#pragma once
/*
 *  Copyright (C) 2024  Brett Terpstra
 *
 *  This program is free software: you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation, either version 3 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License
 *  along with this program.  If not, see <https://www.gnu.org/licenses/>.
 */

#ifndef BLT_GP_PROGRAM_H
#define BLT_GP_PROGRAM_H


#include <cstddef>
#include <functional>
#include <type_traits>
#include <string_view>
#include <string>
#include <utility>
#include <iostream>
#include <algorithm>
#include <memory>
#include <array>
#include <thread>
#include <mutex>
#include <atomic>
#include <condition_variable>
#include <stdexcept>

#include <blt/std/ranges.h>
#include <blt/std/hashmap.h>
#include <blt/std/types.h>
#include <blt/std/utility.h>
#include <blt/std/meta.h>
#include <blt/std/memory.h>
#include <blt/std/thread.h>
#include <blt/gp/fwdecl.h>
#include <blt/gp/typesystem.h>
#include <blt/gp/operations.h>
#include <blt/gp/transformers.h>
#include <blt/gp/selection.h>
#include <blt/gp/tree.h>
#include <blt/gp/stack.h>
#include <blt/gp/config.h>
#include <blt/gp/random.h>
#include "blt/std/format.h"

namespace blt::gp
{
    
    struct argc_t
    {
        blt::u32 argc = 0;
        blt::u32 argc_context = 0;
        
        [[nodiscard]] bool is_terminal() const
        {
            return argc == 0;
        }
    };
    
    struct operator_info
    {
        // types of the arguments
        std::vector<type_id> argument_types;
        // return type of this operator
        type_id return_type;
        // number of arguments for this operator
        argc_t argc;
        // per operator function callable (slow)
        detail::operator_func_t func;
    };
    
    struct operator_storage
    {
        // indexed from return TYPE ID, returns index of operator
        blt::expanding_buffer<std::vector<operator_id>> terminals;
        blt::expanding_buffer<std::vector<operator_id>> non_terminals;
        blt::expanding_buffer<std::vector<std::pair<operator_id, blt::size_t>>> operators_ordered_terminals;
        // indexed from OPERATOR ID (operator number)
        blt::hashset_t<operator_id> static_types;
        std::vector<operator_info> operators;
        std::vector<detail::print_func_t> print_funcs;
        std::vector<detail::destroy_func_t> destroy_funcs;
        std::vector<std::optional<std::string_view>> names;
        
        detail::eval_func_t eval_func;
    };
    
    template<typename Context = detail::empty_t>
    class operator_builder
    {
            friend class gp_program;
            
            friend class blt::gp::detail::operator_storage_test;
        
        public:
            explicit operator_builder(type_provider& system): system(system)
            {}
            
            template<typename... Operators>
            operator_storage& build(Operators& ... operators)
            {
                std::vector<blt::size_t> sizes;
                (sizes.push_back(add_operator(operators)), ...);
                blt::size_t largest = 0;
                for (auto v : sizes)
                    largest = std::max(v, largest);
                
                storage.eval_func = [&operators..., largest](const tree_t& tree, void* context) -> evaluation_context& {
                    const auto& ops = tree.get_operations();
                    const auto& vals = tree.get_values();
                    
                    static thread_local evaluation_context results{};
                    results.values.reset();
                    results.values.reserve(largest);
                    
                    blt::size_t total_so_far = 0;
                    
                    for (const auto& operation : blt::reverse_iterate(ops.begin(), ops.end()))
                    {
                        if (operation.is_value)
                        {
                            total_so_far += stack_allocator::aligned_size(operation.type_size);
                            results.values.copy_from(vals.from(total_so_far), stack_allocator::aligned_size(operation.type_size));
                            continue;
                        }
                        call_jmp_table(operation.id, context, results.values, results.values, operators...);
                    }
                    
                    return results;
                };
                
