!
!     CalculiX - A 3-dimensional finite element program
!              Copyright (C) 1998-2025 Guido Dhondt
!
!     This program is free software; you can redistribute it and/or
!     modify it under the terms of the GNU General Public License as
!     published by the Free Software Foundation(version 2);
!     
!
!     This program is distributed in the hope that it will be useful,
!     but WITHOUT ANY WARRANTY; without even the implied warranty of 
!     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 
!     GNU General Public License for more details.
!
!     You should have received a copy of the GNU General Public License
!     along with this program; if not, write to the Free Software
!     Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
!
!
!      function to evaluate shape function for quad-lin mortar
!     method \f$ shape(\xi,\eta) \f$
!     see phd-thesis Sitzmann Chapter 4.1.
      
!     Author: Saskia Sitzmann
! 
!  [in] xi		xi-coordinate
!  [in] et		eta-coordinate
!  [in] xl		local node coordinates
!  [out] xsj		jacobian vector
!  [out] xs		local derivative of the global coordinates
!  [out] shp		evaluated shape functions and derivatives
!  [in] ns		current slave face
!  [in] pslavdual 	(:,i)coefficients \f$ \alpha_{ij}\f$,
!                       \f$ 1,j=1,..8\f$ for dual shape functions for face i
!  [in] iflag		flag indicating what to compute
!
      subroutine dualshape6tritilde_lin(xi,et,xl,xsj,xs,shp,ns,
     &  pslavdual,iflag)
!
!     iflag=2: calculate the value of the shape functions,
!              their derivatives w.r.t. the local coordinates
!              and the Jacobian vector (local normal to the
!              surface)
!     iflag=3: calculate the value of the shape functions, the
!              value of their derivatives w.r.t. the global
!              coordinates and the Jacobian vector (local normal
!              to the surface)
!
!     shape functions and derivatives for a 6-node quadratic
!     isoparametric triangular element. 0<=xi,et<=1,xi+et<=1 
!
      implicit none
!
      integer i,j,k,iflag,ns
!     
      real*8 shp(7,8),xs(3,7),xsi(2,3),xl(3,8),sh(3),xsj(3),
     &     shpold(7,8),alpha,pslavdual(64,*)
!     
      real*8 xi,et
!
!
!     
!     shape functions and their glocal derivatives for an element
!     described with two local parameters and three global ones.
!     
      alpha=1.0/5.0
!     
!     standard shape functions
!     
      shpold(4,1)=2.d0*(0.5d0-xi-et)*(1.d0-xi-et)
      shpold(4,2)=xi*(2.d0*xi-1.d0)
      shpold(4,3)=et*(2.d0*et-1.d0)
      shpold(4,4)=4.d0*xi*(1.d0-xi-et)
      shpold(4,5)=4.d0*xi*et
      shpold(4,6)=4.d0*et*(1.d0-xi-et) 
!
!     transformed shape functions
!     
      shp(3,1)=1.d0-xi-et
      shp(3,2)=xi
      shp(3,3)=et
      shp(3,4)=4.d0*xi*(1.d0-xi-et)
      shp(3,5)=4.d0*xi*et
      shp(3,6)=4.d0*et*(1.d0-xi-et) 
!
!     Caution: derivatives and exspecially jacobian for untransformed
!     standard basis functions are given
!     needed for consistent integration
!     
!     local derivatives of the shape functions: xi-derivative
!     
      shp(1,1)=4.d0*(xi+et)-3.d0
      shp(1,2)=4.d0*xi-1.d0
      shp(1,3)=0.d0
      shp(1,4)=4.d0*(1.d0-2.d0*xi-et)
      shp(1,5)=4.d0*et
      shp(1,6)=-4.d0*et
!     
!     local derivatives of the shape functions: eta-derivative
!     
      shp(2,1)=4.d0*(xi+et)-3.d0
      shp(2,2)=0.d0
      shp(2,3)=4.d0*et-1.d0
      shp(2,4)=-4.d0*xi
      shp(2,5)=4.d0*xi
      shp(2,6)=4.d0*(1.d0-xi-2.d0*et)          
!     
!     Dual shape functions with transformed std basis
!     
      do i=1,6
         shp(4,i)=0.0
         do j=1,6
            shp(4,i)=shp(4,i)+pslavdual((i-1)*8+j,ns)*shp(3,j)
         enddo 
      enddo
!     
!     computation of the local derivative of the global coordinates
!     (xs)
!     
      do i=1,3
        do j=1,2
           xs(i,j)=0.d0
           do k=1,6
              xs(i,j)=xs(i,j)+xl(i,k)*shp(j,k)
           enddo
        enddo
      enddo
!     
!     computation of the jacobian vector
!     
      xsj(1)=xs(2,1)*xs(3,2)-xs(3,1)*xs(2,2)
      xsj(2)=xs(1,2)*xs(3,1)-xs(3,2)*xs(1,1)
      xsj(3)=xs(1,1)*xs(2,2)-xs(2,1)*xs(1,2)
!     
      if(iflag.eq.2) return
!     
!     computation of the global derivative of the local coordinates
!     (xsi) (inversion of xs)
!     
      xsi(1,1)=xs(2,2)/xsj(3)
      xsi(2,1)=-xs(2,1)/xsj(3)
      xsi(1,2)=-xs(1,2)/xsj(3)
      xsi(2,2)=xs(1,1)/xsj(3)
      xsi(1,3)=-xs(2,2)/xsj(1)
      xsi(2,3)=xs(2,1)/xsj(1)
!     
!     computation of the global derivatives of the shape functions
!     
      do k=1,6
         do j=1,3
            sh(j)=shp(1,k)*xsi(1,j)+shp(2,k)*xsi(2,j)
         enddo
         do j=1,3
            shp(j,k)=sh(j)
         enddo
      enddo
!     
      return
      end
      
