MapLibreParticleLab/include/mln/style/conversion_impl.hpp

#pragma once

#include <mln/style/color_ramp_property_value.hpp>
#include <mln/style/conversion.hpp>
#include <mln/style/expression/image.hpp>
#include <mln/style/layer.hpp>
#include <mln/style/property_value.hpp>
#include <mln/style/rotation.hpp>
#include <mln/style/transition_options.hpp>
#include <mln/util/feature.hpp>
#include <mln/util/geojson.hpp>
#include <mln/util/traits.hpp>

#include <mapbox/compatibility/value.hpp>

#include <array>
#include <chrono>
#include <string>
#include <type_traits>
#include <optional>

namespace mln {
namespace style {

/**
   The `conversion` namespace defines conversions from JSON structures
   conforming to the schema defined by the MapLibre %Style Specification, to the
   various C++ types that form the C++ model of that domain:

       * `std::unique_ptr<Source>`
       * `std::unique_ptr<Layer>`
       * `Filter`
       * `PropertyValue<T>`

   A single template function serves as the public interface:

       template <class T>
       std::optional<T> convert(const Convertible& input, Error& error);

   Where `T` is one of the above types. If the conversion fails, the result is
   empty, and the error parameter includes diagnostic text suitable for
   presentation to a library user. Otherwise, a filled optional is returned.

   `Convertible` is a type that encapsulates a special form of polymorphism over
   various underlying types that can serve as input to the conversion algorithm.
   For instance, on macOS, we need to support conversion from both RapidJSON
   types, and a JSON structure represented with `NSArray`/`NSDictionary`/etc. On
   Qt, we need to support conversion from RapidJSON types and QVariant.

   We don't want to use traditional forms of polymorphism to accomplish this:

     * Compile time polymorphism using a template parameter for the actual value
   type leads to excessive code bloat and long compile times.
     * Runtime polymorphism using virtual methods requires extra heap allocation
   and ubiquitous use of std::unique_ptr, unsuitable for this
   performance-sensitive code.

   Therefore, we're using a custom implementation of runtime polymorphism where
   we manually create and dispatch through a table of function pointers
   (vtable), while keeping the storage for any of the possible underlying types
   inline on the stack, using `std::aligned_storage`.

   For a given underlying type T, an explicit specialization of
   `ConversionTraits<T>` must be provided. This specialization must provide the
   following static methods:

      * `isUndefined(v)` -- returns a boolean indication whether `v` is
   undefined or a JSON null

      * `isArray(v)` -- returns a boolean indicating whether `v` represents a
   JSON array
      * `arrayLength(v)` -- called only if `isArray(v)`; returns a size_t length
      * `arrayMember(v)` -- called only if `isArray(v)`; returns `V` or `V&`

      * `isObject(v)` -- returns a boolean indicating whether `v` represents a
   JSON object
      * `objectMember(v, name)` -- called only if `isObject(v)`; `name` is
   `const char *`; return value:
         * is true when evaluated in a boolean context iff the named member
   exists
         * is convertible to a `V` or `V&` when dereferenced
      * `eachMember(v, [] (const std::string&, const V&) -> std::optional<Error>
   {...})` -- called only if `isObject(v)`; calls the provided lambda once for
   each key and value of the object; short-circuits if any call returns an
   `Error`

      * `toBool(v)` -- returns `optional<bool>`, absence indicating `v` is not a
   JSON boolean
      * `toNumber(v)` -- returns `optional<float>`, absence indicating `v` is
   not a JSON number
      * `toDouble(v)` -- returns `optional<double>`, absence indicating `v` is
   not a JSON number
      * `toString(v)` -- returns `optional<std::string>`, absence indicating `v`
   is not a JSON string
      * `toValue(v)` -- returns `optional<Value>`, a variant type, for generic
   conversion, absence indicating `v` is not a boolean, number, or string.
   Numbers should be converted to unsigned integer, signed integer, or floating
   point, in descending preference.

   In addition, the type T must be move-constructable. And finally,
   `Convertible::Storage`, a typedef for `std::aligned_storage_t`, must be large
   enough to satisfy the memory requirements for any of the possible underlying
   types. (A static assert will fail if this is not the case.)

