C++11: Implementation of list comprehension in SQL-like form

listcompr.cpp

// List comprehension in C++11 in form of SQL-like syntax
// Example code from http://vitiy.info/cpp11-writing-list-comprehension-in-form-of-sql/
// Written by Victor Laskin ([email protected])

#include <iostream>
#include <functional>
#include <vector>
#include <future>
#include <algorithm>
using namespace std;

 // This is for converting lambda / pointer into correponding std::function<>
    
    template <typename Fun>
    struct fn_is_ptr: std::integral_constant<bool, std::is_pointer<Fun>::value
                      && std::is_function< typename std::remove_pointer<Fun>::type>::value>
    {};
    
    template <typename ReturnType, typename... Args, class = typename std::enable_if<fn_is_ptr< ReturnType(*)(Args...) >::value>::type>
    std::function<ReturnType(Args...)> to_fn (ReturnType (*fp)(Args...))
    {
        return std::function<ReturnType(Args...)>(fp);
    }

    
    template <typename Function>
    struct function_traits : public function_traits<decltype(&Function::operator())>
    {};
    
    template <typename ClassType, typename ReturnType, typename... Args>
    struct function_traits<ReturnType(ClassType::*)(Args...) const>
    {
        typedef ReturnType (*pointer)(Args...);
        typedef std::function<ReturnType(Args...)> function;
    };
    
    template <typename Function, class = typename std::enable_if<!fn_is_ptr<Function>::value>::type>
    typename function_traits<Function>::function to_fn(Function& lambda)
    {
        return typename function_traits<Function>::function(lambda);
    }
    
    template <typename Function, class = typename std::enable_if<!fn_is_ptr<Function>::value>::type>
    typename function_traits<Function>::function to_fn(const Function& lambda)
    {
        return typename function_traits<Function>::function(lambda);
    }
    
    // Range of natural integers
    class Naturals
    {
        int min;
        int max;
    public:
        Naturals() : min(1),max(1000) {}
        Naturals(int min, int max) : min(min),max(max) {}
        int at(int i) const { return i + min; } ;
        int size() const { return max - min + 1; } ;
        
        class Iterator {
            int position;
        public:
            Iterator(int _position):position(_position) {}
            int& operator*() { return position; }
            Iterator& operator++() { ++position; return *this; }
            bool operator!=(const Iterator& it) const { return position != it.position; }
        };
        
        Iterator begin() const { return { min }; }
        Iterator end()   const { return { max }; }
    };

    
    // Additional options
    
    class SelectOption {
    private:
        virtual bool imPolymorphic() { return true; }
    };
    
    class SelectOptionLimit : public SelectOption {
    public:
        int limit;
        SelectOptionLimit(int limit) : limit(limit) {}
    };
    
    class SelectOptionSort : public SelectOption {};
    
    class SelectOptionDistict : public SelectOption {};
    
    class SelectContinuation : public SelectOption {
    public:
        vector<int> indexes;
        SelectContinuation(){}
        template <typename... Args>
        SelectContinuation(Args... args) : indexes{ args... } { }
    };


    /// Main iterations

    template <typename Src, typename... Args>
    inline typename std::enable_if< std::is_object<decltype(std::declval<Src>().begin()) >::value, Src&&>::type
    getSource(Src && src, Args&&... args) {
        return std::forward<Src&&>(src);
    }
    
    
    template <typename F, typename... Args, typename FRes = decltype(std::declval<F>()(std::declval<Args>()...))>
    inline typename std::enable_if<std::is_object< decltype(std::declval<F>()(std::declval<Args>()...)) >::value, FRes  >::type
    getSource(F && f, Args&&... args)
    {
        return std::forward<FRes>(f(std::forward<Args>(args)...));
    }
    
    
    template<std::size_t I = 0, typename FuncT, typename... Tp, typename... Args>
    inline typename std::enable_if<I == sizeof...(Tp), bool>::type
    for_each_in_sources(const std::tuple<Tp...> &, FuncT& f, Args&... args)
    {
        return f(args...);
    }
    
