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Filename : indexed.hpp
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// Copyright 2015-2016 Hans Dembinski
//
// Distributed under the Boost Software License, Version 1.0.
// (See accompanying file LICENSE_1_0.txt
// or copy at http://www.boost.org/LICENSE_1_0.txt)

#ifndef BOOST_HISTOGRAM_INDEXED_HPP
#define BOOST_HISTOGRAM_INDEXED_HPP

#include <array>
#include <boost/config.hpp> // BOOST_ATTRIBUTE_NODISCARD
#include <boost/histogram/axis/traits.hpp>
#include <boost/histogram/detail/axes.hpp>
#include <boost/histogram/detail/iterator_adaptor.hpp>
#include <boost/histogram/detail/operators.hpp>
#include <boost/histogram/fwd.hpp>
#include <iterator>
#include <type_traits>
#include <utility>

namespace boost {
namespace histogram {

/** Coverage mode of the indexed range generator.

  Defines options for the iteration strategy.
*/
enum class coverage {
  inner, /*!< iterate over inner bins, exclude underflow and overflow */
  all,   /*!< iterate over all bins, including underflow and overflow */
};

/** Input iterator range over histogram bins with multi-dimensional index.

  The iterator returned by begin() can only be incremented. begin() may only be called
  once, calling it a second time returns the end() iterator. If several copies of the
  input iterators exist, the other copies become invalid if one of them is incremented.
*/
template <class Histogram>
class BOOST_ATTRIBUTE_NODISCARD indexed_range {
private:
  using histogram_type = Histogram;
  static constexpr unsigned buffer_size =
      detail::buffer_size<typename std::decay_t<histogram_type>::axes_type>::value;

public:
  using value_iterator = std::conditional_t<std::is_const<histogram_type>::value,
                                            typename histogram_type::const_iterator,
                                            typename histogram_type::iterator>;
  using value_reference = typename std::iterator_traits<value_iterator>::reference;
  using value_type = typename std::iterator_traits<value_iterator>::value_type;

  class iterator;
  using range_iterator [[deprecated("use iterator instead")]] = iterator; ///< deprecated

  /** Lightweight view to access value and index of current cell.

    The methods provide access to the current cell indices and bins. It acts like a
    pointer to the cell value, and in a limited way also like a reference. To interoperate
    with the algorithms of the standard library, the accessor is implicitly convertible to
    a cell value. Assignments and comparisons are passed through to the cell. An accessor
    is coupled to its parent indexed_range::iterator. Moving the parent iterator
    forward also updates the linked accessor. Accessors are not copyable. They cannot be
    stored in containers, but indexed_range::iterator can be stored.
  */
  class BOOST_ATTRIBUTE_NODISCARD accessor : detail::mirrored<accessor, void> {
  public:
    /// Array-like view into the current multi-dimensional index.
    class index_view {
      using index_pointer = const typename iterator::index_data*;

    public:
      using const_reference = const axis::index_type&;
      using reference [[deprecated("use const_reference instead")]] =
          const_reference; ///< deprecated

      /// implementation detail
      class const_iterator
          : public detail::iterator_adaptor<const_iterator, index_pointer,
                                            const_reference> {
      public:
        const_reference operator*() const noexcept { return const_iterator::base()->idx; }

      private:
        explicit const_iterator(index_pointer i) noexcept
            : const_iterator::iterator_adaptor_(i) {}

        friend class index_view;
      };

      const_iterator begin() const noexcept { return const_iterator{begin_}; }
      const_iterator end() const noexcept { return const_iterator{end_}; }
      std::size_t size() const noexcept {
        return static_cast<std::size_t>(end_ - begin_);
      }
      const_reference operator[](unsigned d) const noexcept { return begin_[d].idx; }
      const_reference at(unsigned d) const { return begin_[d].idx; }

    private:
      /// implementation detail
      index_view(index_pointer b, index_pointer e) : begin_(b), end_(e) {}

      index_pointer begin_, end_;
      friend class accessor;
    };

    // assignment is pass-through
    accessor& operator=(const accessor& o) {
      get() = o.get();
      return *this;
    }

