Remove Abseil dependency to allow cleaner Python bindings

[rulyone]

Hello.

I'm working on Python bindings for using this library, and so far I have accomplished this with a Not so clean solution using C++ lambda expressions to avoid issues with the absl::Status returned by the Reset and Update functions of the StrokeModeler class like this:

py::class_<ink::stroke_model::StrokeModeler>(handle, "StrokeModeler")
        .def(py::init<>())
        .def("Reset", [](ink::stroke_model::StrokeModeler &self) -> py::str {
            auto status = self.Reset();
            if(!status.ok()) {
                return py::str(status.message());
            }
            return py::str("OK");
        })
        .def("Reset", [](ink::stroke_model::StrokeModeler &self, ink::stroke_model::StrokeModelParams params) -> py::str {
            auto status = self.Reset(params);
            if(!status.ok()) {
                return py::str(status.message());
            }
            return py::str("OK");
        })
        .def("Update", [](ink::stroke_model::StrokeModeler &self, const ink::stroke_model::Input& input, std::vector<ink::stroke_model::Result>& result) -> py::str {
            auto status = self.Update(input, result);
            if(!status.ok()) {
                return py::str(status.message());
            }
            return py::str("OK");
        })
        .def("Save", &ink::stroke_model::StrokeModeler::Save)
        .def("Restore", &ink::stroke_model::StrokeModeler::Restore);

Here you can see that the Save and Restore bindings are much clear to translate to python.

Another issue is that for this abseil implementation, I need to add into the pybind11 CMakeLists.txt file all dependencies on this project, eg:

pybind11_add_module(
    smooth_lines 
    wrapper.cpp 
    #The following dependencies are not needed if there weren't an abseil dependency.
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/types.cc
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/params.cc
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/stroke_modeler.cc

    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/wobble_smoother.cc
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/stylus_state_modeler.cc

    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_predictor.cc
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/stroke_end_predictor.cc

    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_filter/axis_predictor.cc
    ${CMAKE_SOURCE_DIR}/ink-stroke-modeler/ink_stroke_modeler/internal/prediction/kalman_filter/kalman_filter.cc
)

If abseil is not a dependency, the CMakeLists.txt file above would be much cleaner.

My suggestion is to use a custom exception class instead of using Abseil library. Not an expert in C++ so I'm not sure if a custom class is the best option, but removing the Abseil dependency would be great for this use case (and might be the same for other bindings).

[sfreilich]

Do the pybind Abseil bindings make this easier?
https://github.com/pybind/pybind11_abseil