Below is the Fortran code for a simple Decimation-in-Frequency, Radix-2, one butterfly Cooley-Tukey FFT followed by a bit-reversing unscrambler.
C
C A COOLEY-TUKEY RADIX-2, DIF FFT PROGRAM
C COMPLEX INPUT DATA IN ARRAYS X AND Y
C C. S. BURRUS, RICE UNIVERSITY, SEPT 1983
C---------------------------------------------------------
SUBROUTINE FFT (X,Y,N,M)
REAL X(1), Y(1)
C--------------MAIN FFT LOOPS-----------------------------
C
N2 = N
DO 10 K = 1, M
N1 = N2
N2 = N2/2
E = 6.283185307179586/N1
A = 0
DO 20 J = 1, N2
C = COS (A)
S = SIN (A)
A = J*E
DO 30 I = J, N, N1
L = I + N2
XT = X(I) - X(L)
X(I) = X(I) + X(L)
YT = Y(I) - Y(L)
Y(I) = Y(I) + Y(L)
X(L) = C*XT + S*YT
Y(L) = C*YT - S*XT
30 CONTINUE
20 CONTINUE
10 CONTINUE
C
C------------DIGIT REVERSE COUNTER-----------------
100 J = 1
N1 = N - 1
DO 104 I=1, N1
IF (I.GE.J) GOXTO 101
XT = X(J)
X(J) = X(I)
X(I) = XT
XT = Y(J)
Y(J) = Y(I)
Y(I) = XT
101 K = N/2
102 IF (K.GE.J) GOTO 103
J = J - K
K = K/2
GOTO 102
103 J = J + K
104 CONTINUE
RETURN
END
Figure: Radix-2, DIF, One Butterfly Cooley-Turkey FFT
