path: root/contrib/restricted/abseil-cpp/absl/container/internal/layout.h

// Copyright 2018 The Abseil Authors. 
// 
// Licensed under the Apache License, Version 2.0 (the "License"); 
// you may not use this file except in compliance with the License. 
// You may obtain a copy of the License at 
// 
//      https://www.apache.org/licenses/LICENSE-2.0 
// 
// Unless required by applicable law or agreed to in writing, software 
// distributed under the License is distributed on an "AS IS" BASIS, 
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
// See the License for the specific language governing permissions and 
// limitations under the License. 
// 
//                           MOTIVATION AND TUTORIAL 
// 
// If you want to put in a single heap allocation N doubles followed by M ints, 
// it's easy if N and M are known at compile time. 
// 
//   struct S { 
//     double a[N]; 
//     int b[M]; 
//   }; 
// 
//   S* p = new S; 
// 
// But what if N and M are known only in run time? Class template Layout to the 
// rescue! It's a portable generalization of the technique known as struct hack. 
// 
//   // This object will tell us everything we need to know about the memory 
//   // layout of double[N] followed by int[M]. It's structurally identical to 
//   // size_t[2] that stores N and M. It's very cheap to create. 
//   const Layout<double, int> layout(N, M); 
// 
//   // Allocate enough memory for both arrays. `AllocSize()` tells us how much 
//   // memory is needed. We are free to use any allocation function we want as 
//   // long as it returns aligned memory. 
//   std::unique_ptr<unsigned char[]> p(new unsigned char[layout.AllocSize()]); 
// 
//   // Obtain the pointer to the array of doubles. 
//   // Equivalent to `reinterpret_cast<double*>(p.get())`. 
//   // 
//   // We could have written layout.Pointer<0>(p) instead. If all the types are 
//   // unique you can use either form, but if some types are repeated you must 
//   // use the index form. 
//   double* a = layout.Pointer<double>(p.get()); 
// 
//   // Obtain the pointer to the array of ints. 
//   // Equivalent to `reinterpret_cast<int*>(p.get() + N * 8)`. 
//   int* b = layout.Pointer<int>(p); 
// 
// If we are unable to specify sizes of all fields, we can pass as many sizes as 
// we can to `Partial()`. In return, it'll allow us to access the fields whose 
// locations and sizes can be computed from the provided information. 
// `Partial()` comes in handy when the array sizes are embedded into the 
// allocation. 
// 
//   // size_t[1] containing N, size_t[1] containing M, double[N], int[M]. 
//   using L = Layout<size_t, size_t, double, int>; 
// 
//   unsigned char* Allocate(size_t n, size_t m) { 
//     const L layout(1, 1, n, m); 
//     unsigned char* p = new unsigned char[layout.AllocSize()]; 
//     *layout.Pointer<0>(p) = n; 
//     *layout.Pointer<1>(p) = m; 
//     return p; 
//   } 
// 
//   void Use(unsigned char* p) { 
//     // First, extract N and M. 
//     // Specify that the first array has only one element. Using `prefix` we 
//     // can access the first two arrays but not more. 
//     constexpr auto prefix = L::Partial(1); 
//     size_t n = *prefix.Pointer<0>(p); 
//     size_t m = *prefix.Pointer<1>(p); 
// 
//     // Now we can get pointers to the payload. 
//     const L layout(1, 1, n, m); 
//     double* a = layout.Pointer<double>(p); 
//     int* b = layout.Pointer<int>(p); 
//   } 
// 
// The layout we used above combines fixed-size with dynamically-sized fields. 
// This is quite common. Layout is optimized for this use case and generates 
// optimal code. All computations that can be performed at compile time are 
// indeed performed at compile time. 
// 
// Efficiency tip: The order of fields matters. In `Layout<T1, ..., TN>` try to 
// ensure that `alignof(T1) >= ... >= alignof(TN)`. This way you'll have no 
// padding in between arrays. 
// 
// You can manually override the alignment of an array by wrapping the type in 
// `Aligned<T, N>`. `Layout<..., Aligned<T, N>, ...>` has exactly the same API 
// and behavior as `Layout<..., T, ...>` except that the first element of the 
// array of `T` is aligned to `N` (the rest of the elements follow without 
// padding). `N` cannot be less than `alignof(T)`. 
// 
// `AllocSize()` and `Pointer()` are the most basic methods for dealing with 
// memory layouts. Check out the reference or code below to discover more. 
// 
//                            EXAMPLE 
// 
//   // Immutable move-only string with sizeof equal to sizeof(void*). The 
//   // string size and the characters are kept in the same heap allocation. 
//   class CompactString { 
//    public: 
//     CompactString(const char* s = "") { 
//       const size_t size = strlen(s); 
//       // size_t[1] followed by char[size + 1]. 
//       const L layout(1, size + 1); 
//       p_.reset(new unsigned char[layout.AllocSize()]); 
//       // If running under ASAN, mark the padding bytes, if any, to catch 
//       // memory errors. 
//       layout.PoisonPadding(p_.get()); 
//       // Store the size in the allocation. 
//       *layout.Pointer<size_t>(p_.get()) = size; 
//       // Store the characters in the allocation. 
//       memcpy(layout.Pointer<char>(p_.get()), s, size + 1); 
//     } 
// 
//     size_t size() const { 
//       // Equivalent to reinterpret_cast<size_t&>(*p). 
//       return *L::Partial().Pointer<size_t>(p_.get()); 
//     } 
// 
//     const char* c_str() const { 
//       // Equivalent to reinterpret_cast<char*>(p.get() + sizeof(size_t)). 
//       // The argument in Partial(1) specifies that we have size_t[1] in front 
//       // of the characters. 
//       return L::Partial(1).Pointer<char>(p_.get()); 
//     } 
// 
//    private: 
//     // Our heap allocation contains a size_t followed by an array of chars. 
//     using L = Layout<size_t, char>; 
//     std::unique_ptr<unsigned char[]> p_; 
//   }; 
// 
//   int main() { 
//     CompactString s = "hello"; 
//     assert(s.size() == 5); 
//     assert(strcmp(s.c_str(), "hello") == 0); 
//   } 
// 
//                               DOCUMENTATION 
// 
// The interface exported by this file consists of: 
// - class `Layout<>` and its public members. 
// - The public members of class `internal_layout::LayoutImpl<>`. That class 
//   isn't intended to be used directly, and its name and template parameter 
//   list are internal implementation details, but the class itself provides 
//   most of the functionality in this file. See comments on its members for 
//   detailed documentation. 
// 
// `Layout<T1,... Tn>::Partial(count1,..., countm)` (where `m` <= `n`) returns a 
// `LayoutImpl<>` object. `Layout<T1,..., Tn> layout(count1,..., countn)` 
// creates a `Layout` object, which exposes the same functionality by inheriting 
// from `LayoutImpl<>`. 
 
