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110 lines
3.4 KiB
Fortran
110 lines
3.4 KiB
Fortran
*DECK RFFTI1
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SUBROUTINE RFFTI1 (N, WA, IFAC)
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C***BEGIN PROLOGUE RFFTI1
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C***PURPOSE Initialize a real and an integer work array for RFFTF1 and
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C RFFTB1.
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C***LIBRARY SLATEC (FFTPACK)
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C***CATEGORY J1A1
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C***TYPE SINGLE PRECISION (RFFTI1-S, CFFTI1-C)
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C***KEYWORDS FFTPACK, FOURIER TRANSFORM
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C***AUTHOR Swarztrauber, P. N., (NCAR)
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C***DESCRIPTION
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C
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C Subroutine RFFTI1 initializes the work arrays WA and IFAC which are
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C used in both RFFTF1 and RFFTB1. The prime factorization of N and a
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C tabulation of the trigonometric functions are computed and stored in
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C IFAC and WA, respectively.
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C
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C Input Argument
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C
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C N the length of the sequence to be transformed.
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C
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C Output Arguments
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C
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C WA a real work array which must be dimensioned at least N.
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C
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C IFAC an integer work array which must be dimensioned at least 15.
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C
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C The same work arrays can be used for both RFFTF1 and RFFTB1 as long
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C as N remains unchanged. Different WA and IFAC arrays are required
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C for different values of N. The contents of WA and IFAC must not be
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C changed between calls of RFFTF1 or RFFTB1.
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C
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C***REFERENCES P. N. Swarztrauber, Vectorizing the FFTs, in Parallel
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C Computations (G. Rodrigue, ed.), Academic Press,
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C 1982, pp. 51-83.
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C***ROUTINES CALLED (NONE)
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C***REVISION HISTORY (YYMMDD)
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C 790601 DATE WRITTEN
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C 830401 Modified to use SLATEC library source file format.
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C 860115 Modified by Ron Boisvert to adhere to Fortran 77 by
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C (a) changing dummy array size declarations (1) to (*),
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C (b) changing references to intrinsic function FLOAT
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C to REAL, and
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C (c) changing definition of variable TPI by using
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C FORTRAN intrinsic functions instead of DATA
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C statements.
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C 881128 Modified by Dick Valent to meet prologue standards.
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C 890531 Changed all specific intrinsics to generic. (WRB)
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900131 Routine changed from subsidiary to user-callable. (WRB)
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C 920501 Reformatted the REFERENCES section. (WRB)
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C***END PROLOGUE RFFTI1
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DIMENSION WA(*), IFAC(*), NTRYH(4)
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SAVE NTRYH
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DATA NTRYH(1),NTRYH(2),NTRYH(3),NTRYH(4)/4,2,3,5/
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C***FIRST EXECUTABLE STATEMENT RFFTI1
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NL = N
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NF = 0
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J = 0
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101 J = J+1
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IF (J-4) 102,102,103
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102 NTRY = NTRYH(J)
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GO TO 104
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103 NTRY = NTRY+2
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104 NQ = NL/NTRY
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NR = NL-NTRY*NQ
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IF (NR) 101,105,101
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105 NF = NF+1
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IFAC(NF+2) = NTRY
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NL = NQ
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IF (NTRY .NE. 2) GO TO 107
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IF (NF .EQ. 1) GO TO 107
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DO 106 I=2,NF
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IB = NF-I+2
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IFAC(IB+2) = IFAC(IB+1)
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106 CONTINUE
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IFAC(3) = 2
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107 IF (NL .NE. 1) GO TO 104
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IFAC(1) = N
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IFAC(2) = NF
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TPI = 8.*ATAN(1.)
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ARGH = TPI/N
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IS = 0
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NFM1 = NF-1
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L1 = 1
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IF (NFM1 .EQ. 0) RETURN
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DO 110 K1=1,NFM1
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IP = IFAC(K1+2)
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LD = 0
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L2 = L1*IP
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IDO = N/L2
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IPM = IP-1
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DO 109 J=1,IPM
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LD = LD+L1
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I = IS
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ARGLD = LD*ARGH
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FI = 0.
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DO 108 II=3,IDO,2
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I = I+2
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FI = FI+1.
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ARG = FI*ARGLD
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WA(I-1) = COS(ARG)
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WA(I) = SIN(ARG)
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108 CONTINUE
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IS = IS+IDO
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109 CONTINUE
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L1 = L2
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110 CONTINUE
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RETURN
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END
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