40 byte recursive-based version of the Euclidean algorithm. Euclid's algorithm is used to find the greatest common divisor of 2 numbers (the largest number that divides both of them without leaving a remainder). The Euclidean algorithm is based on the principle that the greatest common divisor of two numbers does not change if the smaller number is subtracted from the larger number.
For illustration, the gcd(1071, 462) is calculated from the equivalent gcd(462, 1071 % 462) = gcd(462, 147). The latter GCD is calculated from the gcd(147, 462 % 147) = gcd(147, 21), which in turn is calculated from the gcd(21, 147 % 21) = gcd(21, 0) = 21.