cpp-useful-iterators/include/iterator/proxy.h

#pragma once

#include <iterator/facade.h>
#include <iterator/forwards.h>

namespace iterator {
template <typename It, typename Self, typename Cat>
class Proxy : public Facade<Self> {
public:
  static constexpr bool single_pass_iterator = true;

private:
  It impl_;

public:
  Proxy() = default;
  Proxy(It impl) : impl_(impl) {}
  template <typename... Args>
  Proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  bool at_end() const
    requires(has_sentinel<It>)
  {
    return impl() == typename It::sentinel_type();
  }

protected:
  auto impl() const { return impl_; }
};

template <typename It, typename Self>
class Proxy<It, Self, std::forward_iterator_tag> : public Facade<Self> {
private:
  It impl_;

public:
  Proxy() = default;
  Proxy(It impl) : impl_(impl) {}
  template <typename... Args>
  Proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  bool at_end() const
    requires(has_sentinel<It>)
  {
    return impl() == typename It::sentinel_type();
  }

protected:
  auto impl() const { return impl_; }
};

template <typename It, typename Self>
class Proxy<It, Self, std::bidirectional_iterator_tag> : public Facade<Self> {
private:
  It impl_;

public:
  Proxy() = default;
  Proxy(It impl) : impl_(impl) {}
  template <typename... Args>
  Proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  void decrement() { --impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  bool at_end() const
    requires(has_sentinel<It>)
  {
    return impl() == typename It::sentinel_type();
  }

protected:
  auto impl() const { return impl_; }
};

template <typename It, typename Self>
class Proxy<It, Self, std::random_access_iterator_tag> : public Facade<Self> {
public:
  using difference_type = typename std::iterator_traits<It>::difference_type;

private:
  It impl_;

public:
  Proxy() = default;
  Proxy(It impl) : impl_(impl) {}
  template <typename... Args>
  Proxy(Args &&... args) : impl_(std::forward<Args>(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void advance(difference_type off) { impl_ += off; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  difference_type distance_to(Self const & other) const {
    return other.impl_ - impl_;
  }

  friend auto operator-(sentinel_for<It> auto sentinel, Self const & self) {
    return sentinel - self.impl();
  }

  bool at_end() const
    requires(has_sentinel<It>)
  {
    return (typename It::sentinel_type() - impl()) <= 0;
  }

protected:
  auto impl() const { return impl_; }
};
}