Rate Limiter in Javascript

README.md

How to use

Let's say you want to limit some event to 5 occurrences every 20 seconds, you would set up the RateLimit like so:

var limit = new RateLimit(20); // Initialize a 20 second ticklog

// For events that we initiate...
if (limit.count('myevent_id') <= 5)) {
    // Do some stuff
    limit.tick('myevent_id');
} else {
    // Over the limit!
}

// Or events that we respond to...
if (limit.count('someevent_id') > 5) {
    // Respond with failure
} else {
    // Respond with success!
}

Technical details

You could run this forever and ever and the ticklog would remain the same size because it's a fixed length list. As we add new entries to the list, the index wraps around to the beginning (i.e. index % ticklog.length). Every call to count iterates over this fixed length list and performs a dictionary look up for tick count indexed by id.

ratelimit.js

/**
 * Constructor for RateLimit objects
 * @constructor
 * @param {number} [history=5] The size of the tick history
 */
function RateLimit(history) {
    if (!(this instanceof RateLimit)) return new RateLimit(history);
    history = history || 5;

    /**
     * The time this RateLimit object was created
     * @type number
     */
    this.created = parseInt(new Date().getTime() / 1000);
    /**
     * Time of last tick
     * @type number
     */
    this.lasttick = 0;
    /**
     * Stores a bunch of ticks
     * @type array
     */
    this.ticklog = [];

    for (var i = 0; i < history; i++) {
        this.ticklog.push([0, {}]);
    }
}
RateLimit.prototype = {
    /**
     * Checks if the occurrences for this id are within the limit and if true,
     * does a tick and returns true, otherwise returns false.
     * @param id Any kind of identifier
     * @param {number} limit The max occurrences for this identifier
     * @param {number} [now] The time right now
     */
    tick: function(id, now) {
        if (!id) return;
        now = now || parseInt(new Date().getTime() / 1000) - this.created;
        this.lasttick = now;
        var index = now % this.ticklog.length;
        var entry = (this.ticklog[index] &&
                     this.ticklog[index][0] == now)
            ? this.ticklog[index]
            : this.ticklog[index] = [now, {}];
        var timestamp = entry[0],
            tickcount = entry[1];
        tickcount[id] = tickcount[id] ? tickcount[id] + 1: 1;
        return;
    },
    /**
     * Checks if the occurrences for this id are within the limit and if true,
     * does a tick and returns true, otherwise returns false.
     * @param id Any kind of identifier
     * @param {number} limit The max occurrences for this identifier
     * @param {number} [now] The time right now
     * @return {number} The number of ticks by a given id
     */
    count: function(id, now) {
        if (!id) return;
        now = now || parseInt(new Date().getTime() / 1000) - this.created;
        var count = 0;
        for (var i = 0; i < this.ticklog.length; i++) {
            var timestamp = this.ticklog[i][0],
                tickcount = this.ticklog[i][1];
            if (now < timestamp ||
                now - timestamp >= this.ticklog.length) {
                continue;
            }
            if (tickcount[id]) {
                count += tickcount[id];
            }
        }
        return count;
    },
    /**
     * Checks if the occurrences for this id are within the limit and if true,
     * does a tick and returns true, otherwise returns false.
     * @param id Any kind of identifier
     * @param {number} limit The max occurrences for this identifier
     * @param {number} [now] The time right now
     * @return {boolean} True if within the limit
     */
    execute: function(id, limit, now) {
        if (!id) return;
        if (!limit) return;
        now = now || parseInt(new Date().getTime() / 1000) - this.created;
        var success = this.count(id, now) < limit;
        if (success) {
            this.tick(id, now);
        }
        return success;
    }
};