api/array_view.h

/*
 *  Copyright 2015 The WebRTC Project Authors. All rights reserved.
 *
 *  Use of this source code is governed by a BSD-style license
 *  that can be found in the LICENSE file in the root of the source
 *  tree. An additional intellectual property rights grant can be found
 *  in the file PATENTS.  All contributing project authors may
 *  be found in the AUTHORS file in the root of the source tree.
 */

#ifndef API_ARRAY_VIEW_H_
#define API_ARRAY_VIEW_H_

#include <algorithm>
#include <type_traits>

#include "rtc_base/checks.h"
#include "rtc_base/type_traits.h"

namespace rtc {

// tl;dr: rtc::ArrayView is the same thing as gsl::span from the Guideline
//        Support Library.
//
// Many functions read from or write to arrays. The obvious way to do this is
// to use two arguments, a pointer to the first element and an element count:
//
//   bool Contains17(const int* arr, size_t size) {
//     for (size_t i = 0; i < size; ++i) {
//       if (arr[i] == 17)
//         return true;
//     }
//     return false;
//   }
//
// This is flexible, since it doesn't matter how the array is stored (C array,
// std::vector, rtc::Buffer, ...), but it's error-prone because the caller has
// to correctly specify the array length:
//
//   Contains17(arr, arraysize(arr));     // C array
//   Contains17(arr.data(), arr.size());  // std::vector
//   Contains17(arr, size);               // pointer + size
//   ...
//
// It's also kind of messy to have two separate arguments for what is
// conceptually a single thing.
//
// Enter rtc::ArrayView<T>. It contains a T pointer (to an array it doesn't
// own) and a count, and supports the basic things you'd expect, such as
// indexing and iteration. It allows us to write our function like this:
//
//   bool Contains17(rtc::ArrayView<const int> arr) {
//     for (auto e : arr) {
//       if (e == 17)
//         return true;
//     }
//     return false;
//   }
//
// And even better, because a bunch of things will implicitly convert to
// ArrayView, we can call it like this:
//
//   Contains17(arr);                             // C array
//   Contains17(arr);                             // std::vector
//   Contains17(rtc::ArrayView<int>(arr, size));  // pointer + size
//   Contains17(nullptr);                         // nullptr -> empty ArrayView
//   ...
//
// ArrayView<T> stores both a pointer and a size, but you may also use
// ArrayView<T, N>, which has a size that's fixed at compile time (which means
// it only has to store the pointer).
//
// One important point is that ArrayView<T> and ArrayView<const T> are
// different types, which allow and don't allow mutation of the array elements,
// respectively. The implicit conversions work just like you'd hope, so that
// e.g. vector<int> will convert to either ArrayView<int> or ArrayView<const
// int>, but const vector<int> will convert only to ArrayView<const int>.
// (ArrayView itself can be the source type in such conversions, so
// ArrayView<int> will convert to ArrayView<const int>.)
//
// Note: ArrayView is tiny (just a pointer and a count if variable-sized, just
// a pointer if fix-sized) and trivially copyable, so it's probably cheaper to
// pass it by value than by const reference.

namespace impl {

// Magic constant for indicating that the size of an ArrayView is variable
// instead of fixed.
enum : std::ptrdiff_t { kArrayViewVarSize = -4711 };

// Base class for ArrayViews of fixed nonzero size.
template <typename T, std::ptrdiff_t Size>
class ArrayViewBase {
  static_assert(Size > 0, "ArrayView size must be variable or non-negative");

 public:
  ArrayViewBase(T* data, size_t size) : data_(data) {}

  static constexpr size_t size() { return Size; }
  static constexpr bool empty() { return false; }
  T* data() const { return data_; }

 protected:
  static constexpr bool fixed_size() { return true; }

 private:
  T* data_;
};

// Specialized base class for ArrayViews of fixed zero size.
template <typename T>
class ArrayViewBase<T, 0> {
 public:
  explicit ArrayViewBase(T* data, size_t size) {}

  static constexpr size_t size() { return 0; }
  static constexpr bool empty() { return true; }
  T* data() const { return nullptr; }

 protected:
  static constexpr bool fixed_size() { return true; }
};

// Specialized base class for ArrayViews of variable size.
template <typename T>
class ArrayViewBase<T, impl::kArrayViewVarSize> {
 public:
  ArrayViewBase(T* data, size_t size)
      : data_(size == 0 ? nullptr : data), size_(size) {}

  size_t size() const { return size_; }
  bool empty() const { return size_ == 0; }
  T* data() const { return data_; }

 protected:
  static constexpr bool fixed_size() { return false; }

 private:
  T* data_;
  size_t size_;
};

}  // namespace impl

template <typename T, std::ptrdiff_t Size = impl::kArrayViewVarSize>
class ArrayView final : public impl::ArrayViewBase<T, Size> {
 public:
  using value_type = T;
  using const_iterator = const T*;

