defmodule KopoKopo do
  @moduledoc """
  Client helpers for Kopo Kopo payments.

  The client supports OAuth client credentials, M-PESA STK Push incoming
  payments, status verification, and callback signature validation.

  ## Configuration

      config :kopokopo,
        base_url: "https://api.kopokopo.com",
        client_id: System.fetch_env!("KOPOKOPO_CLIENT_ID"),
        client_secret: System.fetch_env!("KOPOKOPO_CLIENT_SECRET"),
        api_key: System.fetch_env!("KOPOKOPO_API_KEY"),
        till_number: System.fetch_env!("KOPOKOPO_TILL_NUMBER"),
        callback_url: "https://example.com/payments/kopokopo/callback",
        user_agent: "my_app/1.0"

  For local sandbox work, set `base_url` to `https://sandbox.kopokopo.com`.
  """

  @default_base_url "https://sandbox.kopokopo.com"
  @default_user_agent "kopokopo/0.1.0"

  defstruct [:status, :url, :body]

  @type t :: %__MODULE__{status: pos_integer() | nil, url: String.t() | nil, body: term()}
  @type payment_result :: {:ok, map()} | {:pending, map()} | {:error, term()}

  @doc """
  Converts an error reason returned by this client into displayable text.
  """
  def error_text(%__MODULE__{body: body}) when is_binary(body), do: body
  def error_text(%__MODULE__{status: status, body: body}), do: "HTTP #{status}: #{inspect(body)}"
  def error_text(reason) when is_binary(reason), do: reason
  def error_text(reason), do: inspect(reason)

  @doc """
  Initiates an M-PESA STK Push payment.

  Expected fields are:

    * `:reference`
    * `:amount`
    * `:phone`
    * `:email`
    * `:name`

  Returns `{:ok, %{location: location, reference: reference}}` when Kopo Kopo
  accepts the payment request.
  """
  @spec initiate_payment(map()) :: {:ok, map()} | {:error, term()}
  def initiate_payment(%{
        reference: reference,
        amount: amount,
        phone: phone,
        email: email,
        name: name
      }) do
    with {:ok, token} <- access_token() do
      body =
        incoming_payment_body(%{
          reference: reference,
          amount: amount,
          phone: phone,
          email: email,
          name: name
        })

      case Req.post("#{base_url()}/api/v2/incoming_payments",
             headers: json_headers(token),
             json: body,
             receive_timeout: 30_000
           ) do
        {:ok, %{status: 201, headers: headers}} ->
          {:ok, %{location: header(headers, "location"), reference: reference}}

        {:ok, %{status: status, body: body}} ->
          {:error, error_message(status, "#{base_url()}/api/v2/incoming_payments", body)}

        {:error, reason} ->
          {:error, inspect(reason)}
      end
    end
  end

  @doc """
  Sends a small STK Push test prompt.

  This uses configured credentials and till details, so it can trigger a real
  M-PESA prompt when pointed at production.
  """
  @spec test_prompt(String.t(), String.t() | number()) :: {:ok, map()} | {:error, term()}
  def test_prompt(phone \\ "0740769596", amount \\ "1") do
    reference = "TEST-#{System.system_time(:second)}"

    initiate_payment(%{
      reference: reference,
      amount: amount,
      phone: phone,
      email: "payments-test@example.com",
      name: "Kopo Kopo Test"
    })
  end

  @doc """
  Checks whether configured credentials can obtain an OAuth access token.
  """
  @spec auth_check() :: {:ok, String.t()} | {:error, term()}
  def auth_check, do: access_token()

  @doc """
  Checks credentials against a temporary base URL.
  """
  @spec auth_check(String.t()) :: {:ok, String.t()} | {:error, term()}
  def auth_check(base_url) when is_binary(base_url) do
    with_temporary_base_url(base_url, &access_token/0)
  end

  @doc """
  Sends a test prompt against a temporary base URL.
  """
  @spec test_prompt(String.t(), String.t(), String.t() | number()) ::
          {:ok, map()} | {:error, term()}
  def test_prompt(base_url, phone, amount) when is_binary(base_url) do
    with_temporary_base_url(base_url, fn -> test_prompt(phone, amount) end)
  end

  @doc """
  Returns the outgoing STK Push payload without making a network request.
  """
  @spec debug_prompt_payload(String.t(), String.t() | number()) :: map()
  def debug_prompt_payload(phone \\ "0740769596", amount \\ "1") do
    reference = "TEST-#{System.system_time(:second)}"

    %{
      url: "#{base_url()}/api/v2/incoming_payments",
      callback_url: callback_url(),
      body:
        incoming_payment_body(%{
          reference: reference,
          amount: amount,
          phone: phone,
          email: "payments-test@example.com",
          name: "Kopo Kopo Test"
        })
    }
  end

