A quick something to compare posts with an elo rating to try and find what your favorite is.

e621_elo_compare.user.js

// ==UserScript==
// @name         e621 elo compare
// @namespace    http://tampermonkey.net/
// @version      1.00001
// @description  try to take over the world!

// @author       Sasquire

// @match        https://e621.net/extensions/post_elo
// @match        http://e621.net/extensions/post_elo

// @grant        GM_addStyle

// ==/UserScript==

const starting_elo = 1000;
const elo_k = 30;

// Clears the page
(() => {
	function clear_node(node){
		while(node.children.length > 0){
			node.removeChild(node.children[0]);
		}
	}

	function remove_toJSON(){
		// This gave me a lot of greif. e621 changes the toJSON
		// methods and creates not optimal JSON.
		// https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/JSON/stringify
		delete Object.prototype.toJSON;
		delete Date.prototype.toJSON;
		delete String.prototype.toJSON;
		delete Array.prototype.toJSON;
		delete Number.prototype.toJSON;

		// Kira I don't know what this did, but it gave me errors. So I removed it.
		jQuery.event.dispatch = () => '';
	}

	clear_node(document.head);
	clear_node(document.body);
	remove_toJSON();

    document.body.innerHTML = `
<div id="main">
    <div id="header"><button id="export">Export</button></div>
    <div id="leftpost"></div>
    <div id="rightpost"></div>
</div>
`;

    GM_addStyle(`
#main {
	height: 100vh;
	display: grid;
	grid-template-columns: auto 20px auto;
	grid-template-rows: 75px auto;
}

#header {
    grid-column: 1 / 3;
    grid-rows: 1 / 1;
}

#leftpost {
    grid-column: 1 / 1;
    grid-rows: 2 / 2;
}

#rightpost {
    grid-column: 3 / 3;
    grid-rows: 2 / 2;
}

#leftpost > *, #rightpost > * {
    max-width: 90%;
	max-height: 90%;
	margin: auto;
	align-self: center;
}
`);
})();

async function get_all_posts(query){
    const posts = {}
    for(let page = 1; true; page++){
        const data = await get_search_results(query, page);
        data.forEach(e => (e.elo = starting_elo));
        data.forEach(e => (e.hit = 0));
        data.forEach(e => (posts[e.id] = e));
        if(data.length != 320){
            console.log('Posts loaded')
            return posts;
        }
    }
}

async function get_search_results(query, page){
	console.log(`Getting page ${page} for [${query}]\nThis could take a while`);
	const url = new URL('https://e621.net/post/index.json');
	url.searchParams.set('tags', query);
	url.searchParams.set('page', page);
	url.searchParams.set('limit', 320);

	const res = await fetch(new Request(url));
	const text = await res.text();
	if(res.status != 200){
		console.log(`Bad status code ${res.status}\n${text}`);
		throw new Error(`Bad status code ${res.status}\n${text}`);
	} else {
		const data = JSON.parse(text);
		return data;
	}
}

function set(id, post){
    const golder = document.getElementById(id);
    // I know its not safe. this is something quick
    if(post.file_ext == 'webm'){
		golder.innerHTML = `
		<video
			poster="${post.sample_url}"
			id="webm-container"
			controls
			loop="true"
		>
			<source src="${post.file_url}" type="video/webm" />
		</video>`;
	} else if(post.file_ext == 'swf') {
		golder.innerHTML = `
		<object>
			<param name="movie" value="${post.file_url}">
			<embed
				type="application/x-shockwave-flash"
				src="${post.file_url}"
				allowscriptaccess="never"
			>
		</object>`;
	} else {
		golder.innerHTML = `<img src="${post.file_url}">`;
	}
}

function compare_two(post1, post2){
    set('leftpost', post1);
    set('rightpost', post2);

    document.getElementById('leftpost').setAttribute('data-id', post1.id);
    document.getElementById('rightpost').setAttribute('data-id', post2.id);
}

function do_elo(rating1, rating2, winner){
    function probability(r1, r2){
        return 1 / (1 + Math.pow(10, (r1 - r2) / 400));
    }

    const p1 = probability(rating2, rating1); // Probability p1 wins
    const p2 = probability(rating1, rating2);

    if(winner == 1){
        const nr1 = rating1 + elo_k * (1 - p1);
        const nr2 = rating2 + elo_k * (0 - p2);
        return [nr1, nr2]
    } else {
        const nr1 = rating1 + elo_k * (0 - p1);
        const nr2 = rating2 + elo_k * (1 - p2);
        return [nr1, nr2];
    }
}

function select2(posts){
    const all_posts = Object.values(posts).sort((a, b) => {
        const hd = a.hit - b.hit;
        if(hd != 0){ return hd; }

        const ed = a.elo - b.elo;
        if(ed != 0){ return ed; }

        return Math.random() > 0.5 ? -1 : 1
    });
    compare_two(all_posts[0], all_posts[1]);
}

get_all_posts('').then(posts => {
    select2(posts);

    document.addEventListener('keydown', e => {
        if(['ArrowLeft', 'ArrowRight', 'ArrowUp', 'ArrowDown', 'Space'].includes(e.key) == false){ return; }
        const get = id => parseInt(document.getElementById(id).getAttribute('data-id'), 10);
        const left_id = get('leftpost');
        const right_id = get('rightpost');
        let r1 = posts[left_id].elo;
        let r2 = posts[right_id].elo;

        if(e.key == 'ArrowLeft'){
            [r1, r2] = do_elo(r1, r2, 1);
        } else if(e.key == 'ArrowRight'){
            [r1, r2] = do_elo(r1, r2, 2);
        } else if(e.key == 'ArrowUp'){
            // Both win
            let _ = null;
            [r1, _] = do_elo(r1, r2, 1);
            [_, r2] = do_elo(r1, r2, 2);
        } else if(e.key == 'ArrowDown'){
            // Both lose
            let _ = null;
            [r1, _] = do_elo(r1, r2, 2);
            [_, r2] = do_elo(r1, r2, 1);
        } else if(e.key == 'Space'){

        }

        posts[left_id].elo = r1;
        posts[right_id].elo = r2;
        posts[left_id].hit++;
        posts[right_id].hit++;

        select2(posts);
    });

    document.getElementById('export').addEventListener('click', () => {
        console.log(Object.values(posts).map(e => ({
            post_id: e.id,
            elo: e.elo
        })).map(e => `${e.post_id} ${e.elo}`))
    });
});