common.h

/*!
 * Copyright 2015-2022 by XGBoost Contributors
 * \file common.h
 * \brief Common utilities
 */
#ifndef XGBOOST_COMMON_COMMON_H_
#define XGBOOST_COMMON_COMMON_H_

#include <xgboost/base.h>
#include <xgboost/logging.h>
#include <xgboost/span.h>

#include <algorithm>
#include <exception>
#include <functional>
#include <limits>
#include <numeric>
#include <sstream>
#include <string>
#include <type_traits>
#include <utility>
#include <vector>

#if defined(__CUDACC__)
#include <thrust/system/cuda/error.h>
#include <thrust/system_error.h>

#define WITH_CUDA() true

#else

#define WITH_CUDA() false

#endif  // defined(__CUDACC__)

namespace dh {
#if defined(__CUDACC__)
/*
 * Error handling  functions
 */
#define safe_cuda(ans) ThrowOnCudaError((ans), __FILE__, __LINE__)

inline cudaError_t ThrowOnCudaError(cudaError_t code, const char *file,
                                    int line) {
  if (code != cudaSuccess) {
    LOG(FATAL) << thrust::system_error(code, thrust::cuda_category(),
                                       std::string{file} + ": " +  // NOLINT
                                       std::to_string(line)).what();
  }
  return code;
}
#endif  // defined(__CUDACC__)
}  // namespace dh

namespace xgboost {
namespace common {
/*!
 * \brief Split a string by delimiter
 * \param s String to be split.
 * \param delim The delimiter.
 */
inline std::vector<std::string> Split(const std::string& s, char delim) {
  std::string item;
  std::istringstream is(s);
  std::vector<std::string> ret;
  while (std::getline(is, item, delim)) {
    ret.push_back(item);
  }
  return ret;
}

template <typename T>
XGBOOST_DEVICE T Max(T a, T b) {
  return a < b ? b : a;
}

// simple routine to convert any data to string
template<typename T>
inline std::string ToString(const T& data) {
  std::ostringstream os;
  os << data;
  return os.str();
}

template <typename T1, typename T2>
XGBOOST_DEVICE T1 DivRoundUp(const T1 a, const T2 b) {
  return static_cast<T1>(std::ceil(static_cast<double>(a) / b));
}

namespace detail {
template <class T, std::size_t N, std::size_t... Idx>
constexpr auto UnpackArr(std::array<T, N> &&arr, std::index_sequence<Idx...>) {
  return std::make_tuple(std::forward<std::array<T, N>>(arr)[Idx]...);
}
}  // namespace detail

template <class T, std::size_t N>
constexpr auto UnpackArr(std::array<T, N> &&arr) {
  return detail::UnpackArr(std::forward<std::array<T, N>>(arr),
                           std::make_index_sequence<N>{});
}

/*
 * Range iterator
 */
class Range {
 public:
  using DifferenceType = int64_t;

  class Iterator {
    friend class Range;

   public:
    XGBOOST_DEVICE DifferenceType operator*() const { return i_; }
    XGBOOST_DEVICE const Iterator &operator++() {
      i_ += step_;
      return *this;
    }
    XGBOOST_DEVICE Iterator operator++(int) {
      Iterator res {*this};
      i_ += step_;
      return res;
    }

    XGBOOST_DEVICE bool operator==(const Iterator &other) const {
      return i_ >= other.i_;
    }
    XGBOOST_DEVICE bool operator!=(const Iterator &other) const {
      return i_ < other.i_;
    }

    XGBOOST_DEVICE void Step(DifferenceType s) { step_ = s; }

   protected:
    XGBOOST_DEVICE explicit Iterator(DifferenceType start) : i_(start) {}
    XGBOOST_DEVICE explicit Iterator(DifferenceType start, DifferenceType step) :
        i_{start}, step_{step} {}

   private:
    int64_t i_;
    DifferenceType step_ = 1;
  };

  XGBOOST_DEVICE Iterator begin() const { return begin_; }  // NOLINT
  XGBOOST_DEVICE Iterator end() const { return end_; }      // NOLINT

  XGBOOST_DEVICE Range(DifferenceType begin, DifferenceType end)
      : begin_(begin), end_(end) {}
  XGBOOST_DEVICE Range(DifferenceType begin, DifferenceType end,
                       DifferenceType step)
      : begin_(begin, step), end_(end) {}

  XGBOOST_DEVICE bool operator==(const Range& other) const {
    return *begin_ == *other.begin_ && *end_ == *other.end_;
  }
  XGBOOST_DEVICE bool operator!=(const Range& other) const {
    return !(*this == other);
  }

  XGBOOST_DEVICE void Step(DifferenceType s) { begin_.Step(s); }

 private:
  Iterator begin_;
  Iterator end_;
};