#ifndef ABSL_CONTAINER_INTERNAL_LAYOUT_H_ 
#define ABSL_CONTAINER_INTERNAL_LAYOUT_H_ 
 
#include <assert.h> 
#include <stddef.h> 
#include <stdint.h> 

#include <ostream> 
#include <string> 
#include <tuple> 
#include <type_traits> 
#include <typeinfo> 
#include <utility> 
 
#include "absl/base/config.h"
#include "absl/meta/type_traits.h" 
#include "absl/strings/str_cat.h" 
#include "absl/types/span.h" 
#include "absl/utility/utility.h" 
 
#ifdef ABSL_HAVE_ADDRESS_SANITIZER
#include <sanitizer/asan_interface.h>
#endif

#if defined(__GXX_RTTI) 
#define ABSL_INTERNAL_HAS_CXA_DEMANGLE 
#endif 
 
#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE 
#include <cxxabi.h> 
#endif 
 
namespace absl { 
ABSL_NAMESPACE_BEGIN
namespace container_internal { 
 
// A type wrapper that instructs `Layout` to use the specific alignment for the 
// array. `Layout<..., Aligned<T, N>, ...>` has exactly the same API 
// and behavior as `Layout<..., T, ...>` except that the first element of the 
// array of `T` is aligned to `N` (the rest of the elements follow without 
// padding). 
// 
// Requires: `N >= alignof(T)` and `N` is a power of 2. 
template <class T, size_t N> 
struct Aligned; 
 
