FlipFaceCoords3D Subroutine

  pure subroutine FlipFaceCoords3D(u,v,flip,u2,v2)
    !! Coordinate form of MortarFaceMap: maps receiver-face coordinates (u,v) to
    !! donor-face coordinates (u2,v2) for each of the eight SELF face flips.
    implicit none
    real(prec),intent(in) :: u,v
    integer,intent(in) :: flip
    real(prec),intent(out) :: u2,v2

    select case(flip)
    case(0)
      u2 = u
      v2 = v
    case(1)
      u2 = -u
      v2 = v
    case(2)
      u2 = -u
      v2 = -v
    case(3)
      u2 = u
      v2 = -v
    case(4)
      u2 = v
      v2 = u
    case(5)
      u2 = -v
      v2 = u
    case(6)
      u2 = -v
      v2 = -u
    case default ! flip == 7
      u2 = v
      v2 = -u
    endselect

  endsubroutine FlipFaceCoords3D