                blt::hashset_t<type_id> has_terminals;
                
                for (const auto& v : blt::enumerate(storage.terminals))
                {
                    if (!v.second.empty())
                        has_terminals.insert(v.first);
                }
                
                for (const auto& op_r : blt::enumerate(storage.non_terminals))
                {
                    if (op_r.second.empty())
                        continue;
                    auto return_type = op_r.first;
                    std::vector<std::pair<operator_id, blt::size_t>> ordered_terminals;
                    for (const auto& op : op_r.second)
                    {
                        // count number of terminals
                        blt::size_t terminals = 0;
                        for (const auto& type : storage.operators[op].argument_types)
                        {
                            if (has_terminals.contains(type))
                                terminals++;
                        }
                        ordered_terminals.emplace_back(op, terminals);
                    }
                    bool found_terminal_inputs = false;
                    bool matches_argc = false;
                    for (const auto& terms : ordered_terminals)
                    {
                        if (terms.second == storage.operators[terms.first].argc.argc)
                            matches_argc = true;
                        if (terms.second != 0)
                            found_terminal_inputs = true;
                        if (matches_argc && found_terminal_inputs)
                            break;
                    }
                    if (!found_terminal_inputs)
                        BLT_ABORT(("Failed to find function with terminal arguments for return type " + std::to_string(return_type)).c_str());
                    if (!matches_argc)
                    {
                        BLT_ABORT(("Failed to find a function which purely translates types "
                                   "(that is all input types are terminals) for return type " + std::to_string(return_type)).c_str());
                    }
                    
                    std::sort(ordered_terminals.begin(), ordered_terminals.end(), [](const auto& a, const auto& b) {
                        return a.second > b.second;
                    });
                    
                    auto first_size = *ordered_terminals.begin();
                    auto iter = ordered_terminals.begin();
                    while (++iter != ordered_terminals.end() && iter->second == first_size.second)
                    {}
                    
                    ordered_terminals.erase(iter, ordered_terminals.end());
                    
                    storage.operators_ordered_terminals[return_type] = ordered_terminals;
                }
                
                return storage;
            }
            
            operator_storage&& grab()
            {
                return std::move(storage);
            }
        
        private:
            template<typename RawFunction, typename Return, typename... Args>
            auto add_operator(operation_t<RawFunction, Return(Args...)>& op)
            {
                auto total_size_required = stack_allocator::aligned_size(sizeof(Return));
                ((total_size_required += stack_allocator::aligned_size(sizeof(Args))), ...);
                
                auto return_type_id = system.get_type<Return>().id();
                auto operator_id = blt::gp::operator_id(storage.operators.size());
                op.id = operator_id;
                
                operator_info info;
                
                if constexpr (sizeof...(Args) > 0)
                {
                    (add_non_context_argument<detail::remove_cv_ref<Args>>(info.argument_types), ...);
                }
                
                info.argc.argc_context = info.argc.argc = sizeof...(Args);
                info.return_type = return_type_id;
                info.func = op.template make_callable<Context>();
                
                ((std::is_same_v<detail::remove_cv_ref<Args>, Context> ? info.argc.argc -= 1 : (blt::size_t) nullptr), ...);
                
                auto& operator_list = info.argc.argc == 0 ? storage.terminals : storage.non_terminals;
                operator_list[return_type_id].push_back(operator_id);
                