namespace internal_layout { 
 
template <class T> 
struct NotAligned {}; 
 
template <class T, size_t N> 
struct NotAligned<const Aligned<T, N>> { 
  static_assert(sizeof(T) == 0, "Aligned<T, N> cannot be const-qualified"); 
}; 
 
template <size_t> 
using IntToSize = size_t; 
 
template <class> 
using TypeToSize = size_t; 
 
template <class T> 
struct Type : NotAligned<T> { 
  using type = T; 
}; 
 
template <class T, size_t N> 
struct Type<Aligned<T, N>> { 
  using type = T; 
}; 
 
template <class T> 
struct SizeOf : NotAligned<T>, std::integral_constant<size_t, sizeof(T)> {}; 
 
template <class T, size_t N> 
struct SizeOf<Aligned<T, N>> : std::integral_constant<size_t, sizeof(T)> {}; 
 
// Note: workaround for https://gcc.gnu.org/PR88115 
template <class T> 
struct AlignOf : NotAligned<T> { 
  static constexpr size_t value = alignof(T); 
}; 
 
template <class T, size_t N> 
struct AlignOf<Aligned<T, N>> { 
  static_assert(N % alignof(T) == 0, 
                "Custom alignment can't be lower than the type's alignment"); 
  static constexpr size_t value = N; 
}; 
 
// Does `Ts...` contain `T`? 
template <class T, class... Ts> 
using Contains = absl::disjunction<std::is_same<T, Ts>...>; 
 
template <class From, class To> 
using CopyConst = 
    typename std::conditional<std::is_const<From>::value, const To, To>::type; 
 
// Note: We're not qualifying this with absl:: because it doesn't compile under 
// MSVC. 
template <class T> 
using SliceType = Span<T>; 
 
// This namespace contains no types. It prevents functions defined in it from 
// being found by ADL. 
namespace adl_barrier { 
 
template <class Needle, class... Ts> 
constexpr size_t Find(Needle, Needle, Ts...) { 
  static_assert(!Contains<Needle, Ts...>(), "Duplicate element type"); 
  return 0; 
} 
 
template <class Needle, class T, class... Ts> 
constexpr size_t Find(Needle, T, Ts...) { 
  return adl_barrier::Find(Needle(), Ts()...) + 1; 
} 
 
constexpr bool IsPow2(size_t n) { return !(n & (n - 1)); } 
 
// Returns `q * m` for the smallest `q` such that `q * m >= n`. 
// Requires: `m` is a power of two. It's enforced by IsLegalElementType below. 
constexpr size_t Align(size_t n, size_t m) { return (n + m - 1) & ~(m - 1); } 
 
constexpr size_t Min(size_t a, size_t b) { return b < a ? b : a; } 
 
constexpr size_t Max(size_t a) { return a; } 
 
template <class... Ts> 
constexpr size_t Max(size_t a, size_t b, Ts... rest) { 
  return adl_barrier::Max(b < a ? a : b, rest...); 
} 
 
template <class T> 
std::string TypeName() { 
  std::string out; 
  int status = 0; 
  char* demangled = nullptr; 
#ifdef ABSL_INTERNAL_HAS_CXA_DEMANGLE 
  demangled = abi::__cxa_demangle(typeid(T).name(), nullptr, nullptr, &status); 
#endif 
  if (status == 0 && demangled != nullptr) {  // Demangling succeeded. 
    absl::StrAppend(&out, "<", demangled, ">"); 
    free(demangled); 
  } else { 
#if defined(__GXX_RTTI) || defined(_CPPRTTI) 
    absl::StrAppend(&out, "<", typeid(T).name(), ">"); 
#endif 
  } 
  return out; 
} 
 
}  // namespace adl_barrier 
 
template <bool C> 
using EnableIf = typename std::enable_if<C, int>::type; 
 
// Can `T` be a template argument of `Layout`? 
template <class T> 
using IsLegalElementType = std::integral_constant< 
    bool, !std::is_reference<T>::value && !std::is_volatile<T>::value && 
              !std::is_reference<typename Type<T>::type>::value && 
              !std::is_volatile<typename Type<T>::type>::value && 
              adl_barrier::IsPow2(AlignOf<T>::value)>; 
 
template <class Elements, class SizeSeq, class OffsetSeq> 
class LayoutImpl; 
 
