Coprimes drill

# Co-prime drill

function gcd(x::Integer, y::Integer)
    while (x > 0) && (y > 0)
        if x < y
            y = y % x
        else
            x = x % y
        end
    end

    return max(x, y)
end

AreCoprime(x::Integer, y::Integer) = (gcd(x, y) == 1)

function main(n_max::Integer)
    x = 9 # n = 0
    y = 10 # n = 1

    for n = BigInt.(2:n_max)
        n % 50000 == 0 && println(n)
        x = y
        y = n^17 + 9

        if !AreCoprime(x, y)
            println("Rule breaks at n = ", n, " as ", x, " and ", y, " are not coprime!")
            return
        end
    end

    println("\nAll cases up to n = ", n_max, " were coprimes...")
end