Function PrintProgressCallback

include/cppoptlib/solver/solver.h:60–147

// is passed as an argument.
template <class FunctionType, class StateType>
auto PrintProgressCallback(std::ostream& output_stream) {
  return [&output_stream](const FunctionType& function, const StateType& state,
                          const Progress<FunctionType, StateType>& progress) {
    const int label_width = 18;  // Width for labels like "Value:", "X Delta:"
    const int num_width = 15;    // Width for numeric values
    const int precision = 6;     // Decimal places for floating-point numbers

    output_stream << std::fixed << std::setprecision(precision);

    // --- Iteration Header ---
    output_stream << "--- Iteration: " << std::setw(5) << std::right
                  << progress.num_iterations << " ---\n";

    // --- Function & State Information ---
    // Read the cached value/gradient from the populated FunctionState.
    // Fall back to one `function()` call only if the state isn't populated
    // (e.g. first iteration with a user-supplied state that didn't go
    // through `Solver::Minimize`'s initial evaluating constructor).
    if constexpr (IsFunctionState<StateType>::value) {
      const auto value = (state.gradient.size() == state.x.size())
                             ? state.value
                             : function(state.x);
      output_stream << std::left << std::setw(label_width)
                    << "  Value:" << std::right << std::setw(num_width) << value
                    << "\n";
    } else {
      output_stream << std::left << std::setw(label_width)
                    << "  Value:" << std::right << std::setw(num_width)
                    << function(state.x) << "\n";
    }

    // Format vector/matrix output using a stringstream to avoid messing up
    // widths easily
    std::stringstream ss_x;
    ss_x << state.x.transpose();

    output_stream << std::left << std::setw(label_width) << "  X:"
                  << " " << ss_x.str() << "\n";

    // --- Gradient (Conditional) ---
    if constexpr (FunctionType::Differentiability >=
                  cppoptlib::function::DifferentiabilityMode::First) {
      typename FunctionType::VectorType gradient;
      if constexpr (IsFunctionState<StateType>::value) {
        if (state.gradient.size() == state.x.size()) {
          gradient = state.gradient;
        } else {
          gradient.resize(state.x.size());
          function(state.x, &gradient);
        }
      } else {
        gradient.resize(state.x.size());
        function(state.x, &gradient);
      }
      std::stringstream ss_grad;
      ss_grad << gradient.transpose();
      output_stream << std::left << std::setw(label_width) << "  Gradient:"
                    << " " << ss_grad.str() << "\n";