// Public base class of `Layout` and the result type of `Layout::Partial()`. 
// 
// `Elements...` contains all template arguments of `Layout` that created this 
// instance. 
// 
// `SizeSeq...` is `[0, NumSizes)` where `NumSizes` is the number of arguments 
// passed to `Layout::Partial()` or `Layout::Layout()`. 
// 
// `OffsetSeq...` is `[0, NumOffsets)` where `NumOffsets` is 
// `Min(sizeof...(Elements), NumSizes + 1)` (the number of arrays for which we 
// can compute offsets). 
template <class... Elements, size_t... SizeSeq, size_t... OffsetSeq> 
class LayoutImpl<std::tuple<Elements...>, absl::index_sequence<SizeSeq...>, 
                 absl::index_sequence<OffsetSeq...>> { 
 private: 
  static_assert(sizeof...(Elements) > 0, "At least one field is required"); 
  static_assert(absl::conjunction<IsLegalElementType<Elements>...>::value, 
                "Invalid element type (see IsLegalElementType)"); 
 
  enum { 
    NumTypes = sizeof...(Elements), 
    NumSizes = sizeof...(SizeSeq), 
    NumOffsets = sizeof...(OffsetSeq), 
  }; 
 
  // These are guaranteed by `Layout`. 
  static_assert(NumOffsets == adl_barrier::Min(NumTypes, NumSizes + 1), 
                "Internal error"); 
  static_assert(NumTypes > 0, "Internal error"); 
 
  // Returns the index of `T` in `Elements...`. Results in a compilation error 
  // if `Elements...` doesn't contain exactly one instance of `T`. 
  template <class T> 
  static constexpr size_t ElementIndex() { 
    static_assert(Contains<Type<T>, Type<typename Type<Elements>::type>...>(), 
                  "Type not found"); 
    return adl_barrier::Find(Type<T>(), 
                             Type<typename Type<Elements>::type>()...); 
  } 
 
  template <size_t N> 
  using ElementAlignment = 
      AlignOf<typename std::tuple_element<N, std::tuple<Elements...>>::type>; 
 
 public: 
  // Element types of all arrays packed in a tuple. 
  using ElementTypes = std::tuple<typename Type<Elements>::type...>; 
 
  // Element type of the Nth array. 
  template <size_t N> 
  using ElementType = typename std::tuple_element<N, ElementTypes>::type; 
 
  constexpr explicit LayoutImpl(IntToSize<SizeSeq>... sizes) 
      : size_{sizes...} {} 
 
  // Alignment of the layout, equal to the strictest alignment of all elements. 
  // All pointers passed to the methods of layout must be aligned to this value. 
  static constexpr size_t Alignment() { 
    return adl_barrier::Max(AlignOf<Elements>::value...); 
  } 
 
  // Offset in bytes of the Nth array. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   assert(x.Offset<0>() == 0);   // The ints starts from 0. 
  //   assert(x.Offset<1>() == 16);  // The doubles starts from 16. 
  // 
  // Requires: `N <= NumSizes && N < sizeof...(Ts)`. 
  template <size_t N, EnableIf<N == 0> = 0> 
  constexpr size_t Offset() const { 
    return 0; 
  } 
 
  template <size_t N, EnableIf<N != 0> = 0> 
  constexpr size_t Offset() const { 
    static_assert(N < NumOffsets, "Index out of bounds"); 
    return adl_barrier::Align( 
        Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * size_[N - 1],
        ElementAlignment<N>::value); 
  } 
 
  // Offset in bytes of the array with the specified element type. There must 
  // be exactly one such array and its zero-based index must be at most 
  // `NumSizes`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   assert(x.Offset<int>() == 0);      // The ints starts from 0. 
  //   assert(x.Offset<double>() == 16);  // The doubles starts from 16. 
  template <class T> 
  constexpr size_t Offset() const { 
    return Offset<ElementIndex<T>()>(); 
  } 
 
  // Offsets in bytes of all arrays for which the offsets are known. 
  constexpr std::array<size_t, NumOffsets> Offsets() const { 
    return {{Offset<OffsetSeq>()...}}; 
  } 
 
