Rate limiting only using a single scalar value per user/actor

Rate Limiter.md

You can use this by importing the function into the console (or your app)

> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ true, 1635358774575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358774575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358774575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ true, 1635358775575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358775575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358775575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358775575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ true, 1635358776575 ]
> x = rateLimit(a, {rps: 1, grace: 2}); a = x[1]; x
[ false, 1635358776575 ]

ratelimiter.js

/**
 * @param {int} oldValue is the old ratelimiter value, or null
 * @param {config} has `rps` for the number of requests per second. `grace` is how many requests to allow bursting.
 * @returns {[success, int]} Boolean for successful request or false to limit. Also, new value for the ratelimiter to cache.
 */
export function rateLimit(oldValue, config) {
  const now = (new Date()).getTime(); // milliseconds of current time. Important that this is accurate!
  const reqMs = 1000 / config.rps; // equivelant request millisecond counter.
  if (oldValue != null && oldValue > now) {
    if (oldValue > now + (reqMs * config.grace)) {
      // Too many requests beyond burst grace
      return [false, oldValue];
    } else {
      // In grace, increment and allow the request
      return [true, (oldValue + reqMs)];
    }
  }
  // Haven't had a request in a while
  return [true, (now + reqMs)];
}