defmodule Bolt.Sips.Router do
  @moduledoc """
  This "driver" works in tandem with Neo4j's [Causal Clustering](https://neo4j.com/docs/operations-manual/current/clustering/>) feature by directing read and write behaviour to appropriate cluster members
  """
  use GenServer
  require Logger

  alias Bolt.Sips.Routing.RoutingTable
  alias Bolt.Sips.{Protocol, ConnectionSupervisor, LoadBalancer, Response, Error}

  defmodule State do
    @moduledoc """
    todo:
    this is work in progress and will be used for defining the state of the Router (Gen)Server
    """
    @type role :: atom

    @type t :: %__MODULE__{
            connections: %{
              role => %{String.t() => non_neg_integer},
              updated_at: non_neg_integer,
              ttl: non_neg_integer
            }
          }

    @enforce_keys [:connections]
    defstruct @enforce_keys
  end

  @no_routing nil
  @routing_table_keys [:read, :write, :route, :updated_at, :ttl, :error]

  def configure(opts), do: GenServer.call(__MODULE__, {:configure, opts})

  def get_connection(role, prefix \\ :direct)

  def get_connection(role, prefix),
    do: GenServer.call(__MODULE__, {:get_connection, role, prefix})

  def terminate_connections(role, prefix \\ :default)

  def terminate_connections(role, prefix),
    do: GenServer.call(__MODULE__, {:terminate_connections, role, prefix})

  def info(), do: GenServer.call(__MODULE__, :info)
  def routing_table(prefix), do: GenServer.call(__MODULE__, {:routing_table_info, prefix})

  @spec start_link(Keyword.t()) :: :ignore | {:error, Keyword.t()} | {:ok, pid()}
  def start_link(init_args) do
    GenServer.start_link(__MODULE__, init_args, name: __MODULE__)
  end

  @impl true
  @spec init(Keyword.t()) :: {:ok, State.t(), {:continue, :post_init}}
  def init(options) do
    {:ok, options, {:continue, :post_init}}
  end

  @impl true
  def handle_call({:configure, opts}, _from, state) do
    prefix = Keyword.get(opts, :prefix, :default)
    %{connections: current_connections} = Map.get(state, prefix, %{connections: %{}})

    %{user_options: user_options, connections: connections} =
      try do
        opts
        |> _configure()
        |> Map.get(prefix)
      rescue
        e in Bolt.Sips.Exception ->
          %{user_options: opts, connections: %{error: e.message}}
      end

    updated_connections = Map.merge(current_connections, connections)

    new_state =
      Map.put(state, prefix, %{user_options: user_options, connections: updated_connections})

    {:reply, new_state, new_state}
  end

  # getting connections for role in [:route, :read, :write]
  @impl true
  def handle_call({:get_connection, role, prefix}, _from, state)
      when role in [:route, :read, :write] do
    with %{connections: connections} <- Map.get(state, prefix),
         {:ok, conn, updated_connections} <- _get_connection(role, connections, prefix) do
      {:reply, {:ok, conn}, put_in(state, [prefix, :connections], updated_connections)}
    else
      e ->
        err_msg = error_no_connection_available_for_role(role, e, prefix)
        {:reply, {:error, err_msg}, state}
    end
  end

  # getting connections for any user defined roles, or: `:direct`
  @impl true
  def handle_call({:get_connection, role, prefix}, _from, state) do
    with %{connections: connections} <- Map.get(state, prefix),
         true <- Map.has_key?(connections, role),
         [url | _none] <- connections |> Map.get(role) |> Map.keys(),
         {:ok, pid} <- ConnectionSupervisor.find_connection(role, url, prefix) do
      {:reply, {:ok, pid}, state}
    else
      e ->
        err_msg = error_no_connection_available_for_role(role, e, prefix)
        {:reply, {:error, err_msg}, state}
    end
  end

