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+// Copyright 2017 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
+//
+//      http://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.
+//
+// -----------------------------------------------------------------------------
+// File: fixed_array.h
+// -----------------------------------------------------------------------------
+//
+// A `FixedArray<T>` represents a non-resizable array of `T` where the length of
+// the array can be determined at run-time. It is a good replacement for
+// non-standard and deprecated uses of `alloca()` and variable length arrays
+// within the GCC extension. (See
+// https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html).
+//
+// `FixedArray` allocates small arrays inline, keeping performance fast by
+// avoiding heap operations. It also helps reduce the chances of
+// accidentally overflowing your stack if large input is passed to
+// your function.
+
+#ifndef ABSL_CONTAINER_FIXED_ARRAY_H_
+#define ABSL_CONTAINER_FIXED_ARRAY_H_
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <cstddef>
+#include <initializer_list>
+#include <iterator>
+#include <limits>
+#include <memory>
+#include <new>
+#include <type_traits>
+
+#include "absl/algorithm/algorithm.h"
+#include "absl/base/dynamic_annotations.h"
+#include "absl/base/internal/throw_delegate.h"
+#include "absl/base/macros.h"
+#include "absl/base/optimization.h"
+#include "absl/base/port.h"
+
+namespace absl {
+
+constexpr static auto kFixedArrayUseDefault = static_cast<size_t>(-1);
+
+// -----------------------------------------------------------------------------
+// FixedArray
+// -----------------------------------------------------------------------------
+//
+// A `FixedArray` provides a run-time fixed-size array, allocating small arrays
+// inline for efficiency and correctness.
+//
+// Most users should not specify an `inline_elements` argument and let
+// `FixedArray<>` automatically determine the number of elements
+// to store inline based on `sizeof(T)`. If `inline_elements` is specified, the
+// `FixedArray<>` implementation will inline arrays of
+// length <= `inline_elements`.
+//
+// Note that a `FixedArray` constructed with a `size_type` argument will
+// default-initialize its values by leaving trivially constructible types
+// uninitialized (e.g. int, int[4], double), and others default-constructed.
+// This matches the behavior of c-style arrays and `std::array`, but not
+// `std::vector`.
+//
+// Note that `FixedArray` does not provide a public allocator; if it requires a
+// heap allocation, it will do so with global `::operator new[]()` and
+// `::operator delete[]()`, even if T provides class-scope overrides for these
+// operators.
+template <typename T, size_t inlined = kFixedArrayUseDefault>
+class FixedArray {
+  static constexpr size_t kInlineBytesDefault = 256;
+
+  // std::iterator_traits isn't guaranteed to be SFINAE-friendly until C++17,
+  // but this seems to be mostly pedantic.
+  template <typename Iter>
+  using EnableIfForwardIterator = typename std::enable_if<
+      std::is_convertible<
+          typename std::iterator_traits<Iter>::iterator_category,
+          std::forward_iterator_tag>::value,
+      int>::type;
+
+ public:
+  // For playing nicely with stl:
+  using value_type = T;
+  using iterator = T*;
+  using const_iterator = const T*;
+  using reverse_iterator = std::reverse_iterator<iterator>;
+  using const_reverse_iterator = std::reverse_iterator<const_iterator>;
+  using reference = T&;
+  using const_reference = const T&;
+  using pointer = T*;
+  using const_pointer = const T*;
+  using difference_type = ptrdiff_t;
+  using size_type = size_t;
+
+  static constexpr size_type inline_elements =
+      inlined == kFixedArrayUseDefault
+          ? kInlineBytesDefault / sizeof(value_type)
+          : inlined;
+
+  // Creates an array object that can store `n` elements.
+  // Note that trivially constructible elements will be uninitialized.
+  explicit FixedArray(size_type n) : rep_(n) {}
+
+  // Creates an array initialized with `n` copies of `val`.
+  FixedArray(size_type n, const value_type& val) : rep_(n, val) {}
+
+  // Creates an array initialized with the elements from the input
+  // range. The array's size will always be `std::distance(first, last)`.
+  // REQUIRES: Iter must be a forward_iterator or better.
+  template <typename Iter, EnableIfForwardIterator<Iter> = 0>
+  FixedArray(Iter first, Iter last) : rep_(first, last) {}
+
+  // Create the array from an initializer_list.
