Every time I use a static method I feel embarassed, like I'm doing something shameful. Or when I see a Pull-Request with a bunch of static methods I think "Why? Why did you have to do it this way?". I dont want to push my beliefs on the others, so I don't decline those PRs. At least they get the work done. However I feel it's time to express why do I hate static methods.

To begin with, a static method is a class method, that can be invoked without instantiating a class. The most common usage I've seen are helper classes (oh my god I do also hate helper classes) and singletones. Let's look at those helper classes from another point of view. We have some set of methods, that, let's say, format a string in different ways. So we have

class Helper
{
   public static formatA() {...}
   public static formatB() {...}
   public static formatC() {...}
}

The thing is, it has absolutely no difference from:

function helper_formatA() {...}
function helper_formatB() {...}
function helper_formatC() {...}

Why would you bother writing this within a class instead of just a set of functions? Is it because you think you are using Object-Oriented Programming and writing class keyword makes your code better? I feel, like using static methods this way is a step away from the ideas of the OOP. This is not an instance of a class anymore.

So we have used this static method somewhere in the code:

// .. within some class
public function foo()
{
    // ...
    Helper::formatA('bar');
}

our module is now dependent on that static class. Can you tell by looking at the class API that it has that dependency? No you don't. It's a hidden dependency that can hit you when you don't expect it. If you extract your class into some library for common usage, you can suddenly find out, that you have to include Helper too. Moreover, this is a tight dependency, you can not substitute a call to this class by some other helper. And the more you use those static methods, the more dependencies you have in your project, that will entangle it like a spider web.

A singletone is absolutely no better. But how do I ensure, that I have just a single connection to my DB, you may ask? Well, you can instantiate it once at the bootstrapping, put it in the service container and reuse it. True, you will end up with more code that before, but if you want your project to be flexible you have to accept abstraction costs.

The same goes for the Helper Formatter example. Take an advantage of passing an instance of a Formatter class as a dependency injection. Use an interface as a type hinting to make it loosely coupled and be easily replacable. And then your method will depend on an abstraction rather than a specific object.

Usage of static methods can probably be explained somehow, the same way as a usage of goto. I used goto once, because of the PHP 'continue 2' memory leak bug. But under normal circustumstances I don't think there is a justification of using a static method.

Project Euler 502 11.04.2015

Hello everyone, I've decided to continue writing posts into my blog, when I deal with something interesting. This time my attention has been drawn to Project Euler problems. I tend to refer to those problems when I'm learning a new programming language. Solving easier problems help to get handy with the syntax and language constructs. So it's Haskell now. I'm not going to tell how Haskell is different from anything I've learned before (Coursera Scala course was the closest), so i'll just proceed with the Problem 502, which I found really intriguing.

The problem itself can be found here. Brief comprehension is: how many castles can you build, that follow the rules: no hanging blocks, total amount of blocks is even. I am currently at the first approach stage. The first approach is usually naive: generate every possible castle, check the validity and calculate how many there are.

But there are several concepts I used, which I find quite interesting. A castle consists of levels, each level consists of blocks and spaces between them. If you encode blocks and spaces as 0's and 1's you get a binary string, which you can convert to decimal. So, a castle is a just list of integers.

The next thing is, how do we generate a level on top of the previus level. And the answer is again, quite simple. If a level has a width of 3, then there are 8 possible combinations of blocks and spaces, which are also numbers from 0 (no blocks, only spaces) to 7 (one block across the whole level). So on the 2x2 grid there are: 1 for the base level + 3 possible combinations (0, 1, 2) of the second level = 4 castles. At this point there will also be invalid castles (with hanging blocks), but we will filter them out later. On a WxH grid we will have 1 + (2 ^ W - 1) ^ (H - 1) castles. As a simple example in the task it is given a 10x13 grid, which would produce ~1.65*10^35 possible castles (Eeek).

The next idea, which will cut the number of computations by a huge amount is a check, if there are any hanging blocks. Because if there is at least one, we can skip this "branch". How do we do it? Let's see, if a level is a binary number, the next level may not be bigger, than that number, because otherwise most significant bit will hover over 0 of the underlying level, which is not allowed. Moreover, every 1 of the new level must have an underlying 1 from the previous level - bitwise AND. (New level) AND (Old level) must be equal to the (New level), otherwise new level is invalid.

So this is basically it, I start with a base level, add numbers from 0 to (2 ^ W) - 1, if they AND with the previous level, and after all I calculate amonut of blocks by grouping them.

import Data.List
import Data.Bits

{-|
  Decimal to binary conversion
 -}
binary :: Integer -> [Integer]
binary 0 = [0]
binary 1 = [1]
binary x = binary (div x 2) ++ [mod x 2]

{-|
  Naive implementation of Euler Problem 502. Generate all castles, check validity, count
-}
castles :: Integer -> Integer -> Integer
castles w h = genericLength [c | c <- (allCastles w h), (even . sum . (map (countBlocks))) c]

{-|
  Generates all possible castles, even invalid ones. Each level is introduced
  with a number, which in binary form represents blocks
-}
allCastles :: Integer -> Integer -> [[Integer]]
allCastles _ 0 = []
allCastles 0 _ = []
allCastles w h = addLevel [[base]] (h-1)
  where base = (2 ^ w)-1
        addLevel castles levelsLeft
            | levelsLeft == 0 = castles
            | otherwise = addLevel ([c ++ [p] | c <- castles, p <- [0..(last c)], (p == p .&. (last c))]) (levelsLeft-1)

{-|
  Counts amount of blocks within a castle row
-}
countBlocks :: Integer -> Integer
countBlocks = genericLength . (filter ((== 1).head)) . group . binary

This is by no means an optimal solution, I couldn't even compute a 10x13 grid from the simpler example within a reasonable amonut of time. The biggest so far was a 9x7 grid for 28+ millions of castles.

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