src/castable.h

// Copyright 2020 The Tint Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef SRC_CASTABLE_H_
#define SRC_CASTABLE_H_

#include <stdint.h>
#include <functional>
#include <utility>

#include "src/traits.h"
#include "src/utils/crc32.h"

#if defined(__clang__)
/// Temporarily disable certain warnings when using Castable API
#define TINT_CASTABLE_PUSH_DISABLE_WARNINGS()                               \
  _Pragma("clang diagnostic push")                                     /**/ \
      _Pragma("clang diagnostic ignored \"-Wundefined-var-template\"") /**/ \
      static_assert(true, "require extra semicolon")

/// Restore disabled warnings
#define TINT_CASTABLE_POP_DISABLE_WARNINGS() \
  _Pragma("clang diagnostic pop") /**/       \
      static_assert(true, "require extra semicolon")
#else
#define TINT_CASTABLE_PUSH_DISABLE_WARNINGS() \
  static_assert(true, "require extra semicolon")
#define TINT_CASTABLE_POP_DISABLE_WARNINGS() \
  static_assert(true, "require extra semicolon")
#endif

TINT_CASTABLE_PUSH_DISABLE_WARNINGS();

namespace tint {

// Forward declaration
class CastableBase;

namespace detail {
template <typename T>
struct TypeInfoOf;

}  // namespace detail

/// Helper macro to instantiate the TypeInfo<T> template for `CLASS`.
#define TINT_INSTANTIATE_TYPEINFO(CLASS)                      \
  TINT_CASTABLE_PUSH_DISABLE_WARNINGS();                      \
  template <>                                                 \
  const tint::TypeInfo tint::detail::TypeInfoOf<CLASS>::info{ \
      &tint::detail::TypeInfoOf<CLASS::TrueBase>::info,       \
      #CLASS,                                                 \
      tint::TypeInfo::HashCodeOf<CLASS>(),                    \
      tint::TypeInfo::CombinedHashCodeOf<CLASS>(),            \
  };                                                          \
  TINT_CASTABLE_POP_DISABLE_WARNINGS()

/// Bit flags that can be passed to the template parameter `FLAGS` of Is() and
/// As().
enum CastFlags {
  /// Disables the static_assert() inside Is(), that compile-time-verifies that
  /// the cast is possible. This flag may be useful for highly-generic template
  /// code that needs to compile for template permutations that generate
  /// impossible casts.
  kDontErrorOnImpossibleCast = 1,
};

/// TypeInfo holds type information for a Castable type.
struct TypeInfo {
  /// The type of a hash code
  using HashCode = uint64_t;

  /// The base class of this type
  const TypeInfo* base;
  /// The type name
  const char* name;
  /// The type hash code
  const HashCode hashcode;
  /// The type hash code or'd with the base class' combined hash code
  const HashCode combined_hashcode;

  /// @param type the test type info
  /// @returns true if the class with this TypeInfo is of, or derives from the
  /// class with the given TypeInfo.
  inline bool Is(const tint::TypeInfo* type) const {
    // Optimization: Check whether the all the bits of the type's hashcode can
    // be found in the combined_hashcode. If a single bit is missing, then we
    // can quickly tell that that this TypeInfo does not derive from `type`.
    if ((combined_hashcode & type->hashcode) != type->hashcode) {
      return false;
    }

    // Walk the base types, starting with this TypeInfo, to see if any of the
    // pointers match `type`.
    for (auto* ti = this; ti != nullptr; ti = ti->base) {
      if (ti == type) {
        return true;
      }
    }
    return false;
  }

  /// @returns true if `type` derives from the class `TO`
  /// @param type the object type to test from, which must be, or derive from
  /// type `FROM`.
  /// @see CastFlags
  template <typename TO, typename FROM, int FLAGS = 0>
  static inline bool Is(const tint::TypeInfo* type) {
    constexpr const bool downcast = std::is_base_of<FROM, TO>::value;
    constexpr const bool upcast = std::is_base_of<TO, FROM>::value;
    constexpr const bool nocast = std::is_same<FROM, TO>::value;
    constexpr const bool assert_is_castable =
        (FLAGS & kDontErrorOnImpossibleCast) == 0;

    static_assert(upcast || downcast || nocast || !assert_is_castable,
                  "impossible cast");

    if (upcast || nocast) {
      return true;
    }

    return type->Is(&Of<std::remove_const_t<TO>>());
  }

  /// @returns the static TypeInfo for the type T
  template <typename T>
  static const TypeInfo& Of() {
    using NO_CV = typename std::remove_cv<T>::type;
    return detail::TypeInfoOf<NO_CV>::info;
  }

  /// @returns a compile-time hashcode for the type `T`.
  /// @note the returned hashcode will have at most 2 bits set, as the hashes
  /// are expected to be used in bloom-filters which will quickly saturate when
  /// multiple hashcodes are bitwise-or'd together.
  template <typename T>
  static constexpr HashCode HashCodeOf() {
    /// Use the compiler's "pretty" function name, which includes the template
    /// type, to obtain a unique hash value.
#ifdef _MSC_VER
    constexpr uint32_t crc = utils::CRC32(__FUNCSIG__);
#else
    constexpr uint32_t crc = utils::CRC32(__PRETTY_FUNCTION__);
#endif
    constexpr uint32_t bit_a = (crc & 63);
    constexpr uint32_t bit_b = ((crc >> 6) & 63);
    return (static_cast<HashCode>(1) << bit_a) |
           (static_cast<HashCode>(1) << bit_b);
  }

