Tukukkk

I began implementing Game of Life for fun, then I challenged myself to implement the game with no loops. I only used STL Algorithms. I don't know how to make pretty GUI's yet but this will print the coordinates of each alive cell. I also do not know if I made a good hashing function. I think it's sufficient for the game but I don't know any theory behind good hashing functions. I glanced through the top questions with c++ and game-of-life and noticed everyone used a 2d array. I figured a hashing container would be more efficient.

#include <unordered_set>

#include <iostream>

#include <vector>

#include <functional>

#include <array>

#include <iterator>

#include <cstddef>

#include <algorithm>

#include <utility>



using Cell = std::pair<int, int>;



namespace std {

  template<>

  struct hash<Cell> {

    std::size_t operator()(const Cell& cell) const {

      const std::hash<int> hasher;

      return hasher(cell.first) & hasher(cell.second);

    }

  };

}



std::ostream& operator<<(std::ostream& out, const Cell& cell) {

  out << '(' << cell.first << ',' << cell.second << ')';

  return out;

}



class Life {

public:

  template<typename InputIt>

  Life(InputIt begin, InputIt end);



  void tick();

  friend std::ostream& operator<<(std::ostream& out, const Life& life);



private:

  std::unordered_set<Cell> grid;

  std::array<Cell, 8> neighbors_of(const Cell& cell) const;

  int n_alive_neighbors(const std::array<Cell, 8>& neighbors) const;

};



template<typename InputIt>

Life::Life(InputIt begin, InputIt end)

  : grid(begin, end) {}



void Life::tick() {

  std::vector<Cell> to_die;

  std::vector<Cell> to_create;

  std::vector<Cell> all_neighbors;



  // find cells that will die

  std::copy_if(grid.begin(), grid.end(), std::back_inserter(to_die),

               [&](const auto& cell){

                 const auto neighbors = neighbors_of(cell);

                 const auto alive_neighbors = n_alive_neighbors(neighbors);

                 return alive_neighbors < 2 || alive_neighbors > 3;

               });



  // collect neighbors of all cells

  std::for_each(grid.begin(), grid.end(),

                [&](const auto& cell){

                  const auto neighbors = neighbors_of(cell);

                  std::copy(neighbors.begin(), neighbors.end(), std::back_inserter(all_neighbors));

                });



  // find cells that will be created

  std::copy_if(all_neighbors.begin(), all_neighbors.end(), std::back_inserter(to_create),

               [&](const auto& cell) {

                 if (grid.find(cell) != grid.end()) return false;

                 const auto neighbors = neighbors_of(cell);

                 const auto alive_neighbors = n_alive_neighbors(neighbors);

                 return alive_neighbors == 3;

               });



  // kill cells

  std::for_each(to_die.begin(), to_die.end(), [&](const auto& cell){ grid.erase(cell); });

  // reproduce cells

  grid.insert(to_create.begin(), to_create.end());

}



std::array<Cell, 8> Life::neighbors_of(const Cell& cell) const {

  return { Cell(cell.first - 1, cell.second + 1),

           Cell(cell.first, cell.second + 1),

           Cell(cell.first + 1, cell.second + 1),

           Cell(cell.first + 1, cell.second),

           Cell(cell.first + 1, cell.second - 1),

           Cell(cell.first, cell.second - 1),

           Cell(cell.first - 1, cell.second - 1),

           Cell(cell.first - 1, cell.second) };

}



int Life::n_alive_neighbors(const std::array<Cell, 8>& neighbors) const {

  return std::count_if(neighbors.begin(), neighbors.end(),

                       [&](const auto& cell){ return grid.find(cell) != grid.end(); });

}



std::ostream& operator<<(std::ostream& out, const Life& life) {

  if (life.grid.empty()) return out;

  out << *life.grid.begin();

  std::for_each(std::next(life.grid.begin()), life.grid.end(),

                [&](const auto& cell){

                  out << 'n' << cell;

                });

  return out;

}



int main() {

  std::array<Cell, 3> blinker {Cell(-1, 0), Cell(0, 0), Cell(1, 0)};

  std::array<Cell, 6> toad {Cell(0, 0), Cell(1, 0), Cell(2, 0),

                            Cell(1, 1), Cell(2, 1), Cell(3, 1)};



  Life life(toad.begin(), toad.end());



  std::cout << life << 'n';

  for (int i = 0; i < 6; ++i) {

    life.tick();

    std::cout << 'n' << life << 'n';

  }

}