clio/src/util/Taggable.hpp

#pragma once

#include "util/Assert.hpp"
#include "util/config/ConfigDefinition.hpp"

#include <boost/algorithm/string/predicate.hpp>
#include <boost/json.hpp>
#include <boost/json/conversion.hpp>
#include <boost/json/value.hpp>
#include <boost/uuid/uuid.hpp>
#include <boost/uuid/uuid_io.hpp>

#include <atomic>
#include <functional>
#include <memory>
#include <optional>
#include <ostream>
#include <string>
#include <string_view>
#include <utility>

namespace util {
namespace impl {

/**
 * @brief A `null` tag generator - does nothing.
 */
struct NullTagGenerator final {};

/**
 * @brief This strategy uses an `atomic_uint64_t` to remain lock free.
 */
struct UIntTagGenerator final {
    using TagType = std::atomic_uint64_t;

    static TagType
    next();
};

/**
 * @brief This strategy uses `boost::uuids::uuid` with a static random generator and a mutex.
 */
struct UUIDTagGenerator final {
    using TagType = boost::uuids::uuid;

    static TagType
    next();
};

}  // namespace impl

/**
 * @brief Represents any tag decorator.
 */
class BaseTagDecorator {
public:
    virtual ~BaseTagDecorator() = default;

    /**
     * @brief Decorates a std::ostream.
     *
     * @param os The stream to decorate
     */
    virtual void
    decorate(std::ostream& os) const = 0;

    /**
     * @brief Support for decorating streams (boost log, cout, etc.).
     *
     * @param os The stream
     * @param decorator The decorator
     * @return The same stream that we were given
     */
    friend std::ostream&
    operator<<(std::ostream& os, BaseTagDecorator const& decorator)
    {
        decorator.decorate(os);
        return os;
    }

    /**
     * @brief Gets the string representation of the tag.
     *
     * @return The string representation of the tag
     */
    [[nodiscard]] std::string
    toString() const
    {
        std::ostringstream oss;
        decorate(oss);
        return std::move(oss).str();
    }
};

/**
 * @brief A decorator that decorates a string (log line) with a unique tag.
 *
 * @tparam Generator The strategy used to generate the tag.
 */
template <typename Generator>
class TagDecorator final : public BaseTagDecorator {
    using ParentType = std::optional<std::reference_wrapper<BaseTagDecorator const>>;
    using TagType = typename Generator::TagType;

    ParentType parent_ = std::nullopt;
    TagType tag_ = Generator::next();

public:
    /**
     * @brief Create a new tag decorator with an optional parent.
     *
     * If the `parent` is specified it will be streamed out as a chain when this decorator will
     * decorate an ostream.
     *
     * Note that if `parent` is specified it is your responsibility that the decorator referred to
     * by `parent` outlives this decorator.
     *
     * @param parent An optional parent tag decorator
     */
    explicit TagDecorator(ParentType parent = std::nullopt) : parent_{parent}
    {
    }

    /**
     * @brief Implementation of the decoration. Chaining tags when parent is available.
     *
     * @param os The stream to output into
     */
    void
    decorate(std::ostream& os) const override
    {
        os << "[";

        if (parent_.has_value())
            parent_->get().decorate(os);

        os << tag_ << "] ";
    }
};

/**
 * @brief Specialization for a nop/null decorator.
 *
 * This generates a pass-through decorate member function which can be optimized away by the
 * compiler.
 */
template <>
class TagDecorator<impl::NullTagGenerator> final : public BaseTagDecorator {
public:
    /**
     * @brief Nop implementation for the decorator.
     *
     * @param os The stream
     */
    void
    decorate([[maybe_unused]] std::ostream& os) const override
    {
        // nop
    }
};

/**
 * @brief A factory for TagDecorator instantiation.
 */
class TagDecoratorFactory final {
    using ParentType = std::optional<std::reference_wrapper<BaseTagDecorator const>>;

    /**
     * @brief Represents the type of tag decorator.
     */
    enum class Type {
        NONE, /**< No decoration and no tag */
        UUID, /**< Tag based on `boost::uuids::uuid`, thread-safe via mutex */
        UINT  /**< atomic_uint64_t tag, thread-safe, lock-free */
    };

    Type type_;                        /*< The type of TagDecorator this factory produces */
    ParentType parent_ = std::nullopt; /*< The parent tag decorator to bind */

    static Type
    getLogTagType(std::string_view style)
    {
        if (boost::iequals(style, "int") || boost::iequals(style, "uint"))
            return TagDecoratorFactory::Type::UINT;

        if (boost::iequals(style, "null") || boost::iequals(style, "none"))
            return TagDecoratorFactory::Type::NONE;

        if (boost::iequals(style, "uuid"))
            return TagDecoratorFactory::Type::UUID;

        ASSERT(false, "log.tag_style does not have valid value");
        std::unreachable();
    }

public:
    ~TagDecoratorFactory() = default;

    /**
     * @brief Instantiates a tag decorator factory from `clio` configuration.
     *
     * @param config The configuration as a json object
     */
    explicit TagDecoratorFactory(util::config::ClioConfigDefinition const& config)
        : type_{getLogTagType(config.get<std::string>("log.tag_style"))}
    {
    }

private:
    TagDecoratorFactory(Type type, ParentType parent) noexcept : type_{type}, parent_{parent}
    {
    }

public:
    /**
     * @brief Instantiates the TagDecorator specified by `type_` with parent bound from `parent_`.
     *
     * @return An instance of the requested decorator
     */
    [[nodiscard]] std::unique_ptr<BaseTagDecorator>
    make() const;

    /**
     * @brief Creates a new tag decorator factory with a bound parent tag decorator.
     *
     * @param parent The parent tag decorator to use
     * @return A new instance of the tag decorator factory
     */
    [[nodiscard]] TagDecoratorFactory
    with(ParentType parent) const noexcept;
};

/**
 * @brief A base class that allows attaching a tag decorator to a subclass.
 */
class Taggable {
    using DecoratorType = std::unique_ptr<BaseTagDecorator>;
    DecoratorType tagDecorator_;

protected:
    /**
     * @brief New Taggable from a specified factory.
     *
     * @param tagFactory The factory to use
     */
    explicit Taggable(util::TagDecoratorFactory const& tagFactory)
        : tagDecorator_{tagFactory.make()}
    {
    }

public:
    virtual ~Taggable() = default;
    Taggable(Taggable&&) = default;

    Taggable&
    operator=(Taggable&&) = default;

    /**
     * @brief Getter for tag decorator.
     *
     * @return Reference to the tag decorator
     */
    [[nodiscard]] BaseTagDecorator const&
    tag() const
    {
        return *tagDecorator_;
    }
};

}  // namespace util