   `Convertible` itself is movable, but not copyable. A moved-from `Convertible`
   is in an invalid state; you must not do anything with it except let it go out
   of scope.
*/
namespace conversion {

template <typename T>
class ConversionTraits;

class Convertible {
public:
    template <typename T>
    // NOLINTNEXTLINE(bugprone-forwarding-reference-overload)
    Convertible(T&& value)
        : vtable(vtableForType<std::decay_t<T>>()) {
        static_assert(sizeof(Storage) >= sizeof(std::decay_t<T>), "Storage must be large enough to hold value type");
        new (static_cast<void*>(&storage)) std::decay_t<T>(std::forward<T>(value));
    }

    Convertible(Convertible&& v) noexcept
        : vtable(v.vtable) {
        // NOLINTNEXTLINE(performance-move-const-arg)
        vtable->move(std::move(v.storage), storage);
    }

    ~Convertible() { vtable->destroy(storage); }

    Convertible& operator=(Convertible&& v) noexcept {
        if (this != &v) {
            vtable->destroy(storage);
            vtable = v.vtable;
            // NOLINTNEXTLINE(performance-move-const-arg)
            vtable->move(std::move(v.storage), storage);
        }
        return *this;
    }

    Convertible() = delete;
    Convertible(const Convertible&) = delete;
    Convertible& operator=(const Convertible&) = delete;

    friend inline bool isUndefined(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->isUndefined(v.storage);
    }

    friend inline bool isArray(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->isArray(v.storage);
    }

    friend inline std::size_t arrayLength(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->arrayLength(v.storage);
    }

    friend inline Convertible arrayMember(const Convertible& v, std::size_t i) {
        assert(v.vtable);
        return v.vtable->arrayMember(v.storage, i);
    }

    friend inline bool isObject(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->isObject(v.storage);
    }

    friend inline std::optional<Convertible> objectMember(const Convertible& v, const char* name) {
        assert(v.vtable);
        return v.vtable->objectMember(v.storage, name);
    }

    friend inline std::optional<Error> eachMember(
        const Convertible& v, const std::function<std::optional<Error>(const std::string&, const Convertible&)>& fn) {
        assert(v.vtable);
        return v.vtable->eachMember(v.storage, fn);
    }

    friend inline std::optional<bool> toBool(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->toBool(v.storage);
    }

    friend inline std::optional<float> toNumber(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->toNumber(v.storage);
    }

    friend inline std::optional<double> toDouble(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->toDouble(v.storage);
    }

    friend inline std::optional<std::string> toString(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->toString(v.storage);
    }

    friend inline std::optional<Value> toValue(const Convertible& v) {
        assert(v.vtable);
        return v.vtable->toValue(v.storage);
    }

    friend inline std::optional<GeoJSON> toGeoJSON(const Convertible& v, Error& error) {
        assert(v.vtable);
        return v.vtable->toGeoJSON(v.storage, error);
    }

private:
#if __ANDROID__
    // Android:     JSValue* or mln::android::Value
    using Storage = std::aligned_storage_t<32, 8>;
#elif __QT__
    // Qt:          JSValue* or QVariant
    using Storage = std::aligned_storage_t<32, 8>;
#else
    // Node:        JSValue* or v8::Local<v8::Value>
    // iOS/macOS:   JSValue* or id
    using Storage = std::aligned_storage_t<8, 8>;
#endif

    struct VTable {
        void (*move)(Storage&& src, Storage& dest);
        void (*destroy)(Storage&);

        bool (*isUndefined)(const Storage&);

        bool (*isArray)(const Storage&);
        std::size_t (*arrayLength)(const Storage&);
        Convertible (*arrayMember)(const Storage&, std::size_t);

        bool (*isObject)(const Storage&);
        std::optional<Convertible> (*objectMember)(const Storage&, const char*);
        std::optional<Error> (*eachMember)(
            const Storage&, const std::function<std::optional<Error>(const std::string&, const Convertible&)>&);

        std::optional<bool> (*toBool)(const Storage&);
        std::optional<float> (*toNumber)(const Storage&);
        std::optional<double> (*toDouble)(const Storage&);
        std::optional<std::string> (*toString)(const Storage&);
        std::optional<Value> (*toValue)(const Storage&);

        // https://github.com/mapbox/mapbox-gl-native/issues/5623
        std::optional<GeoJSON> (*toGeoJSON)(const Storage&, Error&);
    };

    // Extracted this function from the table below to work around a GCC bug
    // with differing visibility settings for capturing lambdas:
    // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80947
    template <typename T>
    static auto vtableEachMember(
        const Storage& s, const std::function<std::optional<Error>(const std::string&, const Convertible&)>& fn) {
        return ConversionTraits<T>::eachMember(reinterpret_cast<const T&>(s), [&](const std::string& k, T&& v) {
            return fn(k, Convertible(std::move(v)));
        });
    }

