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SUBROUTINE ZFFTI1 (N,WA,IFAC)
IMPLICIT DOUBLE PRECISION (A-H,O-Z)
DIMENSION WA(*) ,IFAC(*) ,NTRYH(4)
DATA NTRYH(1),NTRYH(2),NTRYH(3),NTRYH(4)/3,4,2,5/
NL = N
NF = 0
J = 0
101 J = J+1
IF (J.le.4) GO TO 102
GO TO 103
102 NTRY = NTRYH(J)
GO TO 104
103 NTRY = NTRY+2
104 NQ = NL/NTRY
NR = NL-NTRY*NQ
IF (NR.eq.0) GO TO 105
GO TO 101
105 NF = NF+1
IFAC(NF+2) = NTRY
NL = NQ
IF (NTRY .NE. 2) GO TO 107
IF (NF .EQ. 1) GO TO 107
DO 106 I=2,NF
IB = NF-I+2
IFAC(IB+2) = IFAC(IB+1)
106 CONTINUE
IFAC(3) = 2
107 IF (NL .NE. 1) GO TO 104
IFAC(1) = N
IFAC(2) = NF
TPI = 6.28318530717958647692D0
ARGH = TPI/FLOAT(N)
I = 2
L1 = 1
DO 110 K1=1,NF
IP = IFAC(K1+2)
LD = 0
L2 = L1*IP
IDO = N/L2
IDOT = IDO+IDO+2
IPM = IP-1
DO 109 J=1,IPM
I1 = I
WA(I-1) = 1.0D0
WA(I) = 0.0D0
LD = LD+L1
FI = 0.0D0
ARGLD = FLOAT(LD)*ARGH
DO 108 II=4,IDOT,2
I = I+2
FI = FI+1.D0
ARG = FI*ARGLD
WA(I-1) = COS(ARG)
WA(I) = SIN(ARG)
108 CONTINUE
IF (IP .LE. 5) GO TO 109
WA(I1-1) = WA(I-1)
WA(I1) = WA(I)
109 CONTINUE
L1 = L2
110 CONTINUE
RETURN
END
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