include/ftl/static_vector.h

/*
 * Copyright 2020 The Android Open Source Project
 *
 * 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.
 */

#pragma once

#include <ftl/details/array_traits.h>
#include <ftl/initializer_list.h>

#include <algorithm>
#include <cassert>
#include <iterator>
#include <memory>
#include <type_traits>
#include <utility>

namespace android::ftl {

constexpr struct IteratorRangeTag {
} kIteratorRange;

// Fixed-capacity, statically allocated counterpart of std::vector. Like std::array, StaticVector
// allocates contiguous storage for N elements of type T at compile time, but stores at most (rather
// than exactly) N elements. Unlike std::array, its default constructor does not require T to have a
// default constructor, since elements are constructed in place as the vector grows. Operations that
// insert an element (emplace_back, push_back, etc.) fail when the vector is full. The API otherwise
// adheres to standard containers, except the unstable_erase operation that does not preserve order,
// and the replace operation that destructively emplaces.
//
// StaticVector<T, 1> is analogous to an iterable std::optional.
// StaticVector<T, 0> is an error.
//
// Example usage:
//
//   ftl::StaticVector<char, 3> vector;
//   assert(vector.empty());
//
//   vector = {'a', 'b'};
//   assert(vector.size() == 2u);
//
//   vector.push_back('c');
//   assert(vector.full());
//
//   assert(!vector.push_back('d'));
//   assert(vector.size() == 3u);
//
//   vector.unstable_erase(vector.begin());
//   assert(vector == (ftl::StaticVector{'c', 'b'}));
//
//   vector.pop_back();
//   assert(vector.back() == 'c');
//
//   const char array[] = "hi";
//   vector = ftl::StaticVector(array);
//   assert(vector == (ftl::StaticVector{'h', 'i', '\0'}));
//
//   ftl::StaticVector strings = ftl::init::list<std::string>("abc")("123456", 3u)(3u, '?');
//   assert(strings.size() == 3u);
//   assert(strings[0] == "abc");
//   assert(strings[1] == "123");
//   assert(strings[2] == "???");
//
template <typename T, std::size_t N>
class StaticVector final : details::ArrayTraits<T>,
                           details::ArrayIterators<StaticVector<T, N>, T>,
                           details::ArrayComparators<StaticVector> {
  static_assert(N > 0);

  using details::ArrayTraits<T>::construct_at;

  using Iter = details::ArrayIterators<StaticVector, T>;
  friend Iter;

  // There is ambiguity when constructing from two iterator-like elements like pointers:
  // they could be an iterator range, or arguments for in-place construction. Assume the
  // latter unless they are input iterators and cannot be used to construct elements. If
  // the former is intended, the caller can pass an IteratorRangeTag to disambiguate.
  template <typename I, typename Traits = std::iterator_traits<I>>
  using is_input_iterator =
      std::conjunction<std::is_base_of<std::input_iterator_tag, typename Traits::iterator_category>,
                       std::negation<std::is_constructible<T, I>>>;

 public:
  FTL_ARRAY_TRAIT(T, value_type);
  FTL_ARRAY_TRAIT(T, size_type);
  FTL_ARRAY_TRAIT(T, difference_type);

  FTL_ARRAY_TRAIT(T, pointer);
  FTL_ARRAY_TRAIT(T, reference);
  FTL_ARRAY_TRAIT(T, iterator);
  FTL_ARRAY_TRAIT(T, reverse_iterator);

  FTL_ARRAY_TRAIT(T, const_pointer);
  FTL_ARRAY_TRAIT(T, const_reference);
  FTL_ARRAY_TRAIT(T, const_iterator);
  FTL_ARRAY_TRAIT(T, const_reverse_iterator);

  // Creates an empty vector.
  StaticVector() = default;

  // Copies and moves a vector, respectively.
  StaticVector(const StaticVector& other)
      : StaticVector(kIteratorRange, other.begin(), other.end()) {}

  StaticVector(StaticVector&& other) { swap<true>(other); }

  // Copies at most N elements from a smaller convertible vector.
  template <typename U, std::size_t M, typename = std::enable_if_t<M <= N>>
  StaticVector(const StaticVector<U, M>& other)
      : StaticVector(kIteratorRange, other.begin(), other.end()) {}

  // Copies at most N elements from an array.
  template <typename U, std::size_t M>
  explicit StaticVector(U (&array)[M])
      : StaticVector(kIteratorRange, std::begin(array), std::end(array)) {}

  // Copies at most N elements from the range [first, last).
  //
  // IteratorRangeTag disambiguates with initialization from two iterator-like elements.
  //
  template <typename Iterator, typename = std::enable_if_t<is_input_iterator<Iterator>{}>>
  StaticVector(Iterator first, Iterator last) : StaticVector(kIteratorRange, first, last) {
    using V = typename std::iterator_traits<Iterator>::value_type;
    static_assert(std::is_constructible_v<value_type, V>, "Incompatible iterator range");
  }

  template <typename Iterator>
  StaticVector(IteratorRangeTag, Iterator first, Iterator last)
      : size_(std::min(max_size(), static_cast<size_type>(std::distance(first, last)))) {
    std::uninitialized_copy(first, first + size_, begin());
  }

