range.hpp

#ifndef ITER_RANGE_H_
#define ITER_RANGE_H_

#include "internal/iterbase.hpp"

#include <cassert>
#include <exception>
#include <iterator>
#include <type_traits>

namespace iter {
  namespace impl {
    template <typename T>
    class Range;
  }

  template <typename T>
  constexpr impl::Range<T> range(T) noexcept;
  template <typename T>
  constexpr impl::Range<T> range(T, T) noexcept;
  template <typename T>
  constexpr impl::Range<T> range(T, T, T) noexcept;
}

namespace iter {
  namespace detail {
    template <typename T, bool IsFloat = std::is_floating_point<T>::value>
    class RangeIterData;

    // everything except floats
    template <typename T>
    class RangeIterData<T, false> {
     private:
      T value_{};
      T step_{};

     public:
      constexpr RangeIterData() noexcept = default;
      constexpr RangeIterData(T in_value, T in_step) noexcept
          : value_{in_value}, step_{in_step} {}

      constexpr T value() const noexcept {
        return value_;
      }

      constexpr T step() const noexcept {
        return step_;
      }

      void inc() noexcept {
        value_ += step_;
      }

      constexpr bool operator==(const RangeIterData& other) const noexcept {
        return value_ == other.value_;
      }

      constexpr bool operator!=(const RangeIterData& other) const noexcept {
        return !(*this == other);
      }
    };

    // float data
    template <typename T>
    class RangeIterData<T, true> {
     private:
      T start_{};
      T value_{};
      T step_{};
      std::size_t steps_taken_{};

     public:
      constexpr RangeIterData() noexcept = default;
      constexpr RangeIterData(T in_start, T in_step) noexcept
          : start_{in_start}, value_{in_start}, step_{in_step} {}

      constexpr T value() const noexcept {
        return value_;
      }

      constexpr T step() const noexcept {
        return step_;
      }

      void inc() noexcept {
        ++steps_taken_;
        value_ = start_ + (step_ * steps_taken_);
      }

      constexpr bool operator==(const RangeIterData& other) const noexcept {
        // if the difference between the two values is less than the
        // step_ size, they are considered equal
        return (value_ < other.value_ ? other.value_ - value_
                                      : value_ - other.value_)
               < step_;
      }

      constexpr bool operator!=(const RangeIterData& other) const noexcept {
        return !(*this == other);
      }
    };
  }
}

template <typename T>
class iter::impl::Range {
  // see stackoverflow.com/questions/32174186 about why only specializations
  // aren't marked as friend
  template <typename U>
  friend constexpr Range<U> iter::range(U) noexcept;
  template <typename U>
  friend constexpr Range<U> iter::range(U, U) noexcept;
  template <typename U>
  friend constexpr Range<U> iter::range(U, U, U) noexcept;

 private:
  const T start_;
  const T stop_;
  const T step_;

  constexpr Range(T stop) noexcept : start_{0}, stop_{stop}, step_{1} {}

  constexpr Range(T start, T stop, T step = 1) noexcept
      : start_{start}, stop_{stop}, step_{step} {}

  // if val is "before" the stopping point.
  static constexpr bool is_within_range(
      T val, T stop_val, [[maybe_unused]] T step_val) {
    if constexpr (std::is_unsigned<T>{}) {
      return val < stop_val;
    } else {
      return !(step_val > 0 && val >= stop_val)
             && !(step_val < 0 && val <= stop_val);
    }
  }

 public:
  constexpr T start() const noexcept {
    return start_;
  }

  constexpr T stop() const noexcept {
    return stop_;
  }

  constexpr T step() const noexcept {
    return step_;
  }

  constexpr T operator[](std::size_t index) const noexcept {
    return start() + (step() * index);
  }

  constexpr std::size_t size() const noexcept {
    static_assert(!std::is_floating_point_v<T>,
        "range size() not supperted with floating point types");
    if (!is_within_range(start(), stop(), step())) {
      return 0;
    }

    auto diff = stop() - start();
    auto res = diff / step();
    assert(res >= 0);
    auto result = static_cast<std::size_t>(res);
    if (diff % step()) {
      ++result;
    }
    return result;
  }

  // the reference type here is T, which doesn't strictly follow all
  // of the rules, but std::vector<bool>::iterator::reference isn't
  // a reference type either, this isn't any worse

  class Iterator {
   private:
    iter::detail::RangeIterData<T> data;
    bool is_end{};

    // first argument must be regular iterator
    // second argument must be end iterator
    static bool not_equal_to_impl(
        const Iterator& lhs, const Iterator& rhs) noexcept {
      assert(!lhs.is_end);
      assert(rhs.is_end);
      return is_within_range(
          lhs.data.value(), rhs.data.value(), lhs.data.step());
    }

    static bool not_equal_to_end(
        const Iterator& lhs, const Iterator& rhs) noexcept {
      if (rhs.is_end) {
        return not_equal_to_impl(lhs, rhs);
      }
      return not_equal_to_impl(rhs, lhs);
    }

   public:
    using iterator_category = std::forward_iterator_tag;
    using value_type = T;
    using difference_type = std::ptrdiff_t;
    using pointer = value_type*;
    using reference = value_type;

    constexpr Iterator() noexcept = default;

    constexpr Iterator(T in_value, T in_step, bool in_is_end) noexcept
        : data(in_value, in_step), is_end{in_is_end} {}

    constexpr T operator*() const noexcept {
      return data.value();
    }

    constexpr ArrowProxy<T> operator->() const noexcept {
      return {**this};
    }

    Iterator& operator++() noexcept {
      data.inc();
      return *this;
    }

    Iterator operator++(int) noexcept {
      auto ret = *this;
      ++*this;
      return ret;
    }

    // This operator would more accurately read as "in bounds"
    // or "incomplete" because exact comparison with the end
    // isn't good enough for the purposes of this Iterator.
    // There are two odd cases that need to be handled
    //
    // 1) The Range is infinite, such as
    // Range (-1, 0, -1) which would go forever down toward
    // infinitely (theoretically).  If this occurs, the Range
    // will instead effectively be empty
    //
    // 2) (stop_ - start_) % step_ != 0.  For
    // example Range(1, 10, 2).  The iterator will never be
    // exactly equal to the stop_ value.
    //
    // Another way to think about it is that the "end"
    // iterator represents the range of values that are invalid
    // So, if an iterator is not equal to that, it is valid
    //
    // Two end iterators will compare equal
    //
    // Two non-end iterators will compare by their stored values
    bool operator!=(const Iterator& other) const noexcept {
      if (is_end && other.is_end) {
        return false;
      }

      if (!is_end && !other.is_end) {
        return data != other.data;
      }
      return not_equal_to_end(*this, other);
    }

    bool operator==(const Iterator& other) const noexcept {
      return !(*this != other);
    }
  };

  constexpr Iterator begin() const noexcept {
    return {start_, step_, false};
  }

  constexpr Iterator end() const noexcept {
    return {stop_, step_, true};
  }
};

template <typename T>
constexpr iter::impl::Range<T> iter::range(T stop_) noexcept {
  return {stop_};
}

template <typename T>
constexpr iter::impl::Range<T> iter::range(T start_, T stop_) noexcept {
  return {start_, stop_};
}

template <typename T>
constexpr iter::impl::Range<T> iter::range(
    T start_, T stop_, T step_) noexcept {
  return step_ == T(0) ? impl::Range<T>{0}
                       : impl::Range<T>{start_, stop_, step_};
}

#endif