CTITLESAVSPLN -- SPLINE INTERPOLATION
C***********************************************************************
C                 COPYRIGHT ATLANTIC RICHFIELD COMPANY 1991            *
C***********************************************************************
CA
CA   DESIGNER     JAMES SUN
CA   AUTHOR       JAMES SUN
CA   LANGUAGE     FORTRAN
CA   SYSTEM       IBM AND CRAY
CA   WRITTEN      02/08/89
CA
C    REVISED      03/31/89 JCS - REAPPLY MUTE TO THE DEPTH SECTION
C    REVISED      01/20/90 JJC - MODIFIED TO MEET EDP STANDARDS.
C    REVISED      02/05/90 JJC - RENAMED VSPLINE TO SAVSPLN.
C    REVISED      07/24/90 CLJ - ALLOW PREP TO RUN ON THE IBM
C
CA
CA CALLING PROCEDURE:
CA     FUNCTION SAVSPLN(N,XN,FN,S,INDEX,X)
CA
C   CALLING ARGUMENTS
CA
CA     INPUT     N      NO. OF INPUT DATA POINTS.                     I4
CA     INPUT     XN     INPUT ARRAY (INDEPENDENT)                     R4
CA     INPUT     FN     INPUT ARRAY (DEPENDENT )                      R4
CA     INPUT     S      COEFFICIENT ARRAY                             R4
CA     INPUT     INDEX  COEFFICIENT ARRAY                             I4
CA     INPUT     X      INTERPOLATED FN AT THIS X VALUE (INDEPENDENT) R4
C
C
      FUNCTION SAVSPLN(N,XN,FN,S,INDEX,X)                               05660000
C                                                                       05680000
      IMPLICIT   INTEGER(A-Z)
C
      REAL     A
      REAL     B
      REAL     FN
      REAL     HL
      REAL     HNM1
      REAL     H1
      REAL     R6
      REAL     S
      REAL     SAVSPLN
      REAL     X
      REAL     XN
      DIMENSION XN(1),FN(1),S(1),INDEX(1)                               05670000
C                                                                       05680000
C
      R6=1./6.                                                          05690000
      I1=INDEX(1)                                                       05700000
      IF(X .GE. XN(I1)) GO TO 100                                       05710000
      JJ=2                                                              05720000
      I2=INDEX(JJ)                                                      05730000
      H1=XN(I2)-XN(I1)                                                  05740000
      SAVSPLN=FN(I1)+(X-XN(I1))*((FN(I2)-FN(I1))/H1-H1*S(JJ)*R6)        05750000
      RETURN                                                            05760000
  100 IN=INDEX(N)                                                       05770000
      IF(X .LE. XN(IN))GO TO 120                                        05780000
      INM1=INDEX(N-1)                                                   05790000
      HNM1=XN(IN)-XN(INM1)                                              05800000
      SAVSPLN=FN(IN)+(X-XN(IN))*((FN(IN)-FN(INM1))/HNM1+HNM1*S(N-1)*R6) 05810000
      RETURN                                                            05820000
C                                                                       05830000
C
  120 DO 140 I=2,N                                                      05840000
        II=INDEX(I)                                                     05850000
        IF(X .LE. XN(II))GO TO 160                                      05860000
  140 CONTINUE                                                          05870000
  160 L=I-1                                                             05880000
      IL=INDEX(L)                                                       05890000
      ILP1=INDEX(L+1)                                                   05900000
      A=XN(ILP1)-X                                                      05910000
      B=X-XN(IL)                                                        05920000
      HL=XN(ILP1)-XN(IL)                                                05930000
      SAVSPLN=A*S(L)*(A**2/HL-HL)*R6+B*S(L+1)*(B**2/HL-HL)*R6           05940000
     1  +(A*FN(IL)+B*FN(ILP1))/HL                                       05950000
C                                                                       05960000
C
      RETURN                                                            05970000
      END                                                               05980000