text: //===--- ArrayRef.h - Array Reference Wrapper -------------------*- C++ -*-===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  
  // ATen: modified from llvm::ArrayRef.
  // removed llvm-specific functionality
  // removed some implicit const -> non-const conversions that rely on
  // complicated std::enable_if meta-programming
  // removed a bunch of slice variants for simplicity...
  
  #pragma once
  
  #include <c10/util/C++17.h>
  #include <c10/util/Deprecated.h>
  #include <c10/util/Exception.h>
  #include <c10/util/SmallVector.h>
  
  #include <array>
  #include <iterator>
  #include <vector>
  
  namespace c10 {
  
  /// ArrayRef - Represent a constant reference to an array (0 or more elements
  /// consecutively in memory), i.e. a start pointer and a length.  It allows
  /// various APIs to take consecutive elements easily and conveniently.
  ///
  /// This class does not own the underlying data, it is expected to be used in
  /// situations where the data resides in some other buffer, whose lifetime
  /// extends past that of the ArrayRef. For this reason, it is not in general
  /// safe to store an ArrayRef.
  ///
  /// This is intended to be trivially copyable, so it should be passed by
  /// value.
  template <typename T>
  class ArrayRef final {
   public:
    using iterator = const T*;
    using const_iterator = const T*;
    using size_type = size_t;
    using value_type = T;
  
    using reverse_iterator = std::reverse_iterator<iterator>;
  
   private:
    /// The start of the array, in an external buffer.
    const T* Data;
  
    /// The number of elements.
    size_type Length;
  
    void debugCheckNullptrInvariant() {
      TORCH_INTERNAL_ASSERT_DEBUG_ONLY(
          Data != nullptr || Length == 0,
          "created ArrayRef with nullptr and non-zero length! c10::optional relies on this being illegal");
    }
  
   public:
    /// @name Constructors
    /// @{
  
    /// Construct an empty ArrayRef.
    /* implicit */ constexpr ArrayRef() : Data(nullptr), Length(0) {}
  
    /// Construct an ArrayRef from a single element.
    // TODO Make this explicit
    constexpr ArrayRef(const T& OneElt) : Data(&OneElt), Length(1) {}
  
    /// Construct an ArrayRef from a pointer and length.
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA ArrayRef(const T* data, size_t length)
        : Data(data), Length(length) {
      debugCheckNullptrInvariant();
    }
  
    /// Construct an ArrayRef from a range.
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA ArrayRef(const T* begin, const T* end)
        : Data(begin), Length(end - begin) {
      debugCheckNullptrInvariant();
    }
  
    /// Construct an ArrayRef from a SmallVector. This is templated in order to
    /// avoid instantiating SmallVectorTemplateCommon<T> whenever we
    /// copy-construct an ArrayRef.
    template <typename U>
    /* implicit */ ArrayRef(const SmallVectorTemplateCommon<T, U>& Vec)
        : Data(Vec.data()), Length(Vec.size()) {
      debugCheckNullptrInvariant();
    }
  
    /// Construct an ArrayRef from a generic Container.
    template <
        typename Container,
        typename = std::enable_if_t<std::is_same<
            std::remove_const_t<decltype(std::declval<Container>().data())>,
            T*>::value>>
    /* implicit */ ArrayRef(const Container& container)
        : Data(container.data()), Length(container.size()) {
      debugCheckNullptrInvariant();
    }
  
    /// Construct an ArrayRef from a std::vector.
    // The enable_if stuff here makes sure that this isn't used for
    // std::vector<bool>, because ArrayRef can't work on a std::vector<bool>
    // bitfield.
    template <typename A>
    /* implicit */ ArrayRef(const std::vector<T, A>& Vec)
        : Data(Vec.data()), Length(Vec.size()) {
      static_assert(
          !std::is_same<T, bool>::value,
          "ArrayRef<bool> cannot be constructed from a std::vector<bool> bitfield.");
    }
  
