I cannot give an algorithm how to calculate the remainder.
Even less do I know how to generalize it to full Complex.
Since one wants the absolute value of the remainder less
than the absolute value of the divisor the float result
is *rounded* in the real and imaginary components separately.
That is the rmod definition as outlined in my other mail in
this thread. The fact that a remainder is negative for positive
dividend and divisor is moot for complex numbers with their
continuous "sign" in the range 0..2*pi in the polar representation.