#include "vpngate/node_store.h"

#include <sys/stat.h>

#include <filesystem>
#include <fstream>
#include <sstream>

#include "common/error.h"
#include "common/http_get.h"
#include "common/logging.h"
#include "common/metrics.h"

namespace ovg::vpngate {
namespace {

constexpr const char *kMod = "vpngate";

auto *m_refresh_ok = metrics::counter("ovg_vpngate_refresh_success_total",
                                      "Successful node list refreshes");
auto *m_refresh_fail = metrics::counter("ovg_vpngate_refresh_failure_total",
                                        "Failed node list refreshes");
auto *m_nodes = metrics::gauge("ovg_vpngate_nodes",
                               "Nodes in the current snapshot");

}  // namespace

NodeStore::NodeStore(asio::io_context &io, VpnGateConfig cfg)
    : io_(io), cfg_(std::move(cfg)), timer_(io) {
  nodes_ = std::make_shared<const NodeList>();
}

NodeStore::~NodeStore() { stop(); }

void NodeStore::start() {
  // A stale cache is far better than no nodes: it lets us bring a tunnel up
  // while the network fetch is still in flight (or if VPNGate is unreachable).
  std::string body;
  if (load_cache(&body)) {
    ParseResult pr;
    std::string err;
    if (parse_node_list(body, &pr, &err)) {
      auto list = std::make_shared<const NodeList>(std::move(pr.nodes));
      {
        std::lock_guard lk(mu_);
        nodes_ = list;
      }
      m_nodes->set(static_cast<int64_t>(list->size()));
      LOG_INFO(kMod, "loaded {} nodes from cache {}", list->size(),
               cfg_.cache_path);
    } else {
      LOG_WARN(kMod, "cache at {} is unusable: {}", cfg_.cache_path, err);
    }
  }

  refresh_now(nullptr);
  schedule_next();
}

void NodeStore::stop() {
  stopped_.store(true);
  std::error_code ignored;
  timer_.cancel(ignored);
}

void NodeStore::schedule_next() {
  if (stopped_.load()) return;
  timer_.expires_after(cfg_.refresh_interval);
  timer_.async_wait([this](std::error_code ec) {
    if (ec || stopped_.load()) return;
    refresh_now(nullptr);
    schedule_next();
  });
}

void NodeStore::refresh_now(RefreshHandler handler) {
  {
    std::lock_guard lk(mu_);
    if (fetch_in_flight_) {
      // Coalesce: a second caller waits on the fetch already running instead of
      // hammering the API.
      if (handler) waiters_.push_back(std::move(handler));
      return;
    }
    fetch_in_flight_ = true;
    if (handler) waiters_.push_back(std::move(handler));
  }
  try_urls(0, nullptr);
}

void NodeStore::try_urls(size_t index, RefreshHandler handler) {
  if (stopped_.load()) {
    complete(make_error_code(Error::Cancelled), 0);
    return;
  }
  if (index >= cfg_.api_urls.size()) {
    LOG_WARN(kMod, "all {} API endpoints failed", cfg_.api_urls.size());
    m_refresh_fail->inc();
    complete(make_error_code(Error::UpstreamFailure), 0);
    return;
  }

  http::Options opts;
  opts.timeout = cfg_.http_timeout;
  opts.max_bytes = cfg_.max_response_bytes;

  const std::string &url = cfg_.api_urls[index];
  http::async_get(io_, url, opts,
                  [this, index, url](std::error_code ec, http::Response resp) {
                    if (ec) {
                      LOG_WARN(kMod, "fetch {} failed: {}", url, ec.message());
                      try_urls(index + 1, nullptr);
                      return;
                    }
                    if (resp.status != 200) {
                      LOG_WARN(kMod, "fetch {} returned HTTP {}", url,
                               resp.status);
                      try_urls(index + 1, nullptr);
                      return;
                    }
                    LOG_DEBUG(kMod, "fetched {} bytes from {}",
                              resp.body.size(), url);
                    on_body(std::move(resp.body), /*from_cache=*/false, nullptr);
                  });
}

void NodeStore::on_body(std::string body, bool from_cache,
                        RefreshHandler /*handler*/) {
  ParseResult pr;
  std::string err;
  if (!parse_node_list(body, &pr, &err)) {
    LOG_WARN(kMod, "parse failed: {}", err);
    m_refresh_fail->inc();
    complete(make_error_code(Error::ProtocolError), 0);
    return;
  }

  const size_t count = pr.nodes.size();
  auto list = std::make_shared<const NodeList>(std::move(pr.nodes));
  {
    std::lock_guard lk(mu_);
    nodes_ = list;
    last_success_ = std::chrono::system_clock::now();
  }
  m_nodes->set(static_cast<int64_t>(count));
  m_refresh_ok->inc();

  if (!from_cache) save_cache(body);

  LOG_INFO(kMod, "node list refreshed: {} usable nodes", count);
  complete({}, count);
}

void NodeStore::complete(std::error_code ec, size_t count) {
  std::vector<RefreshHandler> waiters;
  {
    std::lock_guard lk(mu_);
    fetch_in_flight_ = false;
    waiters.swap(waiters_);
  }
  for (auto &w : waiters) {
    if (w) asio::post(io_, [w = std::move(w), ec, count] { w(ec, count); });
  }
}

bool NodeStore::load_cache(std::string *body) {
  if (cfg_.cache_path.empty()) return false;

  std::error_code ec;
  const auto path = std::filesystem::path(cfg_.cache_path);
  if (!std::filesystem::exists(path, ec)) return false;

  struct stat st{};
  if (::stat(cfg_.cache_path.c_str(), &st) != 0) return false;

  const auto age = std::chrono::system_clock::now() -
                   std::chrono::system_clock::from_time_t(st.st_mtime);
  if (age > cfg_.cache_max_age) {
    LOG_INFO(kMod, "cache {} is too old ({}h), ignoring", cfg_.cache_path,
             std::chrono::duration_cast<std::chrono::hours>(age).count());
    return false;
  }

  std::ifstream in(cfg_.cache_path, std::ios::binary);
  if (!in) return false;
  std::ostringstream ss;
  ss << in.rdbuf();
  *body = ss.str();
  return !body->empty();
}

void NodeStore::save_cache(const std::string &body) {
  if (cfg_.cache_path.empty()) return;

  std::error_code ec;
  const auto path = std::filesystem::path(cfg_.cache_path);
  if (path.has_parent_path())
    std::filesystem::create_directories(path.parent_path(), ec);

  // Write-then-rename so a crash mid-write cannot leave a truncated cache that
  // we would happily parse on the next start.
  const std::string tmp = cfg_.cache_path + ".tmp";
  {
    std::ofstream out(tmp, std::ios::binary | std::ios::trunc);
    if (!out) {
      LOG_WARN(kMod, "cannot write cache {}", tmp);
      return;
    }
    out.write(body.data(), static_cast<std::streamsize>(body.size()));
    if (!out) {
      LOG_WARN(kMod, "short write to cache {}", tmp);
      return;
    }
  }
  std::filesystem::rename(tmp, path, ec);
  if (ec) LOG_WARN(kMod, "cannot rename cache into place: {}", ec.message());
}

NodeListPtr NodeStore::snapshot() const {
  std::lock_guard lk(mu_);
  return nodes_;
}

bool NodeStore::has_nodes() const {
  std::lock_guard lk(mu_);
  return nodes_ && !nodes_->empty();
}

std::chrono::system_clock::time_point NodeStore::last_success() const {
  std::lock_guard lk(mu_);
  return last_success_;
}

}  // namespace ovg::vpngate