*DECK COSTI
SUBROUTINE COSTI (N, WSAVE)
C***BEGIN PROLOGUE COSTI
C***PURPOSE Initialize a work array for COST.
C***LIBRARY SLATEC (FFTPACK)
C***CATEGORY J1A3
C***TYPE SINGLE PRECISION (COSTI-S)
C***KEYWORDS COSINE FOURIER TRANSFORM, FFTPACK
C***AUTHOR Swarztrauber, P. N., (NCAR)
C***DESCRIPTION
C
C Subroutine COSTI initializes the array WSAVE which is used in
C subroutine COST. The prime factorization of N together with
C a tabulation of the trigonometric functions are computed and
C stored in WSAVE.
C
C Input Parameter
C
C N the length of the sequence to be transformed. The method
C is most efficient when N-1 is a product of small primes.
C
C Output Parameter
C
C WSAVE a work array which must be dimensioned at least 3*N+15.
C Different WSAVE arrays are required for different values
C of N. The contents of WSAVE must not be changed between
C calls of COST.
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 RFFTI
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 PI by using
C FORTRAN intrinsic function ATAN instead of a DATA
C statement.
C 881128 Modified by Dick Valent to meet prologue standards.
C 890531 Changed all specific intrinsics to generic. (WRB)
C 890531 REVISION DATE from Version 3.2
C 891214 Prologue converted to Version 4.0 format. (BAB)
C 920501 Reformatted the REFERENCES section. (WRB)
C***END PROLOGUE COSTI
DIMENSION WSAVE(*)
C***FIRST EXECUTABLE STATEMENT COSTI
IF (N .LE. 3) RETURN
PI = 4.*ATAN(1.)
NM1 = N-1
NP1 = N+1
NS2 = N/2
DT = PI/NM1
FK = 0.
DO 101 K=2,NS2
KC = NP1-K
FK = FK+1.
WSAVE(K) = 2.*SIN(FK*DT)
WSAVE(KC) = 2.*COS(FK*DT)
101 CONTINUE
CALL RFFTI (NM1,WSAVE(N+1))
RETURN
END