  /// @returns the hashcode of the given type, bitwise-or'd with the hashcodes
  /// of all base classes.
  template <typename T>
  static constexpr HashCode CombinedHashCodeOf() {
    if constexpr (std::is_same_v<T, CastableBase>) {
      return HashCodeOf<CastableBase>();
    } else {
      return HashCodeOf<T>() | CombinedHashCodeOf<typename T::TrueBase>();
    }
  }
};

namespace detail {

/// TypeInfoOf contains a single TypeInfo field for the type T.
/// TINT_INSTANTIATE_TYPEINFO() must be defined in a .cpp file for each type
/// `T`.
template <typename T>
struct TypeInfoOf {
  /// The unique TypeInfo for the type T.
  static const TypeInfo info;
};

// Forward declaration
template <typename TO_FIRST, typename... TO_REST>
struct IsAnyOf;

/// A placeholder structure used for template parameters that need a default
/// type, but can always be automatically inferred.
struct Infer;

}  // namespace detail

/// @returns true if `obj` is a valid pointer, and is of, or derives from the
/// class `TO`
/// @param obj the object to test from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline bool Is(FROM* obj) {
  if (obj == nullptr) {
    return false;
  }
  return TypeInfo::Is<TO, FROM, FLAGS>(&obj->TypeInfo());
}

/// @returns true if `obj` is a valid pointer, and is of, or derives from the
/// class `TO`, and pred(const TO*) returns true
/// @param obj the object to test from
/// @param pred predicate function with signature `bool(const TO*)` called iff
/// object is of, or derives from the class `TO`.
/// @see CastFlags
template <typename TO,
          int FLAGS = 0,
          typename FROM = detail::Infer,
          typename Pred = detail::Infer>
inline bool Is(FROM* obj, Pred&& pred) {
  return Is<TO, FLAGS, FROM>(obj) &&
         pred(static_cast<std::add_const_t<TO>*>(obj));
}

/// @returns true if `obj` is of, or derives from any of the `TO`
/// classes.
/// @param obj the object to cast from
template <typename... TO, typename FROM>
inline bool IsAnyOf(FROM* obj) {
  if (!obj) {
    return false;
  }
  // Optimization: Compare the object's combined_hashcode to the bitwise-or of
  // all the tested type's hashcodes. If there's no intersection of bits in the
  // two masks, then we can guarantee that the type is not in `TO`.
  using Helper = detail::IsAnyOf<TO...>;
  auto* type = &obj->TypeInfo();
  auto hashcode = type->combined_hashcode;
  if ((Helper::kHashCodes & hashcode) == 0) {
    return false;
  }
  // Possibly one of the types in `TO`. Continue to testing against each type.
  return Helper::template Exec<FROM>(type);
}

/// @returns obj dynamically cast to the type `TO` or `nullptr` if
/// this object does not derive from `TO`.
/// @param obj the object to cast from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline TO* As(FROM* obj) {
  auto* as_castable = static_cast<CastableBase*>(obj);
  return Is<TO, FLAGS>(obj) ? static_cast<TO*>(as_castable) : nullptr;
}

/// @returns obj dynamically cast to the type `TO` or `nullptr` if
/// this object does not derive from `TO`.
/// @param obj the object to cast from
/// @see CastFlags
template <typename TO, int FLAGS = 0, typename FROM = detail::Infer>
inline const TO* As(const FROM* obj) {
  auto* as_castable = static_cast<const CastableBase*>(obj);
  return Is<TO, FLAGS>(obj) ? static_cast<const TO*>(as_castable) : nullptr;
}

/// CastableBase is the base class for all Castable objects.
/// It is not encouraged to directly derive from CastableBase without using the
/// Castable helper template.
/// @see Castable
class CastableBase {
 public:
  /// Copy constructor
  CastableBase(const CastableBase&) = default;

  /// Destructor
  virtual ~CastableBase() = default;

  /// Copy assignment
  /// @param other the CastableBase to copy
  /// @returns the new CastableBase
  CastableBase& operator=(const CastableBase& other) = default;

  /// @returns the TypeInfo of the object
  virtual const tint::TypeInfo& TypeInfo() const = 0;

  /// @returns true if this object is of, or derives from the class `TO`
  template <typename TO>
  inline bool Is() const {
    return tint::Is<TO>(this);
  }

  /// @returns true if this object is of, or derives from the class `TO` and
  /// pred(const TO*) returns true
  /// @param pred predicate function with signature `bool(const TO*)` called iff
  /// object is of, or derives from the class `TO`.
  template <typename TO, int FLAGS = 0, typename Pred = detail::Infer>
  inline bool Is(Pred&& pred) const {
    return tint::Is<TO, FLAGS>(this, std::forward<Pred>(pred));
  }