+  FixedArray(std::initializer_list<T> init_list)
+      : FixedArray(init_list.begin(), init_list.end()) {}
+
+  ~FixedArray() {}
+
+  // Copy and move construction and assignment are deleted because (1) you can't
+  // copy or move an array, (2) assignment breaks the invariant that the size of
+  // a `FixedArray` never changes, and (3) there's no clear answer as to what
+  // should happen to a moved-from `FixedArray`.
+  FixedArray(const FixedArray&) = delete;
+  void operator=(const FixedArray&) = delete;
+
+  // FixedArray::size()
+  //
+  // Returns the length of the fixed array.
+  size_type size() const { return rep_.size(); }
+
+  // FixedArray::max_size()
+  //
+  // Returns the largest possible value of `std::distance(begin(), end())` for a
+  // `FixedArray<T>`. This is equivalent to the most possible addressable bytes
+  // over the number of bytes taken by T.
+  constexpr size_type max_size() const {
+    return std::numeric_limits<difference_type>::max() / sizeof(value_type);
+  }
+
+  // FixedArray::empty()
+  //
+  // Returns whether or not the fixed array is empty.
+  bool empty() const { return size() == 0; }
+
+  // FixedArray::memsize()
+  //
+  // Returns the memory size of the fixed array in bytes.
+  size_t memsize() const { return size() * sizeof(value_type); }
+
+  // FixedArray::data()
+  //
+  // Returns a const T* pointer to elements of the `FixedArray`. This pointer
+  // can be used to access (but not modify) the contained elements.
+  const_pointer data() const { return AsValue(rep_.begin()); }
+
+  // Overload of FixedArray::data() to return a T* pointer to elements of the
+  // fixed array. This pointer can be used to access and modify the contained
+  // elements.
+  pointer data() { return AsValue(rep_.begin()); }
+  // FixedArray::operator[]
+  //
+  // Returns a reference the ith element of the fixed array.
+  // REQUIRES: 0 <= i < size()
+  reference operator[](size_type i) {
+    assert(i < size());
+    return data()[i];
+  }
+
+  // Overload of FixedArray::operator()[] to return a const reference to the
+  // ith element of the fixed array.
+  // REQUIRES: 0 <= i < size()
+  const_reference operator[](size_type i) const {
+    assert(i < size());
+    return data()[i];
+  }
+
+  // FixedArray::at
+  //
+  // Bounds-checked access.  Returns a reference to the ith element of the
+  // fiexed array, or throws std::out_of_range
+  reference at(size_type i) {
+    if (ABSL_PREDICT_FALSE(i >= size())) {
+      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
+    }
+    return data()[i];
+  }
+
+  // Overload of FixedArray::at() to return a const reference to the ith element
+  // of the fixed array.
+  const_reference at(size_type i) const {
+    if (i >= size()) {
+      base_internal::ThrowStdOutOfRange("FixedArray::at failed bounds check");
+    }
+    return data()[i];
+  }
+
+  // FixedArray::front()
+  //
+  // Returns a reference to the first element of the fixed array.
+  reference front() { return *begin(); }
+
+  // Overload of FixedArray::front() to return a reference to the first element
+  // of a fixed array of const values.
+  const_reference front() const { return *begin(); }
+
+  // FixedArray::back()
+  //
+  // Returns a reference to the last element of the fixed array.
+  reference back() { return *(end() - 1); }
+
+  // Overload of FixedArray::back() to return a reference to the last element
+  // of a fixed array of const values.
+  const_reference back() const { return *(end() - 1); }
+
+  // FixedArray::begin()
+  //
+  // Returns an iterator to the beginning of the fixed array.
+  iterator begin() { return data(); }
+
+  // Overload of FixedArray::begin() to return a const iterator to the
+  // beginning of the fixed array.
+  const_iterator begin() const { return data(); }
+
+  // FixedArray::cbegin()
+  //
+  // Returns a const iterator to the beginning of the fixed array.
+  const_iterator cbegin() const { return begin(); }
+
+  // FixedArray::end()
+  //
+  // Returns an iterator to the end of the fixed array.
+  iterator end() { return data() + size(); }
+
+  // Overload of FixedArray::end() to return a const iterator to the end of the
+  // fixed array.
+  const_iterator end() const { return data() + size(); }
+
+  // FixedArray::cend()
+  //
+  // Returns a const iterator to the end of the fixed array.
+  const_iterator cend() const { return end(); }
+
+  // FixedArray::rbegin()
+  //
+  // Returns a reverse iterator from the end of the fixed array.
