irange.cpp

#include <experimental/ranges/concepts>
#include <experimental/ranges/functional>
#include <experimental/ranges/iterator>

namespace ranges = std::experimental::ranges;

ranges::Integral{I}
class integer_range {
  I lo_ = 0;
  I hi_ = 0;

  struct cursor {
    using difference_type = std::make_signed_t<I>;

    I i_ = 0;

    cursor() = default;
    explicit cursor(I i) : i_{i} {}

    I read() const { return i_; }
    void next() { ++i_; }
    void prev() { --i_; }
    void advance(difference_type n) { i_ += n; }
    difference_type distance_to(const cursor& that) const
    { return that.i_ - i_; }
    bool equal(const cursor& that) const
    { return that.i_ == i_; }
  };

public:
  integer_range() = default;
  integer_range(I lo, I hi) : lo_{lo}, hi_{hi >= lo ? hi : lo} {}

  using iterator = ranges::basic_iterator<cursor>;
  iterator begin() const { return cursor{lo_}; }
  iterator end() const { return cursor{hi_}; }
};

template <ranges::Integral I>
integer_range<I> irange(I hi) {
  return {I{0}, hi};
}

template <ranges::Integral I1, ranges::Integral I2>
integer_range<ranges::common_type_t<I1, I2>> irange(I1 lo, I2 hi) {
  return {lo, hi};
}

static_assert(ranges::View<integer_range<int>>());
static_assert(ranges::RandomAccessRange<integer_range<int>>());
static_assert(sizeof(integer_range<int>::iterator) == sizeof(int));

int sum_for_loop(int lo, int hi) {
  int total = 0;
  for (auto i = lo; i < hi; ++i) {
    total += i;
  }
  return total;
}

int sum_range(int lo, int hi) {
  int total = 0;
  for (auto i : irange(lo, hi)) {
    total += i;
  }
  return total;
}