impose_boundary_conditions Subroutine

   pure subroutine impose_boundary_conditions(self, U)
   !< Impose boundary conditions.
   !<
   !< The boundary conditions are imposed on the primitive variables by means of the ghost cells approach.
   class(euler_1d),              intent(in)    :: self          !< Euler field.
   type(conservative_compressible), intent(inout) :: U(1-self%Ng:) !< Conservative variables.
   ! type(primitive_compressible), intent(inout) :: P(1-self%Ng:) !< Primitive variables.
   integer(I4P)                                :: i             !< Space counter.

   select case(trim(adjustl(self%BC_L)))
      case('TRA') ! trasmissive (non reflective) BC
         do i=1-self%Ng, 0
            ! P(i) = P(-i+1)
            U(i) = U(-i+1)
         enddo
      case('REF') ! reflective BC
         do i=1-self%Ng, 0
            ! P(i)%density  =   P(-i+1)%density
            ! P(i)%velocity = - P(-i+1)%velocity
            ! P(i)%pressure =   P(-i+1)%pressure
            U(i)%density  =   U(-i+1)%density
            U(i)%momentum = - U(-i+1)%momentum
            U(i)%energy   =   U(-i+1)%energy
         enddo
   endselect

   select case(trim(adjustl(self%BC_R)))
      case('TRA') ! trasmissive (non reflective) BC
         do i=self%Ni+1, self%Ni+self%Ng
            ! P(i) = P(self%Ni-(i-self%Ni-1))
            U(i) = U(self%Ni-(i-self%Ni-1))
         enddo
      case('REF') ! reflective BC
         do i=self%Ni+1, self%Ni+self%Ng
            ! P(i)%density  =   P(self%Ni-(i-self%Ni-1))%density
            ! P(i)%velocity = - P(self%Ni-(i-self%Ni-1))%velocity
            ! P(i)%pressure =   P(self%Ni-(i-self%Ni-1))%pressure
            U(i)%density  =   U(self%Ni-(i-self%Ni-1))%density
            U(i)%momentum = - U(self%Ni-(i-self%Ni-1))%momentum
            U(i)%energy   =   U(self%Ni-(i-self%Ni-1))%energy
         enddo
   endselect
   endsubroutine impose_boundary_conditions