///////////////////////////////////////////////////////////////////////////////
//
// Copyright (c) 2015 Microsoft Corporation. All rights reserved.
//
// This code is licensed under the MIT License (MIT).
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
//
///////////////////////////////////////////////////////////////////////////////

#include <UnitTest++/UnitTest++.h>
#include <gsl/span>

#include <iostream>
#include <list>
#include <map>
#include <memory>
#include <string>
#include <vector>
#include <regex>

using namespace std;
using namespace gsl;

namespace
{
struct BaseClass
{
};
struct DerivedClass : BaseClass
{
};
}

SUITE(span_tests)
{
    TEST(default_constructor)
    {
        {
            span<int> s;
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int> cs;
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
            span<int, 0> s;
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int, 0> cs;
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            span<int, 1> s;
            CHECK(s.length() == 1 && s.data() == nullptr); // explains why it can't compile
#endif
        }

        {
            span<int> s{};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int> cs{};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }
    }

    TEST(size_optimization)
    {
        {
            span<int> s;
            CHECK(sizeof(s) == sizeof(int*) + sizeof(ptrdiff_t));
        }

        {
            span<int, 0> s;
            CHECK(sizeof(s) == sizeof(int*));
        }
    }

    TEST(from_nullptr_constructor)
    {
        {
            span<int> s = nullptr;
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int> cs = nullptr;
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
            span<int, 0> s = nullptr;
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int, 0> cs = nullptr;
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            span<int, 1> s = nullptr;
            CHECK(s.length() == 1 && s.data() == nullptr); // explains why it can't compile
#endif
        }

        {
            span<int> s{nullptr};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int> cs{nullptr};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
            span<int*> s{nullptr};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int*> cs{nullptr};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }
    }

    TEST(from_nullptr_length_constructor)
    {
        {
            span<int> s{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int> cs{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
            span<int, 0> s{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int, 0> cs{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }

        {
            auto workaround_macro = []() { span<int, 1> s{ nullptr, static_cast<span<int>::index_type>(0) }; };
            CHECK_THROW(workaround_macro(), fail_fast); 
        }

        {
            auto workaround_macro = []() { span<int> s{nullptr, 1}; };
            CHECK_THROW(workaround_macro(), fail_fast);

            auto const_workaround_macro = []() { span<const int> cs{nullptr, 1}; };
            CHECK_THROW(const_workaround_macro(), fail_fast);
        }

        {
            auto workaround_macro = []() { span<int, 0> s{nullptr, 1}; };
            CHECK_THROW(workaround_macro(), fail_fast);

            auto const_workaround_macro = []() { span<const int, 0> s{nullptr, 1}; };
            CHECK_THROW(const_workaround_macro(), fail_fast);
        }

        {
            span<int*> s{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(s.length() == 0 && s.data() == nullptr);

            span<const int*> cs{nullptr, static_cast<span<int>::index_type>(0)};
            CHECK(cs.length() == 0 && cs.data() == nullptr);
        }
    }

    TEST(from_pointer_length_constructor)
    {
        int arr[4] = {1, 2, 3, 4};

        {
            span<int> s{&arr[0], 2};
            CHECK(s.length() == 2 && s.data() == &arr[0]);
            CHECK(s[0] == 1 && s[1] == 2);
        }

        {
            span<int, 2> s{&arr[0], 2};
            CHECK(s.length() == 2 && s.data() == &arr[0]);
            CHECK(s[0] == 1 && s[1] == 2);
        }

        {
            int* p = nullptr;
            span<int> s{p, static_cast<span<int>::index_type>(0)};
            CHECK(s.length() == 0 && s.data() == nullptr);
        }

        {
            int* p = nullptr;
            auto workaround_macro = [=]() { span<int> s{p, 2}; };
            CHECK_THROW(workaround_macro(), fail_fast);
        }
    }

    TEST(from_pointer_pointer_constructor)
    {
        int arr[4] = {1, 2, 3, 4};

        {
            span<int> s{&arr[0], &arr[2]};
            CHECK(s.length() == 2 && s.data() == &arr[0]);
            CHECK(s[0] == 1 && s[1] == 2);
        }

        {
            span<int, 2> s{&arr[0], &arr[2]};
            CHECK(s.length() == 2 && s.data() == &arr[0]);
            CHECK(s[0] == 1 && s[1] == 2);
        }

        {
            span<int> s{&arr[0], &arr[0]};
            CHECK(s.length() == 0 && s.data() == &arr[0]);
        }

