defmodule Vela.Cache.Config do
  @moduledoc """
  Configuration struct for a Vela cache instance.
  """

  @type eviction_policy :: :lru | :lfu | :fifo | :ttl_only
  @type topology :: module()
  @type backend :: module()

  @type t :: %__MODULE__{
          name: atom(),
          backend: backend(),
          backend_opts: keyword(),
          topology: topology(),
          topology_opts: keyword(),
          near_cache: boolean(),
          near_cache_l1_ttl: non_neg_integer(),
          default_ttl: non_neg_integer() | :infinity,
          max_size: non_neg_integer() | :infinity,
          eviction_policy: eviction_policy(),
          stampede_protection: boolean(),
          stampede_timeout: non_neg_integer(),
          stats_enabled: boolean(),
          telemetry_prefix: [atom()],
          sweep_interval: non_neg_integer()
        }

  defstruct [
    :name,
    backend: Vela.Backend.ETS,
    backend_opts: [],
    topology: Vela.Topology.Local,
    topology_opts: [],
    near_cache: false,
    near_cache_l1_ttl: 60_000,
    default_ttl: :infinity,
    max_size: :infinity,
    eviction_policy: :ttl_only,
    stampede_protection: true,
    stampede_timeout: 5_000,
    stats_enabled: true,
    telemetry_prefix: [:vela, :cache],
    sweep_interval: 30_000
  ]

  @doc """
  Builds a Config from a keyword list, applying library defaults.
  """
  @spec new(atom(), keyword()) :: t()
  def new(name, opts) do
    # Read library-wide defaults from application config
    # e.g. config :vela, default_sweep_interval: 60_000
    app_defaults = Application.get_all_env(:vela)

    fields =
      struct_fields()
      |> Enum.map(fn {key, _default} ->
        value =
          Keyword.get(opts, key) ||
            Keyword.get(app_defaults, key) ||
            Map.get(%__MODULE__{}, key)

        {key, value}
      end)
      |> Keyword.put(:name, name)

    struct(__MODULE__, fields)
  end

  # Returns all struct field names with their defaults
  defp struct_fields do
    %__MODULE__{}
    |> Map.from_struct()
    |> Map.to_list()
  end
end