  // The number of elements in the Nth array. This is the Nth argument of 
  // `Layout::Partial()` or `Layout::Layout()` (zero-based). 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   assert(x.Size<0>() == 3); 
  //   assert(x.Size<1>() == 4); 
  // 
  // Requires: `N < NumSizes`. 
  template <size_t N> 
  constexpr size_t Size() const { 
    static_assert(N < NumSizes, "Index out of bounds"); 
    return size_[N]; 
  } 
 
  // The number of elements in the array with the specified element type. 
  // There must be exactly one such array and its zero-based index must be 
  // at most `NumSizes`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   assert(x.Size<int>() == 3); 
  //   assert(x.Size<double>() == 4); 
  template <class T> 
  constexpr size_t Size() const { 
    return Size<ElementIndex<T>()>(); 
  } 
 
  // The number of elements of all arrays for which they are known. 
  constexpr std::array<size_t, NumSizes> Sizes() const { 
    return {{Size<SizeSeq>()...}}; 
  } 
 
  // Pointer to the beginning of the Nth array. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  //   int* ints = x.Pointer<0>(p); 
  //   double* doubles = x.Pointer<1>(p); 
  // 
  // Requires: `N <= NumSizes && N < sizeof...(Ts)`. 
  // Requires: `p` is aligned to `Alignment()`. 
  template <size_t N, class Char> 
  CopyConst<Char, ElementType<N>>* Pointer(Char* p) const { 
    using C = typename std::remove_const<Char>::type; 
    static_assert( 
        std::is_same<C, char>() || std::is_same<C, unsigned char>() || 
            std::is_same<C, signed char>(), 
        "The argument must be a pointer to [const] [signed|unsigned] char"); 
    constexpr size_t alignment = Alignment(); 
    (void)alignment; 
    assert(reinterpret_cast<uintptr_t>(p) % alignment == 0); 
    return reinterpret_cast<CopyConst<Char, ElementType<N>>*>(p + Offset<N>()); 
  } 
 
  // Pointer to the beginning of the array with the specified element type. 
  // There must be exactly one such array and its zero-based index must be at 
  // most `NumSizes`. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  //   int* ints = x.Pointer<int>(p); 
  //   double* doubles = x.Pointer<double>(p); 
  // 
  // Requires: `p` is aligned to `Alignment()`. 
  template <class T, class Char> 
  CopyConst<Char, T>* Pointer(Char* p) const { 
    return Pointer<ElementIndex<T>()>(p); 
  } 
 
  // Pointers to all arrays for which pointers are known. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  // 
  //   int* ints; 
  //   double* doubles; 
  //   std::tie(ints, doubles) = x.Pointers(p); 
  // 
  // Requires: `p` is aligned to `Alignment()`. 
  // 
  // Note: We're not using ElementType alias here because it does not compile 
  // under MSVC. 
  template <class Char> 
  std::tuple<CopyConst< 
      Char, typename std::tuple_element<OffsetSeq, ElementTypes>::type>*...> 
  Pointers(Char* p) const { 
    return std::tuple<CopyConst<Char, ElementType<OffsetSeq>>*...>( 
        Pointer<OffsetSeq>(p)...); 
  } 
 
  // The Nth array. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  //   Span<int> ints = x.Slice<0>(p); 
  //   Span<double> doubles = x.Slice<1>(p); 
  // 
  // Requires: `N < NumSizes`. 
  // Requires: `p` is aligned to `Alignment()`. 
  template <size_t N, class Char> 
  SliceType<CopyConst<Char, ElementType<N>>> Slice(Char* p) const { 
    return SliceType<CopyConst<Char, ElementType<N>>>(Pointer<N>(p), Size<N>()); 
  } 
 
  // The array with the specified element type. There must be exactly one 
  // such array and its zero-based index must be less than `NumSizes`. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  //   Span<int> ints = x.Slice<int>(p); 
  //   Span<double> doubles = x.Slice<double>(p); 
  // 
  // Requires: `p` is aligned to `Alignment()`. 
  template <class T, class Char> 
  SliceType<CopyConst<Char, T>> Slice(Char* p) const { 
    return Slice<ElementIndex<T>()>(p); 
  } 
 
