defmodule Mv.Mailer do
  @moduledoc """
  Swoosh mailer for transactional emails.

  Use `mail_from/0` for the configured sender address (join confirmation,
  user confirmation, password reset).

  ## Sender identity

  The "from" address is determined by priority:
  1. `MAIL_FROM_EMAIL` / `MAIL_FROM_NAME` environment variables
  2. Settings database (`smtp_from_email`, `smtp_from_name`)
  3. Hardcoded default (`"Mila"`, `"noreply@example.com"`)

  **Important:** On most SMTP servers the sender email must be owned by the
  authenticated SMTP user. Set `smtp_from_email` to the same address as
  `smtp_username` (or an alias allowed by the server).

  ## SMTP adapter configuration

  The SMTP adapter can be configured via:
  - **Environment variables** at boot (`SMTP_HOST`, `SMTP_PORT`, `SMTP_USERNAME`,
    `SMTP_PASSWORD`, `SMTP_PASSWORD_FILE`, `SMTP_SSL`) — configured in `runtime.exs`.
  - **Admin Settings** (database) — read at send time via `Mv.Config.smtp_*()` helpers.
    Settings-based config is passed per-send via `smtp_config/0`.

  ENV takes priority over Settings (same pattern as OIDC and Vereinfacht).
  """
  use Swoosh.Mailer, otp_app: :mv

  import Swoosh.Email
  use Gettext, backend: MvWeb.Gettext, otp_app: :mv

  require Logger

  @email_regex ~r/^[^\s@]+@[^\s@]+\.[^\s@]+$/

  @doc """
  Returns the configured "from" address for transactional emails as `{name, email}`.

  Priority: ENV `MAIL_FROM_NAME`/`MAIL_FROM_EMAIL` > Settings `smtp_from_name`/`smtp_from_email` > defaults.
  """
  @spec mail_from() :: {String.t(), String.t()}
  def mail_from do
    {Mv.Config.mail_from_name(), Mv.Config.mail_from_email() || "noreply@example.com"}
  end

  @doc """
  Sends a test email to the given address. Used from Global Settings SMTP section.

  Returns `{:ok, email}` on success, `{:error, reason}` on failure.
  The `reason` is a classified atom for known error categories, or `{:smtp_error, message}`
  for SMTP-level errors with a human-readable message, or the raw term for unknown errors.
  """
  @spec send_test_email(String.t()) ::
          {:ok, Swoosh.Email.t()} | {:error, atom() | {:smtp_error, String.t()} | term()}
  def send_test_email(to_email) when is_binary(to_email) do
    if valid_email?(to_email) do
      subject = gettext("Mila – Test email")

      body =
        gettext(
          "This is a test email sent from Mila. If you received this, your SMTP configuration is working correctly."
        )

      email =
        new()
        |> from(mail_from())
        |> to(to_email)
        |> subject(subject)
        |> text_body(body)
        |> html_body("<p>#{body}</p>")

      case deliver(email, smtp_config()) do
        {:ok, _} = ok ->
          ok

        {:error, reason} ->
          classified = classify_smtp_error(reason)
          Logger.warning("SMTP test email failed: #{inspect(reason)}")
          {:error, classified}
      end
    else
      {:error, :invalid_email_address}
    end
  end

  def send_test_email(_), do: {:error, :invalid_email_address}

  @doc """
  Builds the per-send SMTP config from `Mv.Config` when SMTP is configured via
  Settings only (not boot-time ENV). Returns an empty list when the mailer is
  already configured at boot (ENV-based), so Swoosh uses the Application config.

  The return value must be a flat keyword list (adapter, relay, port, ...).
  Swoosh merges it with Application config; top-level keys override the mailer's
  default adapter (e.g. Local in dev), so this delivery uses SMTP.
  """
  @spec smtp_config() :: keyword()
  def smtp_config do
    if Mv.Config.smtp_configured?() and not boot_smtp_configured?() do
      host = Mv.Config.smtp_host()
      port = Mv.Config.smtp_port() || 587
      username = Mv.Config.smtp_username()
      password = Mv.Config.smtp_password()
      ssl_mode = Mv.Config.smtp_ssl() || "tls"

      [
        adapter: Swoosh.Adapters.SMTP,
        relay: host,
        port: port,
        ssl: ssl_mode == "ssl",
        tls: if(ssl_mode == "tls", do: :always, else: :never),
        auth: :always,
        username: username,
        password: password,
        # OTP 26+ enforces verify_peer; allow self-signed / internal certs.
        # tls_options: STARTTLS upgrade (port 587); sockopts: direct SSL connect (port 465).
        tls_options: [verify: :verify_none],
        sockopts: [verify: :verify_none]
      ]
      |> Enum.reject(fn {_k, v} -> is_nil(v) end)
    else
      []
    end
  end

  # ---------------------------------------------------------------------------
  # SMTP error classification
  # Maps raw gen_smtp error terms to human-readable atoms / structs.
  # ---------------------------------------------------------------------------

  @doc false
  @spec classify_smtp_error(term()) ::
          :sender_rejected
          | :auth_failed
          | :recipient_rejected
          | :tls_failed
          | :connection_failed
          | {:smtp_error, String.t()}
          | term()
  def classify_smtp_error({:retries_exceeded, {:temporary_failure, _host, :tls_failed}}),
    do: :tls_failed

  def classify_smtp_error({:retries_exceeded, {:network_failure, _host, _}}),
    do: :connection_failed

  def classify_smtp_error({:send, {:permanent_failure, _host, msg}}) do
    str = if is_list(msg), do: List.to_string(msg), else: to_string(msg)
    classify_permanent_failure_message(str)
  end

  def classify_smtp_error(reason), do: reason

  # ---------------------------------------------------------------------------
  # Private helpers
  # ---------------------------------------------------------------------------

  defp classify_permanent_failure_message(str) do
    cond do
      smtp_auth_failure?(str) -> :auth_failed
      smtp_sender_rejected?(str) -> :sender_rejected
      smtp_recipient_rejected?(str) -> :recipient_rejected
      true -> {:smtp_error, String.trim(str)}
    end
  end

  defp smtp_auth_failure?(str),
    do:
      String.contains?(str, "535") or String.contains?(str, "authentication") or
        String.contains?(str, "Authentication")

  defp smtp_sender_rejected?(str),
    do:
      String.contains?(str, "553") or String.contains?(str, "Sender address rejected") or
        String.contains?(str, "not owned")

  defp smtp_recipient_rejected?(str),
    do:
      String.contains?(str, "550") or String.contains?(str, "No such user") or
        String.contains?(str, "no such user") or String.contains?(str, "User unknown")

  # Returns true when the SMTP adapter has been configured at boot time via ENV
  # (i.e. the Application config is already set to the SMTP adapter).
  defp boot_smtp_configured? do
    case Application.get_env(:mv, __MODULE__) do
      config when is_list(config) -> Keyword.get(config, :adapter) == Swoosh.Adapters.SMTP
      _ -> false
    end
  end

  defp valid_email?(email) when is_binary(email) do
    Regex.match?(@email_regex, String.trim(email))
  end

  defp valid_email?(_), do: false
end