SUBROUTINE R3FFT(C,ID,NL,NM,NN,WL,WM,WN,IOPT,ISIG,IORD,IWORK,IERR)****PURPOSE:*       THIS ROUTINE PERFORMS A 3-DIMENSIONAL REAL FOURIER TRANSFORM,*       OF ORDER NL*NM*NN  .****USAGE:*       THE USER IS EXPECTED TO PROVIDE THE DATA IN A 3-DIMENSIONAL*       REAL ARRAY C, DIMENSIONED IN THE CALLING PROGRAM C(ID,NM,NN);*       ID HAS TO BE AN EVEN INTEGER, EQUAL TO NL+2.*       FOR OUTPUT DATA ARRENGEMENT SEE NOTES TO R2FFT HERE ABOVE.*       THIS ROUTINE IS*       INTENDED FOR REPEATED USAGE, THUS SEPARATE SET-UP AND*       AND OPERATING CALLS ARE AVAILABLE: THE USER SHOULD IN ANY CASE*       PERFORM A SET-UP CALL (ISIG=0) PASSING THE PARAMETERS BEFORE*       PERFORMING AN ACTUAL TRANSFORM ( ISIG= +1 OR -1 ); THE USER CAN*       CHOOSE WHETHER TO OBTAIN THE RESULTS OF THE DIRECT TRANSFORM*       IN NATURAL ORDER (ISIG=-1,IORD=1) OR LEAVE THEM IN THE*       BIT-REVERSED  ORDER( ISIG=-1,IORD=0); THIS CHOICE SAVES*       SOME COMPUTER TIME, AND IT IS RECOMMENDED IN CASES DISCUSSED*       IN THE LONG WRITE-UP. ANALOGOUSLY, THE INVERSE TRANSFORM ACCEPTS*       INPUT ( PLEASE NOTE| ) DATA IN NATURAL ORDER ( ISIG=1,IORD=1),*       OR DATA ALREADY SUBJECTED TO A BIT-REVERSAL PERMUTATION( ISIG=1*       IORD=0).*       A SPECIAL TREATMENT IS AVAILABLE TO SPEED UP THE TRANSFORM OF*       SMALL MATRICES. THIS TREATMENT IS ACTIVATED BY THE FLAG IOPT. IN*       THIS CASE THE TABLES FOR THE SECOND DIMENSION ( WM ) ARE LARGER,*       BUT THE INCREASE IN PERFORMANCE IS SUBSTANTIAL WHEN NM<32.****ARGUMENTS :*       INPUT :*       C : ARRAY TO BE TRANSFORMED; DECLARED COMPLEX C(ID,NM,NN) IN THE*           CALLING PROGRAM;*       ID : FIRST DIMENSION OF C IN THE CALLING PROGRAM*            IT HAS TO BE AN EVEN INTEGER .GE. NL+2.*       ISIG : OPTION FLAG : ISIG=0 : SET-UP RUN, C NOT USED*                            ISIG=-1: DIRECT TRANSFORM*                            ISIG=+1: INVERSE TRANSFORM*       WL,WM,WN : INTEGER ARRAYS,USED TO HOST TABLES FOR THE TRANSFORMS*               DIMENSIONED IN THE CALLING PROGRAM AT LEAST (6*NL+14)*               (4*NM+14) AND (4*NN+14) RESPECTIVELY; IF*               IOPT=1, WM MUST BE DIMENSIONED AT LEAST 4*NM*(ID/2+1)+14*               IF ISIG.NE.0, THEY ARE ASSUMED TO HAVE BEEN SET BY A*               PREVIOUS CALL WITH ISIG=0 AND OTHER ARGUMENTS EQUAL, AND*               NEVER HAVE BEEN MODIFIED ;*               WHEN THE CORRESPONDING ORDERS ARE EQUAL, THEY DO NOT*               NEED TO BE DISTINCT*       NL : ORDER OF THE TRANSFORM ALONG THE COLUMNS OF C*            IT HAS TO BE AN EVEN INTEGER.*       NM : ORDER OF THE TRANSFORM ALONG THE ROWS OF C*       NN : ORDER OF THE TRANSFORM ALONG THE THIRD DIMENSION OF C*       IOPT : OPTION FLAG : =0 : NORMAL TREATMENT*                            =1 : SPECIAL TREATMENT FOR IMPROVING*                                 VECTORIZATION ON MATRICES WITH*                                 SMALL NL; REQUIRES LONG WM(SEE);IF*                                 REQUESTED, MUST BE PRESENT IN BOTH*                                 THE SET-UP AND TRANSFORM CALLS;*       IORD : OPTION FLAG : =1 : OUTPUT IN NATURAL ORDER (ISIG=-1)*                                 INPUT IN NATURAL ORDER  (ISIG=+1)*                            =0 : OUTPUT IN BIT-REVERSED ORDER(ISIG=-1)*                                 INPUT IN BIT-REVERSED ORDER(ISIG=+1)*       IWORK : INTEGER ARRAY, USED AS WORK AREA FOR REORDERING IF*               IORD=1; IT MUST BE AT LEAST MAX(NL,NM,NN) WORDS LONG.