*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