/**
 * \brief Transform iterator that takes an index and calls transform operator.
 *
 *   This is CPU-only right now as taking host device function as operator complicates the
 *   code.  For device side one can use `thrust::transform_iterator` instead.
 */
template <typename Fn>
class IndexTransformIter {
  size_t iter_{0};
  Fn fn_;

 public:
  using iterator_category = std::random_access_iterator_tag;  // NOLINT
  using value_type = std::result_of_t<Fn(size_t)>;            // NOLINT
  using difference_type = detail::ptrdiff_t;                  // NOLINT
  using reference = std::add_lvalue_reference_t<value_type>;  // NOLINT
  using pointer = std::add_pointer_t<value_type>;             // NOLINT

 public:
  /**
   * \param op Transform operator, takes a size_t index as input.
   */
  explicit IndexTransformIter(Fn &&op) : fn_{op} {}
  IndexTransformIter(IndexTransformIter const &) = default;
  IndexTransformIter& operator=(IndexTransformIter&&) = default;
  IndexTransformIter& operator=(IndexTransformIter const& that) {
    iter_ = that.iter_;
    return *this;
  }

  value_type operator*() const { return fn_(iter_); }

  auto operator-(IndexTransformIter const &that) const { return iter_ - that.iter_; }
  bool operator==(IndexTransformIter const &that) const { return iter_ == that.iter_; }
  bool operator!=(IndexTransformIter const &that) const { return !(*this == that); }

  IndexTransformIter &operator++() {
    iter_++;
    return *this;
  }
  IndexTransformIter operator++(int) {
    auto ret = *this;
    ++(*this);
    return ret;
  }
  IndexTransformIter &operator+=(difference_type n) {
    iter_ += n;
    return *this;
  }
  IndexTransformIter &operator-=(difference_type n) {
    (*this) += -n;
    return *this;
  }
  IndexTransformIter operator+(difference_type n) const {
    auto ret = *this;
    return ret += n;
  }
  IndexTransformIter operator-(difference_type n) const {
    auto ret = *this;
    return ret -= n;
  }
};

template <typename Fn>
auto MakeIndexTransformIter(Fn&& fn) {
  return IndexTransformIter<Fn>(std::forward<Fn>(fn));
}

int AllVisibleGPUs();

inline void AssertGPUSupport() {
#ifndef XGBOOST_USE_CUDA
    LOG(FATAL) << "XGBoost version not compiled with GPU support.";
#endif  // XGBOOST_USE_CUDA
}

inline void AssertOneAPISupport() {
#ifndef XGBOOST_USE_ONEAPI
    LOG(FATAL) << "XGBoost version not compiled with OneAPI support.";
#endif  // XGBOOST_USE_ONEAPI
}

template <typename Idx, typename Container,
          typename V = typename Container::value_type,
          typename Comp = std::less<V>>
std::vector<Idx> ArgSort(Container const &array, Comp comp = std::less<V>{}) {
  std::vector<Idx> result(array.size());
  std::iota(result.begin(), result.end(), 0);
  auto op = [&array, comp](Idx const &l, Idx const &r) { return comp(array[l], array[r]); };
  XGBOOST_PARALLEL_STABLE_SORT(result.begin(), result.end(), op);
  return result;
}

struct OptionalWeights {
  Span<float const> weights;
  float dft{1.0f};  // fixme: make this compile time constant

  explicit OptionalWeights(Span<float const> w) : weights{w} {}
  explicit OptionalWeights(float w) : dft{w} {}

  XGBOOST_DEVICE float operator[](size_t i) const { return weights.empty() ? dft : weights[i]; }
  auto Empty() const { return weights.empty(); }
};

/**
 * Last index of a group in a CSR style of index pointer.
 */
template <typename Indexable>
XGBOOST_DEVICE size_t LastOf(size_t group, Indexable const &indptr) {
  return indptr[group + 1] - 1;
}

/**
 * \brief A CRTP (curiously recurring template pattern) helper function.
 *
 * https://www.fluentcpp.com/2017/05/19/crtp-helper/
 *
 * Does two things:
 * 1. Makes "crtp" explicit in the inheritance structure of a CRTP base class.
 * 2. Avoids having to `static_cast` in a lot of places.
 *
 * \tparam T The derived class in a CRTP hierarchy.
 */
template <typename T>
struct Crtp {
  T &Underlying() { return static_cast<T &>(*this); }
  T const &Underlying() const { return static_cast<T const &>(*this); }
};

/**
 * \brief C++17 std::as_const
 */
template <typename T>
typename std::add_const<T>::type &AsConst(T &v) noexcept {  // NOLINT(runtime/references)
  return v;
}
}  // namespace common
}  // namespace xgboost
#endif  // XGBOOST_COMMON_COMMON_H_