spfft1.f

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C> @file
C> @brief Perform multiple fast Fourier transforms.
C> @author Iredell @date 96-02-20
 
C> This subprogram performs multiple fast Fourier transforms
C> between complex amplitudes in Fourier space and real values
C> in cyclic physical space.
C>
C> Subprogram spfft1() initializes trigonometric data each call.
C> Use subprogram spfft() to save time and initialize once.
C> This version invokes the IBM ESSL FFT.
C>
C> @note The restrictions on IMAX are that it must be a multiple of 1
C> to 25 factors of two, up to 2 factors of three, and up to 1 factor of
C> five, seven and eleven.
C>
C> @note This subprogram is thread-safe.
C>
C> @param IMAX number of values in the cyclic physical space
C> (see limitations on imax in remarks below.)
C> @param INCW first dimension of the complex amplitude array
C> (INCW >= IMAX/2+1)
C> @param INCG first dimension of the real value array (INCG >= IMAX)
C> @param KMAX number of transforms to perform
C> @param[out] W complex amplitudes if IDIR>0
C> @param[out] G values if IDIR<0
C> @param IDIR direction flag
C> - IDIR>0 to transform from Fourier to physical space
C> - IDIR<0 to transform from physical to Fourier space
C>
C> @author Iredell @date 96-02-20
      SUBROUTINE spfft1(IMAX,INCW,INCG,KMAX,W,G,IDIR)
        IMPLICIT NONE
        INTEGER,INTENT(IN):: IMAX,INCW,INCG,KMAX,IDIR
        COMPLEX,INTENT(INOUT):: W(INCW,KMAX)
        REAL:: WREAL(INCW,KMAX)
        REAL,INTENT(INOUT):: G(INCG,KMAX)
        REAL:: AUX1(25000+INT(0.82*IMAX))
        REAL:: AUX2(20000+INT(0.57*IMAX))
        INTEGER:: NAUX1,NAUX2
C - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
        naux1=25000+int(0.82*imax)
        naux2=20000+int(0.57*imax)
        wreal=real(w)
C - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C  FOURIER TO PHYSICAL TRANSFORM.
        SELECT CASE(idir)
          CASE(1:)
            CALL scrft(1,real(w),incw,g,incg,imax,kmax,-1,1.,
     &                 aux1,naux1,aux2,naux2,0.,0)
            CALL scrft(0,real(w),incw,g,incg,imax,kmax,-1,1.,
     &                 aux1,naux1,aux2,naux2,0.,0)
C - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
C  PHYSICAL TO FOURIER TRANSFORM.
          CASE(:-1)
            CALL srcft(1,g,incg,wreal,incw,imax,kmax,+1,1./imax,
     &               aux1,naux1,aux2,naux2,0.,0)
            CALL srcft(0,g,incg,wreal,incw,imax,kmax,+1,1./imax,
     &               aux1,naux1,aux2,naux2,0.,0)
        END SELECT
        w=cmplx(wreal,0.0)
      SUBROUTINE spfft1(IMAX,INCW,INCG,KMAX,W,G,IDIR) …
      END SUBROUTINE