  // Constructs at most N elements. The template arguments T and N are inferred using the
  // deduction guide defined below. Note that T is determined from the first element, and
  // subsequent elements must have convertible types:
  //
  //   ftl::StaticVector vector = {1, 2, 3};
  //   static_assert(std::is_same_v<decltype(vector), ftl::StaticVector<int, 3>>);
  //
  //   const auto copy = "quince"s;
  //   auto move = "tart"s;
  //   ftl::StaticVector vector = {copy, std::move(move)};
  //
  //   static_assert(std::is_same_v<decltype(vector), ftl::StaticVector<std::string, 2>>);
  //
  template <typename E, typename... Es,
            typename = std::enable_if_t<std::is_constructible_v<value_type, E>>>
  StaticVector(E&& element, Es&&... elements)
      : StaticVector(std::index_sequence<0>{}, std::forward<E>(element),
                     std::forward<Es>(elements)...) {
    static_assert(sizeof...(elements) < N, "Too many elements");
  }

  // Constructs at most N elements in place by forwarding per-element constructor arguments. The
  // template arguments T and N are inferred using the deduction guide defined below. The syntax
  // for listing arguments is as follows:
  //
  //   ftl::StaticVector vector = ftl::init::list<std::string>("abc")()(3u, '?');
  //
  //   static_assert(std::is_same_v<decltype(vector), ftl::StaticVector<std::string, 3>>);
  //   assert(vector.full());
  //   assert(vector[0] == "abc");
  //   assert(vector[1].empty());
  //   assert(vector[2] == "???");
  //
  template <typename U, std::size_t Size, std::size_t... Sizes, typename... Types>
  StaticVector(InitializerList<U, std::index_sequence<Size, Sizes...>, Types...>&& list)
      : StaticVector(std::index_sequence<0, 0, Size>{}, std::make_index_sequence<Size>{},
                     std::index_sequence<Sizes...>{}, list.tuple) {
    static_assert(sizeof...(Sizes) < N, "Too many elements");
  }

  ~StaticVector() { std::destroy(begin(), end()); }

  StaticVector& operator=(const StaticVector& other) {
    StaticVector copy(other);
    swap(copy);
    return *this;
  }

  StaticVector& operator=(StaticVector&& other) {
    clear();
    swap<true>(other);
    return *this;
  }

  // IsEmpty enables a fast path when the vector is known to be empty at compile time.
  template <bool IsEmpty = false>
  void swap(StaticVector&);

  static constexpr size_type max_size() { return N; }
  size_type size() const { return size_; }

  bool empty() const { return size() == 0; }
  bool full() const { return size() == max_size(); }

  iterator begin() { return std::launder(reinterpret_cast<pointer>(data_)); }
  iterator end() { return begin() + size(); }

  using Iter::begin;
  using Iter::end;

  using Iter::cbegin;
  using Iter::cend;

  using Iter::rbegin;
  using Iter::rend;

  using Iter::crbegin;
  using Iter::crend;

  using Iter::last;

  using Iter::back;
  using Iter::front;

  using Iter::operator[];

  // Replaces an element, and returns a reference to it. The iterator must be dereferenceable, so
  // replacing at end() is erroneous.
  //
  // The element is emplaced via move constructor, so type T does not need to define copy/move
  // assignment, e.g. its data members may be const.
  //
  // The arguments may directly or indirectly refer to the element being replaced.
  //
  // Iterators to the replaced element point to its replacement, and others remain valid.
  //
  template <typename... Args>
  reference replace(const_iterator it, Args&&... args) {
    value_type element{std::forward<Args>(args)...};
    std::destroy_at(it);
    // This is only safe because exceptions are disabled.
    return *construct_at(it, std::move(element));
  }

  // Appends an element, and returns an iterator to it. If the vector is full, the element is not
  // inserted, and the end() iterator is returned.
  //
  // On success, the end() iterator is invalidated.
  //
  template <typename... Args>
  iterator emplace_back(Args&&... args) {
    if (full()) return end();
    const iterator it = construct_at(end(), std::forward<Args>(args)...);
    ++size_;
    return it;
  }

  // Appends an element unless the vector is full, and returns whether the element was inserted.
  //
  // On success, the end() iterator is invalidated.
  //
  bool push_back(const value_type& v) {
    // Two statements for sequence point.
    const iterator it = emplace_back(v);
    return it != end();
  }

  bool push_back(value_type&& v) {
    // Two statements for sequence point.
    const iterator it = emplace_back(std::move(v));
    return it != end();
  }

  // Removes the last element. The vector must not be empty, or the call is erroneous.
  //
  // The last() and end() iterators are invalidated.
  //
  void pop_back() { unstable_erase(last()); }