        {
            span<int, 0> s{&arr[0], &arr[0]};
            CHECK(s.length() == 0 && s.data() == &arr[0]);
        }

        // this will fail the std::distance() precondition, which asserts on MSVC debug builds
        //{
        //    auto workaround_macro = [&]() { span<int> s{&arr[1], &arr[0]}; };
        //    CHECK_THROW(workaround_macro(), fail_fast);
        //}

        // this will fail the std::distance() precondition, which asserts on MSVC debug builds
        //{
        //    int* p = nullptr;
        //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
        //    CHECK_THROW(workaround_macro(), fail_fast);
        //}

        {
            int* p = nullptr;
            span<int> s{ p, p };
            CHECK(s.length() == 0 && s.data() == nullptr);
        }

        {
            int* p = nullptr;
            span<int, 0> s{ p, p };
            CHECK(s.length() == 0 && s.data() == nullptr);
        }

        // this will fail the std::distance() precondition, which asserts on MSVC debug builds
        //{
        //    int* p = nullptr;
        //    auto workaround_macro = [&]() { span<int> s{&arr[0], p}; };
        //    CHECK_THROW(workaround_macro(), fail_fast);
        //}
    }

    TEST(from_array_constructor)
    {
        int arr[5] = {1, 2, 3, 4, 5};

        {
            span<int> s{arr};
            CHECK(s.length() == 5 && s.data() == &arr[0]);
        }

        {
            span<int, 5> s{arr};
            CHECK(s.length() == 5 && s.data() == &arr[0]);
        }

        int arr2d[2][3] = { 1, 2, 3, 4, 5, 6 };

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int, 6> s{arr};
        }

        {
            span<int, 0> s{arr};
            CHECK(s.length() == 0 && s.data() == &arr[0]);
        }

        {
            span<int> s{arr2d};
            CHECK(s.length() == 6 && s.data() == &arr2d[0][0]);
            CHECK(s[0] == 1 && s[5] == 6);
        }

        {
            span<int, 0> s{arr2d};
            CHECK(s.length() == 0 && s.data() == &arr2d[0][0]);
        }

        {
            span<int, 6> s{ arr2d };
        }
#endif
        {
            span<int[3]> s{ &(arr2d[0]), 1 };
            CHECK(s.length() == 1 && s.data() == &arr2d[0]);
        }

        int arr3d[2][3][2] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int> s{arr3d};
            CHECK(s.length() == 12 && s.data() == &arr3d[0][0][0]);
            CHECK(s[0] == 1 && s[11] == 12);
        }

        {
            span<int, 0> s{arr3d};
            CHECK(s.length() == 0 && s.data() == &arr3d[0][0][0]);
        }

        {
            span<int, 11> s{arr3d};
        }

        {
            span<int, 12> s{arr3d};
            CHECK(s.length() == 12 && s.data() == &arr3d[0][0][0]);
            CHECK(s[0] == 1 && s[5] == 6);
        }
#endif
        {
            span<int[3][2]> s{&arr3d[0], 1};
            CHECK(s.length() == 1 && s.data() == &arr3d[0]);
        }
    }

    TEST(from_dynamic_array_constructor)
    {
        double(*arr)[3][4] = new double[100][3][4];

        {
            span<double> s(&arr[0][0][0], 10);
            CHECK(s.length() == 10 && s.data() == &arr[0][0][0]);
        }

        delete[] arr;
    }

    TEST(from_std_array_constructor)
    {
        std::array<int, 4> arr = {1, 2, 3, 4};

        {
            span<int> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());

            span<const int> cs{arr};
            CHECK(cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data());
        }

        {
            span<int, 4> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());

            span<const int, 4> cs{arr};
            CHECK(cs.size() == narrow_cast<ptrdiff_t>(arr.size()) && cs.data() == arr.data());
        }

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int, 2> s{arr};
            CHECK(s.size() == 2 && s.data() == arr.data());

            span<const int, 2> cs{arr};
            CHECK(cs.size() == 2 && cs.data() == arr.data());
        }

        {
            span<int, 0> s{arr};
            CHECK(s.size() == 0 && s.data() == arr.data());

            span<const int, 0> cs{arr};
            CHECK(cs.size() == 0 && cs.data() == arr.data());
        }

        {
            span<int, 5> s{arr};
        }

        {
            auto get_an_array = []()->std::array<int, 4> { return{1, 2, 3, 4}; };
            auto take_a_span = [](span<int> s) { static_cast<void>(s); };
            // try to take a temporary std::array
            take_a_span(get_an_array());
        }
#endif

