#include <mbgl/storage/online_file_source.hpp>
#include <mbgl/storage/http_context_base.hpp>
#include <mbgl/storage/network_status.hpp>

#include <mbgl/storage/response.hpp>
#include <mbgl/platform/log.hpp>

#include <mbgl/util/thread.hpp>
#include <mbgl/util/mapbox.hpp>
#include <mbgl/util/exception.hpp>
#include <mbgl/util/chrono.hpp>
#include <mbgl/util/async_task.hpp>
#include <mbgl/util/noncopyable.hpp>
#include <mbgl/util/timer.hpp>

#include <algorithm>
#include <cassert>
#include <set>
#include <unordered_map>

namespace mbgl {

class OnlineFileRequestImpl : public util::noncopyable {
public:
    using Callback = std::function<void (Response)>;

    OnlineFileRequestImpl(const Resource&, Callback, OnlineFileSource::Impl&);
    ~OnlineFileRequestImpl();

    void networkIsReachableAgain(OnlineFileSource::Impl&);

private:
    void schedule(OnlineFileSource::Impl&, bool forceImmediate = false);

    Resource resource;
    HTTPRequestBase* request = nullptr;
    util::Timer timer;
    Callback callback;

    // Counts the number of subsequent failed requests. We're using this value for exponential
    // backoff when retrying requests.
    uint32_t failedRequests = 0;
    Response::Error::Reason failedRequestReason = Response::Error::Reason::Success;
};

class OnlineFileSource::Impl {
public:
    // Dummy parameter is a workaround for a gcc 4.9 bug.
    Impl(int) {
        NetworkStatus::Subscribe(&reachability);
    }

    ~Impl() {
        NetworkStatus::Unsubscribe(&reachability);
    }

    void request(FileRequest* req, Resource resource, Callback callback) {
        pending[req] = std::make_unique<OnlineFileRequestImpl>(resource, callback, *this);
    }

    void cancel(FileRequest* req) {
        pending.erase(req);
    }

private:
    friend OnlineFileRequestImpl;

    void networkIsReachableAgain() {
        for (auto& req : pending) {
            req.second->networkIsReachableAgain(*this);
        }
    }

    std::unordered_map<FileRequest*, std::unique_ptr<OnlineFileRequestImpl>> pending;
    const std::unique_ptr<HTTPContextBase> httpContext { HTTPContextBase::createContext() };
    util::AsyncTask reachability { std::bind(&Impl::networkIsReachableAgain, this) };
};

OnlineFileSource::OnlineFileSource()
    : thread(std::make_unique<util::Thread<Impl>>(
          util::ThreadContext{ "OnlineFileSource", util::ThreadType::Unknown, util::ThreadPriority::Low }, 0)) {
}

OnlineFileSource::~OnlineFileSource() = default;

std::unique_ptr<FileRequest> OnlineFileSource::request(const Resource& resource, Callback callback) {
    Resource res = resource;

    switch (resource.kind) {
    case Resource::Kind::Unknown:
        break;

    case Resource::Kind::Style:
        res.url = mbgl::util::mapbox::normalizeStyleURL(resource.url, accessToken);
        break;

    case Resource::Kind::Source:
        res.url = util::mapbox::normalizeSourceURL(resource.url, accessToken);
        break;

    case Resource::Kind::Glyphs:
        res.url = util::mapbox::normalizeGlyphsURL(resource.url, accessToken);
        break;

    case Resource::Kind::SpriteImage:
    case Resource::Kind::SpriteJSON:
        res.url = util::mapbox::normalizeSpriteURL(resource.url, accessToken);
        break;

    case Resource::Kind::Tile:
        res.url = util::mapbox::normalizeTileURL(resource.url, accessToken);
        break;
    }

    class OnlineFileRequest : public FileRequest {
    public:
        OnlineFileRequest(Resource resource_, FileSource::Callback callback_, util::Thread<OnlineFileSource::Impl>& thread_)
            : thread(thread_),
              workRequest(thread.invokeWithCallback(&OnlineFileSource::Impl::request, callback_, this, resource_)) {
        }

        ~OnlineFileRequest() {
            thread.invoke(&OnlineFileSource::Impl::cancel, this);
        }

        util::Thread<OnlineFileSource::Impl>& thread;
        std::unique_ptr<WorkRequest> workRequest;
    };

    return std::make_unique<OnlineFileRequest>(res, callback, *thread);
}

OnlineFileRequestImpl::OnlineFileRequestImpl(const Resource& resource_, Callback callback_, OnlineFileSource::Impl& impl)
    : resource(resource_),
      callback(callback_) {
    // Force an immediate first request if we don't have an expiration time.
    schedule(impl, !resource.priorExpires);
}

OnlineFileRequestImpl::~OnlineFileRequestImpl() {
    if (request) {
        request->cancel();
    }
}

static Duration errorRetryTimeout(Response::Error::Reason failedRequestReason, uint32_t failedRequests) {
    if (failedRequestReason == Response::Error::Reason::Server) {
        // Retry after one second three times, then start exponential backoff.
        return Seconds(failedRequests <= 3 ? 1 : 1 << std::min(failedRequests - 3, 31u));
    } else if (failedRequestReason == Response::Error::Reason::Connection) {
        // Immediate exponential backoff.
        assert(failedRequests > 0);
        return Seconds(1 << std::min(failedRequests - 1, 31u));
    } else {
        // No error, or not an error that triggers retries.
        return Duration::max();
    }
}

static Duration expirationTimeout(optional<SystemTimePoint> expires) {
    if (expires) {
        return std::max(SystemDuration::zero(), *expires - SystemClock::now());
    } else {
        return Duration::max();
    }
}

void OnlineFileRequestImpl::schedule(OnlineFileSource::Impl& impl, bool forceImmediate) {
    if (request) {
        // There's already a request in progress; don't start another one.
        return;
    }

    // If we're not being asked for a forced refresh, calculate a timeout that depends on how many
    // consecutive errors we've encountered, and on the expiration time, if present.
    Duration timeout = forceImmediate
        ? Duration::zero()
        : std::min(errorRetryTimeout(failedRequestReason, failedRequests),
                   expirationTimeout(resource.priorExpires));

    if (timeout == Duration::max()) {
        return;
    }

    timer.start(timeout, Duration::zero(), [this, &impl] {
        assert(!request);
        request = impl.httpContext->createRequest(resource, [this, &impl](Response response) {
            request = nullptr;

            // If we didn't get various caching headers in the response, continue using the
            // previous values. Otherwise, update the previous values to the new values.

            if (!response.modified) {
                response.modified = resource.priorModified;
            } else {
                resource.priorModified = response.modified;
            }

            if (!response.expires) {
                response.expires = resource.priorExpires;
            } else {
                resource.priorExpires = response.expires;
            }

            if (!response.etag) {
                response.etag = resource.priorEtag;
            } else {
                resource.priorEtag = response.etag;
            }

            if (response.error) {
                failedRequests++;
                failedRequestReason = response.error->reason;
            } else {
                failedRequests = 0;
                failedRequestReason = Response::Error::Reason::Success;
            }

            callback(response);
            schedule(impl);
        });
    });
}

void OnlineFileRequestImpl::networkIsReachableAgain(OnlineFileSource::Impl& impl) {
    // We need all requests to fail at least once before we are going to start retrying
    // them, and we only immediately restart request that failed due to connection issues.
    if (failedRequestReason == Response::Error::Reason::Connection) {
        schedule(impl, true);
    }
}

} // namespace mbgl