    /// Construct an ArrayRef from a std::array
    template <size_t N>
    /* implicit */ constexpr ArrayRef(const std::array<T, N>& Arr)
        : Data(Arr.data()), Length(N) {}
  
    /// Construct an ArrayRef from a C array.
    template <size_t N>
    /* implicit */ constexpr ArrayRef(const T (&Arr)[N]) : Data(Arr), Length(N) {}
  
    /// Construct an ArrayRef from a std::initializer_list.
    /* implicit */ constexpr ArrayRef(const std::initializer_list<T>& Vec)
        : Data(
              std::begin(Vec) == std::end(Vec) ? static_cast<T*>(nullptr)
                                               : std::begin(Vec)),
          Length(Vec.size()) {}
  
    /// @}
    /// @name Simple Operations
    /// @{
  
    constexpr iterator begin() const {
      return Data;
    }
    constexpr iterator end() const {
      return Data + Length;
    }
  
    // These are actually the same as iterator, since ArrayRef only
    // gives you const iterators.
    constexpr const_iterator cbegin() const {
      return Data;
    }
    constexpr const_iterator cend() const {
      return Data + Length;
    }
  
    constexpr reverse_iterator rbegin() const {
      return reverse_iterator(end());
    }
    constexpr reverse_iterator rend() const {
      return reverse_iterator(begin());
    }
  
    /// empty - Check if the array is empty.
    constexpr bool empty() const {
      return Length == 0;
    }
  
    constexpr const T* data() const {
      return Data;
    }
  
    /// size - Get the array size.
    constexpr size_t size() const {
      return Length;
    }
  
    /// front - Get the first element.
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA const T& front() const {
      TORCH_CHECK(
          !empty(), "ArrayRef: attempted to access front() of empty list");
      return Data[0];
    }
  
    /// back - Get the last element.
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA const T& back() const {
      TORCH_CHECK(!empty(), "ArrayRef: attempted to access back() of empty list");
      return Data[Length - 1];
    }
  
    /// equals - Check for element-wise equality.
    constexpr bool equals(ArrayRef RHS) const {
      return Length == RHS.Length && std::equal(begin(), end(), RHS.begin());
    }
  
    /// slice(n, m) - Take M elements of the array starting at element N
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA ArrayRef<T> slice(size_t N, size_t M)
        const {
      TORCH_CHECK(
          N + M <= size(),
          "ArrayRef: invalid slice, N = ",
          N,
          "; M = ",
          M,
          "; size = ",
          size());
      return ArrayRef<T>(data() + N, M);
    }
  
    /// slice(n) - Chop off the first N elements of the array.
    constexpr ArrayRef<T> slice(size_t N) const {
      return slice(N, size() - N);
    }
  
    /// @}
    /// @name Operator Overloads
    /// @{
    constexpr const T& operator[](size_t Index) const {
      return Data[Index];
    }
  
    /// Vector compatibility
    C10_HOST_CONSTEXPR_EXCEPT_WIN_CUDA const T& at(size_t Index) const {
      TORCH_CHECK(
          Index < Length,
          "ArrayRef: invalid index Index = ",
          Index,
          "; Length = ",
          Length);
      return Data[Index];
    }
  
    /// Disallow accidental assignment from a temporary.
    ///
    /// The declaration here is extra complicated so that "arrayRef = {}"
    /// continues to select the move assignment operator.
    template <typename U>
    typename std::enable_if<std::is_same<U, T>::value, ArrayRef<T>>::type&
    operator=(U&& Temporary) = delete;
  
    /// Disallow accidental assignment from a temporary.
    ///
    /// The declaration here is extra complicated so that "arrayRef = {}"
    /// continues to select the move assignment operator.
    template <typename U>
    typename std::enable_if<std::is_same<U, T>::value, ArrayRef<T>>::type&
    operator=(std::initializer_list<U>) = delete;
  
    /// @}
    /// @name Expensive Operations
    /// @{
    std::vector<T> vec() const {
      return std::vector<T>(Data, Data + Length);
    }
  