        {
            auto get_an_array = []() -> std::array<int, 4> { return { 1, 2, 3, 4 }; };
            auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
            // try to take a temporary std::array
            take_a_span(get_an_array());
        }
    }

    TEST(from_const_std_array_constructor)
    {
        const std::array<int, 4> arr = {1, 2, 3, 4};

        {
            span<const int> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
        }

        {
            span<const int, 4> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
        }

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<const int, 2> s{arr};
            CHECK(s.size() == 2 && s.data() == arr.data());
        }

        {
            span<const int, 0> s{arr};
            CHECK(s.size() == 0 && s.data() == arr.data());
        }

        {
            span<const int, 5> s{arr};
        }
#endif

        {
            auto get_an_array = []() -> const std::array<int, 4> { return {1, 2, 3, 4}; };
            auto take_a_span = [](span<const int> s) { static_cast<void>(s); };
            // try to take a temporary std::array
            take_a_span(get_an_array());
        }
    }

    TEST(from_std_array_const_constructor)
    {
        std::array<const int, 4> arr = {1, 2, 3, 4};

        {
            span<const int> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
        }

        {
            span<const int, 4> s{arr};
            CHECK(s.size() == narrow_cast<ptrdiff_t>(arr.size()) && s.data() == arr.data());
        }

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<const int, 2> s{arr};
            CHECK(s.size() == 2 && s.data() == arr.data());
        }

        {
            span<const int, 0> s{arr};
            CHECK(s.size() == 0 && s.data() == arr.data());
        }

        {
            span<const int, 5> s{arr};
        }

        {
            span<int, 4> s{arr};
        }
#endif
    }

    TEST(from_unique_pointer_construction)
    {
        {
            auto ptr = std::make_unique<int>(4);

            {
                span<int> s{ptr};
                CHECK(s.length() == 1 && s.data() == ptr.get());
                CHECK(s[0] == 4);
            }
        }

        {
            auto ptr = std::unique_ptr<int>{nullptr};

            {
                span<int> s{ptr};
                CHECK(s.length() == 0 && s.data() == nullptr);
            }
        }

        {
            auto arr = std::make_unique<int[]>(4);

            for (auto i = 0U; i < 4; i++)
                arr[i] = gsl::narrow_cast<int>(i + 1);

            {
                span<int> s{arr, 4};
                CHECK(s.length() == 4 && s.data() == arr.get());
                CHECK(s[0] == 1 && s[1] == 2);
            }
        }

        {
            auto ptr = std::unique_ptr<int[]>{nullptr};

            {
                span<int> s{ptr, 0};
                CHECK(s.length() == 0 && s.data() == nullptr);
            }
        }
    }

    TEST(from_shared_pointer_construction)
    {
        {
            auto ptr = std::make_shared<int>(4);

            {
                span<int> s{ptr};
                CHECK(s.length() == 1 && s.data() == ptr.get());
                CHECK(s[0] == 4);
            }
        }

        {
            auto ptr = std::shared_ptr<int>{nullptr};

            {
                span<int> s{ptr};
                CHECK(s.length() == 0 && s.data() == nullptr);
            }
        }
    }

    TEST(from_container_constructor)
    {
        std::vector<int> v = {1, 2, 3};
        const std::vector<int> cv = v;

        {
            span<int> s{v};
            CHECK(s.size() == narrow_cast<std::ptrdiff_t>(v.size()) && s.data() == v.data());

            span<const int> cs{v};
            CHECK(cs.size() == narrow_cast<std::ptrdiff_t>(v.size()) && cs.data() == v.data());
        }

        std::string str = "hello";
        const std::string cstr = "hello";

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            span<char> s{str};
            CHECK(s.size() == narrow_cast<std::ptrdiff_t>(str.size()) && s.data() == str.data());
#endif
            span<const char> cs{str};
            CHECK(cs.size() == narrow_cast<std::ptrdiff_t>(str.size()) && cs.data() == str.data());
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            span<char> s{cstr};
#endif                                                          
            span<const char> cs{cstr};
            CHECK(cs.size() == narrow_cast<std::ptrdiff_t>(cstr.size()) &&
                  cs.data() == cstr.data());
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            auto get_temp_vector = []() -> std::vector<int> { return {}; };
            auto use_span = [](span<int> s) { static_cast<void>(s); };
            use_span(get_temp_vector());
#endif                      
        }

