*DECK PASSF
      SUBROUTINE PASSF (NAC, IDO, IP, L1, IDL1, CC, C1, C2, CH, CH2, WA)
C***BEGIN PROLOGUE  PASSF
C***SUBSIDIARY
C***PURPOSE  Calculate the fast Fourier transform of subvectors of
C            arbitrary length.
C***LIBRARY   SLATEC (FFTPACK)
C***TYPE      SINGLE PRECISION (PASSF-S)
C***AUTHOR  Swarztrauber, P. N., (NCAR)
C***ROUTINES CALLED  (NONE)
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           changing dummy array size declarations (1) to (*).
C   881128  Modified by Dick Valent to meet prologue standards.
C   890831  Modified array declarations.  (WRB)
C   891009  Removed unreferenced variable.  (WRB)
C   891214  Prologue converted to Version 4.0 format.  (BAB)
C   900402  Added TYPE section.  (WRB)
C***END PROLOGUE  PASSF
      DIMENSION CH(IDO,L1,*), CC(IDO,IP,*), C1(IDO,L1,*), WA(*),
     +          C2(IDL1,*), CH2(IDL1,*)
C***FIRST EXECUTABLE STATEMENT  PASSF
      IDOT = IDO/2
      IPP2 = IP+2
      IPPH = (IP+1)/2
      IDP = IP*IDO
C
      IF (IDO .LT. L1) GO TO 106
      DO 103 J=2,IPPH
         JC = IPP2-J
         DO 102 K=1,L1
CDIR$ IVDEP
            DO 101 I=1,IDO
               CH(I,K,J) = CC(I,J,K)+CC(I,JC,K)
               CH(I,K,JC) = CC(I,J,K)-CC(I,JC,K)
  101       CONTINUE
  102    CONTINUE
  103 CONTINUE
      DO 105 K=1,L1
CDIR$ IVDEP
         DO 104 I=1,IDO
            CH(I,K,1) = CC(I,1,K)
  104    CONTINUE
  105 CONTINUE
      GO TO 112
  106 DO 109 J=2,IPPH
         JC = IPP2-J
         DO 108 I=1,IDO
CDIR$ IVDEP
            DO 107 K=1,L1
               CH(I,K,J) = CC(I,J,K)+CC(I,JC,K)
               CH(I,K,JC) = CC(I,J,K)-CC(I,JC,K)
  107       CONTINUE
  108    CONTINUE
  109 CONTINUE
      DO 111 I=1,IDO
CDIR$ IVDEP
         DO 110 K=1,L1
            CH(I,K,1) = CC(I,1,K)
  110    CONTINUE
  111 CONTINUE
  112 IDL = 2-IDO
      INC = 0
      DO 116 L=2,IPPH
         LC = IPP2-L
         IDL = IDL+IDO
CDIR$ IVDEP
         DO 113 IK=1,IDL1
            C2(IK,L) = CH2(IK,1)+WA(IDL-1)*CH2(IK,2)
            C2(IK,LC) = -WA(IDL)*CH2(IK,IP)
  113    CONTINUE
         IDLJ = IDL
         INC = INC+IDO
         DO 115 J=3,IPPH
            JC = IPP2-J
            IDLJ = IDLJ+INC
            IF (IDLJ .GT. IDP) IDLJ = IDLJ-IDP
            WAR = WA(IDLJ-1)
            WAI = WA(IDLJ)
CDIR$ IVDEP
            DO 114 IK=1,IDL1
               C2(IK,L) = C2(IK,L)+WAR*CH2(IK,J)
               C2(IK,LC) = C2(IK,LC)-WAI*CH2(IK,JC)
  114       CONTINUE
  115    CONTINUE
  116 CONTINUE
      DO 118 J=2,IPPH
CDIR$ IVDEP
         DO 117 IK=1,IDL1
            CH2(IK,1) = CH2(IK,1)+CH2(IK,J)
  117    CONTINUE
  118 CONTINUE
      DO 120 J=2,IPPH
         JC = IPP2-J
CDIR$ IVDEP
         DO 119 IK=2,IDL1,2
            CH2(IK-1,J) = C2(IK-1,J)-C2(IK,JC)
            CH2(IK-1,JC) = C2(IK-1,J)+C2(IK,JC)
            CH2(IK,J) = C2(IK,J)+C2(IK-1,JC)
            CH2(IK,JC) = C2(IK,J)-C2(IK-1,JC)
  119    CONTINUE
  120 CONTINUE
      NAC = 1
      IF (IDO .EQ. 2) RETURN
      NAC = 0
CDIR$ IVDEP
      DO 121 IK=1,IDL1
         C2(IK,1) = CH2(IK,1)
  121 CONTINUE
      DO 123 J=2,IP
CDIR$ IVDEP
         DO 122 K=1,L1
            C1(1,K,J) = CH(1,K,J)
            C1(2,K,J) = CH(2,K,J)
  122    CONTINUE
  123 CONTINUE
      IF (IDOT .GT. L1) GO TO 127
      IDIJ = 0
      DO 126 J=2,IP
         IDIJ = IDIJ+2
         DO 125 I=4,IDO,2
            IDIJ = IDIJ+2
CDIR$ IVDEP
            DO 124 K=1,L1
               C1(I-1,K,J) = WA(IDIJ-1)*CH(I-1,K,J)+WA(IDIJ)*CH(I,K,J)
               C1(I,K,J) = WA(IDIJ-1)*CH(I,K,J)-WA(IDIJ)*CH(I-1,K,J)
  124       CONTINUE
  125    CONTINUE
  126 CONTINUE
      RETURN
  127 IDJ = 2-IDO
      DO 130 J=2,IP
         IDJ = IDJ+IDO
         DO 129 K=1,L1
            IDIJ = IDJ
CDIR$ IVDEP
            DO 128 I=4,IDO,2
               IDIJ = IDIJ+2
               C1(I-1,K,J) = WA(IDIJ-1)*CH(I-1,K,J)+WA(IDIJ)*CH(I,K,J)
               C1(I,K,J) = WA(IDIJ-1)*CH(I,K,J)-WA(IDIJ)*CH(I-1,K,J)
  128       CONTINUE
  129    CONTINUE
  130 CONTINUE
      RETURN
      END
