[marshray]

It's not the -4 that's weird, it's the modulus and remainder with floating point arithmetic.

[andrewcooke]

agree it's odd, but it's basically 100 * (len - 4) % 121 = 85

(I think?)

(yes, I know about rounding, fractions, different bases - I was just trying to explain why it still worked. sigh.)

[marshray]

No, actually they're different. http://floating-point-gui.de/ http://web.cse.msu.edu/~cse320/Documents/FloatingPoint.pdf

If you know enough about floating point to prove that the rounding will always give you the correct answer, then probably you would have just used the integer representation to begin with.