        {
            auto get_temp_vector = []() -> std::vector<int> { return{}; };
            auto use_span = [](span<const int> s) { static_cast<void>(s); };
            use_span(get_temp_vector());
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            auto get_temp_string = []() -> std::string { return{}; };
            auto use_span = [](span<char> s) { static_cast<void>(s); };
            use_span(get_temp_string());
#endif                         
        }

        {
            auto get_temp_string = []() -> std::string { return {}; };
            auto use_span = [](span<const char> s) { static_cast<void>(s); };
            use_span(get_temp_string());
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            auto get_temp_vector = []() -> const std::vector<int> { return {}; };
            auto use_span = [](span<const char> s) { static_cast<void>(s); };
            use_span(get_temp_vector());
#endif
        }

        {
            auto get_temp_string = []() -> const std::string { return {}; };
            auto use_span = [](span<const char> s) { static_cast<void>(s); };
            use_span(get_temp_string());
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            std::map<int, int> m;
            span<int> s{m};
#endif
        }
    }

    TEST(from_convertible_span_constructor)
    {
        {
            span<DerivedClass> avd;
            span<const DerivedClass> avcd = avd;
            static_cast<void>(avcd);
        }

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            span<DerivedClass> avd;
            span<BaseClass> avb = avd;
            static_cast<void>(avb);
#endif
        }

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int> s;
            span<unsigned int> s2 = s;
            static_cast<void>(s2);
        }

        {
            span<int> s;
            span<const unsigned int> s2 = s;
            static_cast<void>(s2);
        }

        {
            span<int> s;
            span<short> s2 = s;
            static_cast<void>(s2);
        }
#endif
    }

    TEST(copy_move_and_assignment)
    {
        span<int> s1;
        CHECK(s1.empty());

        int arr[] = {3, 4, 5};

        span<const int> s2 = arr;
        CHECK(s2.length() == 3 && s2.data() == &arr[0]);

        s2 = s1;
        CHECK(s2.empty());

        auto get_temp_span = [&]() -> span<int> { return {&arr[1], 2}; };
        auto use_span = [&](span<const int> s) { CHECK(s.length() == 2 && s.data() == &arr[1]); };
        use_span(get_temp_span());

        s1 = get_temp_span();
        CHECK(s1.length() == 2 && s1.data() == &arr[1]);
    }

    TEST(first)
    {
        int arr[5] = {1, 2, 3, 4, 5};

        {
            span<int, 5> av = arr;
            CHECK(av.first<2>().length() == 2);
            CHECK(av.first(2).length() == 2);
        }

        {
            span<int, 5> av = arr;
            CHECK(av.first<0>().length() == 0);
            CHECK(av.first(0).length() == 0);
        }

        {
            span<int, 5> av = arr;
            CHECK(av.first<5>().length() == 5);
            CHECK(av.first(5).length() == 5);
        }

        {
            span<int, 5> av = arr;
#ifdef CONFIRM_COMPILATION_ERRORS
            CHECK(av.first<6>().length() == 6);
            CHECK(av.first<-1>().length() == -1);
#endif
            CHECK_THROW(av.first(6).length(), fail_fast);
        }

        {
            span<int> av;
            CHECK(av.first<0>().length() == 0);
            CHECK(av.first(0).length() == 0);
        }
    }

    TEST(last)
    {
        int arr[5] = {1, 2, 3, 4, 5};

        {
            span<int, 5> av = arr;
            CHECK(av.last<2>().length() == 2);
            CHECK(av.last(2).length() == 2);
        }

        {
            span<int, 5> av = arr;
            CHECK(av.last<0>().length() == 0);
            CHECK(av.last(0).length() == 0);
        }

        {
            span<int, 5> av = arr;
            CHECK(av.last<5>().length() == 5);
            CHECK(av.last(5).length() == 5);
        }

        {
            span<int, 5> av = arr;
#ifdef CONFIRM_COMPILATION_ERRORS
            CHECK(av.last<6>().length() == 6);
#endif    
            CHECK_THROW(av.last(6).length(), fail_fast);
        }

        {
            span<int> av;
            CHECK(av.last<0>().length() == 0);
            CHECK(av.last(0).length() == 0);
        }
    }

    TEST(subspan)
    {
        int arr[5] = {1, 2, 3, 4, 5};

        {
            span<int, 5> av = arr;
            CHECK((av.subspan<2, 2>().length() == 2));
            CHECK(av.subspan(2, 2).length() == 2);
            CHECK(av.subspan(2, 3).length() == 3);
        }

        {
            span<int, 5> av = arr;
            CHECK((av.subspan<0, 0>().length() == 0));
            CHECK(av.subspan(0, 0).length() == 0);
        }

