/* Copyright (C) 2018 Olivier Goffart <ogoffart@woboq.com>

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and
associated documentation files (the "Software"), to deal in the Software without restriction,
including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so,
subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial
portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT
NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES
OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
*/
#pragma once

#include <QtCore/QObject>
#include <QtCore/QEvent>
#include <QtCore/QDebug>

/// Pointer to a method of QObject which takes no arguments and returns nothing.
/// Actually this is a "type-erased" method with various arguments and return
/// value, but it merely represents a generic pointer, and let other code
/// handle the types and memory safety.
using QObjectErasedMethod = void (QObject::*)();

/// This type represents signals defined both in C++ and Rust, and provides
/// handy conversions.
///
/// Internally Qt signals are represented by some ID which must be unique
/// within class hierarchy (not to be confused with indices). So, things
/// like `&QObject::objectNameChanged` are only meaningful as far as they
/// can be converted to some kind of magic representation (`void **`), and
/// those representations can be compared for equality.
///
/// In C++, signals are represented as pointers to member functions, but with
/// types erased down to `void (QObject::*)()`.
/// From `QMetaObject::Connection QObject::connectImpl(...)` documentation:
/// > signal is a pointer to a pointer to a member signal of the sender
///
/// For classes defined in Rust, signals are represented as offsets of
/// corresponding `RustSignal` fields from the base address of their struct.
/// This provides an easy way to guarantee ID uniqueness at a low price of
/// having one bool field per signal.
///
/// # Safety
///
/// Users of `SignalInner` must ensure that they only ever use a
/// signal of one class with instances of that class or its subclasses.
///
/// Erased `cpp_erased_method` is not directly used as a function pointer anyway,
/// so it is safe even if it contains garbage.
///
/// # Further reading
///
///  - http://itanium-cxx-abi.github.io/cxx-abi/abi.html#member-pointers
///  - https://docs.microsoft.com/en-us/cpp/cpp/pointers-to-members?view=vs-2019
union SignalInner {
// No need to be public. Pointer to a signal is exposed via safe public getter.
private:
    /// For signals derived from `RustSignal` Rust structs, e.g. `greeter.name_changed`.
    ptrdiff_t rust_field_offset;
    /// For signals defined in C++ classes, e.g. `&QObject::objectNameChanged`.
    QObjectErasedMethod cpp_erased_method;

public:
    /// Construct signal representation for an arbitrary Qt signal defined in Rust
    /// as an offset of signal's field within Rust struct.
    explicit SignalInner(ptrdiff_t field_offset)
        : rust_field_offset(field_offset)
    {}

    /// Construct signal representation for an arbitrary Qt signal defined in C++.
    ///
    /// Note: this is an implicit conversion.
    template<typename R, typename Type, typename ...Args>
    SignalInner(R (Type::* qt_signal)(Args...))
        // (there is a double indirection in the reinterpret_cast to avoid -Wcast-function-type)
        : cpp_erased_method(*reinterpret_cast<QObjectErasedMethod *>(&qt_signal))
    {}

    /// Qt uses "pointer to a pointer to a member" signal representation inside
    /// `QObject::connect(...)` functions. This little helper encapsulates the
    /// required cast.
    void **asRawSignal() {
        return reinterpret_cast<void **>(&cpp_erased_method);
        // equivalently:
        // return reinterpret_cast<void **>(&rust_field_offset);
    }
};

/// Wrapper for Rust `std::raw::TraitObject` struct.
///
/// Note: `std::raw` is marked unstable as of Rust 1.43.0, so for future
/// compatibility it would be better to box the trait object on the heap,
/// and never manipulate its content directly from C++. For the time being,
/// though, let it be.
struct TraitObject {
    void *data;
    void *vtable;

    /// Nullability check.
    bool isValid() const {
        return data && vtable;
    }

    /// Forget about referenced object.
    ///
    /// If this TraitObject represented a `Box` (owned object) rather than a
    /// `&dyn` reference (borrowed object) then it may cause memory leaks,
    /// unless a copy was made for later proper destruction.
    inline void invalidate() {
        data = nullptr;
        vtable = nullptr;
    }
};

extern "C" QMetaObject *RustObject_metaObject(TraitObject);
extern "C" void RustObject_destruct(TraitObject);

