#include <algorithm>
#include <cerrno>
#include <chrono>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <random>
#include <stdexcept>
#include <string>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <sys/wait.h>
#include <unistd.h>

constexpr int NIL = -1;
constexpr int MAX_RESULT_SLOTS = 4096;

struct Node {
    int32_t value;
    int next;
};

struct SharedData {
    int results[MAX_RESULT_SLOTS];
    Node nodes[1];
};

struct Options {
    size_t size = 100000;
    int max_depth = 3;
    size_t min_size = 4096;
    unsigned seed = 1337;
    bool print = false;
};

struct SortResult {
    int head = NIL;
    uint64_t processes = 1;
};

static void throw_errno(const std::string& what) {
    throw std::runtime_error(what + ": " + std::strerror(errno));
}

static int merge_lists(Node* nodes, int left, int right) {
    if (left == NIL) return right;
    if (right == NIL) return left;

    int head = NIL;
    int tail = NIL;

    auto append = [&](int idx) {
        if (head == NIL) {
            head = idx;
            tail = idx;
        } else {
            nodes[tail].next = idx;
            tail = idx;
        }
    };

    while (left != NIL && right != NIL) {
        if (nodes[left].value <= nodes[right].value) {
            int next = nodes[left].next;
            append(left);
            left = next;
        } else {
            int next = nodes[right].next;
            append(right);
            right = next;
        }
    }

    if (left != NIL) nodes[tail].next = left;
    if (right != NIL) nodes[tail].next = right;
    return head;
}

static void split_list(Node* nodes, int head, int& left, int& right) {
    if (head == NIL || nodes[head].next == NIL) {
        left = head;
        right = NIL;
        return;
    }

    int slow = head;
    int fast = nodes[head].next;

    while (fast != NIL) {
        fast = nodes[fast].next;
        if (fast != NIL) {
            slow = nodes[slow].next;
            fast = nodes[fast].next;
        }
    }

    left = head;
    right = nodes[slow].next;
    nodes[slow].next = NIL;
}

static int local_sort(Node* nodes, int head) {
    if (head == NIL || nodes[head].next == NIL) return head;

    int left = NIL;
    int right = NIL;
    split_list(nodes, head, left, right);
    left = local_sort(nodes, left);
    right = local_sort(nodes, right);
    return merge_lists(nodes, left, right);
}

static size_t list_length(const Node* nodes, int head) {
    size_t n = 0;
    while (head != NIL) {
        ++n;
        head = nodes[head].next;
    }
    return n;
}

static SortResult parallel_sort(SharedData* data,
                                int head,
                                int depth,
                                const Options& opt,
                                int slot) {
    const size_t n = list_length(data->nodes, head);

    if (head == NIL || data->nodes[head].next == NIL ||
        depth >= opt.max_depth || n <= opt.min_size) {
        return {local_sort(data->nodes, head), 1};
    }

    int left = NIL;
    int right = NIL;
    split_list(data->nodes, head, left, right);

    const int left_slot = slot * 2 + 1;
    const int right_slot = slot * 2 + 2;

    if (right_slot >= MAX_RESULT_SLOTS) {
        int sorted_left = local_sort(data->nodes, left);
        int sorted_right = local_sort(data->nodes, right);
        return {merge_lists(data->nodes, sorted_left, sorted_right), 1};
    }

    pid_t left_pid = fork();
    if (left_pid < 0) throw_errno("fork left");

    if (left_pid == 0) {
        try {
            SortResult r = parallel_sort(data, left, depth + 1, opt, left_slot);
            data->results[left_slot] = r.head;
            _exit(0);
        } catch (...) {
            _exit(2);
        }
    }

    pid_t right_pid = fork();
    if (right_pid < 0) throw_errno("fork right");

    if (right_pid == 0) {
        try {
            SortResult r = parallel_sort(data, right, depth + 1, opt, right_slot);
            data->results[right_slot] = r.head;
            _exit(0);
        } catch (...) {
            _exit(2);
        }
    }

    int left_status = 0;
    int right_status = 0;

    while (waitpid(left_pid, &left_status, 0) < 0 && errno == EINTR) {}
    while (waitpid(right_pid, &right_status, 0) < 0 && errno == EINTR) {}

