e-hentai-prefetch-next-page.user.js

// ==UserScript==
// @name        E-Hentai Prefetch Next Page
// @namespace   Violentmonkey Scripts
// @match       https://e-hentai.org/s/*
// @match       https://exhentai.org/s/*
// @grant       none
// @version     0.2.0
// @author      sorz
// @description Prefetch image file on the next page for faster browsing.
// ==/UserScript==

const MAX_PREFETCH_PAGES = 4;

// page no -> response from api_response
const pageRespCache = new Map();

// page no. -> image key
const pageImgKeyCache = new Map();

const currentPageNo = () => history.state.page;

const getPageNoImgKey = (doc) => {
  let [_, page, imgKey] = doc.querySelector('a#next')
    .attributes.onclick.value
    .match(/load_image\((\d+), '([0-9a-f]+)'\)/);
  return [parseInt(page), imgKey];
};

const template = (html) => {
  const template = document.createElement('template');
  template.innerHTML = html;
  return template.content;
}

function prefetchPage(pageNo, n) {
  const onResponse = (resp) => {
    const [nextPageNo, nextImgKey] = getPageNoImgKey(template(resp.n));
    
    if (!pageRespCache.has(pageNo)) {
      // Cache response
      pageRespCache.set(pageNo, resp);
      pageImgKeyCache.set(nextPageNo, nextImgKey);

      // Prefetch image file
      const src = template(resp.i3).querySelector('img#img').src;    
      const img = new Image();
      img.onload = () => console.log('image fetched', 'page', pageNo);
      img.src = src;
    }

    if (n > 0) prefetchPage(nextPageNo, n - 1);
  }
  
  if (pageRespCache.has(pageNo)) {
    onResponse(pageRespCache.get(pageNo));
    return;
  }
  
  const imgKey = pageImgKeyCache.get(pageNo);
  if (!imgKey) {
    logging.warn('missing image key for page', pageNo);
    return;
  }
  const request = {
    method: "showpage",
    gid: gid,
    page: pageNo,
    imgkey: imgKey,
    showkey: showkey
  };
  const xhr = new XMLHttpRequest();
  api_call(xhr, request, () => {
    const resp = api_response(xhr);
    if (resp == false || resp.error != undefined) return;
    onResponse(resp);
  });
}

function prefetchNextNPage(n) {
  let [pageNo, imgKey] = getPageNoImgKey(document);
  pageImgKeyCache.set(pageNo, imgKey);
  prefetchPage(pageNo, n);
}

apply_json_state = new Proxy(apply_json_state, {
  apply: function(target, thisArg, args) {
    Reflect.apply(target, thisArg, args);
    prefetchNextNPage(MAX_PREFETCH_PAGES);
  }
});

load_image = new Proxy(load_image, {
  apply: function(target, thisArg, [page, imgKey]) {
    const resp = pageRespCache.get(page);
    if (!resp) return Reflect.apply(target, thisArg, [page, imgKey]);
    console.log('cached resp found', 'page', page);
    history.pushState(
      { 
        page: resp.p, 
        imgkey: resp.k, 
        json: resp, 
        expire: get_unixtime() + 300 
      },
      document.title,
      base_url + resp.s
    );
    apply_json_state(resp); 
    return false;
  }
});

prefetchNextNPage(MAX_PREFETCH_PAGES);