        {
            span<int, 5> av = arr;
            CHECK((av.subspan<0, 5>().length() == 5));
            CHECK(av.subspan(0, 5).length() == 5);
            CHECK_THROW(av.subspan(0, 6).length(), fail_fast);
            CHECK_THROW(av.subspan(1, 5).length(), fail_fast);
        }

        {
            span<int, 5> av = arr;
            CHECK((av.subspan<4, 0>().length() == 0));
            CHECK(av.subspan(4, 0).length() == 0);
            CHECK(av.subspan(5, 0).length() == 0);
            CHECK_THROW(av.subspan(6, 0).length(), fail_fast);
        }

        {
            span<int> av;
            CHECK((av.subspan<0, 0>().length() == 0));
            CHECK(av.subspan(0, 0).length() == 0);
            CHECK_THROW((av.subspan<1, 0>().length()), fail_fast);
        }

        {
            span<int> av;
            CHECK(av.subspan(0).length() == 0);
            CHECK_THROW(av.subspan(1).length(), fail_fast);
        }

        {
            span<int> av = arr;
            CHECK(av.subspan(0).length() == 5);
            CHECK(av.subspan(1).length() == 4);
            CHECK(av.subspan(4).length() == 1);
            CHECK(av.subspan(5).length() == 0);
            CHECK_THROW(av.subspan(6).length(), fail_fast);
            auto av2 = av.subspan(1);
            for (int i = 0; i < 4; ++i) CHECK(av2[i] == i + 2);
        }

        {
            span<int, 5> av = arr;
            CHECK(av.subspan(0).length() == 5);
            CHECK(av.subspan(1).length() == 4);
            CHECK(av.subspan(4).length() == 1);
            CHECK(av.subspan(5).length() == 0);
            CHECK_THROW(av.subspan(6).length(), fail_fast);
            auto av2 = av.subspan(1);
            for (int i = 0; i < 4; ++i) CHECK(av2[i] == i + 2);
        }
    }

    TEST(at_call)
    {
        int arr[4] = {1, 2, 3, 4};

        {
            span<int> s = arr;
            CHECK(s.at(0) == 1);
            CHECK_THROW(s.at(5), fail_fast);
        }

        {
            int arr2d[2] = {1, 6};
            span<int, 2> s = arr2d;
            CHECK(s.at(0) == 1);
            CHECK(s.at(1) == 6);
            CHECK_THROW(s.at(2) ,fail_fast);
        }
    }

    TEST(operator_function_call)
    {
        int arr[4] = {1, 2, 3, 4};

        {
            span<int> s = arr;
            CHECK(s(0) == 1);
            CHECK_THROW(s(5), fail_fast);
        }

        {
            int arr2d[2] = {1, 6};
            span<int, 2> s = arr2d;
            CHECK(s(0) == 1);
            CHECK(s(1) == 6);
            CHECK_THROW(s(2) ,fail_fast);
        }
    }

    TEST(iterator_default_init)
    {
        span<int>::iterator it1;
        span<int>::iterator it2;
        CHECK(it1 == it2);
    }

    TEST(const_iterator_default_init)
    {
        span<int>::const_iterator it1;
        span<int>::const_iterator it2;
        CHECK(it1 == it2);
    }

    TEST(iterator_conversions)
    {
        span<int>::iterator badIt;
        span<int>::const_iterator badConstIt;
        CHECK(badIt == badConstIt);

        int a[] = { 1, 2, 3, 4 };
        span<int> s = a;

        auto it = s.begin();
        auto cit = s.cbegin();

        CHECK(it == cit);
        CHECK(cit == it);

        span<int>::const_iterator cit2 = it;
        CHECK(cit2 == cit);

        span<int>::const_iterator cit3 = it + 4;
        CHECK(cit3 == s.cend());
    }

    TEST(iterator_comparisons)
    {
        int a[] = { 1, 2, 3, 4 };
        {
            span<int> s = a;
            span<int>::iterator it = s.begin();
            auto it2 = it + 1;
            span<int>::const_iterator cit = s.cbegin();

            CHECK(it == cit);
            CHECK(cit == it);
            CHECK(it == it);
            CHECK(cit == cit);
            CHECK(cit == s.begin());
            CHECK(s.begin() == cit);
            CHECK(s.cbegin() == cit);
            CHECK(it == s.begin());
            CHECK(s.begin() == it);