**        OUTPUT :*       C : TRANSFORMED ARRAY*       WL, WM, WN : ONLY IF ISIG=0, WL,WM AND WN ARE FILLED WITH THE*                     APPROPRIATE TABLES*       IWORK : UNDEFINED*       IERR  : ERROR CODE : =0 : SUCCESSFUL*                            =1 : WRONG ID PARAMETER*                            =2 : PRIME FACTORS DIFFERENT FROM 2,3,5*                                 ARE PRESENT IN DATA DIMENSIONS*                            =3 : TABLES NOT CORRECTLY INITIALIZED*                            =4 : FIRST DIMENSION IS AN ODD NUMBER*      COMPLEX C(*)      INTEGER WL(-14:*),WM(-14:*),WN(-14:*)      INTEGER  IWORK(*)**      INTEGER IDERR,FACERR,TBERR,ODDERR      PARAMETER (IDERR=1,FACERR=2,TBERR=3,ODDERR=4)*      IF(ID.LT.NL+2)THEN        IERR=IDERR        RETURN      ENDIF      NL1=NL/2      IF(NL1*2.NE.NL)THEN        IERR=ODDERR        RETURN      ENDIF      IERR=0       NMPN=NM*NN**      IF(ISIG.EQ.0) THEN        CALL MFFTP(NM,WM,ID/2*IOPT,IERR)        IF(IERR.NE.0)RETURN         CALL MFFTRP(NL,WL(4*NL))        IF(NL1.NE.NM) THEN          CALL MFFTP(NL1,WL,0,IERR)          IF(IERR.NE.0)RETURN        ELSE          CALL MFFTZ0(WM,1,4*NM+14,WL,1)        ENDIF*        IF(NM.EQ.NN) THEN          CALL MFFTZ0(WM,1,4*NM+14,WN,1)        ELSE IF(NN.EQ.NL1) THEN          CALL MFFTZ0(WL,1,4*NL1+14,WN,1)        ELSE          CALL MFFTP(NN,WN,0,IERR)          IF(IERR.NE.0)RETURN        ENDIF        RETURN*      ELSE   IF(ISIG.GT.0) THEN*        IF(IORD.NE.0) THEN          CALL MFFTOM(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM(NM*3),IWORK)          CALL MFFTOV(C,ID/2*NM,1,NN,ID/2*NM,WN(NN*3),IWORK)        ENDIF*        CALL MFFTIV(C,ID/2*NM,1,NN,ID/2*NM,WN,IERR)        IF(IERR.NE.0)RETURN*        IF(IOPT.EQ.0) THEN          CALL MFFTIM(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM,IERR)          IF(IERR.NE.0)RETURN        ELSE          CALL MFFTIS(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM,IERR)          IF(IERR.NE.0)RETURN        ENDIF*        CALL MFFTRI(C,1,ID/2,NL1,NMPN,WL(4*NL))        CALL MFFTOV(C,1,ID/2,NL1,NMPN,WL(NL1*3),IWORK)        CALL MFFTIV(C,1,ID/2,NL1,NMPN,WL,IERR)        IF(IERR.NE.0)RETURN**      ELSE**        CALL MFFTDV(C,1,ID/2,NL1,NMPN,WL,IERR)        IF(IERR.NE.0)RETURN        CALL MFFTOV(C,1,ID/2,NL1,NMPN,WL(NL1*2),IWORK)        CALL MFFTRD(C,1,ID/2,NL1,NMPN,WL(4*NL))**        IF(IOPT.EQ.0) THEN          CALL MFFTDM(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM,IERR)          IF(IERR.NE.0)RETURN        ELSE          CALL MFFTDS(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM,IERR)          IF(IERR.NE.0)RETURN        ENDIF*        CALL MFFTDV(C,ID/2*NM,1,NN,ID/2*NM,WN,IERR)        IF(IERR.NE.0)RETURN*        IF(IORD.NE.0) THEN          CALL MFFTOV(C,ID/2*NM,1,NN,ID/2*NM,WN(NN*2),IWORK)          CALL MFFTOM(C,ID/2,ID/2*NM,1,NM,NN,NL1+1,WM(NM*2),IWORK)        ENDIF*      ENDIF*      END