  @impl true
  def handle_call({:terminate_connections, role, prefix}, _from, state) do
    %{connections: connections} = Map.get(state, prefix, %{})

    with true <- Map.has_key?(connections, role),
         :ok <-
           connections
           |> Map.get(role)
           |> Map.keys()
           |> Enum.each(&ConnectionSupervisor.terminate_connection(role, &1, prefix)) do
      new_connections = Map.delete(connections, role)
      new_state = put_in(state, [prefix, :connections], new_connections)
      {:reply, :ok, new_state}
    else
      _e ->
        {:reply, {:error, :not_found}, state}
    end
  end

  @impl true
  def handle_call(:info, _from, state), do: {:reply, state, state}

  def handle_call({:routing_table_info, prefix}, _from, state) do
    routing_table =
      with connections when not is_nil(connections) <- get_in(state, [prefix, :connections]) do
        Map.take(connections, @routing_table_keys)
      end

    {:reply, routing_table, state}
  end

  @impl true
  @spec handle_continue(:post_init, Keyword.t()) :: {:noreply, map}
  def handle_continue(:post_init, opts), do: {:noreply, _configure(opts)}

  defp _configure(opts) do
    options = Bolt.Sips.Utils.default_config(opts)

    prefix = Keyword.get(options, :prefix, :default)

    ssl_or_sock = if(Keyword.get(options, :ssl), do: :ssl, else: Keyword.get(options, :socket))

    user_options = Keyword.put(options, :socket, ssl_or_sock)
    with_routing? = Keyword.get(user_options, :schema, "bolt") =~ ~r/(^bolt\+routing$)/i

    with {:ok, routing_table} <- get_routing_table(user_options, with_routing?),
         {:ok, connections} <- start_connections(user_options, routing_table) do
      connections = Map.put(connections, :routing_query, routing_table[:routing_query])

      %{prefix => %{user_options: user_options, connections: connections}}
    else
      {:error, msg} ->
        Logger.error("cannot load the routing table. Error: #{msg}")
        %{prefix => %{user_options: user_options, connections: %{error: "Not a router"}}}
    end
  end

  defp get_routing_table(
         %{routing_query: %{params: props, query: query}} = connections,
         _,
         prefix
       ) do
    with {:ok, conn, updated_connections} <- _get_connection(:route, connections, prefix),
         {:ok, %Response{} = results} <- Bolt.Sips.query(conn, query, props) do
      {:ok, Response.first(results), updated_connections}
    else
      {:error, %Error{code: code, message: message}} ->
        err_msg = "#{code}; #{message}"
        Logger.error(err_msg)
        {:error, err_msg}

      {:error, msg, _updated_connections} ->
        Logger.error(msg)
        {:error, :routing_table_not_available}

      e ->
        Logger.error("get_routing_table error: #{inspect(e)}")
        {:error, :routing_table_not_available_at_all}
    end
  end

  defp get_routing_table(_opts, false), do: {:ok, @no_routing}

  defp get_routing_table(opts, _) do
    prefix = Keyword.get(opts, :prefix, :default)

    with {:ok, %Protocol.ConnData{configuration: configuration}} <- Protocol.connect(opts),
         # DON'T>  :ok <- Protocol.disconnect(:stop, conn),
         {_long, short} <- parse_server_version(configuration[:server_version]) do
      {query, params} =
        if Version.match?(short, ">= 3.2.3") do
          props = Keyword.get(opts, :routing_context, %{})
          {"CALL dbms.cluster.routing.getRoutingTable($context)", %{context: props}}
        else
          {"CALL dbms.cluster.routing.getServers()", %{}}
        end

      with {:ok, pid} <- DBConnection.start_link(Protocol, Keyword.delete(opts, :name)),
           {:ok, %Response{} = results} <- Bolt.Sips.query(pid, query, params),
           true <- Process.exit(pid, :normal) do
        table =
          results
          |> Response.first()
          |> Map.put(:routing_query, %{query: query, params: params})

        ttl = Map.get(table, :ttl, 300) * 1000
        # may overwrite the ttl, when desired in exceptional situations: tests, for example.
        ttl = Keyword.get(opts, :ttl, ttl)

