How would you avoid context/class explosion in this case with MSpec? - mspec

Question

I love mspec. It is great for providing key examples in way that is easy to communicate with non technical people but sometimes I find it provides an unnecessary verbosity, specifically an explosion of classes.
Take the following example.
I want to model the movement of a knight piece in chess. Assuming the knight is not near any other piece or the boundaries of the board there are 8 possible moves that knight can have I want to cover each of these possibilities but to be frank I am too lazy to write 8 separate specifications (8 classes). I know that I can be clever with behaviours and inheritance but as I want to cover the 8 valid moves I cannot see how I can do it with out 8 becauses so therefore 8 separate classes.
What is the best way to cover these scenarios with mspec?
Some code.
public class Knight
{
public string Position {get; private set;}
public Knight(string startposition)
{
Position = startposition;
}
public void Move
{
// some logic in here that allows a valid move pattern and sets positions
}
}
What I might do.
[Subject(typeof(Knight),"Valid movement")]
public class when_moving_the_knight
{
Establish that = () => knight =new Knight("D4");
Because of = ()=> knight.Move("B3");
It should_update_position = ()=> knight.Position.ShouldEqual("B3");
It should_not_throw;
/// etc..
}
But not 8 times.

Answer 1

Honestly, I couldn't tell you the best way to do that in MSpec. But I've experienced a similar class explosion problem with MSpec when using it in similar circumstances. I don't know if you have ever tried RSpec. In RSpec contexts and specifications are built up within the confines of executable code. What that means is you can create a data structure, iterate on it, and create several contexts and specs using one block of code. This becomes especially handy when you are trying to specify how something based in mathematics behaves (prime factors,tic tac toe, chess, etc...). A single pattern of behavior can be specified across each member of a set of given and expected values.
This example is written in NSpec, a context/spec framework for C# modeled after RSpec. I purposefully left a failing spec. I just went down this kata far enough to find a place to use iteration. The failing spec forces you to resolve the shortcomings of the naive implementation.
Here's another example of prime factor kata: http://nspec.org/#dolambda
Output:
describe Knight
when moving 2 back and 1 left
when a knight at D4 is moved to B3
knight position should be B3
when a knight at C4 is moved to A3
knight position should be A3 - FAILED - String lengths are both 2. Strings differ at index 0., Expected: "A3", But was: "B3", -----------^
**** FAILURES ****
describe Knight. when moving 2 back and 1 left. when a knight at C4 is moved to A3. knight position should be A3.
String lengths are both 2. Strings differ at index 0., Expected: "A3", But was: "B3", -----------^
at ChessSpecs.describe_Knight.<>c__DisplayClass5.<when_moving_2_back_and_1_left>b__4() in c:\Users\matt\Documents\Visual Studio 2010\Projects\ChessSpecs\ChessSpecs\describe_Knight.cs:line 23
2 Examples, 1 Failed, 0 Pending
Code:
using System.Collections.Generic;
using NSpec;
class describe_Knight : nspec
{
void when_moving_2_back_and_1_left()
{
new Each<string,string> {
{"D4", "B3"},
{"C4", "A3"},
}.Do( (start, moveTo) =>
{
context["when a knight at {0} is moved to {1}".With(start,moveTo)] = () =>
{
before = () =>
{
knight = new Knight(start);
knight.Move(moveTo);
};
it["knight position should be {0}".With(moveTo)] = () => knight.Position.should_be(moveTo);
};
});
}
Knight knight;
}
class Knight
{
public Knight(string position)
{
Position = position;
}
public void Move(string position)
{
Position = "B3";
}
public string Position { get; set; }
}

Answer 2

Just use the Its the way you want to. It should be able to move from here to there, it should be able to move from here(2) to there(2), etc. Very common pattern in rspec but not so much in MSpec because it's generally overused so no one ever talks about it for fear of guiding the wrong way. This is a great spot to use this though. You're describing the behavior of the Knight's moving.
You can describe it even better by being more specific in your Its. It should be able to move up two and to the right one, it should be able to move up two and to the left one. It should not be able to move onto a friendly piece, etc.
Yes, you'll need to put more than one line of code in your It, but that's OK. At least in my opinion.

Answer 3

From what I see your design states that the Knight would throw an exception if moved in an invalid position. In this case I think that your method has two different responsibilities, one for checking a valid move and the other for doing the correct move or throwing. I would suggest to split your method into the two distinct responsibilities.
For this specific case I would extract a method for checking whether a move is valid or not, and then calling it from your move method. Something like that:
public class Knight
{
internal bool CanMove(string position)
{
// Positioning logic here which returns true or false
}
public void Move(string position)
{
if(CanMove(position))
// Actual code for move
else
// Throw an exception or whatever
}
}
this way you could test the logic inside CanMove for testing valid positions for a given Knight (which you can do with a single test class and different "It"s), then make just one test for the Move method to see if it fails when given an invalid position.