                BLT_ASSERT(info.argc.argc_context - info.argc.argc <= 1 && "Cannot pass multiple context as arguments!");
                
                storage.operators.push_back(info);
                storage.print_funcs.push_back([&op](std::ostream& out, stack_allocator& stack) {
                    if constexpr (blt::meta::is_streamable_v<Return>)
                    {
                        out << stack.from<Return>(0);
                        (void) (op); // remove warning
                    } else
                    {
                        out << "[Printing Value on '" << (op.get_name() ? *op.get_name() : "") << "' Not Supported!]";
                    }
                });
                storage.destroy_funcs.push_back([](detail::destroy_t type, stack_allocator& alloc) {
                    switch (type)
                    {
                        case detail::destroy_t::ARGS:
                            alloc.call_destructors<Args...>();
                            break;
                        case detail::destroy_t::RETURN:
                            if constexpr (detail::has_func_drop_v<remove_cvref_t<Return>>)
                            {
                                alloc.from<detail::remove_cv_ref<Return>>(0).drop();
                            }
                            break;
                    }
                });
                storage.names.push_back(op.get_name());
                if (op.is_ephemeral())
                    storage.static_types.insert(operator_id);
                return total_size_required * 2;
            }
            
            template<typename T>
            void add_non_context_argument(decltype(operator_info::argument_types)& types)
            {
                if constexpr (!std::is_same_v<Context, detail::remove_cv_ref<T>>)
                {
                    types.push_back(system.get_type<T>().id());
                }
            }
            
            template<typename Operator>
            static inline void execute(void* context, stack_allocator& write_stack, stack_allocator& read_stack, Operator& operation)
            {
                if constexpr (std::is_same_v<detail::remove_cv_ref<typename Operator::First_Arg>, Context>)
                {
                    write_stack.push(operation(context, read_stack));
                } else
                {
                    write_stack.push(operation(read_stack));
                }
            }
            
            template<blt::size_t id, typename Operator>
            static inline bool call(blt::size_t op, void* context, stack_allocator& write_stack, stack_allocator& read_stack, Operator& operation)
            {
                if (id == op)
                {
                    execute(context, write_stack, read_stack, operation);
                    return false;
                }
                return true;
            }
            
            template<typename... Operators, size_t... operator_ids>
            static inline void call_jmp_table_internal(size_t op, void* context, stack_allocator& write_stack, stack_allocator& read_stack,
                                                       std::integer_sequence<size_t, operator_ids...>, Operators& ... operators)
            {
                if (op >= sizeof...(operator_ids))
                {
                    BLT_UNREACHABLE;
                }
                (call<operator_ids>(op, context, write_stack, read_stack, operators) && ...);
            }
            
            template<typename... Operators>
            static inline void call_jmp_table(size_t op, void* context, stack_allocator& write_stack, stack_allocator& read_stack,
                                              Operators& ... operators)
            {
                call_jmp_table_internal(op, context, write_stack, read_stack, std::index_sequence_for<Operators...>(), operators...);
            }
            
            type_provider& system;
            operator_storage storage;
    };
    
    class gp_program
    {
        public:
            /**
             * Note about context size: This is required as context is passed to every operator in the GP tree, this context will be provided by your
             * call to one of the evaluator functions. This was the nicest way to provide this as C++ lacks reflection
             *
             * @param system type system to use in tree generation
             * @param engine random engine to use throughout the program.
             * @param context_size number of arguments which are always present as "context" to the GP system / operators
             */
            explicit gp_program(type_provider& system, blt::u64 seed):
                    system(system), seed_func([seed]{return seed;})
            { create_threads(); }
            
            explicit gp_program(type_provider& system, blt::u64 seed, prog_config_t config):
                    system(system), seed_func([seed]{return seed;}), config(config)
            { create_threads(); }
            
            explicit gp_program(type_provider& system, std::function<blt::u64()> seed_func):
                    system(system), seed_func(std::move(seed_func))
            { create_threads(); }
            
            explicit gp_program(type_provider& system, std::function<blt::u64()> seed_func, prog_config_t config):
                    system(system), seed_func(std::move(seed_func)), config(config)
            { create_threads(); }
            
            void create_next_generation()
            {
                // should already be empty
                next_pop.clear();
                thread_helper.next_gen_left.store(config.population_size, std::memory_order_release);
                (*thread_execution_service)(0);
            }
            
            void evaluate_fitness()
            {
                evaluate_fitness_internal();
            }
            
