xkcd_unique_random.user.js

// ==UserScript==
// @name         XKCDUniqueRandom
// @namespace    http://numin.it/
// @version      0.1
// @license      MIT
// @description  uniquely samples XKCDs when you press Random
// @author       numinit
// @match        http://xkcd.com/*
// @match        https://xkcd.com/*
// @grant        none
// ==/UserScript==

$(document).ready(function() {
    var setSeed = function(seed) {
        window.localStorage['xkcd_seed'] = seed;
        return seed;
    };
    
    var genSeed = function() {
        var array = new Uint32Array(1);
        window.crypto.getRandomValues(array);
        return array[0];
    };
    
    var getSeed = function() {
        var ret = window.parseInt(window.localStorage['xkcd_seed']);
        return window.isNaN(ret) ? genSeed() : ret;
    };
    
    var setMax = function(max) {
        window.localStorage['xkcd_max'] = max;
        return max;
    }
    
    var getMax = function() {
        var ret = window.parseInt(window.localStorage['xkcd_max']);
        return window.isNaN(ret) ? 0 : ret;
    }
    
    var iterate = function(seed, max) {
        var r = function(s, m) {
            return ((5 * s) + 1) % m;  
        };
        
        // compute the next-highest power of 2
        var m = (1 << (Math.ceil(Math.log2(max)) | 0)) | 0;
        
        // rejection sample
        seed = r(seed, m);
        while (seed >= max) {
            seed = r(seed, m);
        }
        return seed;
    };
    
    if (window.location.pathname === '/') {
        // I don't see a better way to do this... :(
        var match = (/permanent\s+link\s+to\s+this\s+comic:\s+https?:\/\/xkcd\.com\/(\d+)\//gi).exec($('#middleContainer').text());
        
        if (match && match[1]) {
            var seed = genSeed(), max = window.parseInt(match[1]);
 
            // store the max comic
            setMax(max);
            setSeed(seed);
            console.log('XKCDUniqueRandom: new session with max=' + max + ', seed=' + seed);
        } else {
            // We tried?
            setMax(0);
            console.log('XKCDUniqueRandom: couldn\'t find the max comic :(');
        }
    } else {
        var max = getMax(), seed = getSeed();
        if (max > 0) {
            console.log('XKCDUniqueRandom: continued session with max=' + max + ', seed=' + seed);
        } else {
            console.log('XKCDUniqueRandom: no max value yet; seed=' + seed);
        }
    }
    
    // bind a random handler
    $('a[href$="/random/comic/"]').click(function() {
        var seed = getSeed(), max = getMax();
        
        if (!max) {
            return true;
        } else {
            // find the next comic
            console.log('XKCDUniqueRandom: previous seed ' + seed);
            seed = iterate(seed, max);
            if (seed == 403) {
                // you aren't getting away that easily
                seed = iterate(seed, max);
            }
            
            // store the seed for next time
            setSeed(seed);
            
            // go somewhere else. seeds are in 0..max-1 so add 1 to get 1..max
            console.log('XKCDUniqueRandom: next seed ' + seed);
            window.location.pathname = '/' + (seed + 1);
            
            return false;
        }
    });
});