!c Copyright 1981-2016 ECMWF.
!c
!c This software is licensed under the terms of the Apache Licence 
!c Version 2.0 which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
!c
!c In applying this licence, ECMWF does not waive the privileges and immunities 
!c granted to it by virtue of its status as an intergovernmental organisation 
!c nor does it submit to any jurisdiction.
!c

      SUBROUTINE FFT99(A,WORK,TRIGS,IFAX,INC,JUMP,N,LOT,ISIGN)
      DIMENSION A(N),WORK(N),TRIGS(N),IFAX(*)
!C
!C     SUBROUTINE 'FFT99' - MULTIPLE FAST REAL PERIODIC TRANSFORM
!C     SUPERSEDES PREVIOUS ROUTINE 'FFT99'
!C
!C     REAL TRANSFORM OF LENGTH N PERFORMED BY REMOVING REDUNDANT
!C     OPERATIONS FROM COMPLEX TRANSFORM OF LENGTH N
!C
!C     A IS THE ARRAY CONTAINING INPUT & OUTPUT DATA
!C     WORK IS AN AREA OF SIZE (N+1)*MIN(LOT,64)
!C     TRIGS IS A PREVIOUSLY PREPARED LIST OF TRIG FUNCTION VALUES
!C     IFAX IS A PREVIOUSLY PREPARED LIST OF FACTORS OF N
!C     INC IS THE INCREMENT WITHIN EACH DATA 'VECTOR'
!C         (E.G. INC=1 FOR CONSECUTIVELY STORED DATA)
!C     JUMP IS THE INCREMENT BETWEEN THE START OF EACH DATA VECTOR
!C     N IS THE LENGTH OF THE DATA VECTORS
!C     LOT IS THE NUMBER OF DATA VECTORS
!C     ISIGN = +1 FOR TRANSFORM FROM SPECTRAL TO GRIDPOINT
!C           = -1 FOR TRANSFORM FROM GRIDPOINT TO SPECTRAL
!C
!C     ORDERING OF COEFFICIENTS:
!C         A(0),B(0),A(1),B(1),A(2),B(2),...,A(N/2),B(N/2)
!C         WHERE B(0)=B(N/2)=0; (N+2) LOCATIONS REQUIRED
!C
!C     ORDERING OF DATA:
!C         X(N-1),X(0),X(1),X(2),...,X(N-1),X(0)
!C         I.E. EXPLICIT CYCLIC CONTINUITY; (N+2) LOCATIONS REQUIRED
!C
!C     VECTORIZATION IS ACHIEVED ON CRAY BY DOING THE TRANSFORMS
!C     IN PARALLEL
!C
!C     N MUST BE COMPOSED OF FACTORS 2,3 & 5 BUT DOES NOT HAVE TO BE EVEN
!C
!C     DEFINITION OF TRANSFORMS:
!C     -------------------------
!C
!C     ISIGN=+1: X(J)=SUM(K=0,...,N-1)(C(K)*EXP(2*I*J*K*PI/N))
!C         WHERE C(K)=A(K)+I*B(K) AND C(N-K)=A(K)-I*B(K)
!C
!C     ISIGN=-1: A(K)=(1/N)*SUM(J=0,...,N-1)(X(J)*COS(2*J*K*PI/N))
!C               B(K)=-(1/N)*SUM(J=0,...,N-1)(X(J)*SIN(2*J*K*PI/N))
!C
      IF(IFAX(10).NE.N) CALL SET99(TRIGS,IFAX,N)
      NFAX=IFAX(1)
      NX=N+1
      IF (MOD(N,2).EQ.1) NX=N
      NBLOX=1+(LOT-1)/64
      NVEX=LOT-(NBLOX-1)*64
      IF (ISIGN.EQ.-1) GO TO 300
!C
!C     ISIGN=+1, SPECTRAL TO GRIDPOINT TRANSFORM
!C     -----------------------------------------
  100 CONTINUE
      ISTART=1
      DO 220 NB=1,NBLOX
      IA=ISTART
      I=ISTART
      DO 110 J=1,NVEX
      A(I+INC)=0.5*A(I)
      I=I+JUMP
  110 CONTINUE
      IF (MOD(N,2).EQ.1) GO TO 130
      I=ISTART+N*INC
      DO 120 J=1,NVEX
      A(I)=0.5*A(I)
      I=I+JUMP
  120 CONTINUE
  130 CONTINUE
      IA=ISTART+INC
      LA=1
      IGO=+1
!C
      DO 160 K=1,NFAX
      IFAC=IFAX(K+1)
      IERR=-1
      IF (IGO.EQ.-1) GO TO 140
      CALL RPASSM(A(IA),A(IA+LA*INC),WORK(1),WORK(IFAC*LA+1),TRIGS,&
