Extract transform iterator. (#8498)

src/common/common.h

@@ -164,74 +164,6 @@ class Range {
   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() {