        Process.send_after(self(), {:refresh, prefix}, ttl)

        {:ok, table}
      else
        {:error, %Error{message: message}} ->
          Logger.error(message)
          {:error, message}

        _e ->
          "Are you sure you're connected to a Neo4j cluster? The routing table, is not available."
          |> Logger.error()

          {:error, :routing_table_not_available}
      end
    end
  end

  @doc """
  start a new (DB)Connection process, supervised registered under a name following this convention:

  - "role@hostname:port", the `role`, `hostname` and the `port` are collected from the user's
   configuration: `opts`. The `role` parameter is ignored when the `routing_table` parameter represents
   a neo4j map containing the definition for a neo4j cluster! It defaults to: `:direct`, when not specified!
  """
  def start_connections(opts, routing_table)

  def start_connections(opts, routing_table) when is_nil(routing_table) do
    url = "#{opts[:hostname]}:#{opts[:port]}"
    role = Keyword.get(opts, :role, :direct)

    with {:ok, _pid} <- ConnectionSupervisor.start_child(role, url, opts) do
      {:ok, %{role => %{url => 0}}}
    end
  end

  def start_connections(opts, routing_table) do
    connections =
      with %Bolt.Sips.Routing.RoutingTable{roles: roles} = rt <- RoutingTable.parse(routing_table) do
        roles
        |> Enum.reduce(%{}, fn {role, addresses}, acc ->
          addresses
          |> Enum.reduce(acc, fn {address, count}, acc ->
            # interim hack; force the schema to be `bolt`, otherwise the parse is not happening
            url = "bolt://" <> address
            %URI{host: host, port: port} = URI.parse(url)

            # Important!
            # We remove the url from the routing-specific configs, because the port and the address where the
            # socket will be opened, is using the host and the port returned by the routing table, and not by the
            # initial url param. The Utils will overwrite them if the `url` is defined!
            config =
              opts
              |> Keyword.put(:host, String.to_charlist(host))
              |> Keyword.put(:port, port)
              |> Keyword.put(:name, role)
              |> Keyword.put(:hits, count)
              |> Keyword.delete(:url)

            with {:ok, _pid} <- ConnectionSupervisor.start_child(role, address, config) do
              Map.update(acc, role, %{address => 0}, fn urls -> Map.put(urls, address, 0) end)
            else
              _ -> acc
            end
          end)
          |> Map.merge(acc)
        end)
        |> Map.put(:ttl, rt.ttl)
        |> Map.put(:updated_at, rt.updated_at)
      end

    {:ok, connections}
  end

  @with_routing true
  @impl true
  def handle_info({:refresh, prefix}, state) do
    %{connections: connections, user_options: user_options} = Map.get(state, prefix)

    %{ttl: ttl} = connections
    # may overwrite the ttl, when desired in exceptional situations: tests, for example.
    ttl = Keyword.get(user_options, :ttl, ttl)

    state =
      with {:ok, routing_table, _updated_connections} <-
             get_routing_table(connections, @with_routing, prefix),
           {:ok, new_connections} <- start_connections(user_options, routing_table) do
        connections =
          connections
          |> Map.put(:updated_at, Bolt.Sips.Utils.now())
          |> merge_connections_maps(new_connections, prefix)

        ttl = Keyword.get(user_options, :ttl, ttl * 1000)

        Process.send_after(self(), {:refresh, prefix}, ttl)

        new_state = %{user_options: user_options, connections: connections}
        Map.put(state, prefix, new_state)
      else
        e ->
          Logger.error("Cannot create any connections. Error: #{inspect(e)}")
          Map.put(state, prefix, %{user_options: user_options, connections: %{}})
      end

