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SUBROUTINE FOURT(DATA,NN,NDIM,ISIGN,IFORM,WORK)
implicit none
c
c
c the cooley-tukey fast fourier transform in usasi basic fortran
c
c transform(j1,j2,,,,) = sum(data(i1,i2,,,,)*w1**((i1-1)*(j1-1))
c *w2**((i2-1)*(j2-1))*,,,),
c where i1 and j1 run from 1 to nn(1) and w1=exp(isign*2*pi=
c sqrt(-1)/nn(1)), etc. there is no limit on the dimensionality
c (number of subscripts) of the data array. if an inverse
c transform (isign=+1) is performed upon an array of transformed
c (isign=-1) data, the original data will reappear.
c multiplied by nn(1)*nn(2)*,,, the array of input data must be
c in complex format. however, if all imaginary parts are zero (i.e.
c the data are disguised real) running time is cut up to forty per-
c cent. (for fastest transform of real data, nn(1) should be even.)
c the transform values are always complex and are returned in the
c original array of data, replacing the input data. the length
c of each dimension of the data array may be any integer. the
c program runs faster on composite integers than on primes, and is
c particularly fast on numbers rich in factors of two.
c
c timing is in fact given by the following formula. let ntot be the
c total number of points (real or complex) in the data array, that
c is, ntot=nn(1)*nn(2)*... decompose ntot into its prime factors,
c such as 2**k2 * 3**k3 * 5**k5 * ... let sum2 be the sum of all
c the factors of two in ntot, that is, sum2 = 2*k2. let sumf be
c the sum of all other factors of ntot, that is, sumf = 3*k3*5*k5*..
c the time taken by a multidimensional transform on these ntot data
c is t = t0 + ntot*(t1+t2*sum2+t3*sumf). on the cdc 3300 (floating
c point add time = six microseconds), t = 3000 + ntot*(600+40*sum2+
c 175*sumf) microseconds on complex data.
c
c implementation of the definition by summation will run in a time
c proportional to ntot*(nn(1)+nn(2)+...). for highly composite ntot
c the savings offered by this program can be dramatic. a one-dimen-
c sional array 4000 in length will be transformed in 4000*(600+
c 40*(2+2+2+2+2)+175*(5+5+5)) = 14.5 seconds versus about 4000*
c 4000*175 = 2800 seconds for the straightforward technique.
c
c the fast fourier transform places three restrictions upon the
c data.
c 1. the number of input data and the number of transform values
c must be the same.
c 2. both the input data and the transform values must represent
c equispaced points in their respective domains of time and
c frequency. calling these spacings deltat and deltaf, it must be
c true that deltaf=2*pi/(nn(i)*deltat). of course, deltat need not
c be the same for every dimension.
c 3. conceptually at least, the input data and the transform output
c represent single cycles of periodic functions.
c
c the calling sequence is--
c call fourt(data,nn,ndim,isign,iform,work)
c
c data is the array used to hold the real and imaginary parts
c of the data on input and the transform values on output. it
c is a multidimensional floating point array, with the real and
c imaginary parts of a datum stored immediately adjacent in storage
c (such as fortran iv places them). normal fortran ordering is
c expected, the first subscript changing fastest. the dimensions
c are given in the integer array nn, of length ndim. isign is -1
c to indicate a forward transform (exponential sign is -) and +1
c for an inverse transform (sign is +). iform is +1 if the data are
c complex, 0 if the data are real. if it is 0, the imaginary
c parts of the data must be set to zero. as explained above, the
c transform values are always complex and are stored in array data.
c work is an array used for working storage. it is floating point
c real, one dimensional of length equal to twice the largest array
c dimension nn(i) that is not a power of two. if all nn(i) are
c powers of two, it is not needed and may be replaced by zero in the
c calling sequence. thus, for a one-dimensional array, nn(1) odd,
c work occupies as many storage locations as data. if supplied,
c work must not be the same array as data. all subscripts of all
c arrays begin at one.
c
c example 1. three-dimensional forward fourier transform of a
c complex array dimensioned 32 by 25 by 13 in fortran iv.
