GF Ring: implement butterfly operations
GF Ring: implement butterfly operations

Two butterfly operations are implemented:

1. Butterfly computation for Cooley-Tukey FFT algorithm

Perform in-place oprations on two buffers `P`, `Q` with a coefficient `c`

P[i] = P[i] + c \times Q[i]
Q[i] = P[i] - c \times Q[i]

2. Butterfly computation for Gentleman-Sande FFT algorithm

Perform in-place oprations on two buffers `P`, `Q` with a coefficient `c`

P[i] = P[i] + Q[i]
Q[i] = c \times (P[i] - Q[i])

vrancurel/ntl,vrancurel/ntl,vrancurel/ntl