Convert to FiveM button for gta5-mods.com

// ==UserScript==
// @name         GTA5 Mods: Convert to FiveM button
// @namespace    https://www.gta5modsconvertor.com/
// @version      1.0.0
// @description  Adds a "Convert to FiveM" button to vehicle mod pages on gta5-mods.com. It opens gta5modsconvertor.com with the mod link filled in. Independent tool, not affiliated with gta5-mods.com.
// @author       GTA5 Mods Convertor
// @match        https://www.gta5-mods.com/vehicles/*
// @match        https://www.gta5-mods.com/*/vehicles/*
// @grant        none
// @run-at       document-idle
// @noframes
// @homepageURL  https://www.gta5modsconvertor.com/tools/convert-button
// @downloadURL  https://www.gta5modsconvertor.com/gta5-mods-convert-button.user.js
// @updateURL    https://www.gta5modsconvertor.com/gta5-mods-convert-button.user.js
// @license      MIT
// ==/UserScript==

/*
 * WHAT THIS SCRIPT DOES (the whole of it, so you can audit it in a minute)
 *
 * 1. Runs only on gta5-mods.com addresses under /vehicles/ (and the language
 *    versions such as /de/vehicles/).
 * 2. Checks that the page is a single vehicle mod page: the address has a mod
 *    name after /vehicles/, and the page has the mod's download button or its
 *    file container. On listing, tag, sort and numbered pages it does nothing.
 * 3. Adds ONE link, "Convert to FiveM", directly under the page's Download
 *    button. If that button is not where it is expected, the link is shown as
 *    a small fixed button in the bottom right corner instead.
 * 4. The link opens, in a new tab and only when you click it:
 *      https://www.gta5modsconvertor.com/?url=<address of this mod page>&utm_source=userscript
 *    The converter fills the mod link into its input. Nothing is converted
 *    until you press Convert there.
 *
 * WHAT IT DOES NOT DO
 *
 * - No network requests (no fetch, XMLHttpRequest, beacons, images or iframes).
 * - No cookies, no localStorage, no reading of forms, accounts or page text.
 *   The only things it reads are the page address and whether the download
 *   button exists.
 * - No remote code and no external files: everything it runs is in this file.
 * - No userscript-manager permissions (@grant none).
 * - It changes nothing else on the page and never clicks or downloads anything.
 *
 * Independent tool. Not affiliated with or endorsed by gta5-mods.com,
 * Rockstar Games, Take-Two Interactive or Cfx.re.
 */

(function () {
  "use strict";

  var CONVERTER = "https://www.gta5modsconvertor.com/";
  var BUTTON_ID = "g5mc-convert-button";
  var LABEL = "Convert to FiveM";
  var TITLE =
    "Opens gta5modsconvertor.com with this mod's link filled in. " +
    "Independent tool, not part of gta5-mods.com.";

  // Addresses under /vehicles/ that are lists of mods, not a mod.
  var LISTING_NAMES = [
    "tags", "day", "week", "month", "most-liked", "most-downloaded",
    "highest-rated", "latest-uploads", "latest-versions", "featured"
  ];

  /**
   * The mod's name from the page address, or null when the address is not a
   * single vehicle mod. Accepts /vehicles/<name> and /<xx>/vehicles/<name>
   * (two-letter language code), with anything after the name ignored.
   */
  function modSlug(pathname) {
    var parts = pathname.split("/").filter(Boolean);
    if (parts.length && /^[a-z]{2}$/i.test(parts[0])) parts.shift();
    if (parts.length < 2 || parts[0].toLowerCase() !== "vehicles") return null;
    var slug = parts[1];
    if (/^\d+$/.test(slug)) return null; // page number of the listing
    if (LISTING_NAMES.indexOf(slug.toLowerCase()) !== -1) return null;
    if (!/^[A-Za-z0-9%._~-]+$/.test(slug)) return null; // unexpected characters: leave the page alone
    return slug;
  }

  /** The link element. All styling is inline; no stylesheet or asset is added. */
  function buildLink(href, fixed) {
    var a = document.createElement("a");
    a.id = BUTTON_ID;
    a.href = href;
    a.target = "_blank";
    a.rel = "noopener noreferrer";
    a.title = TITLE;
    a.textContent = LABEL;
    var css = [
      "box-sizing:border-box",
      "text-align:center",
      "padding:8px 14px",
      "font:600 14px/1.4 system-ui,-apple-system,'Segoe UI',Roboto,Arial,sans-serif",
      "color:#fff",
      "background:#c2410c",
      "border:1px solid #9a3412",
      "border-radius:4px",
      "text-decoration:none",
      "cursor:pointer"
    ];
    if (fixed) {
      css.push(
        "position:fixed", "right:12px", "bottom:12px", "z-index:2147483000",
        "display:inline-block", "box-shadow:0 2px 8px rgba(0,0,0,.35)"
      );
    } else {
      css.push("display:block", "margin:0 0 10px 0");
    }
    a.setAttribute("style", css.join(";"));
    return a;
  }

  function run() {
    // Never insert twice.
    if (document.getElementById(BUTTON_ID)) return;

    var slug = modSlug(window.location.pathname);
    if (!slug) return;

    // A mod page has a download button and/or a #file container. A listing has neither.
    var download = document.querySelector("#file-container a.btn-download, a.btn-download");
    var fileBox = document.querySelector("#file[data-user-file-id]");
    if (!download && !fileBox) return;

    // The form the converter accepts: English address, no language prefix, no extras.
    var modUrl = "https://www.gta5-mods.com/vehicles/" + slug;
    var href = CONVERTER + "?url=" + encodeURIComponent(modUrl) + "&utm_source=userscript";

    if (download && download.parentNode) {
      // Directly under the Download button, as its next sibling.
      download.parentNode.insertBefore(buildLink(href, false), download.nextSibling);
    } else if (document.body) {
      // Layout differs from what this script knows: small fixed button instead.
      document.body.appendChild(buildLink(href, true));
    }
  }

  try {
    run();
  } catch (e) {
    // If the page is not what this script expects, do nothing at all.
  }
})();