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

#pragma once

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

#include <iterator/detail/macro.h>

namespace iterator {
template <typename It, typename Self, typename Cat>
class proxy : public facade<Self, category::single_pass> {
public:
  using single_pass_iterator = void;

private:
  It impl_;

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

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  SFINAE(detail::has_sentinel_type_v<It>) bool at_end() const {
    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, category::forward> {
private:
  It impl_;

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

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  SFINAE(detail::has_sentinel_type_v<It>) bool at_end() const {
    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, category::bidirectional> {
private:
  It impl_;

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

  decltype(auto) dereference() const { return *impl_; }
  void increment() { ++impl_; }
  void decrement() { --impl_; }
  bool equal_to(Self const & other) const { return impl_ == other.impl_; }
  SFINAE(detail::has_sentinel_type_v<It>) bool at_end() const {
    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, category::random_access> {
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_(FWD(args)...) {}

  decltype(auto) dereference() const { return *impl_; }
  void advance(difference_type off) { impl_ += off; }
  difference_type distance_to(Self const & other) const {
    return other.impl_ - impl_;
  }
  
  template <typename S, REQUIRES((detail::is_sentinel_v<It, S>))>
  friend auto operator-(S sentinel, Self const &self) {
    return sentinel - self.impl();
  }
  
  SFINAE(detail::has_sentinel_type_v<It>) bool at_end() const {
    return (typename It::sentinel_type() - impl()) <= 0;
  }

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

#include <iterator/detail/undef.h>