            /**
             * takes in a reference to a function for the fitness evaluation function (must return a value convertable to double)
             * The lambda must accept a tree for evaluation, and an index (current tree)
             *
             * tree_t& current_tree, blt::size_t index_of_tree
             *
             * Container must be concurrently accessible from multiple threads using operator[]
             *
             * NOTE: 0 is considered the best, in terms of standardized fitness
             */
            template<typename FitnessFunc, typename Crossover, typename Mutation, typename Reproduction, typename CreationFunc = decltype(default_next_pop_creator<Crossover, Mutation, Reproduction>)>
            void generate_population(type_id root_type, FitnessFunc& fitness_function,
                                     Crossover& crossover_selection, Mutation& mutation_selection, Reproduction& reproduction_selection,
                                     CreationFunc& func = default_next_pop_creator<Crossover, Mutation, Reproduction>, bool eval_fitness_now = true)
            {
                using LambdaReturn = typename decltype(blt::meta::lambda_helper(fitness_function))::Return;
                current_pop = config.pop_initializer.get().generate(
                        {*this, root_type, config.population_size, config.initial_min_tree_size, config.initial_max_tree_size});
                if (config.threads == 1)
                {
                    BLT_INFO("Starting with single thread variant!");
                    thread_execution_service = new std::function(
                            [this, &fitness_function, &crossover_selection, &mutation_selection, &reproduction_selection, &func](blt::size_t) {
                                if (thread_helper.evaluation_left > 0)
                                {
                                    for (const auto& ind : blt::enumerate(current_pop.get_individuals()))
                                    {
                                        if constexpr (std::is_same_v<LambdaReturn, bool> || std::is_convertible_v<LambdaReturn, bool>)
                                        {
                                            auto result = fitness_function(ind.second.tree, ind.second.fitness, ind.first);
                                            if (result)
                                                fitness_should_exit = true;
                                        } else
                                        {
                                            fitness_function(ind.second.tree, ind.second.fitness, ind.first);
                                        }
                                        
                                        if (ind.second.fitness.adjusted_fitness > current_stats.best_fitness)
                                            current_stats.best_fitness = ind.second.fitness.adjusted_fitness;
                                        
                                        if (ind.second.fitness.adjusted_fitness < current_stats.worst_fitness)
                                            current_stats.worst_fitness = ind.second.fitness.adjusted_fitness;
                                        
                                        current_stats.overall_fitness = current_stats.overall_fitness + ind.second.fitness.adjusted_fitness;
                                    }
                                    thread_helper.evaluation_left = 0;
                                }
                                if (thread_helper.next_gen_left > 0)
                                {
                                    static thread_local std::vector<tree_t> new_children;
                                    new_children.clear();
                                    auto args = get_selector_args(new_children);
                                    
                                    crossover_selection.pre_process(*this, current_pop, current_stats);
                                    mutation_selection.pre_process(*this, current_pop, current_stats);
                                    reproduction_selection.pre_process(*this, current_pop, current_stats);
                                    
                                    perform_elitism(args);
                                    
                                    while (new_children.size() < config.population_size)
                                        func(args, crossover_selection, mutation_selection, reproduction_selection);
                                    
                                    for (auto& i : new_children)
                                        next_pop.get_individuals().emplace_back(std::move(i));
                                    
