FTL: Add StaticVector<T, N>

/*

 * 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 <algorithm>

#include <cassert>

#include <iterator>

#include <memory>

#include <new>

#include <type_traits>

#include <utility>

namespace android::ftl {

// Fixed-capacity, statically allocated counterpart of std::vector. Akin to 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, such as push_back and emplace, fail when the vector is full. The API otherwise

// adheres to standard containers, except the unstable_erase operation that does not shift elements,

// and the replace operation that destructively emplaces.

//

// StaticVector<T, 1> is analogous to an iterable std::optional, but 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'}));

//

template <typename T, size_t N>

class StaticVector {

    static_assert(N > 0);

    template <typename I>

    using IsInputIterator = std::is_base_of<std::input_iterator_tag,

                                            typename std::iterator_traits<I>::iterator_category>;

public:

    using value_type = T;

    using size_type = size_t;

    using difference_type = ptrdiff_t;

    using pointer = value_type*;

    using reference = value_type&;

    using iterator = pointer;

    using reverse_iterator = std::reverse_iterator<iterator>;

    using const_pointer = const value_type*;

    using const_reference = const value_type&;

    using const_iterator = const_pointer;

    using const_reverse_iterator = std::reverse_iterator<const_iterator>;

    // Creates an empty vector.

    StaticVector() = default;

    // Copies and moves a vector, respectively.

    StaticVector(const StaticVector& other) : StaticVector(other.begin(), other.end()) {}

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

    // Copies at most N elements from a smaller convertible vector.

    template <typename U, size_t M, typename = std::enable_if_t<M <= N>>

    StaticVector(const StaticVector<U, M>& other) : StaticVector(other.begin(), other.end()) {}

    // Copies at most N elements from an array.

    template <typename U, size_t M>

    explicit StaticVector(U (&array)[M]) : StaticVector(std::begin(array), std::end(array)) {}

    // Copies at most N elements from the range [first, last).

    template <typename Iterator, typename = std::enable_if_t<IsInputIterator<Iterator>{}>>

    StaticVector(Iterator first, Iterator last)

          : mSize(std::min(max_size(), static_cast<size_type>(std::distance(first, last)))) {

        std::uninitialized_copy(first, first + mSize, 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. The template arguments T and N are inferred using the

    // deduction guide defined below. Element types must be convertible to the specified T:

    //

    //     ftl::StaticVector vector(std::in_place_type<std::string>, "red", "velvet", "cake");

    //     static_assert(std::is_same_v<decltype(vector), ftl::StaticVector<std::string, 3>>);

    //

    template <typename... Es>

    explicit StaticVector(std::in_place_type_t<T>, Es... elements)

          : StaticVector(std::forward<Es>(elements)...) {}

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

    StaticVector& operator=(const StaticVector& other) {

        StaticVector copy(other);

        swap(copy);

        return *this;

    }

    StaticVector& operator=(StaticVector&& other) {

        std::destroy(begin(), end());

        mSize = 0;

        swap<Empty>(other);

        return *this;

    }

    template <typename = void>

    void swap(StaticVector&);

    size_type max_size() const { return N; }

    size_type size() const { return mSize; }

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

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

    iterator begin() { return std::launder(reinterpret_cast<pointer>(mData)); }

    const_iterator begin() const { return cbegin(); }

    const_iterator cbegin() const { return mut().begin(); }

    iterator end() { return begin() + size(); }

    const_iterator end() const { return cend(); }

    const_iterator cend() const { return mut().end(); }

    reverse_iterator rbegin() { return std::make_reverse_iterator(end()); }

    const_reverse_iterator rbegin() const { return crbegin(); }

    const_reverse_iterator crbegin() const { return mut().rbegin(); }

    reverse_iterator rend() { return std::make_reverse_iterator(begin()); }

    const_reverse_iterator rend() const { return crend(); }

    const_reverse_iterator crend() const { return mut().rend(); }

    iterator last() { return end() - 1; }

    const_iterator last() const { return mut().last(); }

    reference front() { return *begin(); }

    const_reference front() const { return mut().front(); }

    reference back() { return *last(); }

    const_reference back() const { return mut().back(); }

    reference operator[](size_type i) { return *(begin() + i); }

    const_reference operator[](size_type i) const { return mut()[i]; }

    // Replaces an element, and returns an iterator to it. If the vector is full, the element is not