c dimension data(32,25,13),work(50),nn(3)
c complex data
c data nn/32,25,13/
c do 1 i=1,32
c do 1 j=1,25
c do 1 k=1,13
c 1 data(i,j,k)=complex value
c call fourt(data,nn,3,-1,1,work)
c
c example 2. one-dimensional forward transform of a real array of
c length 64 in fortran ii,
c dimension data(2,64)
c do 2 i=1,64
c data(1,i)=real part
c 2 data(2,i)=0.
c call fourt(data,64,1,-1,0,0)
c
c there are no error messages or error halts in this program. the
c program returns immediately if ndim or any nn(i) is less than one.
c
c program by norman brenner from the basic program by charles
c rader, june 1967. the idea for the digit reversal was
c suggested by ralph alter.
c
c this is the fastest and most versatile version of the fft known
c to the author. a program called four2 is available that also
c performs the fast fourier transform and is written in usasi basic
c fortran. it is about one third as long and restricts the
c dimensions of the input array (which must be complex) to be powers
c of two. another program, called four1, is one tenth as long and
c runs two thirds as fast on a one-dimensional complex array whose
c length is a power of two.
c
c reference--
c ieee audio transactions (june 1967), special issue on the fft.
c
C .. Scalar Arguments ..
INTEGER IFORM,ISIGN,NDIM
C ..
C .. Array Arguments ..
REAL DATA(1),WORK(1)
INTEGER NN(1)
C ..
C .. Local Scalars ..
REAL DIFI,DIFR,OLDSI,OLDSR,RTHLF,SUMI,SUMR,T2I,T2R,T3I,T3R,T4I,
+ T4R,TEMPI,TEMPR,THETA,TWOPI,TWOWR,U1I,U1R,U2I,U2R,U3I,U3R,
+ U4I,U4R,W2I,W2R,W3I,W3R,WI,WR,WSTPI,WSTPR
INTEGER I,I1,I1MAX,I1RNG,I2,I2MAX,I3,ICASE,ICONJ,IDIM,IDIV,IF,
+ IFMIN,IFP1,IFP2,IMAX,IMIN,INON2,IPAR,IQUOT,IREM,J,J1,
+ J1MAX,J1MIN,J2,J2MAX,J2MIN,J2RNG,J3,J3MAX,JMAX,JMIN,K1,K2,
+ K3,K4,KDIF,KMIN,KSTEP,L,LMAX,M,MMAX,N,NHALF,NP0,NP1,NP1HF,
+ NP1TW,NP2,NP2HF,NPREV,NTOT,NTWO,NWORK
C ..
C .. Local Arrays ..
INTEGER IFACT(32)
C ..
C .. Intrinsic Functions ..
INTRINSIC COS,MAX0,REAL,SIN
C ..
C .. Data statements ..
DATA NP0/0/,NPREV/0/
DATA TWOPI/6.2831853071796/,RTHLF/0.70710678118655/
C ..
IF (NDIM-1) 232,101,101
101 NTOT = 2
DO 103 IDIM = 1,NDIM
IF (NN(IDIM)) 232,232,102
102 NTOT = NTOT*NN(IDIM)
103 CONTINUE
c
c main loop for each dimension
c
NP1 = 2
DO 231 IDIM = 1,NDIM
N = NN(IDIM)
NP2 = NP1*N
IF (N-1) 232,230,104
c
c is n a power of two and if not, what are its factors
c
104 M = N
NTWO = NP1
IF = 1
IDIV = 2
105 IQUOT = M/IDIV
IREM = M - IDIV*IQUOT
IF (IQUOT-IDIV) 113,106,106
106 IF (IREM) 108,107,108
107 NTWO = NTWO + NTWO
IFACT(IF) = IDIV
IF = IF + 1
M = IQUOT
GO TO 105
108 IDIV = 3
INON2 = IF
109 IQUOT = M/IDIV
IREM = M - IDIV*IQUOT
IF (IQUOT-IDIV) 115,110,110
110 IF (IREM) 112,111,112
111 IFACT(IF) = IDIV
IF = IF + 1
M = IQUOT
GO TO 109
112 IDIV = IDIV + 2
GO TO 109
113 INON2 = IF
IF (IREM) 115,114,115
114 NTWO = NTWO + NTWO
GO TO 116
115 IFACT(IF) = M
c
c separate four cases--
c 1. complex transform or real transform for the 4th, 9th,etc.
