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[jabaws.git] / website / archive / binaries / mac / src / disembl / Tisean_3.0.1 / source_f / slatec / rffti1.f

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+*DECK RFFTI1
+      SUBROUTINE RFFTI1 (N, WA, IFAC)
+C***BEGIN PROLOGUE  RFFTI1
+C***PURPOSE  Initialize a real and an integer work array for RFFTF1 and
+C            RFFTB1.
+C***LIBRARY   SLATEC (FFTPACK)
+C***CATEGORY  J1A1
+C***TYPE      SINGLE PRECISION (RFFTI1-S, CFFTI1-C)
+C***KEYWORDS  FFTPACK, FOURIER TRANSFORM
+C***AUTHOR  Swarztrauber, P. N., (NCAR)
+C***DESCRIPTION
+C
+C   Subroutine RFFTI1 initializes the work arrays WA and IFAC which are
+C   used in both RFFTF1 and RFFTB1.  The prime factorization of N and a
+C   tabulation of the trigonometric functions are computed and stored in
+C   IFAC and WA, respectively.
+C
+C   Input Argument
+C
+C   N       the length of the sequence to be transformed.
+C
+C   Output Arguments
+C
+C   WA      a real work array which must be dimensioned at least N.
+C
+C   IFAC    an integer work array which must be dimensioned at least 15.
+C
+C   The same work arrays can be used for both RFFTF1 and RFFTB1 as long
+C   as N remains unchanged.  Different WA and IFAC arrays are required
+C   for different values of N.  The contents of WA and IFAC must not be
+C   changed between calls of RFFTF1 or RFFTB1.
+C
+C***REFERENCES  P. N. Swarztrauber, Vectorizing the FFTs, in Parallel
+C                 Computations (G. Rodrigue, ed.), Academic Press,
+C                 1982, pp. 51-83.
+C***ROUTINES CALLED  (NONE)
+C***REVISION HISTORY  (YYMMDD)
+C   790601  DATE WRITTEN
+C   830401  Modified to use SLATEC library source file format.
+C   860115  Modified by Ron Boisvert to adhere to Fortran 77 by
+C           (a) changing dummy array size declarations (1) to (*),
+C           (b) changing references to intrinsic function FLOAT
+C               to REAL, and
+C           (c) changing definition of variable TPI by using
+C               FORTRAN intrinsic functions instead of DATA
+C               statements.
+C   881128  Modified by Dick Valent to meet prologue standards.
+C   890531  Changed all specific intrinsics to generic.  (WRB)
+C   891214  Prologue converted to Version 4.0 format.  (BAB)
+C   900131  Routine changed from subsidiary to user-callable.  (WRB)
+C   920501  Reformatted the REFERENCES section.  (WRB)
+C***END PROLOGUE  RFFTI1
+      DIMENSION WA(*), IFAC(*), NTRYH(4)
+      SAVE NTRYH
+      DATA NTRYH(1),NTRYH(2),NTRYH(3),NTRYH(4)/4,2,3,5/
+C***FIRST EXECUTABLE STATEMENT  RFFTI1
+      NL = N
+      NF = 0
+      J = 0
+  101 J = J+1
+      IF (J-4) 102,102,103
+  102 NTRY = NTRYH(J)
+      GO TO 104
+  103 NTRY = NTRY+2
+  104 NQ = NL/NTRY
+      NR = NL-NTRY*NQ
+      IF (NR) 101,105,101
+  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 = 8.*ATAN(1.)
+      ARGH = TPI/N
+      IS = 0
+      NFM1 = NF-1
+      L1 = 1
+      IF (NFM1 .EQ. 0) RETURN
+      DO 110 K1=1,NFM1
+         IP = IFAC(K1+2)
+         LD = 0
+         L2 = L1*IP
+         IDO = N/L2
+         IPM = IP-1
+         DO 109 J=1,IPM
+            LD = LD+L1
+            I = IS
+            ARGLD = LD*ARGH
+            FI = 0.
+            DO 108 II=3,IDO,2
+               I = I+2
+               FI = FI+1.
+               ARG = FI*ARGLD
+               WA(I-1) = COS(ARG)
+               WA(I) = SIN(ARG)
+  108       CONTINUE
+            IS = IS+IDO
+  109    CONTINUE
+         L1 = L2
+  110 CONTINUE
+      RETURN
+      END