    // assignment is pass-through
    template <class T>
    accessor& operator=(const T& x) {
      get() = x;
      return *this;
    }

    /// Returns the cell reference.
    value_reference get() const noexcept { return *(iter_.iter_); }
    /// @copydoc get()
    value_reference operator*() const noexcept { return get(); }
    /// Access fields and methods of the cell object.
    value_iterator operator->() const noexcept { return iter_.iter_; }

    /// Access current index.
    /// @param d axis dimension.
    axis::index_type index(unsigned d = 0) const noexcept {
      return iter_.indices_[d].idx;
    }

    /// Access indices as an iterable range.
    index_view indices() const noexcept {
      assert(iter_.indices_.hist_);
      return {iter_.indices_.begin(), iter_.indices_.end()};
    }

    /// Access current bin.
    /// @tparam N axis dimension.
    template <unsigned N = 0>
    decltype(auto) bin(std::integral_constant<unsigned, N> = {}) const {
      assert(iter_.indices_.hist_);
      return iter_.indices_.hist_->axis(std::integral_constant<unsigned, N>())
          .bin(index(N));
    }

    /// Access current bin.
    /// @param d axis dimension.
    decltype(auto) bin(unsigned d) const {
      assert(iter_.indices_.hist_);
      return iter_.indices_.hist_->axis(d).bin(index(d));
    }

    /** Computes density in current cell.

      The density is computed as the cell value divided by the product of bin widths. Axes
      without bin widths, like axis::category, are treated as having unit bin with.
    */
    double density() const {
      assert(iter_.indices_.hist_);
      double x = 1;
      unsigned d = 0;
      iter_.indices_.hist_->for_each_axis([&](const auto& a) {
        const auto w = axis::traits::width_as<double>(a, this->index(d++));
        x *= w != 0 ? w : 1;
      });
      return get() / x;
    }

    // forward all comparison operators to the value
    bool operator<(const accessor& o) noexcept { return get() < o.get(); }
    bool operator>(const accessor& o) noexcept { return get() > o.get(); }
    bool operator==(const accessor& o) noexcept { return get() == o.get(); }
    bool operator!=(const accessor& o) noexcept { return get() != o.get(); }
    bool operator<=(const accessor& o) noexcept { return get() <= o.get(); }
    bool operator>=(const accessor& o) noexcept { return get() >= o.get(); }

    template <class U>
    bool operator<(const U& o) const noexcept {
      return get() < o;
    }

    template <class U>
    bool operator>(const U& o) const noexcept {
      return get() > o;
    }

    template <class U>
    bool operator==(const U& o) const noexcept {
      return get() == o;
    }

    template <class U>
    bool operator!=(const U& o) const noexcept {
      return get() != o;
    }

    template <class U>
    bool operator<=(const U& o) const noexcept {
      return get() <= o;
    }

    template <class U>
    bool operator>=(const U& o) const noexcept {
      return get() >= o;
    }

    operator value_type() const noexcept { return get(); }

  private:
    accessor(iterator& i) noexcept : iter_(i) {}

    accessor(const accessor&) = default; // only callable by indexed_range::iterator

    iterator& iter_;

    friend class iterator;
  };

  /// implementation detail
  class iterator {
  public:
    using value_type = typename indexed_range::value_type;
    using reference = accessor;

  private:
    struct pointer_proxy {
      reference* operator->() noexcept { return std::addressof(ref_); }
      reference ref_;
    };

  public:
    using pointer = pointer_proxy;
    using difference_type = std::ptrdiff_t;
    using iterator_category = std::forward_iterator_tag;

    reference operator*() noexcept { return *this; }
    pointer operator->() noexcept { return pointer_proxy{operator*()}; }

    iterator& operator++() {
      assert(iter_ < indices_.hist_->end());
      const auto cbeg = indices_.begin();
      auto c = cbeg;
      ++iter_;
      ++c->idx;
      if (c->idx < c->end) return *this;
      while (c->idx == c->end) {
        iter_ += c->end_skip;
        if (++c == indices_.end()) return *this;
        ++c->idx;
      }
      while (c-- != cbeg) {
        c->idx = c->begin;
        iter_ += c->begin_skip;
      }
      return *this;
    }

    iterator operator++(int) {
      auto prev = *this;
      operator++();
      return prev;
    }

    bool operator==(const iterator& x) const noexcept { return iter_ == x.iter_; }
    bool operator!=(const iterator& x) const noexcept { return !operator==(x); }

