/**
 * @description checks if the userAgent is identified as a bot or crawler
 * @param {string} userAgent , navigator user-agent
 * @returns {boolean}
 */
export const isCrawler = userAgent => {
  const crawlerRegex = /googlebot|adsbot-google|headless|prerender|Feedfetcher-Google|bingbot|yandex|baiduspider|Facebot|facebookexternalhit|twitterbot|rogerbot|linkedinbot|embedly|quora link preview|showyoubot|outbrain|pinterest|slackbot|vkShare|W3C_Validator|bufferbot|redditbot|Applebot|WhatsApp|flipboard|tumblr|bitlybot|Discordbot|qwantify|www.google.com\/webmasters\/tools\/richsnippets|cocon/i;
  return crawlerRegex.test(userAgent);
};

/**
 * @description checks if the stylesheets attributes contains styled-components specific attributes
 * @param attributes , represents a NamedNodeMap containing the list of attributes of an HTML style tag
 * @returns {boolean}
 */
export const isStyledComponentStylesheet = attributes => {
  const conditions = ['data-styled', 'data-styled-version'];
  return Object.values(attributes).some(attribute =>
    conditions.some(c => attribute.nodeName === c),
  );
};

/**
 * Fetches styled-components stylesheets and concatenates them to create one Prerender-ready stylesheet.
 * Function based on Mozilla documentation : https://developer.mozilla.org/en-US/docs/Web/API/StyleSheetList
 */
export const appendStyledComponents = () => {
  const css = [].slice
    .call(document.styleSheets)
    .reduce((prevSheet, styleSheet) => {
      if (styleSheet.cssRules) {
        if (isStyledComponentStylesheet(styleSheet.ownerNode.attributes))
          return (
            prevSheet +
            [].slice
              .call(styleSheet.cssRules)
              .reduce((prevRule, cssRule) => prevRule + cssRule.cssText, '')
          );
      }
      return prevSheet;
    }, '');

  const style = document.createElement('style');
  document.head.appendChild(style);
  style.type = 'text/css';
  style.appendChild(document.createTextNode(css));
};

/**
 * apply prerender changes in document head if needed
 */
export const handlePrerender = () => {
  if (isCrawler(navigator.userAgent)) {
    appendStyledComponents();
  }
};