            CHECK(it != it2);
            CHECK(it2 != it);
            CHECK(it != s.end());
            CHECK(it2 != s.end());
            CHECK(s.end() != it);
            CHECK(it2 != cit);
            CHECK(cit != it2);

            CHECK(it < it2);
            CHECK(it <= it2);
            CHECK(it2 <= s.end());
            CHECK(it < s.end());
            CHECK(it <= cit);
            CHECK(cit <= it);
            CHECK(cit < it2);
            CHECK(cit <= it2);
            CHECK(cit < s.end());
            CHECK(cit <= s.end());

            CHECK(it2 > it);
            CHECK(it2 >= it);
            CHECK(s.end() > it2);
            CHECK(s.end() >= it2);
            CHECK(it2 > cit);
            CHECK(it2 >= cit);
        }
    }

    TEST(begin_end)
    {
        {
            int a[] = { 1, 2, 3, 4 };
            span<int> s = a;

            span<int>::iterator it = s.begin();
            span<int>::iterator it2 = std::begin(s);
            CHECK(it == it2);

            it = s.end();
            it2 = std::end(s);
            CHECK(it == it2);
        }

        {
            int a[] = { 1, 2, 3, 4 };
            span<int> s = a;

            auto it = s.begin();
            auto first = it;
            CHECK(it == first);
            CHECK(*it == 1);

            auto beyond = s.end();
            CHECK(it != beyond);
            CHECK_THROW(*beyond, fail_fast);

            CHECK(beyond - first == 4);
            CHECK(first - first == 0);
            CHECK(beyond - beyond == 0);
            
            ++it;
            CHECK(it - first == 1);
            CHECK(*it == 2);
            *it = 22;
            CHECK(*it == 22);
            CHECK(beyond - it == 3);

            it = first;
            CHECK(it == first);
            while (it != s.end())
            {
                *it = 5;
                ++it;
            }

            CHECK(it == beyond);
            CHECK(it - beyond == 0);

            for (auto& n : s)
            {
                CHECK(n == 5);
            }
        }
    }

    TEST(cbegin_cend)
    {
        {
            int a[] = { 1, 2, 3, 4 };
            span<int> s = a;

            span<int>::const_iterator cit = s.cbegin();
            span<int>::const_iterator cit2 = std::cbegin(s);
            CHECK(cit == cit2);

            cit = s.cend();
            cit2 = std::cend(s);
            CHECK(cit == cit2);
        }

        {
            int a[] = {1, 2, 3, 4};
            span<int> s = a;

            auto it = s.cbegin();
            auto first = it;
            CHECK(it == first);
            CHECK(*it == 1);

            auto beyond = s.cend();
            CHECK(it != beyond);
            CHECK_THROW(*beyond, fail_fast);

            CHECK(beyond - first == 4);
            CHECK(first - first == 0);
            CHECK(beyond - beyond == 0);

            ++it;
            CHECK(it - first == 1);
            CHECK(*it == 2);
            CHECK(beyond - it == 3);

            int last = 0;
            it = first;
            CHECK(it == first);
            while (it != s.cend())
            {
                CHECK(*it == last + 1);

                last = *it;
                ++it;
            }

            CHECK(it == beyond);
            CHECK(it - beyond == 0);
        }
    }

    TEST(rbegin_rend)
    {
        {
            int a[] = {1, 2, 3, 4};
            span<int> s = a;

            auto it = s.rbegin();
            auto first = it;
            CHECK(it == first);
            CHECK(*it == 4);

            auto beyond = s.rend();
            CHECK(it != beyond);
            CHECK_THROW(*beyond, fail_fast);

            CHECK(beyond - first == 4);
            CHECK(first - first == 0);
            CHECK(beyond - beyond == 0);

            ++it;
            CHECK(it - first == 1);
            CHECK(*it == 3);
            *it = 22;
            CHECK(*it == 22);
            CHECK(beyond - it == 3);

            it = first;
            CHECK(it == first);
            while (it != s.rend())
            {
                *it = 5;
                ++it;
            }

            CHECK(it == beyond);
            CHECK(it - beyond == 0);

            for (auto& n : s)
            {
                CHECK(n == 5);
            }
        }
    }

    TEST(crbegin_crend)
    {
        {
            int a[] = {1, 2, 3, 4};
            span<int> s = a;

            auto it = s.crbegin();
            auto first = it;
            CHECK(it == first);
            CHECK(*it == 4);

            auto beyond = s.crend();
            CHECK(it != beyond);
            CHECK_THROW(*beyond, fail_fast);