/// "513 reserved for Qt Jambi's DeleteOnMainThread event"
/// We are just re-using this event type for our purposes.
///
/// Source: https://github.com/qtjambi/qtjambi/blob/8ef99da63315945e6ab540cc31d66e5b021b69e4/src/cpp/qtjambi/qtjambidebugevent.cpp#L857
static constexpr int QtJambi_EventType_DeleteOnMainThread = 513;

template <typename Base>
struct RustObject : Base {
    TraitObject rust_object;  // A QObjectPinned<XXX> where XXX is the base trait
    TraitObject ptr_qobject;  // a QObjectPinned<QObject>
    void (*extra_destruct)(QObject *);
    const QMetaObject *metaObject() const override {
        return ptr_qobject.isValid() ? RustObject_metaObject(ptr_qobject) : Base::metaObject();
    }
    int qt_metacall(QMetaObject::Call _c, int _id, void **_a) override {
        _id = Base::qt_metacall(_c, _id, _a);
        if (_id < 0)
            return _id;
        const QMetaObject *mo = metaObject();
        if (_c == QMetaObject::InvokeMetaMethod || _c == QMetaObject::RegisterMethodArgumentMetaType) {
            int methodCount = mo->methodCount();
            if (_id < methodCount)
                mo->d.static_metacall(this, _c, _id, _a);
            _id -= methodCount;
        } else if ((_c >= QMetaObject::ReadProperty && _c <=
#if QT_VERSION < QT_VERSION_CHECK(6, 0, 0)
                QMetaObject::QueryPropertyUser
#else
                QMetaObject::ResetProperty
#endif
            ) || _c == QMetaObject::RegisterPropertyMetaType) {
            int propertyCount = mo->propertyCount();
            if (_id < propertyCount)
                mo->d.static_metacall(this, _c, _id, _a);
            _id -= propertyCount;
        }
        return _id;
    }
    bool event(QEvent *event) override {
        if (ptr_qobject.isValid() && event->type() == QtJambi_EventType_DeleteOnMainThread) {
            // This event is sent by rust when we are deleted.
            ptr_qobject.invalidate(); // so the destructor don't recurse
            delete this;
            return true;
        }
        return Base::event(event);
    }
    ~RustObject() {
        auto r = ptr_qobject;
        ptr_qobject.invalidate();
        if (extra_destruct)
            extra_destruct(this);
        if (r.isValid())
            RustObject_destruct(r);
    }
};

struct RustQObjectDescriptor {
    size_t size;
    const QMetaObject *baseMetaObject;
    QObject *(*create)(const TraitObject *, const TraitObject *);
    void (*qmlConstruct)(void *, const TraitObject *, const TraitObject *, void (*extra_destruct)(QObject *));
    TraitObject (*get_rust_refcell)(QObject *); // Possible optimisation: make this an offset

    /// Get singleton-per-type descriptor.
    template<typename T>
    static const RustQObjectDescriptor *instance();
};

template<typename T>
const RustQObjectDescriptor *RustQObjectDescriptor::instance() {
    static RustQObjectDescriptor desc {
        /*size*/ sizeof(T),
        /*baseMetaObject*/ &T::staticMetaObject,
        /*create*/ [](
                const TraitObject *self_pinned,
                const TraitObject *self_ptr
        ) -> QObject * {
            auto q = new T();
            q->ptr_qobject = *self_ptr;
            q->rust_object = *self_pinned;
            return q;
        },
        /*qmlConstruct*/ [](
                void *data,
                const TraitObject *self_pinned,
                const TraitObject *self_ptr,
                void (*extra_destruct)(QObject *)
        ) {
            auto *q = new (data) T();
            q->rust_object = *self_pinned;
            q->ptr_qobject = *self_ptr;
            q->extra_destruct = extra_destruct;
        },
        /*get_rust_refcell*/ [](QObject *q) {
            return static_cast<T *>(q)->ptr_qobject;
        }
    };
    return &desc;
}