[falcolas]

Array size of 5 produces a mask of 0100

Write index of 8 = 1000

Masking those together to create a write position 1000 & 0100 = 0

Doesn't work out correctly, got 0, would expect to get 2. In fact, you could never get a write position of 1, 2, or 3 with an array size of 5.

[dllthomas]

This is not the problem.

You are assuming still using &, which is very obviously incorrect with a very obvious fix (%5) which is still wrong because of behavior at overflow.

[falcolas]

I'm answering the specific problem posed by the OP, given his other comments in this thread.

[EDIT] Resolved internal concerns about size calculations.

[dllthomas]

You might have misread the OP. He's not asking why using & for the mask requires a 2^n sized buffer. He's asking why bother using & when it imposes these additional constraints on us. A part of the answer is that the constraints are already there with this approach, even if you pick a different function for masking than f(i,n) = i & (n - 1) -- which point OP only recently understood due to a misreading of the article.