/**
 * WordPress dependencies
 */
import { flushSync, useEffect, useState } from '@wordpress/element';
import { createQueue } from '@wordpress/priority-queue';

type AsyncListConfig = {
	step: number;
};

/**
 * Returns the first items from list that are present on state.
 *
 * @param list  New array.
 * @param state Current state.
 * @return First items present in state.
 */
function getFirstItemsPresentInState< T >( list: T[], state: T[] ): T[] {
	const firstItems = [];

	for ( let i = 0; i < list.length; i++ ) {
		const item = list[ i ];
		if ( ! state.includes( item ) ) {
			break;
		}

		firstItems.push( item );
	}

	return firstItems;
}

/**
 * React hook returns an array which items get asynchronously appended from a source array.
 * This behavior is useful if we want to render a list of items asynchronously for performance reasons.
 *
 * @param list   Source array.
 * @param config Configuration object.
 *
 * @return Async array.
 */
function useAsyncList< T >(
	list: T[],
	config: AsyncListConfig = { step: 1 }
): T[] {
	const { step = 1 } = config;
	const [ current, setCurrent ] = useState< T[] >( [] );

	useEffect( () => {
		// On reset, we keep the first items that were previously rendered.
		let firstItems = getFirstItemsPresentInState( list, current );
		if ( firstItems.length < step ) {
			firstItems = firstItems.concat(
				list.slice( firstItems.length, step )
			);
		}
		setCurrent( firstItems );

		const asyncQueue = createQueue();
		for ( let i = firstItems.length; i < list.length; i += step ) {
			asyncQueue.add( {}, () => {
				flushSync( () => {
					setCurrent( ( state ) => [
						...state,
						...list.slice( i, i + step ),
					] );
				} );
			} );
		}

		return () => asyncQueue.reset();
	}, [ list ] );

	return current;
}

export default useAsyncList;