Allow incrementing by other than 1

README.md

@@ -137,6 +137,20 @@ rate: '100/h'
 
 *Note:* the rate is parsed ahead of time, so this notation doesn't affect performance.
 
+### `incr`
+
+How much to increment the usage for a given call. This is useful in scenarios where incrementing by
+one is not the whole story.  For example, for rate limiting computation time.  Some operations take
+longer than other so counting operations doesn't work.  Rather, use `incr` to track and limit the
+actual compute time.
+
+You would typically specify a custom function:
+
+```js
+// rate-limit based on the cost of an operation
+key: function(x) { return x.cost; }
+```
+
 ## HTTP middleware
 
 This package  contains a pre-built middleware,

lib/options.js

@@ -22,7 +22,12 @@ var getLimit = function(opts){
   return opts.limit;
 };
 
-var getWindow = function(opts){
+var getIncr = function (opts) {
+  if(typeof opts.incr === 'function') return opts.incr;
+  return function () { return opts.incr || 1; };
+};
+
+var getWindow = function (opts) {
   if(getRate(opts)) return moment.duration(1, getMatches(opts)[2]) / 1000;
   if(typeof opts.window === 'function') return opts.window();
   return opts.window;
@@ -38,6 +43,7 @@ var validate = function(opts){
   assert.equal(typeof getKey(opts), 'function', 'Invalid key: ' + opts.key);
   if(opts.rate) assert.ok(getMatches(opts), 'Invalid rate: ' + getRate(opts));
   assert.equal(typeof getLimit(opts), 'number', 'Invalid limit: ' + getLimit(opts));
+  assert.equal(typeof getIncr(opts), 'function', 'Invalid incr: ' + opts.incr);
   assert.equal(typeof getWindow(opts), 'number', 'Invalid window: ' + getWindow(opts));
   assert.notEqual(getLimit(opts), 0,  'Invalid rate limit: ' + getRate(opts));
   assert.notEqual(getWindow(opts), 0, 'Invalid rate window: ' + getRate(opts));
@@ -50,6 +56,7 @@ canonical = function(opts) {
     key: getKey(opts),
     rate: getRate.bind(null, opts),
     limit: getLimit.bind(null, opts),
+    incr: getIncr(opts),
     window: getWindow.bind(null, opts)
   };
 };

lib/rate-limiter.js

@@ -4,12 +4,13 @@ module.exports = function(opts) {
   opts = options.canonical(opts);
   return function(request, callback) {
     var key = opts.key(request);
+    var incr = opts.incr(request);
     var tempKey = 'ratelimittemp:' + key;
     var realKey = 'ratelimit:' + key;
     opts.redis.multi()
          .setex(tempKey, opts.window(), 0)
          .renamenx(tempKey, realKey)
-         .incr(realKey)
+         .incrby(realKey, incr)
          .ttl(realKey)
          .exec(function(err, results) {
            if(err) {