Implementing Memoize in Delphi 2009 – Community Blogs – Embarcadero Community

I think it was Stefan Glienke who pointed me at the Memoisation pattern: [WayBack] Implementing Memoize in Delphi 2009 – Community Blogs – Embarcadero Community

Or in laymans terms: caching with generic functions and dictionaries.

Note the Archive.is link is way better readable than the WayBack link, so below I’ve saved parts only in Archive.is.

It points to two other interesting reads:

• a series on Map/Reduce in Delphi using generics:
  • [Archive.is] Building a Generic Statistics Library, Part 1: Interface – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 2: Unit Tests – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 4: Map – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 4: Map – Community Blogs – Embarcadero Community
  • [Archive.is] Building a Generic Statistics Library, Part 5: Implementation – Community Blogs – Embarcadero Community
• a benchmark class I already had forgotten about which is very useful in Unit Testing (or perhaps better phrased as Integration Testing) of algorithm speeds: [WayBack] Entropy Overload: Anonymous methods in testing / profiling situations written by Barry Kelly (ex Delphi compiler guru)

Later I found it was indeed Stefan who pointed me to the above links and also came up with this very small Memoisation example which is based on Spring4D:

and guess what? because we have interfaced based dictionary this is super easy, let me just hack an example, 5mins6:52 PM

function BuildCache: Tfunc<string,string>;
var
 cache: IDictionary<string,string>;
begin
 cache := TCollections.CreateDictionary<string,string>;
 Result :=
   function(s: string): string
   begin
     if not cache.TryGetValue(s, Result) then
     begin
       Sleep(1000); // hard work! ;)
       Result := ReverseString(s);
       cache.Add(s, Result);
     end;
   end;
end;

Since the returned anonymous method captures the dictionary its kept inside and once the anonymous method goes out of scope the dictionary is also cleaned, want to reset the cache? Just create a new one and throw the old one away.

Need thread safety? just add

System.TMonitoy.Enter/Leave(cache.AsObject);

–jeroen