                                    thread_helper.next_gen_left = 0;
                                }
                            });
                } else
                {
                    BLT_INFO("Starting thread execution service!");
                    std::scoped_lock lock(thread_helper.thread_function_control);
                    thread_execution_service = new std::function(
                            [this, &fitness_function, &crossover_selection, &mutation_selection, &reproduction_selection, &func](blt::size_t id) {
                                thread_helper.barrier.wait();
                                if (thread_helper.evaluation_left > 0)
                                {
                                    while (thread_helper.evaluation_left > 0)
                                    {
                                        blt::size_t size = 0;
                                        blt::size_t begin = 0;
                                        blt::size_t end = thread_helper.evaluation_left.load(std::memory_order_relaxed);
                                        do
                                        {
                                            size = std::min(end, config.evaluation_size);
                                            begin = end - size;
                                        } while (!thread_helper.evaluation_left.compare_exchange_weak(end, end - size,
                                                                                                      std::memory_order::memory_order_relaxed,
                                                                                                      std::memory_order::memory_order_relaxed));
                                        for (blt::size_t i = begin; i < end; i++)
                                        {
                                            auto& ind = current_pop.get_individuals()[i];
                                            
                                            
                                            if constexpr (std::is_same_v<LambdaReturn, bool> || std::is_convertible_v<LambdaReturn, bool>)
                                            {
                                                auto result = fitness_function(ind.tree, ind.fitness, i);
                                                if (result)
                                                    fitness_should_exit = true;
                                            } else
                                            {
                                                fitness_function(ind.tree, ind.fitness, i);
                                            }
                                            
                                            auto old_best = current_stats.best_fitness.load(std::memory_order_relaxed);
                                            while (ind.fitness.adjusted_fitness > old_best &&
                                                   !current_stats.best_fitness.compare_exchange_weak(old_best, ind.fitness.adjusted_fitness,
                                                                                                     std::memory_order_relaxed,
                                                                                                     std::memory_order_relaxed));
                                            
                                            auto old_worst = current_stats.worst_fitness.load(std::memory_order_relaxed);
                                            while (ind.fitness.adjusted_fitness < old_worst &&
                                                   !current_stats.worst_fitness.compare_exchange_weak(old_worst, ind.fitness.adjusted_fitness,
                                                                                                      std::memory_order_relaxed,
                                                                                                      std::memory_order_relaxed));
                                            
                                            auto old_overall = current_stats.overall_fitness.load(std::memory_order_relaxed);
                                            while (!current_stats.overall_fitness.compare_exchange_weak(old_overall,
                                                                                                        ind.fitness.adjusted_fitness + old_overall,
                                                                                                        std::memory_order_relaxed,
                                                                                                        std::memory_order_relaxed));
                                        }
                                    }
                                }
                                if (thread_helper.next_gen_left > 0)
                                {
                                    static thread_local std::vector<tree_t> new_children;
                                    new_children.clear();
                                    auto args = get_selector_args(new_children);
                                    if (id == 0)
                                    {
                                        crossover_selection.pre_process(*this, current_pop, current_stats);
                                        if (&crossover_selection != &mutation_selection)
                                            mutation_selection.pre_process(*this, current_pop, current_stats);
                                        if (&crossover_selection != &reproduction_selection)
                                            reproduction_selection.pre_process(*this, current_pop, current_stats);
                                        
                                        perform_elitism(args);
                                        
                                        for (auto& i : new_children)
                                            next_pop.get_individuals().emplace_back(std::move(i));
                                        thread_helper.next_gen_left -= new_children.size();
                                        new_children.clear();
                                    }
                                    thread_helper.barrier.wait();
                                    
                                    while (thread_helper.next_gen_left > 0)
                                    {
                                        blt::size_t size = 0;
                                        blt::size_t end = thread_helper.next_gen_left.load(std::memory_order_relaxed);
                                        do
                                        {
                                            size = std::min(end, config.evaluation_size);
                                        } while (!thread_helper.next_gen_left.compare_exchange_weak(end, end - size,
                                                                                                    std::memory_order::memory_order_relaxed,
                                                                                                    std::memory_order::memory_order_relaxed));
                                        
                                        while (new_children.size() < size)
                                            func(args, crossover_selection, mutation_selection, reproduction_selection);
                                        