c dimensions.
c 2. real transform for the 2nd or 3rd dimension. method--
c transform half the data, supplying the other half by con-
c jugate symmetry.
c 3. real transform for the 1st dimension, n odd. method--
c set the imaginary parts to zero.
c 4. real transform for the 1st dimension, n even. method--
c transform a complex array of length n/2 whose real parts
c are the even numbered real values and whose imaginary parts
c are the odd numbered real values. separate and supply
c the second half by conjugate symmetry.
c
116 ICASE = 1
IFMIN = 1
I1RNG = NP1
IF (IDIM-4) 117,122,122
117 IF (IFORM) 118,118,122
118 ICASE = 2
I1RNG = NP0* (1+NPREV/2)
IF (IDIM-1) 119,119,122
119 ICASE = 3
I1RNG = NP1
IF (NTWO-NP1) 122,122,120
120 ICASE = 4
IFMIN = 2
NTWO = NTWO/2
N = N/2
NP2 = NP2/2
NTOT = NTOT/2
I = 1
DO 121 J = 1,NTOT
DATA(J) = DATA(I)
I = I + 2
121 CONTINUE
c
c shuffle data by bit reversal, since n=2**k. as the shuffling
c can be done by simple interchange, no working array is needed
c
122 IF (NTWO-NP2) 132,123,123
123 NP2HF = NP2/2
J = 1
DO 131 I2 = 1,NP2,NP1
IF (J-I2) 124,127,127
124 I1MAX = I2 + NP1 - 2
DO 126 I1 = I2,I1MAX,2
DO 125 I3 = I1,NTOT,NP2
J3 = J + I3 - I2
TEMPR = DATA(I3)
TEMPI = DATA(I3+1)
DATA(I3) = DATA(J3)
DATA(I3+1) = DATA(J3+1)
DATA(J3) = TEMPR
DATA(J3+1) = TEMPI
125 CONTINUE
126 CONTINUE
127 M = NP2HF
128 IF (J-M) 130,130,129
129 J = J - M
M = M/2
IF (M-NP1) 130,128,128
130 J = J + M
131 CONTINUE
GO TO 142
c
c shuffle data by digit reversal for general n
c
132 NWORK = 2*N
DO 141 I1 = 1,NP1,2
DO 140 I3 = I1,NTOT,NP2
J = I3
DO 138 I = 1,NWORK,2
IF (ICASE-3) 133,134,133
133 WORK(I) = DATA(J)
WORK(I+1) = DATA(J+1)
GO TO 135
134 WORK(I) = DATA(J)
WORK(I+1) = 0.
135 IFP2 = NP2
IF = IFMIN
136 IFP1 = IFP2/IFACT(IF)
J = J + IFP1
IF (J-I3-IFP2) 138,137,137
137 J = J - IFP2
IFP2 = IFP1
IF = IF + 1
IF (IFP2-NP1) 138,138,136
138 CONTINUE
I2MAX = I3 + NP2 - NP1
I = 1
DO 139 I2 = I3,I2MAX,NP1
DATA(I2) = WORK(I)
DATA(I2+1) = WORK(I+1)
I = I + 2
139 CONTINUE
140 CONTINUE
141 CONTINUE
c
c main loop for factors of two. perform fourier transforms of
c length four, with one of length two if needed. the twiddle factor
c w=exp(isign*2*pi*sqrt(-1)*m/(4*mmax)). check for w=isign*sqrt(-1)
c and repeat for w=w*(1+isign*sqrt(-1))/sqrt(2).
