#ifndef _DATABASE_HPP
#define _DATABASE_HPP

#include <boost/json.hpp>

#include <cstddef>
#include <string>
#include <vector>
#include <queue>
#include <optional>
#include <mutex>
#include <memory>
#include <initializer_list>

#include <pqxx/pqxx>

namespace bserv {

using raw_db_connection_type = pqxx::connection;
using raw_db_transaction_type = pqxx::work;

class db_field {
private:
    pqxx::field field_;
public:
    db_field(const pqxx::field& field) : field_{field} {}
    const char* c_str() const { return field_.c_str(); }
    template <typename Type>
    Type as() const { return field_.as<Type>(); }
};

class db_row {
private:
    pqxx::row row_;
public:
    db_row(const pqxx::row& row) : row_{row} {}
    std::size_t size() const { return row_.size(); }
    db_field operator[](std::size_t idx) const { return row_[idx]; }
};

class db_result {
private:
    pqxx::result result_;
public:
    class const_iterator {
    private:
        pqxx::result::const_iterator iterator_;
    public:
        const_iterator(
            const pqxx::result::const_iterator& iterator
        ) : iterator_{iterator} {}
        const_iterator& operator++() { ++iterator_; return *this; }
        bool operator==(const const_iterator& rhs) const { return iterator_ == rhs.iterator_; }
        bool operator!=(const const_iterator& rhs) const { return iterator_ != rhs.iterator_; }
        db_row operator*() const { return *iterator_; }
    };
    db_result() = default;
    db_result(const pqxx::result& result) : result_{result} {}
    const_iterator begin() const { return result_.begin(); }
    const_iterator end() const { return result_.end(); }
    std::string query() const { return result_.query(); }
};

class db_connection_manager;

class db_connection {
private:
    db_connection_manager& mgr_;
    std::shared_ptr<raw_db_connection_type> conn_;
public:
    db_connection(
        db_connection_manager& mgr,
        std::shared_ptr<raw_db_connection_type> conn)
    : mgr_{mgr}, conn_{conn} {}
    // non-copiable, non-assignable
    db_connection(const db_connection&) = delete;
    db_connection& operator=(const db_connection&) = delete;
    // during the destruction, it should put itself back to the 
    // manager's queue
    ~db_connection();
    raw_db_connection_type& get() { return *conn_; }
};

// provides the database connection pool functionality
class db_connection_manager {
private:
    std::queue<std::shared_ptr<raw_db_connection_type>> queue_;
    // this lock is for manipulating the `queue_`
    mutable std::mutex queue_lock_;
    // since C++ 17 doesn't provide the semaphore functionality,
    // mutex is used to mimic it. (boost provides it)
    // if there are no available connections, trying to lock on
    // it will cause blocking.
    mutable std::mutex counter_lock_;
    friend db_connection;
public:
    db_connection_manager(const std::string& conn_str, int n) {
        for (int i = 0; i < n; ++i)
            queue_.emplace(
                std::make_shared<raw_db_connection_type>(conn_str));
    }
    // if there are no available database connections, this function
    // blocks until there is any;
    // otherwise, this function returns a pointer to `db_connection`.
    std::shared_ptr<db_connection> get_or_block() {
        // `counter_lock_` must be acquired first.
        // exchanging this statement with the next will cause dead-lock,
        // because if the request is blocked by `counter_lock_`,
        // the destructor of `db_connection` will not be able to put
        // itself back due to the `queue_lock_` has already been acquired
        // by this request!
        counter_lock_.lock();
        // `queue_lock_` is acquired so that only one thread will
        // modify the `queue_`
        std::lock_guard<std::mutex> lg{queue_lock_};
        std::shared_ptr<raw_db_connection_type> conn = queue_.front();
        queue_.pop();
        // if there are no connections in the `queue_`,
        // `counter_lock_` remains to be locked
        // so that the following requests will be blocked
        if (queue_.size() != 0) counter_lock_.unlock();
        return std::make_shared<db_connection>(*this, conn);
    }
};

inline db_connection::~db_connection() {
    std::lock_guard<std::mutex> lg{mgr_.queue_lock_};
    mgr_.queue_.emplace(conn_);
    // if this is the first available connection back to the queue,
    // `counter_lock_` is unlocked so that the blocked requests will
    // be notified
    if (mgr_.queue_.size() == 1)
        mgr_.counter_lock_.unlock();
}

// **************************************************************************

class db_parameter {
public:
    virtual ~db_parameter() = default;
    virtual std::string get_value(raw_db_transaction_type&) = 0;
};

class db_name : public db_parameter {
private:
    std::string value_;
public:
    db_name(const std::string& value)
    : value_{value} {}
    std::string get_value(raw_db_transaction_type& tx) {
        return tx.quote_name(value_);
    }
};

template <typename Type>
class db_value : public db_parameter {
private:
    Type value_;
public:
    db_value(const Type& value)
    : value_{value} {}
    std::string get_value(raw_db_transaction_type&) {
        return std::to_string(value_);
    }
};

template <>
class db_value<std::string> : public db_parameter {
private:
    std::string value_;
public:
    db_value(const std::string& value)
    : value_{value} {}
    std::string get_value(raw_db_transaction_type& tx) {
        return tx.quote(value_);
    }
};

