scrolling */
  isScrolling: boolean;
  /** Cache of measured virtual items */
  measurementsCache: Array<VirtualItem>;
  /** Current dimensions of the scroll container */
  scrollRect: Rect | null;
  /** Current scroll offset position */
  scrollOffset: number | null;
  /** Current scroll direction when scrolling */
  scrollDirection: ScrollDirection | null;
  /** Cache of DOM element references */
  elementsCache: Map<Key, TItemElement>;
  /** Current visible range of items */
  range: { startIndex: number; endIndex: number } | null;
}

/**
 * Get the current list of virtual items that should be rendered
 * @returns Array of VirtualItem objects representing visible items
 */
getVirtualItems(): Array<VirtualItem>;

/**
 * Get the indexes of items that need to be rendered (including overscan)
 * @returns Array of item indexes to render
 */
getVirtualIndexes(): Array<number>;

/**
 * Calculate the currently visible range of items
 * @returns Object with startIndex and endIndex, or null if no items visible
 */
calculateRange(): { startIndex: number; endIndex: number } | null;

import { Virtualizer } from '@tanstack/react-virtual';

// Create virtualizer instance
const virtualizer = new Virtualizer({
  count: 1000,
  getScrollElement: () => document.getElementById('scroll-container'),
  estimateSize: () => 50,
  scrollToFn: (offset, { behavior }, instance) => {
    instance.scrollElement?.scrollTo({ top: offset, behavior });
  },
  observeElementRect: (instance, cb) => {
    // ResizeObserver implementation
    const observer = new ResizeObserver(entries => {
      const entry = entries[0];
      if (entry) {
        cb({ width: entry.contentRect.width, height: entry.contentRect.height });
      }
    });
    if (instance.scrollElement) observer.observe(instance.scrollElement as Element);
    return () => observer.disconnect();
  },
  observeElementOffset: (instance, cb) => {
    // Scroll event implementation
    const handler = () => cb((instance.scrollElement as Element).scrollTop, true);
    instance.scrollElement?.addEventListener('scroll', handler);
    return () => instance.scrollElement?.removeEventListener('scroll', handler);
  }
});

// Get items to render
const virtualItems = virtualizer.getVirtualItems();
virtualItems.forEach(item => {
  console.log(`Render item ${item.index} at position ${item.start}`);
});

/**
 * Measure a DOM element and update the virtualizer's size cache
 * @param node - The DOM element to measure, or null to clean up disconnected elements
 */
measureElement(node: TItemElement | null | undefined): void;

/**
 * Manually resize a specific item by index
 * @param index - Index of the item to resize
 * @param size - New size of the item in pixels
 */
resizeItem(index: number, size: number): void;

/**
 * Get the total size of all virtualized content
 * @returns Total height (vertical) or width (horizontal) in pixels
 */
getTotalSize(): number;

/**
 * Force a complete remeasurement of all items
 * Clears the size cache and triggers recalculation
 */
measure(): void;

// Measure element during render
function VirtualItem({ virtualizer, item, data }) {
  return (
    <div
      data-index={item.index}
      ref={(node) => virtualizer.measureElement(node)}
      style={{
        position: 'absolute',
        top: 0,
        left: 0,
        width: '100%',
        transform: `translateY(${item.start}px)`,
      }}
    >
      {data[item.index]}
    </div>
  );
}

// Manual resizing after content change
function updateItemContent(virtualizer: Virtualizer, index: number, newContent: string) {
  // Update content...
  
  // If we know the new size, update it directly
  const newSize = estimateContentHeight(newContent);
  virtualizer.resizeItem(index, newSize);
  
  // Or trigger complete remeasurement
  virtualizer.measure();
}

// Get container height for styling
function VirtualContainer({ virtualizer }) {
  const totalSize = virtualizer.getTotalSize();
  
  return (
    <div style={{
      height: `${totalSize}px`,
      position: 'relative',
    }}>
      {/* Virtual items */}
    </div>
  );
}

/**
 * Scroll to a specific item index
 * @param index - Index of the item to scroll to
 * @param options - Scroll behavior options
 */
scrollToIndex(index: number, options?: ScrollToIndexOptions): void;

/**
 * Scroll to a specific offset position
 * @param toOffset - Target scroll position in pixels
 * @param options - Scroll behavior options
 */
scrollToOffset(toOffset: number, options?: ScrollToOffsetOptions): void;

/**
 * Scroll by a relative amount
 * @param delta - Amount to scroll in pixels (positive = down/right, negative = up/left)
 * @param options - Scroll behavior options
 */
scrollBy(delta: number, options?: ScrollToOffsetOptions): void;

interface ScrollToIndexOptions {
  /** How to align the item within the viewport */
  align?: ScrollAlignment;
  /** Scroll behavior (smooth or instant) */
  behavior?: ScrollBehavior;
}

interface ScrollToOffsetOptions {
  /** How to align the offset within the viewport */
  align?: ScrollAlignment;
  /** Scroll behavior (smooth or instant) */
  behavior?: ScrollBehavior;
}

type ScrollAlignment = 'start' | 'center' | 'end' | 'auto';
type ScrollBehavior = 'auto' | 'smooth';