         INC,1,JUMP,NX,NVEX,N,IFAC,LA,IERR)
      GO TO 150
  140 CONTINUE
      CALL RPASSM(WORK(1),WORK(LA+1),A(IA),A(IA+IFAC*LA*INC),TRIGS,&
         1,INC,NX,JUMP,NVEX,N,IFAC,LA,IERR)
  150 CONTINUE
      IF (IERR.NE.0) GO TO 500
      LA=IFAC*LA
      IGO=-IGO
      IA=ISTART+INC
  160 CONTINUE
!C
!C     IF NECESSARY, COPY RESULTS BACK TO A
!C     ------------------------------------
      IF (MOD(NFAX,2).EQ.0) GO TO 190
      IBASE=1
      JBASE=IA
      DO 180 JJ=1,NVEX
      I=IBASE
      J=JBASE
      DO 170 II=1,N
      A(J)=WORK(I)
      I=I+1
      J=J+INC
  170 CONTINUE
      IBASE=IBASE+NX
      JBASE=JBASE+JUMP
  180 CONTINUE
  190 CONTINUE
!C
!C     FILL IN CYCLIC BOUNDARY VALUES
!C     ------------------------------
      IX=ISTART
      IZ=ISTART+N*INC
!DIR$ IVDEP
      DO 200 J=1,NVEX
      A(IX)=A(IZ)
      A(IZ+INC)=A(IX+INC)
      IX=IX+JUMP
      IZ=IZ+JUMP
  200 CONTINUE
!C
      ISTART=ISTART+NVEX*JUMP
      NVEX=64
  220 CONTINUE
      RETURN
!C
!C     ISIGN=-1, GRIDPOINT TO SPECTRAL TRANSFORM
!C     -----------------------------------------
  300 CONTINUE
      ISTART=1
      DO 410 NB=1,NBLOX
      IA=ISTART+INC
      LA=N
      IGO=+1
!C
      DO 340 K=1,NFAX
      IFAC=IFAX(NFAX+2-K)
      LA=LA/IFAC
      IERR=-1
      IF (IGO.EQ.-1) GO TO 320
      CALL QPASSM(A(IA),A(IA+IFAC*LA*INC),WORK(1),WORK(LA+1),TRIGS,&
         INC,1,JUMP,NX,NVEX,N,IFAC,LA,IERR)
      GO TO 330
  320 CONTINUE
      CALL QPASSM(WORK(1),WORK(IFAC*LA+1),A(IA),A(IA+LA*INC),TRIGS,&
         1,INC,NX,JUMP,NVEX,N,IFAC,LA,IERR)
  330 CONTINUE
      IF (IERR.NE.0) GO TO 500
      IGO=-IGO
      IA=ISTART+INC
  340 CONTINUE
!C
!C     IF NECESSARY, COPY RESULTS BACK TO A
!C     ------------------------------------
      IF (MOD(NFAX,2).EQ.0) GO TO 370
      IBASE=1
      JBASE=IA
      DO 360 JJ=1,NVEX
      I=IBASE
      J=JBASE
      DO 350 II=1,N
      A(J)=WORK(I)
      I=I+1
      J=J+INC
  350 CONTINUE
      IBASE=IBASE+NX
      JBASE=JBASE+JUMP
  360 CONTINUE
  370 CONTINUE
!C
!C     SHIFT A(0) & FILL IN ZERO IMAG PARTS
!C     ------------------------------------
      IX=ISTART
      DO 380 J=1,NVEX
      A(IX)=A(IX+INC)
      A(IX+INC)=0.0
      IX=IX+JUMP
  380 CONTINUE
      IF (MOD(N,2).EQ.1) GO TO 400
      IZ=ISTART+(N+1)*INC
      DO 390 J=1,NVEX
      A(IZ)=0.0
      IZ=IZ+JUMP
  390 CONTINUE
  400 CONTINUE
!C
      ISTART=ISTART+NVEX*JUMP
      NVEX=64
  410 CONTINUE
      RETURN
!C
!C     ERROR MESSAGES
!C     --------------
  500 CONTINUE
      GO TO (510,530,550) IERR
  510 CONTINUE
      WRITE(6,520) NVEX
  520 FORMAT('1VECTOR LENGTH =',I4,', GREATER THAN 64')
      GO TO 570
  530 CONTINUE
      WRITE(6,540) IFAC
  540 FORMAT( '1FACTOR =',I3,', NOT CATERED FOR')
      GO TO 570
  550 CONTINUE
      WRITE(6,560) IFAC
  560 FORMAT('1FACTOR =',I3,', ONLY CATERED FOR IF LA*IFAC=N')
  570 CONTINUE
      RETURN
      END