template <>
class db_value<bool> : public db_parameter {
private:
    bool value_;
public:
    db_value(const bool& value)
    : value_{value} {}
    std::string get_value(raw_db_transaction_type&) {
        return value_ ? "true" : "false";
    }
};

namespace db_internal {

template <typename Param>
std::shared_ptr<db_parameter> convert_parameter(
    const Param& param) {
    return std::make_shared<db_value<Param>>(param);
}

template <typename Param>
std::shared_ptr<db_parameter> convert_parameter(
    const db_value<Param>& param) {
    return std::make_shared<db_value<Param>>(param);
}

inline std::shared_ptr<db_parameter> convert_parameter(
    const char* param) {
    return std::make_shared<db_value<std::string>>(param);
}

inline std::shared_ptr<db_parameter> convert_parameter(
    const db_name& param) {
    return std::make_shared<db_name>(param);
}

template <typename ...Params>
std::vector<std::string> convert_parameters(
    raw_db_transaction_type& tx, std::shared_ptr<Params>... params) {
    return {params->get_value(tx)...};
}

// *************************************

class db_field_holder {
protected:
    std::string name_;
public:
    db_field_holder(const std::string& name)
    : name_{name} {}
    virtual ~db_field_holder() = default;
    virtual void add(
        const db_row& row, std::size_t field_idx,
        boost::json::object& obj) = 0;
};

template <typename Type>
class db_field : public db_field_holder {
public:
    using db_field_holder::db_field_holder;
    void add(
        const db_row& row, std::size_t field_idx,
        boost::json::object& obj) {
        obj[name_] = row[field_idx].as<Type>();
    }
};

template <>
class db_field<std::string> : public db_field_holder {
public:
    using db_field_holder::db_field_holder;
    void add(
        const db_row& row, std::size_t field_idx,
        boost::json::object& obj) {
        obj[name_] = row[field_idx].c_str();
    }
};

}  // db_internal

template <typename Type>
std::shared_ptr<db_internal::db_field_holder> make_db_field(
    const std::string& name) {
    return std::make_shared<db_internal::db_field<Type>>(name);
}

class invalid_operation_exception : public std::exception {
private:
    std::string msg_;
public:
    invalid_operation_exception(const std::string& msg)
    : msg_{msg} {}
    const char* what() const noexcept { return msg_.c_str(); }
};

class db_relation_to_object {
private:
    std::vector<std::shared_ptr<db_internal::db_field_holder>> fields_;
public:
    db_relation_to_object(
        const std::initializer_list<
            std::shared_ptr<db_internal::db_field_holder>>& fields)
    : fields_{fields} {}
    boost::json::object convert_row(const db_row& row) {
        boost::json::object obj;
        for (std::size_t i = 0; i < fields_.size(); ++i)
            fields_[i]->add(row, i, obj);
        return obj;
    }
    std::vector<boost::json::object> convert_to_vector(
        const db_result& result) {
        std::vector<boost::json::object> results;
        for (const auto& row : result)
            results.emplace_back(convert_row(row));
        return results;
    }
    std::optional<boost::json::object> convert_to_optional(
        const db_result& result) {
        // result.size() == 0
        if (result.begin() == result.end()) return std::nullopt;
        auto iterator = result.begin();
        auto first = iterator;
        // result.size() == 1
        if (++iterator == result.end())
            return convert_row(*first);
        // result.size() > 1
        throw invalid_operation_exception{
            "too many objects to convert"};
    }
};

class db_transaction {
private:
    raw_db_transaction_type tx_;
public:
    db_transaction(
        std::shared_ptr<db_connection> connection_ptr
    ) : tx_{connection_ptr->get()} {}
    // non-copiable, non-assignable
    db_transaction(const db_transaction&) = delete;
    db_transaction& operator=(const db_transaction&) = delete;
    // Usage:
    // exec("select * from ? where ? = ? and first_name = 'Name??'",
    //      db_name("auth_user"), db_name("is_active"), db_value<bool>(true));
    // -> SQL: select * from "auth_user" where "is_active" = true and first_name = 'Name?'
    // ======================================================================================
    // exec("select * from ? where ? = ? and first_name = ?",
    //      db_name("auth_user"), db_name("is_active"), false, "Name??");
    // -> SQL: select * from "auth_user" where "is_active" = false and first_name = 'Name??'
    // ======================================================================================
    // Note: "?" is the placeholder for parameters, and "??" will be converted to "?" in SQL.
    //       But, "??" in the parameters remains.
    template <typename ...Params>
    db_result exec(const std::string& s, const Params&... params) {
        std::vector<std::string> param_vec =
            db_internal::convert_parameters(
                tx_, db_internal::convert_parameter(params)...);
        std::size_t idx = 0;
        std::string query;
        for (std::size_t i = 0; i < s.length(); ++i) {
            if (s[i] == '?') {
                if (i + 1 < s.length() && s[i + 1] == '?') {
                    query += s[++i];
                } else {
                    if (idx < param_vec.size()) {
                        query += param_vec[idx++];
                    } else throw std::out_of_range{"too few parameters"};
                }
            } else query += s[i];
        }
        if (idx != param_vec.size())
            throw invalid_operation_exception{"too many parameters"};
        return tx_.exec(query);
    }
    void commit() { tx_.commit(); }
    void abort() { tx_.abort(); }
};


// TODO: add support for time conversions between postgresql and c++, use timestamp?
//       what about time zone?

}  // bserv

#endif  // _DATABASE_HPP