C     SUBROUTINE FFT99B - POSTPROCESSING STEP FOR FFT99, ISIGN=-1
C     (GRIDPOINT TO SPECTRAL TRANSFORM)
C
      SUBROUTINE FFT99B(WORK,A,TRIGS,INC,JUMP,N,LOT)
      DIMENSION WORK(N),A(N),TRIGS(N)
C
      NH=N/2
      NX=N
      INK=INC+INC
C
C     A(0)   A(N/2)
      SCALE=1.0/FLOAT(N)
      IA=1
      IB=2
      JA=1
      JB=N*INC+1
CDIR$ IVDEP
      DO 10 L=1,LOT
      A(JA)=SCALE*(WORK(IA)+WORK(IB))
      A(JA+INC)=0.0
      IA=IA+NX
      IB=IB+NX
      JA=JA+JUMP
      JB=JB+JUMP
   10 CONTINUE
C
C     REMAINING WAVENUMBERS
      SCALE=0.5*SCALE
      IABASE=3
      IBBASE=N-1
      JABASE=2*INC+1
      JBBASE=(N-2)*INC+1
C
      DO 30 K=3,NH,2
      IA=IABASE
      IB=IBBASE
      JA=JABASE
      JB=JBBASE
      C=TRIGS(N+K)
      S=TRIGS(N+K+1)
CDIR$ IVDEP
      DO 20 L=1,LOT
      A(JA)=SCALE*((WORK(IA)+WORK(IB))
     *   +(C*(WORK(IA+1)+WORK(IB+1))+S*(WORK(IA)-WORK(IB))))
      A(JB)=SCALE*((WORK(IA)+WORK(IB))
     *   -(C*(WORK(IA+1)+WORK(IB+1))+S*(WORK(IA)-WORK(IB))))
      A(JA+INC)=SCALE*((C*(WORK(IA)-WORK(IB))-S*(WORK(IA+1)+WORK(IB+1)))
     *    +(WORK(IB+1)-WORK(IA+1)))
      A(JB+INC)=SCALE*((C*(WORK(IA)-WORK(IB))-S*(WORK(IA+1)+WORK(IB+1)))
     *    -(WORK(IB+1)-WORK(IA+1)))
      IA=IA+NX
      IB=IB+NX
      JA=JA+JUMP
      JB=JB+JUMP
   20 CONTINUE
      IABASE=IABASE+2
      IBBASE=IBBASE-2
      JABASE=JABASE+INK
      JBBASE=JBBASE-INK
   30 CONTINUE
C
      IF (IABASE.NE.IBBASE) GO TO 50
C     WAVENUMBER N/4 (IF IT EXISTS)
      IA=IABASE
      JA=JABASE
      SCALE=2.0*SCALE
CDIR$ IVDEP
      DO 40 L=1,LOT
      A(JA)=SCALE*WORK(IA)
      A(JA+INC)=-SCALE*WORK(IA+1)
      IA=IA+NX
      JA=JA+JUMP
   40 CONTINUE
C
   50 CONTINUE
      RETURN
      END