import {
    createVirtualizer as coreCreateVirtualizer,
    observeElementRect,
} from '@tanstack/svelte-virtual';
import type { VirtualItem as CoreVirtualItem } from '@tanstack/virtual-core';
import { get } from 'svelte/store';

export interface VirtualItem {
    index: number;
    start: number;
    size: number;
    end: number;
    key: string | number;
}

export interface VirtualizerOptions {
    /** Total number of items in the data array */
    count: number;
    /** Function to estimate the size of an item at a given index */
    estimateSize: (index: number) => number;
    /** Number of extra items to render outside viewport (default: 5) */
    overscan?: number;
    /** Function to get the key of an item at a given index (defaults to index) */
    getItemKey?: (index: number) => string | number;
    /** Optional margin in pixels for scroll calculations */
    scrollMargin?: number;
}

/**
 * Creates a reactive virtualizer using Svelte 5 runes and TanStack's core library.
 *
 * @example
 * ```ts
 * const virtualizer = createVirtualizer(() => ({
 *   count: items.length,
 *   estimateSize: () => 80,
 *   overscan: 5,
 * }));
 *
 * // In template:
 * // <div bind:this={virtualizer.scrollElement}>
 * //   {#each virtualizer.items as item}
 * //     <div style="transform: translateY({item.start}px)">
 * //       {items[item.index]}
 * //     </div>
 * //   {/each}
 * // </div>
 * ```
 */
export function createVirtualizer(
    optionsGetter: () => VirtualizerOptions,
) {
    let element = $state<HTMLElement | null>(null);

    const internalStore = coreCreateVirtualizer({
        get count() {
            return optionsGetter().count;
        },
        get estimateSize() {
            return optionsGetter().estimateSize;
        },
        get overscan() {
            return optionsGetter().overscan ?? 5;
        },
        get scrollMargin() {
            return optionsGetter().scrollMargin;
        },
        get getItemKey() {
            return optionsGetter().getItemKey ?? (i => i);
        },
        getScrollElement: () => element,
        observeElementRect: observeElementRect,
    });

    const state = $derived(get(internalStore));

    const virtualItems = $derived(
        state.getVirtualItems().map((item: CoreVirtualItem): VirtualItem => ({
            index: item.index,
            start: item.start,
            size: item.size,
            end: item.end,
            key: typeof item.key === 'bigint' ? Number(item.key) : item.key,
        })),
    );

    return {
        get items() {
            return virtualItems;
        },

        get totalSize() {
            return state.getTotalSize();
        },

        get scrollOffset() {
            return state.scrollOffset ?? 0;
        },

        get scrollElement() {
            return element;
        },
        set scrollElement(el) {
            element = el;
        },

        scrollToIndex: (idx: number, opt?: { align?: 'start' | 'center' | 'end' | 'auto' }) =>
            state.scrollToIndex(idx, opt),

        scrollToOffset: (off: number) => state.scrollToOffset(off),

        measureElement: (el: HTMLElement) => state.measureElement(el),
    };
}

export type Virtualizer = ReturnType<typeof createVirtualizer>;