    template<std::size_t I = 0, typename FuncT, typename... Tp, typename... Args>
    inline typename std::enable_if< I < sizeof...(Tp), bool>::type
    for_each_in_sources(const std::tuple<Tp...>& t, FuncT& f, Args&... args)
    {
        bool isFinished;
        auto&& src = getSource(std::get<I>(t), args...);
        for(auto& element : src)
        {
            isFinished = for_each_in_sources<I + 1, FuncT, Tp...>(t, f, args..., element);
            if (isFinished) break;
        }
        return isFinished;
    }
 
    
    template<std::size_t I = 0, typename FuncT, typename... Tp, typename... Args>
    inline typename std::enable_if<I == sizeof...(Tp), bool>::type
    for_each_in_sources_indexed(const std::tuple<Tp...> &, FuncT& f, vector<int>& indexes, Args&&... args)
    {
        return f(args...);
    }
    
    
    template<std::size_t I = 0, typename FuncT, typename... Tp, typename... Args>
    inline typename std::enable_if< I < sizeof...(Tp), bool>::type
    for_each_in_sources_indexed(const std::tuple<Tp...>& t, FuncT& f, vector<int>& indexes, Args&&... args)
    {
        bool isFinished;
        auto&& src = getSource(std::get<I>(t), args...);
        int size = src.size();
        int i = indexes[I];
        if (I != 0) if (i >= size) i = 0;
        if (i >= size) return false;
        for (; i < size; i++)
        {
            isFinished = for_each_in_sources_indexed<I + 1, FuncT, Tp...>(t, f, indexes, args..., src.at(i));
            if (isFinished) break;
        }
        indexes[I] = ++i;
        return isFinished;
    }
    
    template <typename R, typename... Args, typename... Sources, typename... Options>
    vector<R> select(const std::function<R(Args...)>& f,                ///< output expression
                     const std::tuple<Sources...>& sources,             ///< list of sources
                     const std::function<bool(Args...)>& filter,        ///< composition of filters
                     const Options&... options                          ///< other options and flags
                     )
    {
        int limit = -1;
        bool isDistinct = false;
        bool isSorted = false;
        
        vector<int>* indexes = nullptr;
        
        for_each_argument([&](const SelectOption& option){
            if (auto opt = dynamic_cast<const SelectOptionLimit*>(&option)) {  limit = opt->limit; };
            if (dynamic_cast<const SelectOptionSort*>(&option)) { isSorted = true; };
            if (dynamic_cast<const SelectOptionDistict*>(&option)) { isDistinct = true; };
            if (auto opt = dynamic_cast<const SelectContinuation*>(&option)) { indexes = &( (const_cast<SelectContinuation*>(opt))->indexes ); };
        }, (options)...);
        
        vector<R> result;
        int count = 0;
        auto process = [&](const Args&... args){
            if (filter(args...))
            {
                result.push_back(f(args...));
                count++;
            }
            return (count == limit); // isFinished
        };
        
        if (indexes == nullptr)
            for_each_in_sources(sources, process);
        else
        {
            if (indexes->size() == 0)
            {
                indexes->resize(std::tuple_size<std::tuple<Sources...>>::value);
                std::fill(indexes->begin(), indexes->end(), 0);
            }
            
            for_each_in_sources_indexed(sources, process, *indexes);
        }
        
        // sort results
        if ((isDistinct) || (isSorted))
            std::sort(result.begin(), result.end());
        
        // remove duplicates
        if (isDistinct)
        {
            auto last = std::unique(result.begin(), result.end());
            result.erase(last, result.end());
        }
        
        return result;
    }
    
    template <typename R, typename... Args, typename... Sources, typename... Options>
    inline vector<R> select(std::function<R(Args...)> f,
                     const std::tuple<Sources...>& sources,
                     const Options&... options)
    {
        return select(f, sources, to_fn([](Args... args){ return true; }), options...);
    }
    
