defmodule Mv.Smtp.ConfigBuilder do
  @moduledoc """
  Builds Swoosh/gen_smtp SMTP adapter options from connection parameters.

  Single source of truth for TLS/sockopts logic (port 587 vs 465):
  - Port 587 (STARTTLS): `gen_tcp` is used first; `sockopts` must NOT contain `:verify`.
  - Port 465 (implicit SSL): initial connection is `ssl:connect`; `sockopts` must contain `:verify`.

  Used by `config/runtime.exs` (boot-time ENV) and `Mv.Mailer.smtp_config/0` (Settings-only).
  """

  @doc """
  Builds the keyword list of Swoosh SMTP adapter options.

  Options (keyword list):
  - `:host` (required) — relay hostname
  - `:port` (required) — port number (e.g. 587 or 465)
  - `:ssl_mode` (required) — `"tls"` or `"ssl"`
  - `:verify_mode` (required) — `:verify_peer` or `:verify_none`
  - `:username` (optional)
  - `:password` (optional)

  Nil values are stripped from the result.
  """
  @spec build_opts(keyword()) :: keyword()
  def build_opts(opts) do
    host = Keyword.fetch!(opts, :host)
    port = Keyword.fetch!(opts, :port)
    username = Keyword.get(opts, :username)
    password = Keyword.get(opts, :password)
    ssl_mode = Keyword.fetch!(opts, :ssl_mode)
    verify_mode = Keyword.fetch!(opts, :verify_mode)

    base_opts = [
      adapter: Swoosh.Adapters.SMTP,
      relay: host,
      port: port,
      username: username,
      password: password,
      ssl: ssl_mode == "ssl",
      tls: if(ssl_mode == "tls", do: :always, else: :never),
      auth: :always,
      # tls_options: used for STARTTLS (587). For 465, gen_smtp uses sockopts for initial ssl:connect.
      tls_options: [verify: verify_mode]
    ]

    # Port 465: initial connection is ssl:connect; pass verify in sockopts.
    # Port 587: initial connection is gen_tcp; sockopts must NOT contain verify (gen_tcp rejects it).
    opts =
      if ssl_mode == "ssl" do
        Keyword.put(base_opts, :sockopts, verify: verify_mode)
      else
        base_opts
      end

    Enum.reject(opts, fn {_k, v} -> is_nil(v) end)
  end
end