+  reverse_iterator rbegin() { return reverse_iterator(end()); }
+
+  // Overload of FixedArray::rbegin() to return a const reverse iterator from
+  // the end of the fixed array.
+  const_reverse_iterator rbegin() const {
+    return const_reverse_iterator(end());
+  }
+
+  // FixedArray::crbegin()
+  //
+  // Returns a const reverse iterator from the end of the fixed array.
+  const_reverse_iterator crbegin() const { return rbegin(); }
+
+  // FixedArray::rend()
+  //
+  // Returns a reverse iterator from the beginning of the fixed array.
+  reverse_iterator rend() { return reverse_iterator(begin()); }
+
+  // Overload of FixedArray::rend() for returning a const reverse iterator
+  // from the beginning of the fixed array.
+  const_reverse_iterator rend() const {
+    return const_reverse_iterator(begin());
+  }
+
+  // FixedArray::crend()
+  //
+  // Returns a reverse iterator from the beginning of the fixed array.
+  const_reverse_iterator crend() const { return rend(); }
+
+  // FixedArray::fill()
+  //
+  // Assigns the given `value` to all elements in the fixed array.
+  void fill(const T& value) { std::fill(begin(), end(), value); }
+
+  // Relational operators. Equality operators are elementwise using
+  // `operator==`, while order operators order FixedArrays lexicographically.
+  friend bool operator==(const FixedArray& lhs, const FixedArray& rhs) {
+    return absl::equal(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());
+  }
+
+  friend bool operator!=(const FixedArray& lhs, const FixedArray& rhs) {
+    return !(lhs == rhs);
+  }
+
+  friend bool operator<(const FixedArray& lhs, const FixedArray& rhs) {
+    return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(),
+                                        rhs.end());
+  }
+
+  friend bool operator>(const FixedArray& lhs, const FixedArray& rhs) {
+    return rhs < lhs;
+  }
+
+  friend bool operator<=(const FixedArray& lhs, const FixedArray& rhs) {
+    return !(rhs < lhs);
+  }
+
+  friend bool operator>=(const FixedArray& lhs, const FixedArray& rhs) {
+    return !(lhs < rhs);
+  }
+
+ private:
+  // HolderTraits
+  //
+  // Wrapper to hold elements of type T for the case where T is an array type.
+  // If 'T' is an array type, HolderTraits::type is a struct with a 'T v;'.
+  // Otherwise, HolderTraits::type is simply 'T'.
+  //
+  // Maintainer's Note: The simpler solution would be to simply wrap T in a
+  // struct whether it's an array or not: 'struct Holder { T v; };', but
+  // that causes some paranoid diagnostics to misfire about uses of data(),
+  // believing that 'data()' (aka '&rep_.begin().v') is a pointer to a single
+  // element, rather than the packed array that it really is.
+  // e.g.:
+  //
+  //     FixedArray<char> buf(1);
+  //     sprintf(buf.data(), "foo");
+  //
+  //     error: call to int __builtin___sprintf_chk(etc...)
+  //     will always overflow destination buffer [-Werror]
+  //
+  class HolderTraits {
+    template <typename U>
+    struct SelectImpl {
+      using type = U;
+      static pointer AsValue(type* p) { return p; }
+    };
+
+    // Partial specialization for elements of array type.
+    template <typename U, size_t N>
+    struct SelectImpl<U[N]> {
+      struct Holder { U v[N]; };
+      using type = Holder;
+      static pointer AsValue(type* p) { return &p->v; }
+    };
+    using Impl = SelectImpl<value_type>;
+
+   public:
+    using type = typename Impl::type;
+
+    static pointer AsValue(type *p) { return Impl::AsValue(p); }
+
+    // TODO(billydonahue): fix the type aliasing violation
+    // this assertion hints at.
+    static_assert(sizeof(type) == sizeof(value_type),
+                  "Holder must be same size as value_type");
+  };
+
+  using Holder = typename HolderTraits::type;
+  static pointer AsValue(Holder *p) { return HolderTraits::AsValue(p); }
+
+  // InlineSpace
+  //
+  // Allocate some space, not an array of elements of type T, so that we can
+  // skip calling the T constructors and destructors for space we never use.
+  // How many elements should we store inline?
+  //   a. If not specified, use a default of kInlineBytesDefault bytes (This is
+  //   currently 256 bytes, which seems small enough to not cause stack overflow
+  //   or unnecessary stack pollution, while still allowing stack allocation for
+  //   reasonably long character arrays).
