/*
  Copyright 2019-2021 David Robillard <d@drobilla.net>

  Permission to use, copy, modify, and/or distribute this software for any
  purpose with or without fee is hereby granted, provided that the above
  copyright notice and this permission notice appear in all copies.

  THIS SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
  WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
  MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
  ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
  WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
  ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
  OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/

#ifndef SERD_OPTIONAL_HPP
#define SERD_OPTIONAL_HPP

#include <cassert>
#include <type_traits>
#include <utility>

namespace serd {

/**
   @defgroup serdpp_optional Optional
   @ingroup serdpp
   @{
*/

struct ConstructNullOptional {};

/// Special tag for constructing an unset Optional
struct Nullopt {
  enum class Construct { internal };

  explicit constexpr Nullopt(Construct) {}
};

/**
   A simple optional wrapper around a wrapped type with a pointer-like API

   This works like a typical optional type, but only works with Wrapper types,
   and exploits the fact that these are interally just pointers to avoid adding
   space overhead for an "is_set" flag, like a generic optional class would.

   Types must explicitly opt-in to being optional by providing a constructor
   that takes a single ContructNullOptional argument.  This constructor should
   only be used by the Optional implementation, which guarantees that such an
   object will not be used except by calling its cobj() method.
*/
template<class T>
class Optional
{
public:
  /// The type of the underlying C object
  using CType = typename T::CType;

  /// Constructs an empty optional
  Optional() = default;

  /// Constructs an empty optional
  // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
  Optional(Nullopt) {}

  /// Constructs an optional that contains the given value
  // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
  Optional(T value)
    : _value{std::move(value)}
  {}

  /// Constructs an optional that contains a converted value
  template<
    typename U,
    typename = typename std::enable_if<std::is_convertible<U, T>::value>::type>
  // NOLINTNEXTLINE(google-explicit-constructor, hicpp-explicit-conversions)
  Optional(U&& value)
    : _value{std::forward<U>(value)}
  {}

  /// Destroys any contained value
  void reset() { _value = T{nullptr}; }

  /// Accesses the contained value
  const T& operator*() const
  {
    assert(_value.cobj());
    return _value;
  }

  /// Accesses the contained value
  T& operator*()
  {
    assert(_value.cobj());
    return _value;
  }

  /// Accesses the contained value
  const T* operator->() const
  {
    assert(_value.cobj());
    return &_value;
  }

  /// Accesses the contained value
  T* operator->()
  {
    assert(_value.cobj());
    return &_value;
  }

  /// Tests if optional objects are equal
  bool operator==(const Optional& optional)
  {
    return (!*this && !optional) ||
           (*this && optional && _value == optional._value);
  }

  /// Tests if optional objects are not equal
  bool operator!=(const Optional& optional) { return !operator==(optional); }

  /// Returns true if this optional contains a value
  explicit operator bool() const { return _value.cobj(); }

  /// Returns true if this optional does not contain a value
  bool operator!() const { return !_value.cobj(); }

  /// Return a pointer to the underlying C object, or null
  CType* cobj() { return _value.cobj(); }

  /// Return a pointer to the underlying C object, or null
  const CType* cobj() const { return _value.cobj(); }

private:
  T _value{nullptr};
};

/// Creates an optional object from `value`
template<class T>
constexpr Optional<std::decay_t<T>>
make_optional(T&& value)
{
  return Optional<T>{std::forward<T>(value)};
}

/// Creates an optional object with a value constructed in-place from `args`
template<class T, class... Args>
constexpr Optional<T>
make_optional(Args&&... args)
{
  return Optional<T>{std::forward<Args>(args)...};
}

/// Constant that represents an empty optional
static constexpr Nullopt nullopt{Nullopt::Construct::internal};

/**
   @}
*/

} // namespace serd

#endif // SERD_OPTIONAL_HPP