defmodule MobBluetooth do
  @moduledoc """
  Bluetooth Classic (BR/EDR) — device-level discovery, pairing, and
  cross-profile session management.

  Profile-specific operations live in submodules:

    * `MobBluetooth.Hfp` — Hands-Free Profile (audio + vendor AT commands).
      Use this for headsets, PTT-equipped earpieces, etc.
    * `MobBluetooth.Spp` — Serial Port Profile (RFCOMM byte streams).
      Use this for legacy serial-over-Bluetooth devices (Arduino HC-05,
      OBD-II readers, marine GPS, industrial sensors).

  (HID is not supported on Android — receiving HID input requires
  input-method/HID-host privileges Android denies ordinary apps.)

  ## API style

  Same as the rest of Mob: callbacks return `socket` unchanged, results
  arrive in `handle_info/2` as messages. There are two families, and they
  do **not** share a uniform arity:

  **Device-level events** are tagged `:bt` and carry no session id:

      {:bt, :discovery_started}                  # 2-tuple, no payload
      {:bt, :discovery_finished}
      {:bt, :discovery_cancelled}
      {:bt, :discovered, device}                 # 3-tuple, device map
      {:bt, :paired, device}
      {:bt, :pair_failed, %{address: addr, reason: atom}}
      {:bt, :unpaired, device}
      {:bt, :paired_list, [device]}
      {:bt, :error, payload}

  **Profile events** are tagged by profile (`:bt_hfp`, `:bt_spp`) — not
  `:bt`. Once a session exists they carry its integer `session_id` as the
  third element; pre-session failures omit it:

      {:bt_hfp, :connected, session_id, payload}     # 4-tuple, has session
      {:bt_hfp, :connect_failed, %{address: addr, reason: atom}}  # 3-tuple, no session yet
      {:bt_hfp, :disconnected, session_id, reason}

  The profile submodules document their own event sets.

  ## Permissions

  Bluetooth requires runtime permissions on Android 12+ (API 31+):

    * `:bluetooth_scan` — for `start_discovery/1`
    * `:bluetooth_connect` — for `pair/2`, `connect/*`, `disconnect/2`

  Request via `Mob.Permissions.request/2` before calling MobBluetooth functions.

  ## iOS

  Bluetooth Classic on iOS requires Apple's MFi (Made for iPhone)
  certification — a paid, NDA-gated program. MobBluetooth is **Android-only**.
  All functions return `{:error, :unsupported}` synchronously on iOS.
  For iOS-equivalent custom-hardware connectivity, use `Mob.Ble`.

  ## Pairing flow

  Two pairing modes, auto-selected by whether `:pin` is given:

      # System UI flow — Android shows a system pairing dialog
      socket = MobBluetooth.pair(socket, device)

      # Programmatic — PIN supplied via API, no UI
      socket = MobBluetooth.pair(socket, device, pin: "0000")

  If the programmatic PIN fails or the device requires UI confirmation
  (e.g. numeric comparison), Android falls back to the system UI
  automatically.

  ## Disconnect

  One canonical disconnect for any profile session:

      MobBluetooth.disconnect(socket, session_id)

  The framework looks up which profile owns the session_id and routes
  to the right profile-disconnect internally. Emits a profile-specific
  disconnect event (`{:bt_hfp, :disconnected, session_id, reason}` etc).
  """

  @typedoc "An opaque session identifier for an active profile connection."
  @type session_id :: pos_integer()

  @typedoc "A discovered or paired Bluetooth device."
  @type device :: %{
          required(:address) => String.t(),
          required(:name) => String.t(),
          optional(:bond_state) => :none | :bonding | :bonded,
          optional(:device_class) => non_neg_integer(),
          optional(:uuids) => [String.t()]
        }

  # ─────────────────────────────────────────────────────────────
  # Public API
  # ─────────────────────────────────────────────────────────────

  @doc """
  List currently paired (bonded) Bluetooth devices.

  Result arrives as `{:bt, :paired_list, [device]}`.
  """
  @spec list_paired(socket :: term()) :: term()
  def list_paired(socket) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      :mob_bluetooth_nif.bt_list_paired()
      socket
    end
  end

  @doc """
  Begin Bluetooth Classic discovery. Discovered devices arrive as
  individual `{:bt, :discovered, device}` messages, terminated by
  `{:bt, :discovery_finished}`.

  Discovery typically runs ~12 seconds on Android.
  """
  @spec start_discovery(socket :: term()) :: term()
  def start_discovery(socket) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      :mob_bluetooth_nif.bt_start_discovery()
      socket
    end
  end

  @doc """
  Cancel an in-progress discovery.
  """
  @spec cancel_discovery(socket :: term()) :: term()
  def cancel_discovery(socket) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      :mob_bluetooth_nif.bt_cancel_discovery()
      socket
    end
  end

  @doc """
  Pair (bond) with a Bluetooth device.

  Without `:pin`, Android shows the system pairing dialog (user enters
  PIN). With `:pin`, attempts programmatic pairing using the supplied
  PIN; falls back to system UI if the device demands user confirmation.

  Result arrives as one of:

    * `{:bt, :paired, device}`
    * `{:bt, :pair_failed, %{address: String.t(), reason: atom()}}`
  """
  @spec pair(socket :: term(), device(), keyword()) :: term()
  def pair(socket, device, opts \\ []) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      pin = Keyword.get(opts, :pin)
      json = encode_pair(device, pin)
      :mob_bluetooth_nif.bt_pair(json)
      socket
    end
  end

  @doc """
  Remove an existing pairing (bond).

  Result: `{:bt, :unpaired, device}`.
  """
  @spec unpair(socket :: term(), device()) :: term()
  def unpair(socket, device) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      json = encode_device(device)
      :mob_bluetooth_nif.bt_unpair(json)
      socket
    end
  end

  @doc """
  Disconnect a profile session by `session_id`.

  Works for any profile (`MobBluetooth.Hfp`, `MobBluetooth.Spp`) — the
  framework dispatches internally based on which profile owns the session.

  Emits a profile-specific disconnect event:

    * `{:bt_hfp, :disconnected, session_id, reason}`
    * `{:bt_spp, :disconnected, session_id, reason}`
  """
  @spec disconnect(socket :: term(), session_id()) :: term()
  def disconnect(socket, session_id) when is_integer(session_id) do
    if MobBluetooth.Platform.unsupported?(MobBluetooth.Platform.current()) do
      {:error, :unsupported}
    else
      :mob_bluetooth_nif.bt_disconnect(session_id)
      socket
    end
  end

  # Internal JSON helpers, exposed `@doc false` so the test suite can
  # exercise the encoded shape directly (the public functions all dead-end
  # in a NIF call). Nil-safe per the VendorUsb playbook.
  # ─────────────────────────────────────────────────────────────

  @doc false
  @spec encode_pair(device(), String.t() | nil) :: binary()
  def encode_pair(device, nil), do: encode_device(device)

  def encode_pair(device, pin) when is_binary(pin) do
    device |> Map.put(:pin, pin) |> encode_device()
  end

  @doc false
  @spec encode_device(map()) :: binary()
  def encode_device(device) do
    device
    |> Map.new()
    |> drop_nil_values()
    |> :json.encode()
    |> IO.iodata_to_binary()
  end

  defp drop_nil_values(map) do
    :maps.filter(fn _k, v -> v != nil end, map)
  end
end