Question

I’m implementing a cache on a function that gets called hundreds of millions of times. Cache size is tens of millions of items.
It is currently implemented using a Dictionary, and lookups in it take a significant amount of time.

Is it possible to get a reference to the whole pair in the Dictionary, not just the value, so I can check if a value exists, check it (and, possibly, update it) if it does using a single lookup?

Currently, I have something like this:

int val;
if (cache.TryGetValue(key, out val))
    if (val < newVal) cache[key] = newVal;
    else return val;
else
    cache.Add(key, newVal);

I would like to get this:

Pair pair = cache.GetPair(key);
if (pair != null)
    if (pair.Value < newVal) pair.Value = newVal;
    else return pair.Value;
else
    cache.Add(key, newVal);

If there is an alternative data structure that allows this, I’d be glad to hear about it too.

Thanks in advance!

Answer 1

This is inspired by Mare Infinitus’ answer. Assuming your cache variable is now a Dictionary<string, int> you might change it into a Dictionary<string, MutableInt32> where MutableInt32 is written like this:

// wraps an int that may change
class MutableInt32
{
  public int Value;
}

Then you could change your code to

MutableInt32 val;
if (cache.TryGetValue(key, out val))
  if (val.Value < newVal) val.Value = newVal;
  else ...