  // Construct an ArrayView from a pointer and a length.
  template <typename U>
  ArrayView(U* data, size_t size)
      : impl::ArrayViewBase<T, Size>::ArrayViewBase(data, size) {
    RTC_DCHECK_EQ(size == 0 ? nullptr : data, this->data());
    RTC_DCHECK_EQ(size, this->size());
    RTC_DCHECK_EQ(!this->data(),
                  this->size() == 0);  // data is null iff size == 0.
  }

  // Construct an empty ArrayView. Note that fixed-size ArrayViews of size > 0
  // cannot be empty.
  ArrayView() : ArrayView(nullptr, 0) {}
  ArrayView(std::nullptr_t)  // NOLINT
      : ArrayView() {}
  ArrayView(std::nullptr_t, size_t size)
      : ArrayView(static_cast<T*>(nullptr), size) {
    static_assert(Size == 0 || Size == impl::kArrayViewVarSize, "");
    RTC_DCHECK_EQ(0, size);
  }

  // Construct an ArrayView from an array.
  template <typename U, size_t N>
  ArrayView(U (&array)[N])  // NOLINT
      : ArrayView(array, N) {
    static_assert(Size == N || Size == impl::kArrayViewVarSize,
                  "Array size must match ArrayView size");
  }

  // (Only if size is fixed.) Construct an ArrayView from any type U that has a
  // static constexpr size() method whose return value is equal to Size, and a
  // data() method whose return value converts implicitly to T*. In particular,
  // this means we allow conversion from ArrayView<T, N> to ArrayView<const T,
  // N>, but not the other way around. We also don't allow conversion from
  // ArrayView<T> to ArrayView<T, N>, or from ArrayView<T, M> to ArrayView<T,
  // N> when M != N.
  template <
      typename U,
      typename std::enable_if<Size != impl::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(U& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {
    static_assert(U::size() == Size, "Sizes must match exactly");
  }

  // (Only if size is variable.) Construct an ArrayView from any type U that
  // has a size() method whose return value converts implicitly to size_t, and
  // a data() method whose return value converts implicitly to T*. In
  // particular, this means we allow conversion from ArrayView<T> to
  // ArrayView<const T>, but not the other way around. Other allowed
  // conversions include
  // ArrayView<T, N> to ArrayView<T> or ArrayView<const T>,
  // std::vector<T> to ArrayView<T> or ArrayView<const T>,
  // const std::vector<T> to ArrayView<const T>,
  // rtc::Buffer to ArrayView<uint8_t> or ArrayView<const uint8_t>, and
  // const rtc::Buffer to ArrayView<const uint8_t>.
  template <
      typename U,
      typename std::enable_if<Size == impl::kArrayViewVarSize &&
                              HasDataAndSize<U, T>::value>::type* = nullptr>
  ArrayView(U& u)  // NOLINT
      : ArrayView(u.data(), u.size()) {}

  // Indexing and iteration. These allow mutation even if the ArrayView is
  // const, because the ArrayView doesn't own the array. (To prevent mutation,
  // use a const element type.)
  T& operator[](size_t idx) const {
    RTC_DCHECK_LT(idx, this->size());
    RTC_DCHECK(this->data());
    return this->data()[idx];
  }
  T* begin() const { return this->data(); }
  T* end() const { return this->data() + this->size(); }
  const T* cbegin() const { return this->data(); }
  const T* cend() const { return this->data() + this->size(); }

  ArrayView<T> subview(size_t offset, size_t size) const {
    return offset < this->size()
               ? ArrayView<T>(this->data() + offset,
                              std::min(size, this->size() - offset))
               : ArrayView<T>();
  }
  ArrayView<T> subview(size_t offset) const {
    return subview(offset, this->size());
  }
};

// Comparing two ArrayViews compares their (pointer,size) pairs; it does *not*
// dereference the pointers.
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator==(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
  return a.data() == b.data() && a.size() == b.size();
}
template <typename T, std::ptrdiff_t Size1, std::ptrdiff_t Size2>
bool operator!=(const ArrayView<T, Size1>& a, const ArrayView<T, Size2>& b) {
  return !(a == b);
}

// Variable-size ArrayViews are the size of two pointers; fixed-size ArrayViews
// are the size of one pointer. (And as a special case, fixed-size ArrayViews
// of size 0 require no storage.)
static_assert(sizeof(ArrayView<int>) == 2 * sizeof(int*), "");
static_assert(sizeof(ArrayView<int, 17>) == sizeof(int*), "");
static_assert(std::is_empty<ArrayView<int, 0>>::value, "");

template <typename T>
inline ArrayView<T> MakeArrayView(T* data, size_t size) {
  return ArrayView<T>(data, size);
}

}  // namespace rtc

#endif  // API_ARRAY_VIEW_H_