defmodule DaProductApp.GlobalSettings do
  @moduledoc """
  Caches global settings from `global_table` in ShukriaMms database.

  - Loads settings at startup
  - Periodically refreshes (default 10 minutes)
  - Exposes `get_all/0`, `get/1` and `refresh/0` API

  Settings are stored in a public ETS table (`read_concurrency: true`) so that
  reader processes (`get/1`, `get_all/0`) fetch values concurrently and
  lock-free, without serializing through the GenServer. The GenServer is the
  sole writer: it loads/refreshes the data and writes it into ETS.

  The cache is a flat `%{string_key => value}` map so `get_all/0` stays
  backward compatible with existing consumers (e.g.
  `TransactionRules.merge_settings_with_global/2`) that expect scalar values.

  `global_table` rows are `{id, config_key, config_value, group, created_at,
  updated_at}` - there is no separate "extended value" column; any list-style
  config (e.g. `POS_ENTRY_BLOCKED`) stores its comma-separated value directly
  in `config_value`.
  """

  use GenServer
  require Logger
  alias DaProductApp.Repos.ShukriaMmsRepo
  import Ecto.Query

  @name __MODULE__
  @table __MODULE__
  # Single key under which the full settings map is stored in ETS.
  @settings_key :settings
  @refresh_ms Application.get_env(:da_product_app, :global_settings_refresh_ms, :timer.minutes(10))

  # Public API
  def start_link(_opts) do
    GenServer.start_link(__MODULE__, %{}, name: @name)
  end

  def get_all do
    case :ets.lookup(@table, @settings_key) do
      [{@settings_key, map}] -> map
      _ -> %{}
    end
  rescue
    ArgumentError ->
      # Table not yet created (e.g. process not started) - fail safe to empty.
      %{}
  end

  def get(key) do
    Map.get(get_all(), to_string(key))
  end

  def refresh do
    GenServer.cast(@name, :refresh)
  end

  # Callbacks
  @impl true
  def init(_args) do
    :ets.new(@table, [:named_table, :set, :public, read_concurrency: true])
    store(load_once(%{}))

    if is_integer(@refresh_ms) and @refresh_ms > 0 do
      :timer.send_interval(@refresh_ms, :refresh)
    end

    {:ok, %{}}
  end

  @impl true
  def handle_cast(:refresh, state) do
    store(load_once(get_all()))
    {:noreply, state}
  end

  @impl true
  def handle_info(:refresh, state) do
    store(load_once(get_all()))
    {:noreply, state}
  end

  defp store(map) do
    :ets.insert(@table, {@settings_key, map})
    map
  end

  defp load_once(current_state) do
    query = from(g in "global_table", select: {g.config_key, g.config_value})

    case ShukriaMmsRepo.all(query) do
      rows when is_list(rows) ->
        Enum.reduce(rows, %{}, fn {k, v}, acc ->
          Map.put(acc, to_string(k), v)
        end)

      _ ->
        Logger.error("GlobalSettings: unexpected response loading settings, keeping last-known state")
        current_state
    end
  rescue
    e ->
      Logger.error("GlobalSettings load error: #{inspect(e)} - keeping last-known state")
      current_state
  end
end