  // Removes all elements.
  //
  // All iterators are invalidated.
  //
  void clear() {
    std::destroy(begin(), end());
    size_ = 0;
  }

  // Erases an element, but does not preserve order. Rather than shifting subsequent elements,
  // this moves the last element to the slot of the erased element.
  //
  // The last() and end() iterators, as well as those to the erased element, are invalidated.
  //
  void unstable_erase(const_iterator it) {
    std::destroy_at(it);
    if (it != last()) {
      // Move last element and destroy its source for destructor side effects. This is only
      // safe because exceptions are disabled.
      construct_at(it, std::move(back()));
      std::destroy_at(last());
    }
    --size_;
  }

 private:
  // Recursion for variadic constructor.
  template <std::size_t I, typename E, typename... Es>
  StaticVector(std::index_sequence<I>, E&& element, Es&&... elements)
      : StaticVector(std::index_sequence<I + 1>{}, std::forward<Es>(elements)...) {
    construct_at(begin() + I, std::forward<E>(element));
  }

  // Base case for variadic constructor.
  template <std::size_t I>
  explicit StaticVector(std::index_sequence<I>) : size_(I) {}

  // Recursion for in-place constructor.
  //
  // Construct element I by extracting its arguments from the InitializerList tuple. ArgIndex
  // is the position of its first argument in Args, and ArgCount is the number of arguments.
  // The Indices sequence corresponds to [0, ArgCount).
  //
  // The Sizes sequence lists the argument counts for elements after I, so Size is the ArgCount
  // for the next element. The recursion stops when Sizes is empty for the last element.
  //
  template <std::size_t I, std::size_t ArgIndex, std::size_t ArgCount, std::size_t... Indices,
            std::size_t Size, std::size_t... Sizes, typename... Args>
  StaticVector(std::index_sequence<I, ArgIndex, ArgCount>, std::index_sequence<Indices...>,
               std::index_sequence<Size, Sizes...>, std::tuple<Args...>& tuple)
      : StaticVector(std::index_sequence<I + 1, ArgIndex + ArgCount, Size>{},
                     std::make_index_sequence<Size>{}, std::index_sequence<Sizes...>{}, tuple) {
    construct_at(begin() + I, std::move(std::get<ArgIndex + Indices>(tuple))...);
  }

  // Base case for in-place constructor.
  template <std::size_t I, std::size_t ArgIndex, std::size_t ArgCount, std::size_t... Indices,
            typename... Args>
  StaticVector(std::index_sequence<I, ArgIndex, ArgCount>, std::index_sequence<Indices...>,
               std::index_sequence<>, std::tuple<Args...>& tuple)
      : size_(I + 1) {
    construct_at(begin() + I, std::move(std::get<ArgIndex + Indices>(tuple))...);
  }

  size_type size_ = 0;
  std::aligned_storage_t<sizeof(value_type), alignof(value_type)> data_[N];
};

// Deduction guide for array constructor.
template <typename T, std::size_t N>
StaticVector(T (&)[N]) -> StaticVector<std::remove_cv_t<T>, N>;

// Deduction guide for variadic constructor.
template <typename T, typename... Us, typename V = std::decay_t<T>,
          typename = std::enable_if_t<(std::is_constructible_v<V, Us> && ...)>>
StaticVector(T&&, Us&&...) -> StaticVector<V, 1 + sizeof...(Us)>;

// Deduction guide for in-place constructor.
template <typename T, std::size_t... Sizes, typename... Types>
StaticVector(InitializerList<T, std::index_sequence<Sizes...>, Types...>&&)
    -> StaticVector<T, sizeof...(Sizes)>;

template <typename T, std::size_t N>
template <bool IsEmpty>
void StaticVector<T, N>::swap(StaticVector& other) {
  auto [to, from] = std::make_pair(this, &other);
  if (from == this) return;

  // Assume this vector has fewer elements, so the excess of the other vector will be moved to it.
  auto [min, max] = std::make_pair(size(), other.size());

  // No elements to swap if moving into an empty vector.
  if constexpr (IsEmpty) {
    assert(min == 0);
  } else {
    if (min > max) {
      std::swap(from, to);
      std::swap(min, max);
    }

    // Swap elements [0, min).
    std::swap_ranges(begin(), begin() + min, other.begin());

    // No elements to move if sizes are equal.
    if (min == max) return;
  }

  // Move elements [min, max) and destroy their source for destructor side effects.
  const auto [first, last] = std::make_pair(from->begin() + min, from->begin() + max);
  std::uninitialized_move(first, last, to->begin() + min);
  std::destroy(first, last);

  std::swap(size_, other.size_);
}

template <typename T, std::size_t N>
inline void swap(StaticVector<T, N>& lhs, StaticVector<T, N>& rhs) {
  lhs.swap(rhs);
}

}  // namespace android::ftl