    // replaced, and the end iterator is returned.

    template <typename... Args>

    iterator replace(const_iterator cit, Args&&... args) {

        if (full()) return end();

        // Append element, and move into place if not last.

        emplace_back(std::forward<Args>(args)...);

        if (cit != last()) unstable_erase(cit);

        return const_cast<iterator>(cit);

    }

    // 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.

    template <typename... Args>

    iterator emplace_back(Args&&... args) {

        if (full()) return end();

        const iterator it = construct_at(end(), std::forward<Args>(args)...);

        ++mSize;

        return it;

    }

    // 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.

    void unstable_erase(const_iterator it) {

        std::destroy_at(it);

        if (it != last()) {

            // Move last element and destroy its source for destructor side effects.

            construct_at(it, std::move(back()));

            std::destroy_at(last());

        }

        --mSize;

    }

    bool push_back(value_type v) {

        // Two statements for sequence point.

        const iterator it = emplace_back(std::move(v));

        return it != end();

    }

    void pop_back() { unstable_erase(last()); }

private:

    struct Empty {};

    StaticVector& mut() const { return *const_cast<StaticVector*>(this); }

    // Recursion for variadic constructor.

    template <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 <size_t I>

    explicit StaticVector(std::index_sequence<I>) : mSize(I) {}

    // TODO: Replace with std::construct_at in C++20.

    template <typename... Args>

    static pointer construct_at(const_iterator it, Args&&... args) {

        void* const ptr = const_cast<void*>(static_cast<const void*>(it));

        return new (ptr) value_type{std::forward<Args>(args)...};

    }

    size_type mSize = 0;

    std::aligned_storage_t<sizeof(value_type), alignof(value_type)> mData[N];

};

// Deduction guide for array constructor.

template <typename T, 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, typename... Us>

StaticVector(std::in_place_type_t<T>, Us&&...) -> StaticVector<T, sizeof...(Us)>;

template <typename T, size_t N>

template <typename E>

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 (std::is_same_v<E, Empty>) {

        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(mSize, other.mSize);

}

template <typename T, size_t N>

inline void swap(StaticVector<T, N>& lhs, StaticVector<T, N>& rhs) {

    lhs.swap(rhs);

}

// TODO: Replace with operator<=> in C++20.

template <typename T, size_t N, size_t M>

inline bool operator==(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return lhs.size() == rhs.size() && std::equal(lhs.begin(), lhs.end(), rhs.begin());

}

template <typename T, size_t N, size_t M>

inline bool operator<(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return std::lexicographical_compare(lhs.begin(), lhs.end(), rhs.begin(), rhs.end());

}

template <typename T, size_t N, size_t M>

inline bool operator>(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return rhs < lhs;

}

template <typename T, size_t N, size_t M>

inline bool operator!=(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return !(lhs == rhs);

}

template <typename T, size_t N, size_t M>

inline bool operator>=(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return !(lhs < rhs);

}

template <typename T, size_t N, size_t M>

inline bool operator<=(const StaticVector<T, N>& lhs, const StaticVector<T, M>& rhs) {

    return !(rhs < lhs);

}

} // namespace android::ftl

BasedOnStyle: Google

AccessModifierOffset: -2

AllowShortFunctionsOnASingleLine: Inline

BinPackParameters: false

ColumnLimit: 100

CommentPragmas: NOLINT:.*

ConstructorInitializerAllOnOneLineOrOnePerLine: true

ConstructorInitializerIndentWidth: 2

ContinuationIndentWidth: 8

IndentWidth: 4

cc_test {

    name: "ftl_test",

    test_suites: ["device-tests"],

    sanitize: {

        address: true,

    },

    srcs: [

        "StaticVector_test.cpp",

    ],

    cflags: [

        "-Wall",

        "-Werror",

        "-Wextra",

        "-Wpedantic",

    ],

}

/*

 * 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.

 */

#include <ftl/StaticVector.h>

#include <gtest/gtest.h>

#include <algorithm>

#include <iterator>

#include <string>

#include <utility>

using namespace std::string_literals;

namespace android::test {

using ftl::StaticVector;

// Keep in sync with example usage in header file.

