c     kspectrum (http://www.meso-star.com/en_Products.html) - This file is part of kspectrum
c     Copyright (C) 2008-2015 - Méso-Star - Vincent Eymet
c     
c     This file must be used under the terms of the CeCILL license.
c     This source file is licensed as described in the file COPYING, which
c     you should have received as part of this distribution.  The terms
c     are also available at
c     http://www.cecill.info/licences/Licence_CeCILL_V2.1-en.txt
c     
      subroutine lin_interp(x1,x2,x,f1,f2,f)
      implicit none
c
c     Purpose: linear interpolation of quantity "f" between f1 and f2
c
c     Inputs:
c       + x1: absicca where f1 is given
c       + x2: absicca where f2 is given
c       + x : absiccas where f is interpolated
c       + f1: quantity f defined at x1
c       + f2: quantity f defined at x2
c
c     Output: f is the quantity interpolated between f1 and f2
c
      double precision x1,x2,x,f1,f2,f

      f=f1+(f2-f1)/(x2-x1)*(x-x1)

      return
      end