    {:noreply, state}
  end

  def handle_info(req, state) do
    Logger.warning("An unusual request: #{inspect(req)}")
    {:noreply, state}
  end

  @server_version_stringex ~r/Neo4j\/(?<M>\d+)\.(?<m>\d+)\.(?<p>\d+)/
  @spec parse_server_version(map) :: {binary, <<_::16, _::_*8>>}
  @doc """
  parse the version string received from the server, while considering the lack of the
  patch number in some situations

  ## Examples

  iex> Bolt.Sips.Router.parse_server_version(%{"server" => "Neo4j/3.5.0"})
  {"Neo4j/3.5.0", "3.5.0"}

  iex> Bolt.Sips.Router.parse_server_version(%{"server" => "Neo4j/3.5"})
  {"Neo4j/3.5", "3.5.0"}

  iex> Bolt.Sips.Router.parse_server_version(%{"server" => "Neo4j/3.5.10"})
  {"Neo4j/3.5.10", "3.5.10"}

  iex> Bolt.Sips.Router.parse_server_version(%{"server" => "Neo4j/3.5.11.1"})
  {"Neo4j/3.5.11.1", "3.5.11"}

  """
  def parse_server_version(%{"server" => server_version_string}) do
    %{"M" => major, "m" => minor, "p" => patch} =
      @server_version_stringex
      |> Regex.named_captures(server_version_string <> ".0")

    {server_version_string, "#{major}.#{minor}.#{patch}"}
  end

  def parse_server_version(some_version),
    do: raise(ArgumentError, "not a Neo4J version info: " <> inspect(some_version))

  defp error_no_connection_available_for_role(role, _e, prefix \\ :default)

  defp error_no_connection_available_for_role(role, _e, prefix) do
    "no connection exists with this role: #{role} (prefix: #{prefix})"
  end

  @routing_roles ~w{read write route}a
  @spec merge_connections_maps(any(), any(), any()) :: any()
  def merge_connections_maps(current_connections, new_connections, prefix \\ :default)

  def merge_connections_maps(current_connections, new_connections, prefix) do
    @routing_roles
    |> Enum.flat_map(fn role ->
      new_urls = Map.keys(new_connections[role])

      current_connections[role]
      |> Map.keys()
      |> Enum.flat_map(fn url -> remove_old_urls(role, url, new_urls) end)
    end)
    |> close_connections(prefix)

    @routing_roles
    |> Enum.reduce(current_connections, fn role, acc ->
      Map.put(acc, role, new_connections[role])
    end)
  end

  defp remove_old_urls(role, url, urls), do: if(url in urls, do: [], else: [{role, url}])

  # [
  #   read: "localhost:7689",
  #   write: "localhost:7687",
  #   write: "localhost:7690",
  #   route: "localhost:7688",
  #   route: "localhost:7689"
  # ]
  defp close_connections(connections, prefix) do
    connections
    |> Enum.each(fn {role, url} ->
      with {:ok, _pid} = r <- ConnectionSupervisor.terminate_connection(role, url, prefix) do
        r
      else
        {:error, :not_found} ->
          Logger.debug("#{role}: #{url}; not a valid connection/process. It can't be terminated")
      end
    end)
  end

  @spec _get_connection(role :: String.t() | atom, state :: map, prefix :: atom) ::
          {:ok, pid, map} | {:error, any, map}
  defp _get_connection(role, connections, prefix) do
    with true <- Map.has_key?(connections, role),
         {:ok, url} <-
           LoadBalancer.least_reused_url(Map.get(connections, role)),
         {:ok, pid} <- ConnectionSupervisor.find_connection(role, url, prefix) do
      {_, updated_connections} =
        connections
        |> get_and_update_in([role, url], fn hits -> {hits, hits + 1} end)

      {:ok, pid, updated_connections}
    else
      e ->
        err_msg = error_no_connection_available_for_role(role, e)
        {:error, err_msg, connections}
    end
  end
end