export type PagerState =
	| { kind: "at-page"; page: number }
	| { kind: "loading"; page: number }
	| { kind: "failed"; page: number };

export interface OffsetPager<T> {
	readonly state: PagerState;
	readonly items: T[];
	readonly limit: number;
	readonly hasNext: boolean;
	select: (target: number) => Promise<void>;
}

export { discoveredPageCount, pageCount } from "$lib/pagination";

// size the last page to its remainder, or a full page when total is unknown
export const pageRows = (page: number, limit: number, total?: number): number =>
	total === undefined ? limit : Math.min(limit, Math.max(1, total - (page - 1) * limit));

export const createOffsetPager = <T>(
	initial: T[],
	limit: number,
	load: (offset: number) => Promise<T[]>
): OffsetPager<T> => {
	let state = $state<PagerState>({ kind: "at-page", page: 1 });
	let pages = $state<Record<number, T[]>>({ 1: initial });

	// point to the target page immediately so the skeleton and footer follow the click
	const select = async (target: number) => {
		if (state.kind === "loading" || target < 1 || target === state.page) return;

		const currentPage = state.page;
		state = { kind: "loading", page: target };
		try {
			if (!pages[target]) {
				pages = { ...pages, [target]: await load((target - 1) * limit) };
			}
			state = { kind: "at-page", page: target };
		} catch {
			state = { kind: "failed", page: currentPage };
		}
	};

	return {
		get state() {
			return state;
		},
		get items() {
			return pages[state.page] ?? [];
		},
		get limit() {
			return limit;
		},
		get hasNext() {
			// assume an in-flight page is full so the footer keeps its shape
			return (pages[state.page]?.length ?? limit) >= limit;
		},
		select
	};
};