+  //   b. Never use 0 length arrays (not ISO C++)
+  //
+  template <size_type N, typename = void>
+  class InlineSpace {
+   public:
+    Holder* data() { return reinterpret_cast<Holder*>(space_.data()); }
+    void AnnotateConstruct(size_t n) const { Annotate(n, true); }
+    void AnnotateDestruct(size_t n) const { Annotate(n, false); }
+
+   private:
+#ifndef ADDRESS_SANITIZER
+    void Annotate(size_t, bool) const { }
+#else
+    void Annotate(size_t n, bool creating) const {
+      if (!n) return;
+      const void* bot = &left_redzone_;
+      const void* beg = space_.data();
+      const void* end = space_.data() + n;
+      const void* top = &right_redzone_ + 1;
+      // args: (beg, end, old_mid, new_mid)
+      if (creating) {
+        ANNOTATE_CONTIGUOUS_CONTAINER(beg, top, top, end);
+        ANNOTATE_CONTIGUOUS_CONTAINER(bot, beg, beg, bot);
+      } else {
+        ANNOTATE_CONTIGUOUS_CONTAINER(beg, top, end, top);
+        ANNOTATE_CONTIGUOUS_CONTAINER(bot, beg, bot, beg);
+      }
+    }
+#endif  // ADDRESS_SANITIZER
+
+    using Buffer =
+        typename std::aligned_storage<sizeof(Holder), alignof(Holder)>::type;
+
+    ADDRESS_SANITIZER_REDZONE(left_redzone_);
+    std::array<Buffer, N> space_;
+    ADDRESS_SANITIZER_REDZONE(right_redzone_);
+  };
+
+  // specialization when N = 0.
+  template <typename U>
+  class InlineSpace<0, U> {
+   public:
+    Holder* data() { return nullptr; }
+    void AnnotateConstruct(size_t) const {}
+    void AnnotateDestruct(size_t) const {}
+  };
+
+  // Rep
+  //
+  // A const Rep object holds FixedArray's size and data pointer.
+  //
+  class Rep : public InlineSpace<inline_elements> {
+   public:
+    Rep(size_type n, const value_type& val) : n_(n), p_(MakeHolder(n)) {
+      std::uninitialized_fill_n(p_, n, val);
+    }
+
+    explicit Rep(size_type n) : n_(n), p_(MakeHolder(n)) {
+      // Loop optimizes to nothing for trivially constructible T.
+      for (Holder* p = p_; p != p_ + n; ++p)
+        // Note: no parens: default init only.
+        // Also note '::' to avoid Holder class placement new operator.
+        ::new (static_cast<void*>(p)) Holder;
+    }
+
+    template <typename Iter>
+    Rep(Iter first, Iter last)
+        : n_(std::distance(first, last)), p_(MakeHolder(n_)) {
+      std::uninitialized_copy(first, last, AsValue(p_));
+    }
+
+    ~Rep() {
+      // Destruction must be in reverse order.
+      // Loop optimizes to nothing for trivially destructible T.
+      for (Holder* p = end(); p != begin();) (--p)->~Holder();
+      if (IsAllocated(size())) {
+        ::operator delete[](begin());
+      } else {
+        this->AnnotateDestruct(size());
+      }
+    }
+    Holder* begin() const { return p_; }
+    Holder* end() const { return p_ + n_; }
+    size_type size() const { return n_; }
+
+   private:
+    Holder* MakeHolder(size_type n) {
+      if (IsAllocated(n)) {
+        return Allocate(n);
+      } else {
+        this->AnnotateConstruct(n);
+        return this->data();
+      }
+    }
+
+    Holder* Allocate(size_type n) {
+      return static_cast<Holder*>(::operator new[](n * sizeof(Holder)));
+    }
+
+    bool IsAllocated(size_type n) const { return n > inline_elements; }
+
+    const size_type n_;
+    Holder* const p_;
+  };
+
+
+  // Data members
+  Rep rep_;
+};
+
+template <typename T, size_t N>
+constexpr size_t FixedArray<T, N>::inline_elements;
+
+template <typename T, size_t N>
+constexpr size_t FixedArray<T, N>::kInlineBytesDefault;
+
+}  // namespace absl
+#endif  // ABSL_CONTAINER_FIXED_ARRAY_H_