            CHECK(beyond - first == 4);
            CHECK(first - first == 0);
            CHECK(beyond - beyond == 0);

            ++it;
            CHECK(it - first == 1);
            CHECK(*it == 3);
            CHECK(beyond - it == 3);

            it = first;
            CHECK(it == first);
            int last = 5;
            while (it != s.crend())
            {
                CHECK(*it == last - 1);
                last = *it;

                ++it;
            }

            CHECK(it == beyond);
            CHECK(it - beyond == 0);
        }
    }

    TEST(comparison_operators)
    {
        {
            span<int> s1 = nullptr;
            span<int> s2 = nullptr;
            CHECK(s1 == s2);
            CHECK(!(s1 != s2));
            CHECK(!(s1 < s2));
            CHECK(s1 <= s2);
            CHECK(!(s1 > s2));
            CHECK(s1 >= s2);
            CHECK(s2 == s1);
            CHECK(!(s2 != s1));
            CHECK(!(s2 < s1));
            CHECK(s2 <= s1);
            CHECK(!(s2 > s1));
            CHECK(s2 >= s1);
        }

        {
            int arr[] = {2, 1};
            span<int> s1 = arr;
            span<int> s2 = arr;

            CHECK(s1 == s2);
            CHECK(!(s1 != s2));
            CHECK(!(s1 < s2));
            CHECK(s1 <= s2);
            CHECK(!(s1 > s2));
            CHECK(s1 >= s2);
            CHECK(s2 == s1);
            CHECK(!(s2 != s1));
            CHECK(!(s2 < s1));
            CHECK(s2 <= s1);
            CHECK(!(s2 > s1));
            CHECK(s2 >= s1);
        }

        {
            int arr[] = {2, 1}; // bigger

            span<int> s1 = nullptr;
            span<int> s2 = arr;

            CHECK(s1 != s2);
            CHECK(s2 != s1);
            CHECK(!(s1 == s2));
            CHECK(!(s2 == s1));
            CHECK(s1 < s2);
            CHECK(!(s2 < s1));
            CHECK(s1 <= s2);
            CHECK(!(s2 <= s1));
            CHECK(s2 > s1);
            CHECK(!(s1 > s2));
            CHECK(s2 >= s1);
            CHECK(!(s1 >= s2));
        }

        {
            int arr1[] = {1, 2};
            int arr2[] = {1, 2};
            span<int> s1 = arr1;
            span<int> s2 = arr2;

            CHECK(s1 == s2);
            CHECK(!(s1 != s2));
            CHECK(!(s1 < s2));
            CHECK(s1 <= s2);
            CHECK(!(s1 > s2));
            CHECK(s1 >= s2);
            CHECK(s2 == s1);
            CHECK(!(s2 != s1));
            CHECK(!(s2 < s1));
            CHECK(s2 <= s1);
            CHECK(!(s2 > s1));
            CHECK(s2 >= s1);
        }

        {
            int arr[] = {1, 2, 3};

            span<int> s1 = {&arr[0], 2}; // shorter
            span<int> s2 = arr; // longer

            CHECK(s1 != s2);
            CHECK(s2 != s1);
            CHECK(!(s1 == s2));
            CHECK(!(s2 == s1));
            CHECK(s1 < s2);
            CHECK(!(s2 < s1));
            CHECK(s1 <= s2);
            CHECK(!(s2 <= s1));
            CHECK(s2 > s1);
            CHECK(!(s1 > s2));
            CHECK(s2 >= s1);
            CHECK(!(s1 >= s2));
        }

        {
            int arr1[] = {1, 2}; // smaller
            int arr2[] = {2, 1}; // bigger

            span<int> s1 = arr1;
            span<int> s2 = arr2;

            CHECK(s1 != s2);
            CHECK(s2 != s1);
            CHECK(!(s1 == s2));
            CHECK(!(s2 == s1));
            CHECK(s1 < s2);
            CHECK(!(s2 < s1));
            CHECK(s1 <= s2);
            CHECK(!(s2 <= s1));
            CHECK(s2 > s1);
            CHECK(!(s1 > s2));
            CHECK(s2 >= s1);
            CHECK(!(s1 >= s2));
        }
    }

    TEST(as_bytes)
    {
        int a[] = {1, 2, 3, 4};

        {
            span<const int> s = a;
            CHECK(s.length() == 4);
            span<const byte> bs = as_bytes(s);
            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
            CHECK(bs.length() == s.length_bytes());
        }