// Scroll to specific items
function scrollToTop(virtualizer: Virtualizer) {
  virtualizer.scrollToIndex(0, { align: 'start' });
}

function scrollToBottom(virtualizer: Virtualizer) {
  virtualizer.scrollToIndex(virtualizer.options.count - 1, { align: 'end' });
}

function scrollToMiddle(virtualizer: Virtualizer) {
  const middleIndex = Math.floor(virtualizer.options.count / 2);
  virtualizer.scrollToIndex(middleIndex, { align: 'center', behavior: 'smooth' });
}

// Scroll by relative amounts
function scrollPage(virtualizer: Virtualizer, direction: 'up' | 'down') {
  const pageSize = virtualizer.scrollRect?.height ?? 400;
  const delta = direction === 'down' ? pageSize : -pageSize;
  virtualizer.scrollBy(delta, { behavior: 'smooth' });
}

// Scroll to specific positions
function scrollToPosition(virtualizer: Virtualizer, position: number) {
  virtualizer.scrollToOffset(position, { align: 'start' });
}

/**
 * Get the scroll offset needed to show a specific item
 * @param index - Index of the item
 * @param align - How to align the item (default: 'auto')
 * @returns Tuple of [offset, actualAlignment] or undefined if item not found
 */
getOffsetForIndex(index: number, align?: ScrollAlignment): [number, ScrollAlignment] | undefined;

/**
 * Calculate the aligned offset for a given position
 * @param toOffset - Target offset position
 * @param align - Alignment mode
 * @param itemSize - Size of the item being aligned (optional)
 * @returns Calculated offset position
 */
getOffsetForAlignment(toOffset: number, align: ScrollAlignment, itemSize?: number): number;

/**
 * Find the virtual item at a specific scroll offset
 * @param offset - Scroll offset position
 * @returns VirtualItem at that position, or undefined if none found
 */
getVirtualItemForOffset(offset: number): VirtualItem | undefined;

/**
 * Extract the item index from a DOM element's data-index attribute
 * @param node - The DOM element to inspect
 * @returns The item index, or -1 if not found or invalid
 */
indexFromElement(node: TItemElement): number;

// Handle click events on virtual items
function handleItemClick(event: MouseEvent, virtualizer: Virtualizer) {
  const element = event.target as HTMLElement;
  const index = virtualizer.indexFromElement(element);
  if (index >= 0) {
    console.log(`Clicked item at index ${index}`);
  }
}

// Find item at current scroll position
function getCurrentItem(virtualizer: Virtualizer) {
  const currentOffset = virtualizer.scrollOffset ?? 0;
  const item = virtualizer.getVirtualItemForOffset(currentOffset);
  return item;
}

// Calculate positions for custom scroll animations
function animateToItem(virtualizer: Virtualizer, targetIndex: number) {
  const offsetInfo = virtualizer.getOffsetForIndex(targetIndex, 'center');
  if (offsetInfo) {
    const [targetOffset] = offsetInfo;
    // Implement custom animation to targetOffset
    animateScrollTo(targetOffset);
  }
}

export interface VirtualizerOptions<TScrollElement extends Element | Window, TItemElement extends Element> {
  // Required configuration
  count: number;
  getScrollElement: () => TScrollElement | null;
  estimateSize: (index: number) => number;
  scrollToFn: (offset: number, options: ScrollOptions, instance: Virtualizer<TScrollElement, TItemElement>) => void;
  observeElementRect: (instance: Virtualizer<TScrollElement, TItemElement>, cb: (rect: Rect) => void) => void | (() => void);
  observeElementOffset: (instance: Virtualizer<TScrollElement, TItemElement>, cb: (offset: number, isScrolling: boolean) => void) => void | (() => void);
  
  // Optional configuration with defaults
  debug?: boolean; // false
  initialRect?: Rect; // { width: 0, height: 0 }
  onChange?: (instance: Virtualizer<TScrollElement, TItemElement>, sync: boolean) => void;
  measureElement?: (element: TItemElement, entry: ResizeObserverEntry | undefined, instance: Virtualizer<TScrollElement, TItemElement>) => number;
  overscan?: number; // 1
  horizontal?: boolean; // false
  paddingStart?: number; // 0
  paddingEnd?: number; // 0
  scrollPaddingStart?: number; // 0
  scrollPaddingEnd?: number; // 0
  initialOffset?: number | (() => number); // 0
  getItemKey?: (index: number) => Key; // (index) => index
  rangeExtractor?: (range: Range) => Array<number>; // defaultRangeExtractor
  scrollMargin?: number; // 0
  gap?: number; // 0
  indexAttribute?: string; // 'data-index'
  initialMeasurementsCache?: Array<VirtualItem>; // []
  lanes?: number; // 1
  isScrollingResetDelay?: number; // 150
  useScrollendEvent?: boolean; // false
  enabled?: boolean; // true
  isRtl?: boolean; // false
  useAnimationFrameWithResizeObserver?: boolean; // false
}

export interface ScrollOptions {
  adjustments?: number;
  behavior?: ScrollBehavior;
}