c
142 IF (NTWO-NP1) 174,174,143
143 NP1TW = NP1 + NP1
IPAR = NTWO/NP1
144 IF (IPAR-2) 149,146,145
145 IPAR = IPAR/4
GO TO 144
146 DO 148 I1 = 1,I1RNG,2
DO 147 K1 = I1,NTOT,NP1TW
K2 = K1 + NP1
TEMPR = DATA(K2)
TEMPI = DATA(K2+1)
DATA(K2) = DATA(K1) - TEMPR
DATA(K2+1) = DATA(K1+1) - TEMPI
DATA(K1) = DATA(K1) + TEMPR
DATA(K1+1) = DATA(K1+1) + TEMPI
147 CONTINUE
148 CONTINUE
149 MMAX = NP1
150 IF (MMAX-NTWO/2) 151,174,174
151 LMAX = MAX0(NP1TW,MMAX/2)
DO 173 L = NP1,LMAX,NP1TW
M = L
IF (MMAX-NP1) 156,156,152
152 THETA = -TWOPI*REAL(L)/REAL(4*MMAX)
IF (ISIGN) 154,153,153
153 THETA = -THETA
154 WR = COS(THETA)
WI = SIN(THETA)
155 W2R = WR*WR - WI*WI
W2I = 2.*WR*WI
W3R = W2R*WR - W2I*WI
W3I = W2R*WI + W2I*WR
156 DO 169 I1 = 1,I1RNG,2
KMIN = I1 + IPAR*M
IF (MMAX-NP1) 157,157,158
157 KMIN = I1
158 KDIF = IPAR*MMAX
159 KSTEP = 4*KDIF
IF (KSTEP-NTWO) 160,160,169
160 DO 168 K1 = KMIN,NTOT,KSTEP
K2 = K1 + KDIF
K3 = K2 + KDIF
K4 = K3 + KDIF
IF (MMAX-NP1) 161,161,164
161 U1R = DATA(K1) + DATA(K2)
U1I = DATA(K1+1) + DATA(K2+1)
U2R = DATA(K3) + DATA(K4)
U2I = DATA(K3+1) + DATA(K4+1)
U3R = DATA(K1) - DATA(K2)
U3I = DATA(K1+1) - DATA(K2+1)
IF (ISIGN) 162,163,163
162 U4R = DATA(K3+1) - DATA(K4+1)
U4I = DATA(K4) - DATA(K3)
GO TO 167
163 U4R = DATA(K4+1) - DATA(K3+1)
U4I = DATA(K3) - DATA(K4)
GO TO 167
164 T2R = W2R*DATA(K2) - W2I*DATA(K2+1)
T2I = W2R*DATA(K2+1) + W2I*DATA(K2)
T3R = WR*DATA(K3) - WI*DATA(K3+1)
T3I = WR*DATA(K3+1) + WI*DATA(K3)
T4R = W3R*DATA(K4) - W3I*DATA(K4+1)
T4I = W3R*DATA(K4+1) + W3I*DATA(K4)
U1R = DATA(K1) + T2R
U1I = DATA(K1+1) + T2I
U2R = T3R + T4R
U2I = T3I + T4I
U3R = DATA(K1) - T2R
U3I = DATA(K1+1) - T2I
IF (ISIGN) 165,166,166
165 U4R = T3I - T4I
U4I = T4R - T3R
GO TO 167
166 U4R = T4I - T3I
U4I = T3R - T4R
167 DATA(K1) = U1R + U2R
DATA(K1+1) = U1I + U2I
DATA(K2) = U3R + U4R
DATA(K2+1) = U3I + U4I
DATA(K3) = U1R - U2R
DATA(K3+1) = U1I - U2I
DATA(K4) = U3R - U4R
DATA(K4+1) = U3I - U4I
168 CONTINUE
KDIF = KSTEP
KMIN = 4* (KMIN-I1) + I1
GO TO 159
169 CONTINUE
M = M + LMAX
IF (M-MMAX) 170,170,173
170 IF (ISIGN) 171,172,172
171 TEMPR = WR
WR = (WR+WI)*RTHLF
WI = (WI-TEMPR)*RTHLF
GO TO 155
172 TEMPR = WR
WR = (WR-WI)*RTHLF
WI = (TEMPR+WI)*RTHLF
GO TO 155
173 CONTINUE
IPAR = 3 - IPAR
MMAX = MMAX + MMAX
GO TO 150
c
c main loop for factors not equal to two. apply the twiddle factor
c w=exp(isign*2*pi*sqrt(-1)*(j1-1)*(j2-j1)/(ifp1+ifp2)), then
c perform a fourier transform of length ifact(if), making use of
c conjugate symmetries.
