ADAS Subroutine h4ftsp

C
       subroutine h4ftsp( istdim  ,
     &                    x       , xa   , n    , yaa  , y    , dy    ,
     &                    i0      , c1   , c2   , c3   , c4   , isw
     &                  )            
c-----------------------------------------------------------------------
c
c  **************** fortran77 subroutine: h4ftsp.for *********************
c
c  purpose: obtain the value from a spline interpolation
c
c  calling program: various
c
c  
c  input : (i*4)  istdim  = dimensionality for splining arrays
c
c  input : (r*8)  x       = required x-value
c  input : (r*8)  xa(i)   = x-values
c  input : (i*4)  n       = number of values
c  input : (r*8)  yaa(i)  = y-values (possibly stored as multiple sets)
c  input : (i*4)  i0      = starting index(-1) in yaa array of required input set
c  input : (r*8)  c1(i,j) = 1st spline coefficient precursor
c  input : (r*8)  c2(i,j) = 2nd spline coefficient precursor
c  input : (r*8)  c3(i,j) = 3rd spline coefficient precursor
c  input : (r*8)  c4(i,j) = 4th spline coefficient precursor
c  input : (i*4)  isw     = .le.0  ordinary     spline interpolation
c                         = .gt.0  logarithmic  spline interpolation
c
c  output: (r*8)  y       = returned y-value
c  output: (r*8)  dy      = returned derivative
c
c  routines:
c          routine    source    brief description
c          -------------------------------------------------------------
c          i4unit     adas      fetch unit number for output of messages
c
c  author:  Hugh P. Summers, University of Strathclyde
c           JA7.08
c           Tel.: +44 (0)141-548-4196
c
c  date:    24 July 2002
c
c
c  version: 1.1   Hugh Summers  24/07/02
c  modified:      first release
c
c-----------------------------------------------------------------------
      INTEGER             I0,          ISTDIM,      ISW,         N
      REAL*8              C1(ISTDIM,ISTDIM-1),      C2(ISTDIM,ISTDIM-1)
      REAL*8              C3(ISTDIM,ISTDIM-1),      C4(ISTDIM,ISTDIM-1)
      REAL*8              DY,          X,           XA(ISTDIM),  Y
      REAL*8              YAA(*)