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Gelbooru "Index" Button

Adds an "Index" button on post pages, allowing you to return to a search starting from the ID of the current post you're on

// ==UserScript==
// @name         Gelbooru "Index" Button
// @namespace    https://gelbooru.com/
// @version      1.0
// @description  Adds an "Index" button on post pages, allowing you to return to a search starting from the ID of the current post you're on
// @match        https://gelbooru.com/index.php?page=post&s=view*
// @icon         https://gelbooru.com/favicon.ico
// @grant        none
// @license MIT
// ==/UserScript==

(function() {
    'use strict';

    if (location.hostname !== 'gelbooru.com') return;

    const url = new URL(window.location.href);
    const urlParams = url.searchParams;

    const currentId = urlParams.get('id');
    if (!currentId) return; // If no post ID, do nothing

    urlParams.delete('id');
    urlParams.set('s', 'list');

    const rawTags = urlParams.get('tags') || '';
    let decodedTags = decodeURIComponent(rawTags);

    const newIdSearch = 'id:=<' + currentId;

    const idSearchRegex = /id:=<\d+/i;
    if (idSearchRegex.test(decodedTags)) {
        decodedTags = decodedTags.replace(idSearchRegex, newIdSearch);
    } else {
        decodedTags = decodedTags.trim();
        if (decodedTags) decodedTags += ' ';
        decodedTags += newIdSearch;
    }

    urlParams.set('tags', decodedTags.trim());

    const finalUrl = `${url.origin}${url.pathname}?${urlParams.toString()}`;

    const box = document.createElement('div');
    box.className = 'alert alert-info';
    box.style.textAlign = 'center';
    box.style.margin = '10px 0px';
    box.style.padding = '10px';

    const indexLink = document.createElement('a');
    indexLink.textContent = 'Index';
    indexLink.href = finalUrl;

    box.appendChild(indexLink);

    const navContainers = document.querySelectorAll('.alert.alert-info');
    if (navContainers.length > 0) {
        navContainers[navContainers.length - 1].insertAdjacentElement('afterend', box);
    } else {
        const mainBody = document.querySelector('.mainBodyPadding');
        if (mainBody) mainBody.appendChild(box);
    }
})();