  // All arrays with known sizes. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()]; 
  // 
  //   Span<int> ints; 
  //   Span<double> doubles; 
  //   std::tie(ints, doubles) = x.Slices(p); 
  // 
  // Requires: `p` is aligned to `Alignment()`. 
  // 
  // Note: We're not using ElementType alias here because it does not compile 
  // under MSVC. 
  template <class Char> 
  std::tuple<SliceType<CopyConst< 
      Char, typename std::tuple_element<SizeSeq, ElementTypes>::type>>...> 
  Slices(Char* p) const { 
    // Workaround for https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63875 (fixed 
    // in 6.1). 
    (void)p; 
    return std::tuple<SliceType<CopyConst<Char, ElementType<SizeSeq>>>...>( 
        Slice<SizeSeq>(p)...); 
  } 
 
  // The size of the allocation that fits all arrays. 
  // 
  //   // int[3], 4 bytes of padding, double[4]. 
  //   Layout<int, double> x(3, 4); 
  //   unsigned char* p = new unsigned char[x.AllocSize()];  // 48 bytes 
  // 
  // Requires: `NumSizes == sizeof...(Ts)`. 
  constexpr size_t AllocSize() const { 
    static_assert(NumTypes == NumSizes, "You must specify sizes of all fields"); 
    return Offset<NumTypes - 1>() + 
        SizeOf<ElementType<NumTypes - 1>>::value * size_[NumTypes - 1];
  } 
 
  // If built with --config=asan, poisons padding bytes (if any) in the 
  // allocation. The pointer must point to a memory block at least 
  // `AllocSize()` bytes in length. 
  // 
  // `Char` must be `[const] [signed|unsigned] char`. 
  // 
  // Requires: `p` is aligned to `Alignment()`. 
  template <class Char, size_t N = NumOffsets - 1, EnableIf<N == 0> = 0> 
  void PoisonPadding(const Char* p) const { 
    Pointer<0>(p);  // verify the requirements on `Char` and `p` 
  } 
 
  template <class Char, size_t N = NumOffsets - 1, EnableIf<N != 0> = 0> 
  void PoisonPadding(const Char* p) const { 
    static_assert(N < NumOffsets, "Index out of bounds"); 
    (void)p; 
#ifdef ABSL_HAVE_ADDRESS_SANITIZER
    PoisonPadding<Char, N - 1>(p); 
    // The `if` is an optimization. It doesn't affect the observable behaviour. 
    if (ElementAlignment<N - 1>::value % ElementAlignment<N>::value) { 
      size_t start = 
          Offset<N - 1>() + SizeOf<ElementType<N - 1>>::value * size_[N - 1];
      ASAN_POISON_MEMORY_REGION(p + start, Offset<N>() - start); 
    } 
#endif 
  } 
 
  // Human-readable description of the memory layout. Useful for debugging. 
  // Slow. 
  // 
  //   // char[5], 3 bytes of padding, int[3], 4 bytes of padding, followed 
  //   // by an unknown number of doubles. 
  //   auto x = Layout<char, int, double>::Partial(5, 3); 
  //   assert(x.DebugString() == 
  //          "@0<char>(1)[5]; @8<int>(4)[3]; @24<double>(8)"); 
  // 
  // Each field is in the following format: @offset<type>(sizeof)[size] (<type> 
  // may be missing depending on the target platform). For example, 
  // @8<int>(4)[3] means that at offset 8 we have an array of ints, where each 
  // int is 4 bytes, and we have 3 of those ints. The size of the last field may 
  // be missing (as in the example above). Only fields with known offsets are 
  // described. Type names may differ across platforms: one compiler might 
  // produce "unsigned*" where another produces "unsigned int *". 
  std::string DebugString() const { 
    const auto offsets = Offsets(); 
    const size_t sizes[] = {SizeOf<ElementType<OffsetSeq>>::value...};
    const std::string types[] = { 
        adl_barrier::TypeName<ElementType<OffsetSeq>>()...}; 
    std::string res = absl::StrCat("@0", types[0], "(", sizes[0], ")"); 
    for (size_t i = 0; i != NumOffsets - 1; ++i) { 
      absl::StrAppend(&res, "[", size_[i], "]; @", offsets[i + 1], types[i + 1], 
                      "(", sizes[i + 1], ")"); 
    } 
    // NumSizes is a constant that may be zero. Some compilers cannot see that 
    // inside the if statement "size_[NumSizes - 1]" must be valid. 
    int last = static_cast<int>(NumSizes) - 1; 
    if (NumTypes == NumSizes && last >= 0) { 
      absl::StrAppend(&res, "[", size_[last], "]"); 
    } 
    return res; 
  } 
 