c
174 IF (NTWO-NP2) 175,201,201
175 IFP1 = NTWO
IF = INON2
NP1HF = NP1/2
176 IFP2 = IFACT(IF)*IFP1
J1MIN = NP1 + 1
IF (J1MIN-IFP1) 177,177,184
177 DO 183 J1 = J1MIN,IFP1,NP1
THETA = -TWOPI*REAL(J1-1)/REAL(IFP2)
IF (ISIGN) 179,178,178
178 THETA = -THETA
179 WSTPR = COS(THETA)
WSTPI = SIN(THETA)
WR = WSTPR
WI = WSTPI
J2MIN = J1 + IFP1
J2MAX = J1 + IFP2 - IFP1
DO 182 J2 = J2MIN,J2MAX,IFP1
I1MAX = J2 + I1RNG - 2
DO 181 I1 = J2,I1MAX,2
DO 180 J3 = I1,NTOT,IFP2
TEMPR = DATA(J3)
DATA(J3) = DATA(J3)*WR - DATA(J3+1)*WI
DATA(J3+1) = TEMPR*WI + DATA(J3+1)*WR
180 CONTINUE
181 CONTINUE
TEMPR = WR
WR = WR*WSTPR - WI*WSTPI
WI = TEMPR*WSTPI + WI*WSTPR
182 CONTINUE
183 CONTINUE
184 THETA = -TWOPI/REAL(IFACT(IF))
IF (ISIGN) 186,185,185
185 THETA = -THETA
186 WSTPR = COS(THETA)
WSTPI = SIN(THETA)
J2RNG = IFP1* (1+IFACT(IF)/2)
DO 200 I1 = 1,I1RNG,2
DO 199 I3 = I1,NTOT,NP2
J2MAX = I3 + J2RNG - IFP1
DO 197 J2 = I3,J2MAX,IFP1
J1MAX = J2 + IFP1 - NP1
DO 193 J1 = J2,J1MAX,NP1
J3MAX = J1 + NP2 - IFP2
DO 192 J3 = J1,J3MAX,IFP2
JMIN = J3 - J2 + I3
JMAX = JMIN + IFP2 - IFP1
I = 1 + (J3-I3)/NP1HF
IF (J2-I3) 187,187,189
187 SUMR = 0.
SUMI = 0.
DO 188 J = JMIN,JMAX,IFP1
SUMR = SUMR + DATA(J)
SUMI = SUMI + DATA(J+1)
188 CONTINUE
WORK(I) = SUMR
WORK(I+1) = SUMI
GO TO 192
189 ICONJ = 1 + (IFP2-2*J2+I3+J3)/NP1HF
J = JMAX
SUMR = DATA(J)
SUMI = DATA(J+1)
OLDSR = 0.
OLDSI = 0.
J = J - IFP1
190 TEMPR = SUMR
TEMPI = SUMI
SUMR = TWOWR*SUMR - OLDSR + DATA(J)
SUMI = TWOWR*SUMI - OLDSI + DATA(J+1)
OLDSR = TEMPR
OLDSI = TEMPI
J = J - IFP1
IF (J-JMIN) 191,191,190
191 TEMPR = WR*SUMR - OLDSR + DATA(J)
TEMPI = WI*SUMI
WORK(I) = TEMPR - TEMPI
WORK(ICONJ) = TEMPR + TEMPI
TEMPR = WR*SUMI - OLDSI + DATA(J+1)
TEMPI = WI*SUMR
WORK(I+1) = TEMPR + TEMPI
WORK(ICONJ+1) = TEMPR - TEMPI
192 CONTINUE
193 CONTINUE
IF (J2-I3) 194,194,195
194 WR = WSTPR
WI = WSTPI
GO TO 196
195 TEMPR = WR
WR = WR*WSTPR - WI*WSTPI
WI = TEMPR*WSTPI + WI*WSTPR
196 TWOWR = WR + WR
197 CONTINUE
I = 1
I2MAX = I3 + NP2 - NP1
DO 198 I2 = I3,I2MAX,NP1
DATA(I2) = WORK(I)
DATA(I2+1) = WORK(I+1)
I = I + 2
198 CONTINUE
199 CONTINUE
200 CONTINUE
IF = IF + 1
IFP1 = IFP2
IF (IFP1-NP2) 176,201,201
c
c complete a real transform in the 1st dimension, n even, by con-
c jugate symmetries.
