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modular arithmetic

mathematics (Or "clock arithmetic") A kind of integer

arithmetic that reduces all numbers to one of a fixed set

[0..N-1] (this would be "modulo N arithmetic") by effectively

repeatedly adding or subtracting N (the "modulus") until the

result is within this range.

The original mathematical usage considers only _equivalence_

modulo N. The numbers being compared can take any values,

what matters is whether they differ by a multiple of N.

Computing usage however, considers modulo to be an operator

that returns the remainder after integer division of its first

argument by its second.

Ordinary "clock arithmetic" is like modular arithmetic except

that the range is [1..12] whereas modulo 12 would be [0..11].