 private: 
  // Arguments of `Layout::Partial()` or `Layout::Layout()`. 
  size_t size_[NumSizes > 0 ? NumSizes : 1]; 
}; 
 
template <size_t NumSizes, class... Ts> 
using LayoutType = LayoutImpl< 
    std::tuple<Ts...>, absl::make_index_sequence<NumSizes>, 
    absl::make_index_sequence<adl_barrier::Min(sizeof...(Ts), NumSizes + 1)>>; 
 
}  // namespace internal_layout 
 
// Descriptor of arrays of various types and sizes laid out in memory one after 
// another. See the top of the file for documentation. 
// 
// Check out the public API of internal_layout::LayoutImpl above. The type is 
// internal to the library but its methods are public, and they are inherited 
// by `Layout`. 
template <class... Ts> 
class Layout : public internal_layout::LayoutType<sizeof...(Ts), Ts...> { 
 public: 
  static_assert(sizeof...(Ts) > 0, "At least one field is required"); 
  static_assert( 
      absl::conjunction<internal_layout::IsLegalElementType<Ts>...>::value, 
      "Invalid element type (see IsLegalElementType)"); 
 
  // The result type of `Partial()` with `NumSizes` arguments. 
  template <size_t NumSizes> 
  using PartialType = internal_layout::LayoutType<NumSizes, Ts...>; 
 
  // `Layout` knows the element types of the arrays we want to lay out in 
  // memory but not the number of elements in each array. 
  // `Partial(size1, ..., sizeN)` allows us to specify the latter. The 
  // resulting immutable object can be used to obtain pointers to the 
  // individual arrays. 
  // 
  // It's allowed to pass fewer array sizes than the number of arrays. E.g., 
  // if all you need is to the offset of the second array, you only need to 
  // pass one argument -- the number of elements in the first array. 
  // 
  //   // int[3] followed by 4 bytes of padding and an unknown number of 
  //   // doubles. 
  //   auto x = Layout<int, double>::Partial(3); 
  //   // doubles start at byte 16. 
  //   assert(x.Offset<1>() == 16); 
  // 
  // If you know the number of elements in all arrays, you can still call 
  // `Partial()` but it's more convenient to use the constructor of `Layout`. 
  // 
  //   Layout<int, double> x(3, 5); 
  // 
  // Note: The sizes of the arrays must be specified in number of elements, 
  // not in bytes. 
  // 
  // Requires: `sizeof...(Sizes) <= sizeof...(Ts)`. 
  // Requires: all arguments are convertible to `size_t`. 
  template <class... Sizes> 
  static constexpr PartialType<sizeof...(Sizes)> Partial(Sizes&&... sizes) { 
    static_assert(sizeof...(Sizes) <= sizeof...(Ts), ""); 
    return PartialType<sizeof...(Sizes)>(absl::forward<Sizes>(sizes)...); 
  } 
 
  // Creates a layout with the sizes of all arrays specified. If you know 
  // only the sizes of the first N arrays (where N can be zero), you can use 
  // `Partial()` defined above. The constructor is essentially equivalent to 
  // calling `Partial()` and passing in all array sizes; the constructor is 
  // provided as a convenient abbreviation. 
  // 
  // Note: The sizes of the arrays must be specified in number of elements, 
  // not in bytes. 
  constexpr explicit Layout(internal_layout::TypeToSize<Ts>... sizes) 
      : internal_layout::LayoutType<sizeof...(Ts), Ts...>(sizes...) {} 
}; 
 
}  // namespace container_internal 
ABSL_NAMESPACE_END
}  // namespace absl 
 
#endif  // ABSL_CONTAINER_INTERNAL_LAYOUT_H_