    if (!WIFEXITED(left_status) || WEXITSTATUS(left_status) != 0) {
        throw std::runtime_error("left child failed");
    }
    if (!WIFEXITED(right_status) || WEXITSTATUS(right_status) != 0) {
        throw std::runtime_error("right child failed");
    }

    return {merge_lists(data->nodes, data->results[left_slot], data->results[right_slot]),
            static_cast<uint64_t>((1ULL << (depth + 2)) - 1ULL)};
}

static Options parse_args(int argc, char** argv) {
    Options opt;

    for (int i = 1; i < argc; ++i) {
        std::string s = argv[i];

        auto value = [&](const std::string& name) -> std::string {
            if (i + 1 >= argc) throw std::runtime_error("missing value for " + name);
            return argv[++i];
        };

        if (s == "--size" || s == "-n") {
            opt.size = std::stoull(value(s));
        } else if (s == "--depth" || s == "-d") {
            opt.max_depth = std::stoi(value(s));
        } else if (s == "--min-size" || s == "-m") {
            opt.min_size = std::stoull(value(s));
        } else if (s == "--seed") {
            opt.seed = static_cast<unsigned>(std::stoul(value(s)));
        } else if (s == "--print") {
            opt.print = true;
        } else if (s == "--help" || s == "-h") {
            std::cout << "Usage: ./shm_sort [--size N] [--depth D] [--min-size M] "
                      << "[--seed S] [--print]\n";
            std::exit(0);
        } else {
            throw std::runtime_error("unknown argument: " + s);
        }
    }

    if (opt.max_depth < 0) throw std::runtime_error("depth must be non-negative");
    if (opt.size == 0) throw std::runtime_error("size must be positive");
    return opt;
}

static bool is_sorted_list(const Node* nodes, int head, size_t expected) {
    size_t seen = 0;
    int32_t prev = 0;
    bool first = true;

    while (head != NIL) {
        if (!first && prev > nodes[head].value) return false;
        first = false;
        prev = nodes[head].value;
        head = nodes[head].next;
        ++seen;
    }

    return seen == expected;
}

int main(int argc, char** argv) {
    int shmid = -1;
    SharedData* data = reinterpret_cast<SharedData*>(-1);

    try {
        Options opt = parse_args(argc, argv);
        const size_t shm_size = sizeof(SharedData) + sizeof(Node) * (opt.size - 1);

        shmid = shmget(IPC_PRIVATE, shm_size, IPC_CREAT | 0600);
        if (shmid < 0) throw_errno("shmget");

        data = static_cast<SharedData*>(shmat(shmid, nullptr, 0));
        if (data == reinterpret_cast<SharedData*>(-1)) throw_errno("shmat");

        std::fill(data->results, data->results + MAX_RESULT_SLOTS, NIL);

        std::mt19937 rng(opt.seed);
        std::uniform_int_distribution<int32_t> dist(-100000000, 100000000);

        for (size_t i = 0; i < opt.size; ++i) {
            data->nodes[i].value = dist(rng);
            data->nodes[i].next = (i + 1 == opt.size) ? NIL : static_cast<int>(i + 1);
        }

        const auto t1 = std::chrono::steady_clock::now();
        SortResult result = parallel_sort(data, 0, 0, opt, 0);
        const auto t2 = std::chrono::steady_clock::now();

        const double elapsed = std::chrono::duration<double>(t2 - t1).count();
        const bool ok = is_sorted_list(data->nodes, result.head, opt.size);

        if (opt.print) {
            for (int cur = result.head; cur != NIL; cur = data->nodes[cur].next) {
                std::cout << data->nodes[cur].value << ' ';
            }
            std::cout << '\n';
        }

        std::cerr << "STAT: program=shm size=" << opt.size
                  << " depth=" << opt.max_depth
                  << " min_size=" << opt.min_size
                  << " valid=" << (ok ? 1 : 0)
                  << " time=" << elapsed << " sec\n";

        shmdt(data);
        shmctl(shmid, IPC_RMID, nullptr);
        return ok ? 0 : 3;
    } catch (const std::exception& e) {
        if (data != reinterpret_cast<SharedData*>(-1)) shmdt(data);
        if (shmid >= 0) shmctl(shmid, IPC_RMID, nullptr);
        std::cerr << "ERROR: " << e.what() << "\n";
        return 1;
    }
}