    template <typename T>
    static VTable* vtableForType() {
        using Traits = ConversionTraits<T>;
        static VTable vtable = {
            [](Storage&& src, Storage& dest) { new (static_cast<void*>(&dest)) T(reinterpret_cast<T&&>(src)); },
            [](Storage& s) { reinterpret_cast<T&>(s).~T(); },
            [](const Storage& s) { return Traits::isUndefined(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::isArray(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::arrayLength(reinterpret_cast<const T&>(s)); },
            [](const Storage& s, std::size_t i) {
                return Convertible(Traits::arrayMember(reinterpret_cast<const T&>(s), i));
            },
            [](const Storage& s) { return Traits::isObject(reinterpret_cast<const T&>(s)); },
            [](const Storage& s, const char* key) {
                std::optional<T> member = Traits::objectMember(reinterpret_cast<const T&>(s), key);
                if (member) {
                    return std::optional<Convertible>(Convertible(std::move(*member)));
                } else {
                    return std::optional<Convertible>();
                }
            },
            vtableEachMember<T>,
            [](const Storage& s) { return Traits::toBool(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::toNumber(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::toDouble(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::toString(reinterpret_cast<const T&>(s)); },
            [](const Storage& s) { return Traits::toValue(reinterpret_cast<const T&>(s)); },
            [](const Storage& s, Error& err) {
                return Traits::toGeoJSON(reinterpret_cast<const T&>(s), err);
            }};
        return &vtable;
    }

    VTable* vtable;
    Storage storage;
};

template <class T, class... Args>
std::optional<T> convert(const Convertible& value, Error& error, Args&&... args) {
    return Converter<T>()(value, error, std::forward<Args>(args)...);
}

template <>
struct ValueFactory<ColorRampPropertyValue> {
    static Value make(const ColorRampPropertyValue& value) { return value.getExpression().serialize(); }
};

template <>
struct ValueFactory<TransitionOptions> {
    static Value make(const TransitionOptions& value) { return value.serialize(); }
};

template <>
struct ValueFactory<Color> {
    static Value make(const Color& color) { return color.serialize(); }
};

template <>
struct ValueFactory<Padding> {
    static Value make(const Padding& padding) { return padding.serialize(); }
};

template <>
struct ValueFactory<VariableAnchorOffsetCollection> {
    static Value make(const VariableAnchorOffsetCollection& variableAnchorOffset) {
        return variableAnchorOffset.serialize();
    }
};

template <typename T>
struct ValueFactory<T, typename std::enable_if_t<(!std::is_enum_v<T> && !is_linear_container<T>::value)>> {
    static Value make(const T& arg) { return {arg}; }
};

template <typename T>
struct ValueFactory<T, typename std::enable_if_t<std::is_enum_v<T>>> {
    static Value make(T arg) { return {Enum<T>::toString(arg)}; }
};

template <typename T>
struct ValueFactory<T, typename std::enable_if_t<is_linear_container<T>::value>> {
    static Value make(const T& arg) {
        mapbox::base::ValueArray result;
        result.reserve(arg.size());
        for (const auto& item : arg) {
            result.emplace_back(ValueFactory<std::decay_t<decltype(item)>>::make(item));
        }
        return result;
    }
};

template <>
struct ValueFactory<Position> {
    static Value make(const Position& position) {
        return ValueFactory<std::array<float, 3>>::make(position.getSpherical());
    }
};

template <>
struct ValueFactory<Rotation> {
    static Value make(const Rotation& rotation) { return {rotation.getAngle()}; }
};

template <>
struct ValueFactory<float> {
    static Value make(float f) { return f; }
};

template <typename T>
Value makeValue(T&& arg) {
    return ValueFactory<std::decay_t<T>>::make(std::forward<T>(arg));
}

template <typename T>
StyleProperty makeStyleProperty(const PropertyValue<T>& value) {
    return value.match(
        [](const Undefined&) -> StyleProperty { return {}; },
        [](const Color& c) -> StyleProperty { return {makeValue(c), StyleProperty::Kind::Expression}; },
        [](const Padding& p) -> StyleProperty { return {makeValue(p), StyleProperty::Kind::Expression}; },
        [](const VariableAnchorOffsetCollection& v) -> StyleProperty {
            return {makeValue(v), StyleProperty::Kind::Expression};
        },
        [](const PropertyExpression<T>& fn) -> StyleProperty {
            return {fn.getExpression().serialize(), StyleProperty::Kind::Expression};
        },
        [](const auto& t) -> StyleProperty { return {makeValue(t), StyleProperty::Kind::Constant}; });
}

inline StyleProperty makeStyleProperty(const TransitionOptions& value) {
    if (!value.isDefined()) return {};
    return {makeValue(value), StyleProperty::Kind::Transition};
}

inline StyleProperty makeStyleProperty(const ColorRampPropertyValue& value) {
    if (value.isUndefined()) return {};
    return {makeValue(value), StyleProperty::Kind::Expression};
}

} // namespace conversion
} // namespace style
} // namespace mln