c
201 GO TO (230,220,230,202) ICASE
202 NHALF = N
N = N + N
THETA = -TWOPI/REAL(N)
IF (ISIGN) 204,203,203
203 THETA = -THETA
204 WSTPR = COS(THETA)
WSTPI = SIN(THETA)
WR = WSTPR
WI = WSTPI
IMIN = 3
JMIN = 2*NHALF - 1
GO TO 207
205 J = JMIN
DO 206 I = IMIN,NTOT,NP2
SUMR = (DATA(I)+DATA(J))/2.
SUMI = (DATA(I+1)+DATA(J+1))/2.
DIFR = (DATA(I)-DATA(J))/2.
DIFI = (DATA(I+1)-DATA(J+1))/2.
TEMPR = WR*SUMI + WI*DIFR
TEMPI = WI*SUMI - WR*DIFR
DATA(I) = SUMR + TEMPR
DATA(I+1) = DIFI + TEMPI
DATA(J) = SUMR - TEMPR
DATA(J+1) = -DIFI + TEMPI
J = J + NP2
206 CONTINUE
IMIN = IMIN + 2
JMIN = JMIN - 2
TEMPR = WR
WR = WR*WSTPR - WI*WSTPI
WI = TEMPR*WSTPI + WI*WSTPR
207 IF (IMIN-JMIN) 205,208,211
208 IF (ISIGN) 209,211,211
209 DO 210 I = IMIN,NTOT,NP2
DATA(I+1) = -DATA(I+1)
210 CONTINUE
211 NP2 = NP2 + NP2
NTOT = NTOT + NTOT
J = NTOT + 1
IMAX = NTOT/2 + 1
212 IMIN = IMAX - 2*NHALF
I = IMIN
GO TO 214
213 DATA(J) = DATA(I)
DATA(J+1) = -DATA(I+1)
214 I = I + 2
J = J - 2
IF (I-IMAX) 213,215,215
215 DATA(J) = DATA(IMIN) - DATA(IMIN+1)
DATA(J+1) = 0.
IF (I-J) 217,219,219
216 DATA(J) = DATA(I)
DATA(J+1) = DATA(I+1)
217 I = I - 2
J = J - 2
IF (I-IMIN) 218,218,216
218 DATA(J) = DATA(IMIN) + DATA(IMIN+1)
DATA(J+1) = 0.
IMAX = IMIN
GO TO 212
219 DATA(1) = DATA(1) + DATA(2)
DATA(2) = 0.
GO TO 230
c
c complete a real transform for the 2nd or 3rd dimension by
c conjugate symmetries.
c
220 IF (I1RNG-NP1) 221,230,230
221 DO 229 I3 = 1,NTOT,NP2
I2MAX = I3 + NP2 - NP1
DO 228 I2 = I3,I2MAX,NP1
IMIN = I2 + I1RNG
IMAX = I2 + NP1 - 2
JMAX = 2*I3 + NP1 - IMIN
IF (I2-I3) 223,223,222
222 JMAX = JMAX + NP2
223 IF (IDIM-2) 226,226,224
224 J = JMAX + NP0
DO 225 I = IMIN,IMAX,2
DATA(I) = DATA(J)
DATA(I+1) = -DATA(J+1)
J = J - 2
225 CONTINUE
226 J = JMAX
DO 227 I = IMIN,IMAX,NP0
DATA(I) = DATA(J)
DATA(I+1) = -DATA(J+1)
J = J - NP0
227 CONTINUE
228 CONTINUE
229 CONTINUE
c
c end of loop on each dimension
c
230 NP0 = NP1
NP1 = NP2
NPREV = N
231 CONTINUE
232 RETURN
c
c revision history---
c
c january 1978 deleted references to the *cosy cards and
c added revision history
c-----------------------------------------------------------------------
END

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