energy Function

   elemental function density(self, energy, pressure, speed_of_sound, temperature) result(density_)
   !< Return density.
   class(eos_compressible), intent(in)           :: self           !< Equation of state.
   real(R8P),               intent(in), optional :: energy         !< Specific internal energy value.
   real(R8P),               intent(in), optional :: pressure       !< Pressure value.
   real(R8P),               intent(in), optional :: speed_of_sound !< Speed of sound value.
   real(R8P),               intent(in), optional :: temperature    !< Temperature value.
   real(R8P)                                     :: density_       !< Density value.

   density_ = 0._R8P
   if (present(energy).and.present(pressure)) then
      density_ = pressure / ((self%g_ - 1._R8P) * energy)
   elseif (present(pressure).and.present(speed_of_sound)) then
      density_ = self%g_ * pressure / (speed_of_sound * speed_of_sound)
   elseif (present(pressure).and.present(temperature)) then
      density_ = pressure / (self%R_ * temperature)
   endif
   endfunction density

   pure function description(self, prefix) result(desc)
   !< Return a pretty-formatted object description.
   class(eos_compressible), intent(in)           :: self             !< Equation of state.
   character(*),            intent(in), optional :: prefix           !< Prefixing string.
   character(len=:), allocatable                 :: desc             !< Description.
   character(len=:), allocatable                 :: prefix_          !< Prefixing string, local variable.
   character(len=1), parameter                   :: NL=new_line('a') !< New line character.

   prefix_ = '' ; if (present(prefix)) prefix_ = prefix
   desc = ''
   desc = desc//prefix_//'cp  = '//trim(str(n=self%cp_))//NL
   desc = desc//prefix_//'cv  = '//trim(str(n=self%cv_))
   endfunction description

   elemental function energy(self, density, pressure, temperature) result(energy_)
   !< Return specific internal energy.
   class(eos_compressible), intent(in)           :: self        !< Equation of state.
   real(R8P),               intent(in), optional :: density     !< Density value.
   real(R8P),               intent(in), optional :: pressure    !< Pressure value.
   real(R8P),               intent(in), optional :: temperature !< Temperature value.
   real(R8P)                                     :: energy_     !< Energy value.

   energy_ = 0._R8P
   if (present(density).and.present(pressure)) then
      energy_ = pressure / ((self%g_ - 1._R8P) * density)
   elseif (present(temperature)) then
      energy_ = self%cv() * temperature
   endif
   endfunction energy