c  Mahc_Evolve: Numerical Evolution of the General Relativistic Hydro Equations
c  Copyright (C) 2001 Mark Miller

/*@@
  @file      slopelim.F
  @date      December 1999
  @author    Mark Miller
  @desc
     Slope limiters.
  @enddesc
@@*/

#include "cctk.h"

/*@@
  @routine    vis_contrib
  @date       December 1999
  @author     Mark Miller
  @desc
     Slope limiters.
  @enddesc
  @calls
  @calledby
  @history
  @endhistory
@@*/

      subroutine slopelim(slopel,sloper,order,fm1,f,fp1,fp2,tvd)
      implicit none
      CCTK_REAL slopel,sloper,fm1,f,fp1,fp2
      integer order,tvd
 
c     LOCAL VARS
      CCTK_REAL m_mhalf,m_phalf,m_p3half,half,mprod
      half = 0.5d0
      if (order .eq. 1) then
         slopel = 0.0d0
         sloper = 0.0d0
         return
      endif

      m_mhalf = f - fm1
      m_phalf = fp1 - f
      m_p3half = fp2 - fp1

      if (tvd .eq. 1) then
c        left slope
         mprod = m_mhalf * m_phalf
         slopel = (half + sign(half,mprod)) 
         if(mprod .eq. 0.0d0) slopel = 0.0d0
         if(m_mhalf .ge. 0.0d0) then
            slopel = slopel * min(m_mhalf,m_phalf)
         else
            slopel = slopel * max(m_mhalf,m_phalf)
         endif
c        right slope
         mprod = m_p3half * m_phalf
         sloper = (half + sign(half,mprod)) 
         if(mprod .eq. 0.0d0) sloper = 0.0d0
         if(m_p3half .ge. 0.0d0) then
            sloper = sloper * min(m_phalf,m_p3half)
         else
            sloper = sloper * max(m_phalf,m_p3half)
         endif
      else
         slopel = m_phalf
         sloper = m_phalf
      endif


      return
      end