using Printf
import Base.GMP.MPZ: add!, sub!, mul!, fdiv_r!, gcd!

# single-line comments

#=
multi-line comments
=#

# https://stackoverflow.com/a/69000702
function pollard_rho(n::BigInt)::Union{Some{BigInt}, Nothing}
    x, y, d = BigInt(2), BigInt(2), BigInt(1)

    while d == big"1"
        mul!(x, x); add!(x, big"1"); fdiv_r!(x, n)
        mul!(y, y); add!(y, big"1"); fdiv_r!(y, n)
        mul!(y, y); add!(y, big"1"); fdiv_r!(y, n)
        sub!(d, x, y); gcd!(d, abs(d), n)
    end

    if d != n
        return Some(d)
    else
        return nothing
    end
end

if abspath(PROGRAM_FILE) == @__FILE__
    n = parse(BigInt, ARGS[1])
    result = pollard_rho(n)
    try
        p = something(result)
        @printf "%d = %d * %d\n" n p (n÷p)
    catch e
        @printf "%d is prime\n" n
    end
end