import getCourseTableRows from './getCourseTableRows';

const NEXT_PAGE_BUTTON_SELECTOR = '#next_nav_link';
const PREV_PAGE_BUTTON_SELECTOR = '#prev_nav_link';
/**
 * Represents all the states that we care about when autoloading the next page of courses
 */
export enum AutoLoadStatus {
    LOADING = 'LOADING',
    IDLE = 'IDLE',
    ERROR = 'ERROR',
}

let isLoading = false;
let nextPageURL = getNextButton(document)?.href;

/**
 * This will scrape the pagination buttons from the course list and use them to load the next page
 * and then return the table rows from the next page
 * @returns a tuple of the current LoadStatus (whether are currently loading the next page, or if we have reached the end of the course catalog,
 * or if there was an error loading the next page) and an array of the table rows from the next page (or an empty array
 * if we have reached the end of the course catalog
 */
export async function loadNextCourseCatalogPage(): Promise<[AutoLoadStatus, HTMLTableRowElement[]]> {
    // if there is no more nextPageURL, then we have reached the end of the course catalog, so we can stop
    if (!nextPageURL) {
        return [AutoLoadStatus.IDLE, []];
    }
    // remove the next button so that we don't load the same page twice
    removePaginationButtons(document);
    if (isLoading) {
        // if we are already loading the next page, then we don't need to do anything
        return [AutoLoadStatus.LOADING, []];
    }

    // begin loading the next page
    isLoading = true;
    try {
        const response = await fetch(nextPageURL);
        const html = await response.text();
        const parser = new DOMParser();
        const newDocument = parser.parseFromString(html, 'text/html');

        // extract the table rows from the document of the next page
        const tableRows = getCourseTableRows(newDocument);
        if (!tableRows) {
            return [AutoLoadStatus.ERROR, []];
        }

        // extract the next page url from the document of the next page, so when we scroll again we can use that
        nextPageURL = getNextButton(newDocument)?.href;
        isLoading = false;
        return [AutoLoadStatus.IDLE, Array.from(tableRows)];
    } catch (e) {
        console.error(e);
        return [AutoLoadStatus.ERROR, []];
    }
}

/**
 * Scrapes the next button from the document
 * @param doc  the document to get the next button from
 * @returns the next button from the document
 */
function getNextButton(doc: Document) {
    return doc.querySelector<HTMLAnchorElement>(NEXT_PAGE_BUTTON_SELECTOR);
}

/**
 * Removes the next and previous buttons from the document so that we don't load the same page twice
 * @param doc the document to remove the next and previous buttons from
 */
export function removePaginationButtons(doc: Document) {
    const nextButton = doc.querySelectorAll<HTMLAnchorElement>(NEXT_PAGE_BUTTON_SELECTOR);
    nextButton.forEach(button => button.remove());
    const prevButton = doc.querySelectorAll<HTMLAnchorElement>(PREV_PAGE_BUTTON_SELECTOR);
    prevButton.forEach(button => button.remove());
}