  /// @returns true if this object is of, or derives from any of the `TO`
  /// classes.
  template <typename... TO>
  inline bool IsAnyOf() const {
    return tint::IsAnyOf<TO...>(this);
  }

  /// @returns this object dynamically cast to the type `TO` or `nullptr` if
  /// this object does not derive from `TO`.
  /// @see CastFlags
  template <typename TO, int FLAGS = 0>
  inline TO* As() {
    return tint::As<TO, FLAGS>(this);
  }

  /// @returns this object dynamically cast to the type `TO` or `nullptr` if
  /// this object does not derive from `TO`.
  /// @see CastFlags
  template <typename TO, int FLAGS = 0>
  inline const TO* As() const {
    return tint::As<const TO, FLAGS>(this);
  }

 protected:
  CastableBase() = default;
};

/// Castable is a helper to derive `CLASS` from `BASE`, automatically
/// implementing the Is() and As() methods, along with a #Base type alias.
///
/// Example usage:
///
/// ```
/// class Animal : public Castable<Animal> {};
///
/// class Sheep : public Castable<Sheep, Animal> {};
///
/// Sheep* cast_to_sheep(Animal* animal) {
///    // You can query whether a Castable is of the given type with Is<T>():
///    printf("animal is a sheep? %s", animal->Is<Sheep>() ? "yes" : "no");
///
///    // You can always just try the cast with As<T>().
///    // If the object is not of the correct type, As<T>() will return nullptr:
///    return animal->As<Sheep>();
/// }
/// ```
template <typename CLASS, typename BASE = CastableBase>
class Castable : public BASE {
 public:
  // Inherit the `BASE` class constructors.
  using BASE::BASE;

  /// A type alias for `CLASS` to easily access the `BASE` class members.
  /// Base actually aliases to the Castable instead of `BASE` so that you can
  /// use Base in the `CLASS` constructor.
  using Base = Castable;

  /// A type alias for `BASE`.
  using TrueBase = BASE;

  /// @returns the TypeInfo of the object
  const tint::TypeInfo& TypeInfo() const override {
    return TypeInfo::Of<CLASS>();
  }

  /// @returns true if this object is of, or derives from the class `TO`
  /// @see CastFlags
  template <typename TO, int FLAGS = 0>
  inline bool Is() const {
    return tint::Is<TO, FLAGS>(static_cast<const CLASS*>(this));
  }

  /// @returns true if this object is of, or derives from the class `TO` and
  /// pred(const TO*) returns true
  /// @param pred predicate function with signature `bool(const TO*)` called iff
  /// object is of, or derives from the class `TO`.
  template <int FLAGS = 0, typename Pred = detail::Infer>
  inline bool Is(Pred&& pred) const {
    using TO =
        typename std::remove_pointer<traits::ParameterType<Pred, 0>>::type;
    return tint::Is<TO, FLAGS>(static_cast<const CLASS*>(this),
                               std::forward<Pred>(pred));
  }

  /// @returns true if this object is of, or derives from any of the `TO`
  /// classes.
  template <typename... TO>
  inline bool IsAnyOf() const {
    return tint::IsAnyOf<TO...>(static_cast<const CLASS*>(this));
  }

  /// @returns this object dynamically cast to the type `TO` or `nullptr` if
  /// this object does not derive from `TO`.
  /// @see CastFlags
  template <typename TO, int FLAGS = 0>
  inline TO* As() {
    return tint::As<TO, FLAGS>(this);
  }

  /// @returns this object dynamically cast to the type `TO` or `nullptr` if
  /// this object does not derive from `TO`.
  /// @see CastFlags
  template <typename TO, int FLAGS = 0>
  inline const TO* As() const {
    return tint::As<const TO, FLAGS>(this);
  }
};

namespace detail {
/// Helper for Castable::IsAnyOf
template <typename TO_FIRST, typename... TO_REST>
struct IsAnyOf {
  /// The bitwise-or of all typeinfo hashcodes
  static constexpr auto kHashCodes =
      TypeInfo::HashCodeOf<TO_FIRST>() | IsAnyOf<TO_REST...>::kHashCodes;

  /// @param type castable object type to test
  /// @returns true if `obj` is of, or derives from any of `[TO_FIRST,
  /// ...TO_REST]`
  template <typename FROM>
  static bool Exec(const TypeInfo* type) {
    return TypeInfo::Is<TO_FIRST, FROM>(type) ||
           IsAnyOf<TO_REST...>::template Exec<FROM>(type);
  }
};
/// Terminal specialization
template <typename TO>
struct IsAnyOf<TO> {
  /// The bitwise-or of all typeinfo hashcodes
  static constexpr auto kHashCodes = TypeInfo::HashCodeOf<TO>();

  /// @param type castable object type to test
  /// @returns true if `obj` is of, or derives from TO
  template <typename FROM>
  static bool Exec(const TypeInfo* type) {
    return TypeInfo::Is<TO, FROM>(type);
  }
};
}  // namespace detail

}  // namespace tint

TINT_CASTABLE_POP_DISABLE_WARNINGS();

#endif  // SRC_CASTABLE_H_