#include <map>  // Component-entity system in 24 lines of C++11. 2013 rlyeh, MIT licensed.
#include <set>  // Code fragment from kult engine     -     https://github.com/r-lyeh/kult
enum SUBSYSTEM_MODE { JOIN = 0, MERGE = 1, EXCLUDE = 2 };
template<typename T> std::set<unsigned> &system()
{   static std::set<unsigned> entities; return entities; }
template<typename T, int MODE> std::set<unsigned> subsystem( const std::set<unsigned> &B )
{   std::set<unsigned> newset; const std::set<unsigned> &A = system<T>();            // union first,
    /**/ if (MODE == MERGE)   { newset = B; for( auto &id : A ) newset.insert(id); } // then difference,
    else if (MODE == EXCLUDE) { newset = B; for( auto &id : A ) newset.erase (id); } // then intersection
    else if(A.size() < B.size()) { for( auto &id : A ) if(B.find(id) != B.end()) newset.insert( id ); }
                            else { for( auto &id : B ) if(A.find(id) != A.end()) newset.insert( id ); }
    return newset; }
template<typename T> std::map< unsigned, T > &components()
{   static std::map< unsigned, T > objects; return objects;  }
template<typename T> bool has( unsigned id )
{   return components<T>().find( id ) != components<T>().end(); }
template<typename T> decltype(T::value_type) &get( unsigned id )
{   static decltype(T::value_type) invalid, reset;
    return has<T>(id) ? components<T>()[id].value_type : invalid = reset; }
template<typename T> decltype(T::value_type) &add( unsigned id )
{   return system<T>().insert( id ), components<T>()[id] = components<T>()[id], get<T>(id); }
template<typename T> bool del( unsigned id )
{   return add<T>(id), components<T>().erase( id ), system<T>().erase( id ), !has<T>( id ); }
template<typename T, int NAME> struct component { T value_type; };

// example:
#include <cassert>
#include <string>
#include <iostream>
// gamedev types and constants
typedef std::pair<int, int>    vec2i;
typedef std::pair<float,float> vec2f;
const vec2f zero = { 0.f, 0.f }, one = { 1.f, 1.f };
// component aliases
using friendly = component<        bool, 'team' >;
using health   = component<         int, 'heal' >;
using mana     = component<         int, 'mana' >;
using coins    = component<         int, 'coin' >;
using name     = component< std::string, 'name' >;
using position = component<       vec2f, 'pos2' >;
// sugars
template<class T, class U>                   std::set< unsigned > join() { return subsystem<T,JOIN>( system<U>() );   }
template<class T, class U, class V>          std::set< unsigned > join() { return subsystem<T,JOIN>( join<U,V>() );   }
template<class T, class U, class V, class W> std::set< unsigned > join() { return subsystem<T,JOIN>( join<U,V,W>() ); }
template<class T> std::set< unsigned > exclude( const std::set<unsigned> &B ) { return subsystem<T,EXCLUDE>(B); }
int main()
{
    // entities
    int none = 0, player = 1, enemy = 2;

    // components
    assert( !has<name>(player) );
    assert( !has<position>(player) );
    assert( !has<coins>(enemy) );
    assert( !has<health>(enemy) );

    add<name>(player) = "Hero";
    add<position>(player) = zero;
    add<health>(player) = 100;
    add<coins>(player) = 200;
    add<mana>(player) = 4000;
    add<friendly>(player) = true;

    add<name>(enemy) = "Orc";
    add<position>(enemy) = one;
    add<health>(enemy) = 200;
    add<coins>(enemy) = 50;
    add<mana>(enemy) = 10;

    assert( get<health>(player) == 100 ); // :>

    assert(  has<name>(player) );
    assert( !has<vec2i>(player) );
    assert(  has<position>(player) );
    assert(  has<health>(player) );

    assert( get<name>(player) == "Hero" );
    assert( get<position>(player) == zero );
    assert( get<health>(player) == 100 );

    // systems; here we intersect a system of all elements with <name> and <position>.
    assert( (join<name, position>().size() == 2) );

    // systems; render game state
    auto display = []() {
        std::cout << "- ";
        for( auto &id : join<name, coins, health, position>() ) {
            std::cout
                << get<name>(id) << " at "
                << "(" << get<position>(id).first << "," << get<position>(id).second << ")"
                << " " << get<health>(id) << "HP"
                << " " << get<coins>(id) << "$, ";
        }
        std::cout << std::endl;
    };

    display();

    // systems; simulate movement
    for( auto &id : join<name, position>() ) {
        std::cout << get<name>(id) << " says: im moving!" << std::endl;
        vec2f &pos = get<position>(id);
        pos.first += 10;
        pos.second ++;
    }

    // systems; simulate a spell bomb in entities of any type
    for( auto &id : system<mana>() ) {
        std::cout << "spellboomb!!!" << std::endl;
        get<mana>(id) -= 50;
    }

    // systems; simulate a powerup (+$100) for all players
    for( auto &id : join<name, coins, friendly>() ) {
        get<coins>(id) += 100;
        std::cout << get<name>(id) << " says: money! :)" << std::endl;
    }

    // systems; simulate a poison (-50%HP) to all entities that are not friendly (so enemies)
    for( auto &id : exclude<friendly>( join<name, health>() ) ) {
        get<health>(id) *= 0.5;
        std::cout << get<name>(id) << " says: ugh! poisoned :(" << std::endl;
    }

    display();

    assert( get<health>(player) == 100+0 );
    assert( get<health>(enemy) == 200/2 );
    assert( get<coins>(player) == 200+100 );
    assert( get<coins>(enemy) == 50+0 );
    assert( get<mana>(player) == 4000-50 );
    assert( get<mana>(enemy) == 10-50 );

    assert( del<position>(player) );
    assert( !has<position>(player) );
    assert( del<name>(player) );
    assert( !has<name>(player) );

    assert( (join<name, position>().size() == 1) );
}