                                        {
                                            std::scoped_lock lock(thread_helper.thread_generation_lock);
                                            for (auto& i : new_children)
                                            {
                                                if (next_pop.get_individuals().size() < config.population_size)
                                                    next_pop.get_individuals().emplace_back(i);
                                            }
                                        }
                                    }
                                }
                                thread_helper.barrier.wait();
                            });
                    thread_helper.thread_function_condition.notify_all();
                }
                if (eval_fitness_now)
                    evaluate_fitness_internal();
            }
            
            void reset_program(type_id root_type, bool eval_fitness_now = true)
            {
                current_generation = 0;
                current_pop = config.pop_initializer.get().generate(
                        {*this, root_type, config.population_size, config.initial_min_tree_size, config.initial_max_tree_size});
                if (eval_fitness_now)
                    evaluate_fitness_internal();
            }
            
            void next_generation()
            {
                BLT_ASSERT_MSG(next_pop.get_individuals().size() == config.population_size, ("pop size: " + std::to_string(next_pop.get_individuals().size())).c_str());
                current_pop = std::move(next_pop);
                current_generation++;
            }
            
            inline auto& get_current_pop()
            {
                return current_pop;
            }
            
            template<blt::size_t size>
            std::array<blt::size_t, size> get_best_indexes()
            {
                std::array<blt::size_t, size> arr;
                
                std::vector<std::pair<blt::size_t, double>> values;
                values.reserve(current_pop.get_individuals().size());
                
                for (const auto& ind : blt::enumerate(current_pop.get_individuals()))
                    values.emplace_back(ind.first, ind.second.fitness.adjusted_fitness);
                
                std::sort(values.begin(), values.end(), [](const auto& a, const auto& b) {
                    return a.second > b.second;
                });
                
                for (blt::size_t i = 0; i < size; i++)
                    arr[i] = values[i].first;
                
                return arr;
            }
            
            template<blt::size_t size>
            auto get_best_trees()
            {
                return convert_array<std::array<std::reference_wrapper<tree_t>, size>>(get_best_indexes<size>(),
                                                                                       [this](auto&& arr, blt::size_t index) -> tree_t& {
                                                                                           return current_pop.get_individuals()[arr[index]].tree;
                                                                                       },
                                                                                       std::make_integer_sequence<blt::size_t, size>());
            }
            
            template<blt::size_t size>
            auto get_best_individuals()
            {
                return convert_array<std::array<std::reference_wrapper<individual>, size>>(get_best_indexes<size>(),
                                                                                           [this](auto&& arr, blt::size_t index) -> individual& {
                                                                                               return current_pop.get_individuals()[arr[index]];
                                                                                           },
                                                                                           std::make_integer_sequence<blt::size_t, size>());
            }
            
            [[nodiscard]] bool should_terminate() const
            {
                return current_generation >= config.max_generations || fitness_should_exit;
            }
            
            [[nodiscard]] bool should_thread_terminate() const
            {
                return thread_helper.lifetime_over;
            }
            
            [[nodiscard]] random_t& get_random() const;
            
            [[nodiscard]] inline type_provider& get_typesystem()
            {
                return system;
            }
            
            inline operator_id select_terminal(type_id id)
            {
                // we wanted a terminal, but could not find one, so we will select from a function that has a terminal
                if (storage.terminals[id].empty())
                    return select_non_terminal_too_deep(id);
                return get_random().select(storage.terminals[id]);
            }
            
            inline operator_id select_non_terminal(type_id id)
            {
                // non-terminal doesn't exist, return a terminal. This is useful for types that are defined only to have a random value, nothing more.
                // was considering an std::optional<> but that would complicate the generator code considerably. I'll mark this as a TODO for v2
                if (storage.non_terminals[id].empty())
                    return select_terminal(id);
                return get_random().select(storage.non_terminals[id]);
            }
            
            inline operator_id select_non_terminal_too_deep(type_id id)
            {
                // this should probably be an error.
                if (storage.operators_ordered_terminals[id].empty())
                    BLT_ABORT("An impossible state has been reached. Please consult the manual. Error 43");
                return get_random().select(storage.operators_ordered_terminals[id]).first;
            }
            