TEST(StaticVector, Example) {

    ftl::StaticVector<char, 3> vector;

    EXPECT_TRUE(vector.empty());

    vector = {'a', 'b'};

    EXPECT_EQ(vector.size(), 2u);

    vector.push_back('c');

    EXPECT_TRUE(vector.full());

    EXPECT_FALSE(vector.push_back('d'));

    EXPECT_EQ(vector.size(), 3u);

    vector.unstable_erase(vector.begin());

    EXPECT_EQ(vector, (ftl::StaticVector{'c', 'b'}));

    vector.pop_back();

    EXPECT_EQ(vector.back(), 'c');

    const char array[] = "hi";

    vector = ftl::StaticVector(array);

    EXPECT_EQ(vector, (ftl::StaticVector{'h', 'i', '\0'}));

}

TEST(StaticVector, Construct) {

    {

        // Default constructor.

        StaticVector<std::string, 2> vector;

        EXPECT_TRUE(vector.empty());

    }

    {

        // Array constructor.

        const float kFloats[] = {.1f, .2f, .3f};

        StaticVector vector(kFloats);

        EXPECT_EQ(vector, (StaticVector{.1f, .2f, .3f}));

    }

    {

        // Iterator constructor.

        const char chars[] = "abcdef";

        std::string string(chars);

        StaticVector<char, sizeof(chars)> vector(string.begin(), string.end());

        EXPECT_STREQ(vector.begin(), chars);

    }

    {

        // Variadic constructor with same types.

        StaticVector vector = {1, 2, 3};

        static_assert(std::is_same_v<decltype(vector), StaticVector<int, 3>>);

        EXPECT_EQ(vector, (StaticVector{1, 2, 3}));

    }

    {

        // Variadic constructor with different types.

        const auto copy = "quince"s;

        auto move = "tart"s;

        StaticVector vector = {copy, std::move(move)};

        static_assert(std::is_same_v<decltype(vector), StaticVector<std::string, 2>>);

        EXPECT_EQ(vector, (StaticVector{"quince"s, "tart"s}));

    }

    {

        // In-place constructor with same types.

        StaticVector vector(std::in_place_type<std::string>, "red", "velvet", "cake");

        static_assert(std::is_same_v<decltype(vector), StaticVector<std::string, 3>>);

        EXPECT_EQ(vector, (StaticVector{"red"s, "velvet"s, "cake"s}));

    }

    {

        // In-place constructor with different types.

        const auto copy = "red"s;

        auto move = "velvet"s;

        std::initializer_list<char> list = {'c', 'a', 'k', 'e'};

        StaticVector vector(std::in_place_type<std::string>, copy.c_str(), std::move(move), list);

        static_assert(std::is_same_v<decltype(vector), StaticVector<std::string, 3>>);

        EXPECT_EQ(vector, (StaticVector{"red"s, "velvet"s, "cake"s}));

    }

}

TEST(StaticVector, String) {

    StaticVector<char, 10> chars;

    char c = 'a';

    std::generate_n(std::back_inserter(chars), chars.max_size(), [&c] { return c++; });

    chars.back() = '\0';

    EXPECT_STREQ(chars.begin(), "abcdefghi");

    // Constructor takes iterator range.

    const char kString[] = "123456";

    StaticVector<char, 10> string(std::begin(kString), std::end(kString));

    EXPECT_STREQ(string.begin(), "123456");

    EXPECT_EQ(string.size(), 7u);

    // Similar to emplace, but replaces rather than inserts.

    const auto it = string.replace(string.begin() + 5, '\0');

    EXPECT_NE(it, string.end());

    EXPECT_STREQ(string.begin(), "12345");

    swap(chars, string);

    EXPECT_STREQ(chars.begin(), "12345");

    EXPECT_STREQ(string.begin(), "abcdefghi");

}

TEST(StaticVector, CopyableElement) {

    struct Pair {

        const int a, b;

        bool operator==(Pair p) const { return p.a == a && p.b == b; }

    };

    StaticVector<Pair, 5> pairs;

    EXPECT_TRUE(pairs.empty());

    EXPECT_EQ(pairs.max_size(), 5u);

    for (size_t i = 0; i < pairs.max_size(); ++i) {

        EXPECT_EQ(pairs.size(), i);

        const int a = static_cast<int>(i) * 2;

        const auto it = pairs.emplace_back(a, a + 1);

