## 22 January 2006

### The hard piece - solved

What can I do as a kata tonight? Oh yes, the unsolved problem from this week's Dojo, of course!

Well, it was not unsolvable at all, you just have to cool down and try to write down things progressively.

The solution

The idea I had that evening is that, finding the position of the upper left corner of the ith subgrid in a grid, should be as difficult as finding the position of the ith square in a grid. Make as if the big grid is a small one with subgrids in each square.

So here are the formulas (perhaps it is clearer than my previous "explaining" phrase?):

# returns a square position (x, y) for the upper left corner
# of the "grid_nb"th subgrid in a grid
def get_subgrid grid_nb

square = Grid.new(@width / @sub_width, @height / @sub_height).get_square grid_nb
x = 1 + @sub_width*(square.x_pos - 1)
y = 1 + @sub_height*(square.y_pos - 1)
end

and

# returns a square position (x, y) for the "square_nb"th square in a grid
def get_square square_nb
x = 1 + (square_nb - 1)% @width
y = 1 + (square_nb - 1)/ @width
Square.new x, y
end

OK, and for the crazy ones, the formula that precisely calculates the position of the upper left corner of the "grid_nb" subgrid in a (w * w) grid with (subw*subh) subgrids (for instance, you can have a 6x6 grid with 6 2x3 subgrids):

1 + subw*((1 + (grid_nb - 1)% ((w / subw)) - 1) + w * (1 + subh*((1 + (grid_nb - 1)/ w) - 1))

No wonder why we couldn't get it rigth at once! Indeed, it took me slightly more than one hour to do that with 5 tests and 31 assertions in Ruby.

TDD as an unconscious habit

Thinking about the discussion I had about TDD after the Dojo, I feel that TDD should really become an unconscious skill. You have to train yourself consciously then to master it without thinking about it. You know the classical circle for learning:
1. Something I don't know I don't know - unconscious lack of skill
2. Something I know I don't know - conscious lack of skill
3. Somethind I know I know - conscious knowledge
4. Something I don't know I know - unconscious knowledge
Regarding TDD, the specific thing that should become inconscious is the exact rythm of what you are doing:
• either defining test cases - green bar
• either implementing test cases - green bar
• either implementing code to pass the test cases - red bar
• either refactoring code - green bar
• either refactoring test cases - green bar
Practising the Dojo with an attentive animator or practising pair programming with a merciless mate is a great way to get the habit.