Tuesday, September 02, 2008

Arrays to the rescue!

Some time ago I blogged about Haskell Generics and how they simplified my code of a little RPG game I'm writing. My data types were designed with lists. For example a character has traits, and traits are a list of Characteristic objects, that have a name (Strength, Dexterity, etc.) and values. The Generics code helped me to deal with finding the right item in the list depending on the name, updating its value, etc. Records were no good because you can pass the generated function as an accessor but not as a mutator.

I have some unit tests that generate random characters, and I thought they were pretty slow, which I thought was due to the random number generator. Then I ran the tests with profiling on. Guess what? The Generics code amounted for a huge amount of the time spent. And I'd say the underlying list operation, with loads of list updates, were not great either.

So I got rid of the Generics code, and I decided to replace my lists by something else. In the Java world (my day time job) I rely heavily on Maps. I thought it was a bit overkill in this case, so I turned to the Haskell array package. And there was light!

You see, Arrays in Haskell are a different beast that arrays in other languages. You can use different types for the indices, not only integers. So since the data type representing the names of characteristics is a simple enumeration, I could use that as the array index. So the characters traits are an array with the names as indices:

data Characteristic = Strength | Dexterity | ...
deriving (Show,Read,Eq,Enum,Bounded,Ord,Ix)

Ix is the type class that an array index type must implement

type CharacteristicRatings=Array Characteristic Rating

data Character = Character {

And Rating follow the same pattern, since a rating has the current value for the characteristic, the normal value, etc.

Then the array operations make it easy enough to read and modify values: ! to get the value at the specified index, // to update. I get type safety and expressiveness (no need to even use toEnum and fromEnum), and performance...

Performance? With the Generics code and lists, my tests took 8 seconds. With arrays and no Generics, 0.26s. Nuff said.