            inline operator_info& get_operator_info(operator_id id)
            {
                return storage.operators[id];
            }
            
            inline detail::print_func_t& get_print_func(operator_id id)
            {
                return storage.print_funcs[id];
            }
            
            inline detail::destroy_func_t& get_destroy_func(operator_id id)
            {
                return storage.destroy_funcs[id];
            }
            
            inline std::optional<std::string_view> get_name(operator_id id)
            {
                return storage.names[id];
            }
            
            inline std::vector<operator_id>& get_type_terminals(type_id id)
            {
                return storage.terminals[id];
            }
            
            inline std::vector<operator_id>& get_type_non_terminals(type_id id)
            {
                return storage.non_terminals[id];
            }
            
            inline bool is_static(operator_id id)
            {
                return storage.static_types.contains(static_cast<blt::size_t>(id));
            }
            
            inline void set_operations(operator_storage op)
            {
                storage = std::move(op);
            }
            
            inline detail::eval_func_t& get_eval_func()
            {
                return storage.eval_func;
            }
            
            [[nodiscard]] inline auto get_current_generation() const
            {
                return current_generation.load();
            }
            
            [[nodiscard]] inline auto& get_population_stats()
            {
                return current_stats;
            }
            
            ~gp_program()
            {
                thread_helper.lifetime_over = true;
                thread_helper.barrier.notify_all();
                thread_helper.thread_function_condition.notify_all();
                for (auto& thread : thread_helper.threads)
                {
                    if (thread->joinable())
                        thread->join();
                }
                auto* cpy = thread_execution_service.load(std::memory_order_acquire);
                thread_execution_service = nullptr;
                delete cpy;
            }
            
            void kill()
            {
                thread_helper.lifetime_over = true;
            }
        
        private:
            type_provider& system;
            
            operator_storage storage;
            population_t current_pop;
            population_stats current_stats{};
            population_t next_pop;
            std::atomic_uint64_t current_generation = 0;
            std::atomic_bool fitness_should_exit = false;
            
            std::function<blt::u64()> seed_func;
            prog_config_t config{};
            
            struct concurrency_storage
            {
                std::vector<std::unique_ptr<std::thread>> threads;
                
                std::mutex thread_function_control;
                std::mutex thread_generation_lock;
                std::condition_variable thread_function_condition{};
                
                std::atomic_uint64_t evaluation_left = 0;
                std::atomic_uint64_t next_gen_left = 0;
                
                std::atomic_bool lifetime_over = false;
                blt::barrier barrier;
                
                explicit concurrency_storage(blt::size_t threads): barrier(threads, lifetime_over)
                {}
            } thread_helper{config.threads == 0 ? std::thread::hardware_concurrency() : config.threads};
            
            // for convenience, shouldn't decrease performance too much
            std::atomic<std::function<void(blt::size_t)>*> thread_execution_service = nullptr;
            
            inline selector_args get_selector_args(std::vector<tree_t>& next_pop_trees)
            {
                return {*this, next_pop_trees, current_pop, current_stats, config, get_random()};
            }
            
            template<typename Return, blt::size_t size, typename Accessor, blt::size_t... indexes>
            inline Return convert_array(std::array<blt::size_t, size>&& arr, Accessor&& accessor,
                                        std::integer_sequence<blt::size_t, indexes...>)
            {
                return Return{accessor(arr, indexes)...};
            }
            
            void create_threads();
            
            void evaluate_fitness_internal()
            {
                current_stats.clear();
                thread_helper.evaluation_left.store(current_pop.get_individuals().size(), std::memory_order_release);
                (*thread_execution_service)(0);
                
                current_stats.average_fitness = current_stats.overall_fitness / static_cast<double>(config.population_size);
            }
    };
    
}

#endif //BLT_GP_PROGRAM_H