    // make iterator ready for C++17 sentinels
    bool operator==(const value_iterator& x) const noexcept { return iter_ == x; }
    bool operator!=(const value_iterator& x) const noexcept { return !operator==(x); }

    // useful for iterator debugging
    std::size_t offset() const noexcept { return iter_ - indices_.hist_->begin(); }

  private:
    iterator(value_iterator i, histogram_type& h) : iter_(i), indices_(&h) {}

    value_iterator iter_;

    struct index_data {
      axis::index_type idx, begin, end;
      std::size_t begin_skip, end_skip;
    };

    struct indices_t : private std::array<index_data, buffer_size> {
      using base_type = std::array<index_data, buffer_size>;
      using pointer = index_data*;
      using const_pointer = const index_data*;

      indices_t(histogram_type* h) noexcept : hist_{h} {}

      using base_type::operator[];
      unsigned size() const noexcept { return hist_->rank(); }
      pointer begin() noexcept { return base_type::data(); }
      const_pointer begin() const noexcept { return base_type::data(); }
      pointer end() noexcept { return begin() + size(); }
      const_pointer end() const noexcept { return begin() + size(); }

      histogram_type* hist_;
    } indices_;

    friend class indexed_range;
  };

  indexed_range(histogram_type& hist, coverage cov)
      : begin_(hist.begin(), hist), end_(hist.end(), hist) {
    begin_.indices_.hist_->for_each_axis([ca = begin_.indices_.begin(), cov,
                                          stride = std::size_t{1},
                                          this](const auto& a) mutable {
      using opt = axis::traits::get_options<std::decay_t<decltype(a)>>;
      constexpr axis::index_type under = opt::test(axis::option::underflow);
      constexpr axis::index_type over = opt::test(axis::option::overflow);
      const auto size = a.size();

      // -1 if underflow and cover all, else 0
      ca->begin = cov == coverage::all ? -under : 0;
      // size + 1 if overflow and cover all, else size
      ca->end = cov == coverage::all ? size + over : size;
      ca->idx = ca->begin;

      // if axis has *flow and coverage::all OR axis has no *flow:
      //   begin + under == 0, size + over - end == 0
      // if axis has *flow and coverage::inner:
      //   begin + under == 1, size + over - end == 1
      ca->begin_skip = static_cast<std::size_t>(ca->begin + under) * stride;
      ca->end_skip = static_cast<std::size_t>(size + over - ca->end) * stride;
      begin_.iter_ += ca->begin_skip;

      stride *= size + under + over;
      ++ca;
    });
  }

  iterator begin() noexcept { return begin_; }
  iterator end() noexcept { return end_; }

private:
  iterator begin_, end_;
};

/** Generates an indexed range of <a
  href="https://en.cppreference.com/w/cpp/named_req/ForwardIterator">forward iterators</a>
  over the histogram cells.

  Use this in a range-based for loop:

  ```
  for (auto&& x : indexed(hist)) { ... }
  ```

  This generates an optimized loop which is nearly always faster than a hand-written loop
  over the histogram cells. The iterators dereference to an indexed_range::accessor, which
  has methods to query the current indices and bins and acts like a pointer to the cell
  value. The returned iterators are forward iterators. They can be stored in a container,
  but may not be used after the life-time of the histogram ends.

  @returns indexed_range

  @param hist Reference to the histogram.
  @param cov  Iterate over all or only inner bins (optional, default: inner).
 */
template <class Histogram>
auto indexed(Histogram&& hist, coverage cov = coverage::inner) {
  return indexed_range<std::remove_reference_t<Histogram>>{std::forward<Histogram>(hist),
                                                           cov};
}

} // namespace histogram
} // namespace boost

#endif