    // EVIL WAY
    
    #define SELECT(X) select(to_fn(X),
    //#define FROM(...) std::forward_as_tuple(__VA_ARGS__)
    #define FROM(...) std::make_tuple(__VA_ARGS__)
    #define WHERE(...) ,to_fn(__VA_ARGS__)
    #define SORT ,SelectOptionSort()
    #define DISTINCT ,SelectOptionDistict()
    #define LIMIT(X) ,SelectOptionLimit(X))
    #define NOLIMIT LIMIT(-1)


    template <typename R, typename... Args, typename... Sources, typename... Options>
    std::function<vector<R>()> select_lazy(const std::function<R(Args...)>& f,                ///< output expression
                                           const std::tuple<Sources...>& sources,             ///< list of sources
                                           const std::function<bool(Args...)>& filter,        ///< composition of filters
                                           const Options&... options                          ///< other options and flags
                                           )
    {
        return to_fn([=](){ return select(f, sources, filter, options...); });
    }

    
    template <typename R, typename... Args, typename... Sources, typename... Options>
    std::function<vector<R>(const SelectContinuation& job)> select_concurrent(
                                           const std::function<R(Args...)>& f,                ///< output expression
                                           const std::tuple<Sources...>& sources,             ///< list of sources
                                           const std::function<bool(Args...)>& filter,        ///< composition of filters
                                           const Options&... options                          ///< other options and flags
                                           )
    {
        return to_fn([=](const SelectContinuation& job){ return select(f, sources, filter, job, options...); });
    }
 
    #define CONCURRENTSELECT(X) select_concurrent(to_fn(X),
    #define LAZYSELECT(X) select_lazy(to_fn(X),
    #define LAZY(X) ,(X)


    /// Call function for each argument
    template <class F, class... Args>
    void for_each_argument(F f, Args&&... args) {
        (void)(int[]){(f(forward<Args>(args)), 0)...};
    }

int main() {

     // EXAMPLES

    auto print = [](vector<int>& list){ cout << "List: "; for (int x : list) cout << x << " "; cout << endl;};

    cout << "10 naturals" << endl;
	auto result = SELECT([](int x){ return x; }) FROM(Naturals()) LIMIT(10);
    print(result);

    cout << "Distinct test" << endl;
    result = SELECT([](int x, int y){ return x+y; }) FROM(Naturals(), Naturals()) 
             WHERE([](int x, int y){ return (x*x + y*y < 25); }) DISTINCT LIMIT(10);
    print(result);
    
    cout << "Intersection" << endl;
    vector<int> ints = {11,2,1,5,6,7};
    vector<int> ints2 = {3,4,5,7,8,11};
        
    result = SELECT([](int x, int y){ return x; })
             FROM(ints, ints2)
             WHERE([](int x, int y){ return (x == y); }) SORT NOLIMIT;
    print(result);

    // Simple map operation as list comprehension
    cout << "Map" << endl;
    result = SELECT([](int x){ return x + 5; }) FROM(ints) NOLIMIT;
    print(result);
    
    cout << "Pythagorean Triplets" << endl;
    SelectContinuation state;

    auto lazypyth = LAZYSELECT([&](int x, int y, int z){ return make_tuple(z,y,x); })
        FROM(Naturals(1,100000000), 
             [](int x){ return Naturals(1,x); }, 
             [](int x,int y){ return Naturals(1,y); })
        WHERE([&](int x, int y, int z){  return x*x == y*y + z*z; })
        LAZY(state) LIMIT(5);
        
    auto pyth = lazypyth();
    cout << "Part1:" << endl;
    for (auto line: pyth) cout << " -> " << get<0>(line) << " " << get<1>(line) << " " << get<2>(line) << endl;
    pyth = lazypyth();
    cout << "Part2:" << endl;
    for (auto line: pyth) cout << " -> " << get<0>(line) << " " << get<1>(line) << " " << get<2>(line) << endl;

    return 0;
}

really good