        {
            span<int> s;
            auto bs = as_bytes(s);
            CHECK(bs.length() == s.length());
            CHECK(bs.length() == 0);
            CHECK(bs.size_bytes() == 0);
            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
            CHECK(bs.data() == nullptr);
        }

        {
            span<int> s = a;
            auto bs = as_bytes(s);
            CHECK(static_cast<const void*>(bs.data()) == static_cast<const void*>(s.data()));
            CHECK(bs.length() == s.length_bytes());
        }
    }

    TEST(as_writeable_bytes)
    {
        int a[] = {1, 2, 3, 4};

        {
#ifdef CONFIRM_COMPILATION_ERRORS
            // you should not be able to get writeable bytes for const objects
            span<const int> s = a;
            CHECK(s.length() == 4);
            span<const byte> bs = as_writeable_bytes(s);
            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
            CHECK(bs.length() == s.length_bytes());
#endif
        }

        {
            span<int> s;
            auto bs = as_writeable_bytes(s);
            CHECK(bs.length() == s.length());
            CHECK(bs.length() == 0);
            CHECK(bs.size_bytes() == 0);
            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
            CHECK(bs.data() == nullptr);
        }

        {
            span<int> s = a;
            auto bs = as_writeable_bytes(s);
            CHECK(static_cast<void*>(bs.data()) == static_cast<void*>(s.data()));
            CHECK(bs.length() == s.length_bytes());
        }
    }

    TEST(fixed_size_conversions)
    {
        int arr[] = {1, 2, 3, 4};

        // converting to an span from an equal size array is ok
        span<int, 4> s4 = arr;
        CHECK(s4.length() == 4);

        // converting to dynamic_range is always ok
        {
            span<int> s = s4;
            CHECK(s.length() == s4.length());
            static_cast<void>(s);
        }

// initialization or assignment to static span that REDUCES size is NOT ok
#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int, 2> s = arr;
        }
        {
            span<int, 2> s2 = s4;
            static_cast<void>(s2);
        }
#endif

        // even when done dynamically
        {
            span<int> s = arr;
            auto f = [&]() {
                span<int, 2> s2 = s;
                static_cast<void>(s2);
            };
            CHECK_THROW(f(), fail_fast);
        }

        // but doing so explicitly is ok

        // you can convert statically
        {
            span<int, 2> s2 = {arr, 2};
            static_cast<void>(s2);
        }
        {
            span<int, 1> s1 = s4.first<1>();
            static_cast<void>(s1);
        }

        // ...or dynamically
        {
            // NB: implicit conversion to span<int,1> from span<int>
            span<int, 1> s1 = s4.first(1);
            static_cast<void>(s1);
        }

        // initialization or assignment to static span that requires size INCREASE is not ok.
        int arr2[2] = {1, 2};

#ifdef CONFIRM_COMPILATION_ERRORS
        {
            span<int, 4> s3 = arr2;
        }
        {
            span<int, 2> s2 = arr2;
            span<int, 4> s4a = s2;
        }
#endif
        {
            auto f = [&]() {
                span<int, 4> _s4 = {arr2, 2};
                static_cast<void>(_s4);
            };                     
            CHECK_THROW(f(), fail_fast);
        }

        // this should fail - we are trying to assign a small dynamic span to a fixed_size larger one
        span<int> av = arr2;
        auto f = [&]() {
            span<int, 4> _s4 = av;
            static_cast<void>(_s4);            
        };
        CHECK_THROW(f(), fail_fast);
    }

    TEST(interop_with_std_regex)
    {
        char lat[] = { '1', '2', '3', '4', '5', '6', 'E', 'F', 'G' };
        span<char> s = lat;
        auto f_it = s.begin() + 7;

        std::match_results<span<char>::iterator> match;

        std::regex_match(s.begin(), s.end(), match, std::regex(".*"));
        CHECK(match.ready());
        CHECK(!match.empty());
        CHECK(match[0].matched);
        CHECK(match[0].first == s.begin());
        CHECK(match[0].second == s.end());

        std::regex_search(s.begin(), s.end(), match, std::regex("F"));
        CHECK(match.ready());
        CHECK(!match.empty());
        CHECK(match[0].matched);
        CHECK(match[0].first == f_it);
        CHECK(match[0].second == (f_it + 1));
    }

    TEST(interop_with_gsl_at)
    {
        int arr[5] = {1, 2, 3, 4, 5};
        span<int> s{arr};
        CHECK(at(s,0) == 1 && at(s,1) == 2);
    }
}

int main(int, const char* []) { return UnitTest::RunAllTests(); }