In my Binary Decision Diagram Library, the performance bottleneck was the uniqueness cache. By reducing the number of cache lookup, it is possible to greatly improve the performances.

Common pattern

In order to test if the key is already in the cache, the usual pattern is to use key in cache. This leads to the following code:

function cachedValue(key) {
  if (key in cache) {
    return cache[key];
  } else {
    var result;
    /* generate result */
    return cache[key] = result;
  }
}

Ideal cache lookup

The expensive operation with a cache is to traverse the data structure in order to find the position where the element should be. In the previous code, there are three independent lookups: key in cache and 2x cache[key].

In order to improve performance, the ideal would be to make that expensive lookup once, and do the test/get/set directly at that position.

function cachedValue(key) {
  var position = cache.lookup(key);
  if (cache.has(position)) {
    return cache.get(position);
  } else {
    var result;
    /* generate result */
    cache.set(position, result);
  }
}

Javascript Workaround

However, there is no API to directly manipulate the lookup and position. We have to find workarounds.

We remark that cache[key] will either return the value or undefined if not present in the cache. The belonging test no longer requires a full lookup, you only have to compare the value with undefined.

function cachedValue(key) {
  var result = cache[key];
  if (result === undefined) {
    /* generate result */
    cache[key] = result;
  }
  return result;
}

There is only one drawback: it is no longer possible to use undefined as a value in your cache.

Without temporary

If the result can be generated inline and your cache will never contains falsy values, you can use a shorter version. It should not impact performance however.

function cachedValue(key) {
  return cache[key] || (cache[key] = /* generate result */);
}

Conclusion

There are still two lookups done in case of a cache miss. If you find a way to factor the code in a way that removes it, I'm really interested in knowing :)

At the time of writing, the described technique is faster on all the tested browsers. As performance in browser rapidly evolve, check out the jsPerf entry before you decide to use it in your application.

If you liked this article, you might be interested in my Twitter feed as well.
 
  • Animesh

    Would be better if you put a Bloom Filter to check for cache keys.

  • http://blog.vjeux.com/ Vjeux

    It depends. A bloom filter is good when check is very expensive such as data stored outside RAM. Here our cache is fully in RAM, therefore I believe that it would add additional computation that help.

 

Related Posts

  • September 24, 2011 Javascript: Cyclic Object Detection (4)
    URLON.stringify() suffer from a problem, when passed an object that contains a cycle, it will never stop. This article shows 3 techniques in order to detect if an object is cyclical. Edit the object: Mark In order to detect a cycle in an object, the method we learn at school is to mark each v...
  • September 25, 2011 Javascript Object Difference (5)
    This article is about a difference algorithm. It extracts changes from one version of an object to another. It helps storing a smaller amount of information. Template In a project, I have a template object with all the default settings for a widget. var template = { achievement: { ...
  • August 19, 2011 Javascript – Stupid Idea: Hoisting at the end (0)
    JSLint imposes us to do manual hoisting of variables. What if we did it but at the end of the function? :P How you write function print_array (array) { var length = array.length; for (var i = 0; i < length; ++i) { var elem = array[i]; console.log(elem); } } How ...
  • April 5, 2012 Climb – Property-based dispatch in functional languages (1)
    ELS Presentation | A Generic and Dynamic Approach to Image Processing | Chaining Operators & Component Trees | Property-based dispatch in functional languages This is the third (and last) presentation about my work on Climb at the LRDE. During the first one I tackled genericity on data s...
  • October 8, 2011 Copy SQL Row Changing ID (2)
    I've come across an SQL issue. I need to make a fake spell for the WoW database. However creating one from scratch is too annoying, there are like 30 non-NULL values to fill. Instead what I want is to copy an existing spell with the new id. It appeared to be less trivial than expected. Working...