import { useEffect, useMemo, useRef, useState } from "react";

import { useCommand } from "./useCommand.js";

export type UseScrollableTextOptions = {
	textLength: number;
	viewportHeight: number;
	signature: string;
};

export type UseScrollableTextResult = {
	scrollOffset: number;
	canScroll: boolean;
	maxScrollOffset: number;
	pageStep: number;
};

export function useScrollableText({
	textLength,
	viewportHeight,
	signature,
}: UseScrollableTextOptions): UseScrollableTextResult {
	let [scrollOffset, setScrollOffset] = useState(0);

	let lastSignatureRef = useRef(signature);
	useEffect(() => {
		if (lastSignatureRef.current !== signature) {
			lastSignatureRef.current = signature;
			setScrollOffset(0);
		}
	}, [signature]);

	let maxScrollOffset = useMemo(() => Math.max((textLength || 1) - viewportHeight, 0), [textLength, viewportHeight]);

	useEffect(() => {
		setScrollOffset((offset) => Math.min(offset, maxScrollOffset));
	}, [maxScrollOffset]);

	let canScroll = textLength > viewportHeight && viewportHeight > 0;
	let pageStep = useMemo(() => Math.max(viewportHeight - 1, 1), [viewportHeight]);

	useCommand("up", () => setScrollOffset((offset) => Math.max(offset - 1, 0)), canScroll);
	useCommand("down", () => setScrollOffset((offset) => Math.min(offset + 1, maxScrollOffset)), canScroll);
	useCommand("pageup", () => setScrollOffset((offset) => Math.max(offset - pageStep, 0)), canScroll);
	useCommand("pagedown", () => setScrollOffset((offset) => Math.min(offset + pageStep, maxScrollOffset)), canScroll);

	return { scrollOffset, canScroll, maxScrollOffset, pageStep };
}