const Apify = require('apify');

const sleepPromised = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

const logError = (msg, e) => {
    console.log(`ERROR: ${msg}`);
    console.error(e);
};
const logInfo = (msg) => console.log(`INFO: ${msg}`);
const logDebug = (msg) => console.log(`DEBUG: ${msg}`);

/**
 * Method scrolls page to xpos, ypos.
 */
const scrollTo = (page, elementToScroll, scrollToHeight) => page.evaluate((elementToScroll, scrollToHeight) => {
    const scrollable = document.querySelector(elementToScroll);
    scrollable.scrollTop = scrollToHeight;
}, elementToScroll, scrollToHeight);

/**
 * Method returns info about page scroll
 */
const getPageScrollInfo = (page, elementToScroll) => page.evaluate((elementToScroll) => {
    return {
        scrollHeight: document.querySelector(elementToScroll).scrollHeight,
        scrollTop: document.querySelector(elementToScroll).scrollTop,
        clientHeight: document.querySelector(elementToScroll).clientHeight,
    };
}, elementToScroll);

/**
 * Scroll to down page until infinite scroll ends or reaches maxHeight
 * @param page - instance of crawled page
 * @param maxHeight - max height of document to scrollscrollHeight
 * @param elementToScroll - CSS selector of element where we want to scroll, default is 'body'
 * @return {Promise.<void>}
 */
module.exports = async (page, maxHeight, elementToScroll = 'body') => {
    const maybeResourceTypesInfiniteScroll = ['xhr', 'fetch', 'websocket', 'other'];
    const stringifyScrollInfo = (scrollInfo) => {
        return `scrollTop=${scrollInfo.scrollTop}, ` +
            `clientHeight=${scrollInfo.clientHeight}, ` +
            `scrollHeight=${scrollInfo.scrollHeight}, ` +
            `maxHeight=${maxHeight}`;
    };
    const defaultScrollDelay = 2000;

    // Catch and count all pages request for resources
    const resourcesStats = {
        requested: 0,
        finished: 0,
        failed: 0,
        forgotten: 0,
    };
    const pendingRequests = {};
    page.on('request', (msg) => {
        if (maybeResourceTypesInfiniteScroll.includes(msg.resourceType)) {
            pendingRequests[msg._requestId] = Date.now();
            resourcesStats.requested++;
        }
    });
    page.on('requestfailed', (msg) => {
        if (maybeResourceTypesInfiniteScroll.includes(msg.resourceType)) {
            if (pendingRequests[msg._requestId]) {
                delete pendingRequests[msg._requestId];
                resourcesStats.failed++;
            }
        }
    });
    page.on('requestfinished', (msg) => {
        if (maybeResourceTypesInfiniteScroll.includes(msg.resourceType)) {
            if (pendingRequests[msg._requestId]) {
                delete pendingRequests[msg._requestId];
                resourcesStats.finished++;
            }
        }
    });

    try {
        await page.waitForSelector(elementToScroll);
        let scrollInfo = await getPageScrollInfo(page, elementToScroll);
        logInfo(`Infinite scroll started (${stringifyScrollInfo(scrollInfo)}).`);

        while (true) {
            scrollInfo = await getPageScrollInfo(page, elementToScroll);

            // Forget pending resources that didn't finish loading in time
            const now = Date.now();
            const timeout = 30000; // TODO: use resourceTimeout
            Object.keys(pendingRequests)
                .forEach((requestId) => {
                    if (pendingRequests[requestId] + timeout < now) {
                        delete pendingRequests[requestId];
                        resourcesStats.forgotten++;
                    }
                });

            logDebug(`Infinite scroll stats (${stringifyScrollInfo(scrollInfo)} resourcesStats=${JSON.stringify(resourcesStats)}).`);

            const pendingRequestsCount = resourcesStats.requested - (resourcesStats.finished + resourcesStats.failed + resourcesStats.forgotten);
            if (pendingRequestsCount === 0) {
                // If the page is scrolled to the very bottom or beyond maximum height, we are done
                if (scrollInfo.scrollTop + scrollInfo.clientHeight >= Math.min(scrollInfo.scrollHeight, maxHeight)) break;
                // Otherwise we try to scroll down
                await scrollTo(page, elementToScroll, scrollInfo.scrollHeight);
            }

            await sleepPromised(defaultScrollDelay);
        }
        // Scroll back up, otherwise the screenshot of the browser would only show the bottom of
        // the page
        await scrollTo(page, elementToScroll, scrollInfo.scrollHeight);

        logInfo(`Infinite scroll finished (${stringifyScrollInfo(scrollInfo)} resourcesStats=${JSON.stringify(resourcesStats)})`);
    } catch (err) {
        logError('An exception thrown in infiniteScroll()', err);
    }
};