        ASSERT_NE(it, pairs.end());

        EXPECT_EQ(*it, (Pair{a, a + 1}));

    }

    EXPECT_TRUE(pairs.full());

    EXPECT_EQ(pairs.size(), 5u);

    // Insertion fails if the vector is full.

    const auto it = pairs.emplace_back(10, 11);

    EXPECT_EQ(it, pairs.end());

    EXPECT_EQ(pairs, (StaticVector{Pair{0, 1}, Pair{2, 3}, Pair{4, 5}, Pair{6, 7}, Pair{8, 9}}));

    // Constructor takes at most N elements.

    StaticVector<int, 6> sums = {0, 0, 0, 0, 0, -1};

    EXPECT_TRUE(sums.full());

    // Random-access iterators comply with standard.

    std::transform(pairs.begin(), pairs.end(), sums.begin(), [](Pair p) { return p.a + p.b; });

    EXPECT_EQ(sums, (StaticVector{1, 5, 9, 13, 17, -1}));

    sums.pop_back();

    std::reverse(sums.begin(), sums.end());

    EXPECT_EQ(sums, (StaticVector{17, 13, 9, 5, 1}));

}

TEST(StaticVector, MovableElement) {

    // Construct std::string elements in-place from C-style strings. Without std::in_place_type, the

    // element type would be deduced from the first element, i.e. const char*.

    StaticVector strings(std::in_place_type<std::string>, "", "", "", "cake", "velvet", "red", "");

    strings.pop_back();

    EXPECT_EQ(strings.max_size(), 7u);

    EXPECT_EQ(strings.size(), 6u);

    // Erase "cake" and append a substring copy.

    {

        auto it = std::find_if(strings.begin(), strings.end(),

                               [](const auto& s) { return !s.empty(); });

        ASSERT_FALSE(it == strings.end());

        EXPECT_EQ(*it, "cake");

        strings.unstable_erase(it);

        // Construct std::string from first 4 characters of C-style string.

        it = strings.emplace_back("cakewalk", 4u);

        ASSERT_NE(it, strings.end());

        EXPECT_EQ(*it, "cake"s);

    }

    strings[1] = "quince"s;

    // Replace last empty string with "tart".

    {

        const auto rit = std::find(strings.rbegin(), strings.rend(), std::string());

        ASSERT_FALSE(rit == strings.rend());

        std::initializer_list<char> list = {'t', 'a', 'r', 't'};

        const auto it = strings.replace(rit.base() - 1, list);

        EXPECT_NE(it, strings.end());

    }

    strings.front().assign("pie");

    EXPECT_EQ(strings, (StaticVector{"pie"s, "quince"s, "tart"s, "red"s, "velvet"s, "cake"s}));

}

TEST(StaticVector, ReverseTruncate) {

    StaticVector<std::string, 10> strings("pie", "quince", "tart", "red", "velvet", "cake");

    EXPECT_FALSE(strings.full());

    for (auto it = strings.begin(); it != strings.end(); ++it) {

        strings.replace(it, strings.back());

        strings.pop_back();

    }

    EXPECT_EQ(strings, (StaticVector{"cake"s, "velvet"s, "red"s}));

}

TEST(StaticVector, Sort) {

    StaticVector<std::string, 7> strings("pie", "quince", "tart", "red", "velvet", "cake");

    EXPECT_FALSE(strings.full());

    auto sorted = std::move(strings);

    EXPECT_TRUE(strings.empty());

    std::sort(sorted.begin(), sorted.end());

    EXPECT_EQ(sorted, (StaticVector{"cake"s, "pie"s, "quince"s, "red"s, "tart"s, "velvet"s}));

    // Constructor takes array reference.

    {

        const char* kStrings[] = {"cake", "lie"};

        strings = StaticVector(kStrings);

    }

    EXPECT_GT(sorted, strings);

    swap(sorted, strings);

    EXPECT_LT(sorted, strings);

    // Append remaining elements, such that "pie" is the only difference.

    sorted.replace(sorted.end(), "quince");

    for (const char* str : {"red", "tart", "velvet"}) sorted.emplace_back(str);

    EXPECT_NE(sorted, strings);