// ----------------------------------------------------------------------------
// -                        Open3D: www.open3d.org                            -
// ----------------------------------------------------------------------------
// Copyright (c) 2018-2023 www.open3d.org
// SPDX-License-Identifier: MIT
// ----------------------------------------------------------------------------

#pragma once

namespace open3d {
namespace core {

/// This class is from: PyTorch's aten/src/ATen/detail/FunctionTraits.h
/// Fallback, anything with an operator()
template <typename T>
struct FunctionTraits : public FunctionTraits<decltype(&T::operator())> {};

/// Pointers to class members that are themselves functors.
/// For example, in the following code:
///
///     template <typename func_t>
///     struct S {
///         func_t f;
///     };
///     template <typename func_t>
///     S<func_t> make_s(func_t f) {
///         return S<func_t> { .f = f };
///     }
///
///     auto s = make_s([] (int, float) -> double { /* ... */ });
///
///     FunctionTraits<decltype(&s::f)> traits;
template <typename ClassType, typename T>
struct FunctionTraits<T ClassType::*> : public FunctionTraits<T> {};

// Const class member functions
template <typename ClassType, typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType (ClassType::*)(Args...) const>
    : public FunctionTraits<ReturnType(Args...)> {};

// Reference types
template <typename T>
struct FunctionTraits<T&> : public FunctionTraits<T> {};
template <typename T>
struct FunctionTraits<T*> : public FunctionTraits<T> {};

// Free functions
template <typename ReturnType, typename... Args>
struct FunctionTraits<ReturnType(Args...)> {
    // arity is the number of arguments.
    enum { arity = sizeof...(Args) };

    typedef std::tuple<Args...> ArgsTuple;
    typedef ReturnType result_type;

    template <size_t i>
    struct arg {
        typedef typename std::tuple_element<i, std::tuple<Args...>>::type type;
        // the i-th argument is equivalent to the i-th tuple element of a tuple
        // composed of those arguments.
    };
};

template <typename T>
struct NullaryFunctionTraits {
    using traits = FunctionTraits<T>;
    using res_t = typename traits::result_type;
};

template <typename T>
struct UnaryFunctionTraits {
    using traits = FunctionTraits<T>;
    using res_t = typename traits::result_type;
    using arg0_t = typename traits::template arg<0>::type;
};

template <typename T>
struct BinaryFunctionTraits {
    using traits = FunctionTraits<T>;
    using res_t = typename traits::result_type;
    using arg0_t = typename traits::template arg<0>::type;
    using arg1_t = typename traits::template arg<1>::type;
};

}  // namespace core
}  // namespace open3d