§Euclidean division

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Given two integers a and b, with b ≠ 0, there exist unique integers q and r such that
    a = bq + r
and
    0 ≤ r < |b|

In mathematics the result of the modulo operation is the remainder of the Euclidean division.

In C, the % operator is often mistakenly referred as modulo operator, but it in fact is the remainder operator, satisfying:

(a / b) * b + a % b = a

Haskell is a language exposes both modulo and remainder operator

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(x `quot` y)*y + (x `rem` y) == x
(x `div` y)*y + (x `mod` y) == x

If the divisor is of power of 2, a quick way calculating the modulo is to using & (b-1), which assumes two’s complement, but it’s ubiquitous anyway.

§Reference