type_casters.h

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#ifndef SRC_TYPE_CASTERS_H_
#define SRC_TYPE_CASTERS_H_

#include <pybind11/cast.h>
#include <pybind11/numpy.h>

#include <Rcs_MatNd.h>

/*
 * pybind11 type caster for RCS MatNd.
 *
 * A lot of this is adapted from the Eigen typecasters coming with pybind11.
 */
namespace pybind11
{
namespace detail
{

// pybind11 numpy array type with the same shape as the MatNd storage
typedef pybind11::array_t<double, pybind11::array::forcecast | pybind11::array::c_style> ArrayMatNd;

/*
 * Populate a MatNd so that it views the given numpy array's storage.
 */
static MatNd MatNd_fromNumpy(ArrayMatNd& nparray)
{
    // load dimensions
    unsigned int m = 1;
    unsigned int n = 1;
    if (nparray.ndim() >= 1) {
        m = nparray.shape(0);
    }
    if (nparray.ndim() == 2) {
        n = nparray.shape(1);
    }
    return MatNd_fromPtr(m, n, const_cast<double*>(nparray.data()));
}

/*
 * Create a numpy array from the given MatNd.
 *
 * If parent is NULL (ie default initialized handle), the data will be copied
 * If parent is None (python), an unguarded reference will be held
 * If parent is any other object, it is assumed that that object own mat.
 */
static handle MatNd_toNumpy(const MatNd* mat, handle parent = handle(), bool writeable = true)
{
    // create with dimensions, optimizing if mat->n = 1
    ArrayMatNd array;
    if (mat->n == 1) {
        // just a vector
        array = ArrayMatNd(mat->m, mat->ele, parent);
    }
    else {
        // a regular matrix
        array = ArrayMatNd({mat->m, mat->n}, mat->ele, parent);
    }
    
    // set read-only if needed
    if (!writeable) {
        array_proxy(array.ptr())->flags &= ~detail::npy_api::NPY_ARRAY_WRITEABLE_;
    }
    
    // release the handle to keep up the reference count!
    return array.release();
}

/*
 * Create a numpy array holding a reference to the given MatNd
 */
static handle MatNd_toNumpyRef(const MatNd* mat, handle parent = none())
{
    return MatNd_toNumpy(mat, parent, false);
}

/*
 * Create a numpy array holding a reference to the given MatNd
 */
static handle MatNd_toNumpyRef(MatNd* mat, handle parent = none())
{
    return MatNd_toNumpy(mat, parent, true);
}

/*
 * Create a numpy array taking ownership of the given MatNd
 */
static handle MatNd_toNumpyOwn(MatNd* mat)
{
    capsule caps(mat, [](void* ref) { MatNd_destroy((MatNd*) ref); });
    return MatNd_toNumpy(mat, caps);
}


template<>
struct type_caster<MatNd>
{
public:
    /**
     * Function signature for documentation
     */
#if PYBIND11_VERSION_MINOR >= 3
    static constexpr auto name = _("MatNd");
#else
    static PYBIND11_DESCR name() { return type_descr(_("MatNd")); }
#endif


protected:
    // local value
    MatNd value;
    
    // a reference to the numpy array backing value
    ArrayMatNd copyOrRef;
    
    // a double type caster used to process scalars as 1x1 matrices
    make_caster<double> scalar_caster;
    
    // true if we're None a.k.a. NULL
    bool none = false;

public:
    /**
     * Access to the internal loaded value store.
     */
    operator MatNd*()
    {
        if (none) {
            return NULL;
        }
        return &value;
    }
    
    operator MatNd&()
    {
        if (none) {
            throw value_error("Cannot convert None to a MatNd ref");
        }
        
        return value;
    }
    
    template<typename T_> using cast_op_type = pybind11::detail::cast_op_type<T_>;
    
    /**
     * Conversion part 1 (Python->C++): convert a PyObject into a MatNd
     * instance or return false upon failure. The second argument
     * indicates whether implicit conversions should be applied.
     */
    bool load(handle src, bool convert)
    {
        if (!convert && !isinstance<ArrayMatNd>(src)) {
            return false;
        }
        if (src.is_none()) {
            // allow none and pass it as NULL
            none = true;
            return true;
        }
        
        if (scalar_caster.load(src, convert)) {
            // scalars should be handled as 1x1 mats. ArrayMatNd::ensure is capable of this,
            // but would allocate memory which is unnecessary.
            value.ele = cast_op<double*>(scalar_caster);
            value.m = 1;
            value.n = 1;
            value.size = 1;
            value.stackMem = true;
            return true;
        }
        
        // ensure and cast if needed. This copies only if needed
        copyOrRef = ArrayMatNd::ensure(src);
        
        // we can only handle arrays of 2 or less dimensions
        auto dims = copyOrRef.ndim();
        if (dims > 2) {
            return false;
        }
        
        // fill value so that it uses the same memory as copyOrRef
        value = MatNd_fromNumpy(copyOrRef);
        return true;
    }
    
    /**
     * Conversion part 2 (C++ -> Python): convert a MatNd instance into
     * a Python object. The second and third arguments are used to
     * indicate the return value policy and parent object (for
     * ``return_value_policy::reference_internal``).
     */
    static handle cast(MatNd* src, return_value_policy policy, handle parent)
    {
        switch (policy) {
            case return_value_policy::take_ownership:
            case return_value_policy::automatic:
                // this is not entirely correct, since usually we wouldn't support this for pointers at all.
                // however, when used in a tuple, we will always get this policy, so we need to deal with it.
                // and after a fashion, moving a pointer is essentially the same as taking ownership
            case return_value_policy::move:
                
                return MatNd_toNumpyOwn(src);
            case return_value_policy::copy:
                return MatNd_toNumpy(src);
            case return_value_policy::reference:
            case return_value_policy::automatic_reference:
                return MatNd_toNumpyRef(src);
            case return_value_policy::reference_internal:
                return MatNd_toNumpyRef(src, parent);
            default:
                pybind11_fail("Invalid return_value_policy for Rcs MatNd type");
        };
    }
    
    static handle cast(const MatNd* src, return_value_policy policy, handle parent)
    {
        switch (policy) {
            case return_value_policy::copy:
                return MatNd_toNumpy(src);
            case return_value_policy::reference:
            case return_value_policy::automatic_reference:
                return MatNd_toNumpyRef(src);
            case return_value_policy::reference_internal:
                return MatNd_toNumpyRef(src, parent);
            default:
                // can also not take ownership of a const matrix
                pybind11_fail("Invalid return_value_policy for Rcs MatNd type");
        };
    }
};

}
} // namespace pybind11::detail


#endif /* SRC_TYPE_CASTERS_H_ */