    /// @}
  };
  
  template <typename T>
  std::ostream& operator<<(std::ostream& out, ArrayRef<T> list) {
    int i = 0;
    out << "[";
    for (auto e : list) {
      if (i++ > 0)
        out << ", ";
      out << e;
    }
    out << "]";
    return out;
  }
  
  /// @name ArrayRef Convenience constructors
  /// @{
  
  /// Construct an ArrayRef from a single element.
  template <typename T>
  ArrayRef<T> makeArrayRef(const T& OneElt) {
    return OneElt;
  }
  
  /// Construct an ArrayRef from a pointer and length.
  template <typename T>
  ArrayRef<T> makeArrayRef(const T* data, size_t length) {
    return ArrayRef<T>(data, length);
  }
  
  /// Construct an ArrayRef from a range.
  template <typename T>
  ArrayRef<T> makeArrayRef(const T* begin, const T* end) {
    return ArrayRef<T>(begin, end);
  }
  
  /// Construct an ArrayRef from a SmallVector.
  template <typename T>
  ArrayRef<T> makeArrayRef(const SmallVectorImpl<T>& Vec) {
    return Vec;
  }
  
  /// Construct an ArrayRef from a SmallVector.
  template <typename T, unsigned N>
  ArrayRef<T> makeArrayRef(const SmallVector<T, N>& Vec) {
    return Vec;
  }
  
  /// Construct an ArrayRef from a std::vector.
  template <typename T>
  ArrayRef<T> makeArrayRef(const std::vector<T>& Vec) {
    return Vec;
  }
  
  /// Construct an ArrayRef from a std::array.
  template <typename T, std::size_t N>
  ArrayRef<T> makeArrayRef(const std::array<T, N>& Arr) {
    return Arr;
  }
  
  /// Construct an ArrayRef from an ArrayRef (no-op) (const)
  template <typename T>
  ArrayRef<T> makeArrayRef(const ArrayRef<T>& Vec) {
    return Vec;
  }
  
  /// Construct an ArrayRef from an ArrayRef (no-op)
  template <typename T>
  ArrayRef<T>& makeArrayRef(ArrayRef<T>& Vec) {
    return Vec;
  }
  
  /// Construct an ArrayRef from a C array.
  template <typename T, size_t N>
  ArrayRef<T> makeArrayRef(const T (&Arr)[N]) {
    return ArrayRef<T>(Arr);
  }
  
  // WARNING: Template instantiation will NOT be willing to do an implicit
  // conversions to get you to an c10::ArrayRef, which is why we need so
  // many overloads.
  
  template <typename T>
  bool operator==(c10::ArrayRef<T> a1, c10::ArrayRef<T> a2) {
    return a1.equals(a2);
  }
  
  template <typename T>
  bool operator!=(c10::ArrayRef<T> a1, c10::ArrayRef<T> a2) {
    return !a1.equals(a2);
  }
  
  template <typename T>
  bool operator==(const std::vector<T>& a1, c10::ArrayRef<T> a2) {
    return c10::ArrayRef<T>(a1).equals(a2);
  }
  
  template <typename T>
  bool operator!=(const std::vector<T>& a1, c10::ArrayRef<T> a2) {
    return !c10::ArrayRef<T>(a1).equals(a2);
  }
  
  template <typename T>
  bool operator==(c10::ArrayRef<T> a1, const std::vector<T>& a2) {
    return a1.equals(c10::ArrayRef<T>(a2));
  }
  
  template <typename T>
  bool operator!=(c10::ArrayRef<T> a1, const std::vector<T>& a2) {
    return !a1.equals(c10::ArrayRef<T>(a2));
  }
  
  using IntArrayRef = ArrayRef<int64_t>;
  
  // This alias is deprecated because it doesn't make ownership
  // semantics obvious.  Use IntArrayRef instead!
  C10_DEFINE_DEPRECATED_USING(IntList, ArrayRef<int64_t>)
  
  } // namespace c10
  