  @doc """
  Verifies a payment status URL returned by `initiate_payment/1`.
  """
  @spec verify(String.t()) :: payment_result()
  def verify(location) when is_binary(location) and location != "" do
    with {:ok, token} <- access_token() do
      case Req.get(location, headers: json_headers(token), receive_timeout: 30_000) do
        {:ok, %{status: 200, body: %{"data" => data}}} ->
          payment_result(data)

        {:ok, %{status: status, body: body}} ->
          {:error, error_message(status, location, body)}

        {:error, reason} ->
          {:error, inspect(reason)}
      end
    end
  end

  def verify(_), do: {:error, "Missing Kopo Kopo payment request URL."}

  @doc """
  Normalizes Kopo Kopo payment status response data into `:ok`, `:pending`, or
  `:error` tuples.
  """
  @spec payment_result(map()) :: payment_result()
  def payment_result(%{"attributes" => %{"status" => "Success"}} = data), do: {:ok, data}

  def payment_result(%{"attributes" => %{"status" => status}} = data)
      when status in ["Pending", "Received", "Initiated"] do
    {:pending, data}
  end

  def payment_result(%{"attributes" => %{"status" => status, "event" => event}}) do
    {:error, event_error(event) || "Payment status: #{status}"}
  end

  def payment_result(_), do: {:error, "Unexpected Kopo Kopo payment response."}

  @doc """
  Validates a callback body against the `x-kopokopo-signature` header.
  """
  @spec valid_signature?(binary(), binary()) :: boolean()
  def valid_signature?(body, signature) when is_binary(body) and is_binary(signature) do
    expected_signature =
      :crypto.mac(:hmac, :sha256, api_key(), body)
      |> Base.encode16(case: :lower)

    secure_compare(String.downcase(signature), expected_signature)
  end

  def valid_signature?(_body, _signature), do: false

  @doc """
  Returns the configured callback URL.

  If `:callback_url` is configured, it is used directly. Otherwise the URL is
  built from `:site_url` and `:callback_path`.
  """
  @spec callback_url() :: String.t()
  def callback_url do
    configured = Application.get_env(:kopokopo, :callback_url)

    if configured do
      configured
    else
      Path.join(site_url(), callback_path())
    end
  end

  defp access_token do
    body = %{
      "client_id" => client_id(),
      "client_secret" => client_secret(),
      "grant_type" => "client_credentials"
    }

    case Req.post("#{base_url()}/oauth/token",
           headers: [
             {"Content-Type", "application/x-www-form-urlencoded"},
             {"User-Agent", user_agent()}
           ],
           form: body,
           receive_timeout: 30_000
         ) do
      {:ok, %{status: 200, body: %{"access_token" => token}}} ->
        {:ok, token}

      {:ok, %{status: status, body: body}} ->
        {:error, error_message(status, "#{base_url()}/oauth/token", body)}

      {:error, reason} ->
        {:error, inspect(reason)}
    end
  end

  defp json_headers(token) do
    [
      {"Accept", "application/json"},
      {"Content-Type", "application/json"},
      {"Authorization", "Bearer #{token}"},
      {"User-Agent", user_agent()}
    ]
  end

  defp incoming_payment_body(%{
         reference: reference,
         amount: amount,
         phone: phone,
         email: email,
         name: name
       }) do
    %{
      "payment_channel" => "M-PESA STK Push",
      "till_number" => till_number(),
      "subscriber" => %{
        "first_name" => first_name(name),
        "last_name" => last_name(name),
        "phone_number" => normalize_phone(phone),
        "email" => email
      },
      "amount" => %{
        "currency" => "KES",
        "value" => format_amount(amount)
      },
      "metadata" => %{
        "reference" => reference,
        "customer_email" => email,
        "notes" => "Order #{reference}"
      },
      "_links" => %{
        "callback_url" => callback_url()
      }
    }
  end

  defp with_temporary_base_url(base_url, fun) do
    previous = Application.get_env(:kopokopo, :base_url)
    Application.put_env(:kopokopo, :base_url, base_url)

    try do
      fun.()
    after
      if previous do
        Application.put_env(:kopokopo, :base_url, previous)
      else
        Application.delete_env(:kopokopo, :base_url)
      end
    end
  end

  defp header(headers, name) do
    headers
    |> Enum.find_value(fn {key, values} ->
      if String.downcase(to_string(key)) == name do
        List.wrap(values) |> List.first()
      end
    end)
  end

  defp error_message(%{"error_message" => message}), do: message
  defp error_message(%{"message" => message}), do: message
  defp error_message(%{"error_description" => message}), do: message
  defp error_message(%{"error" => error}), do: error
  defp error_message(body) when is_binary(body), do: body
  defp error_message(body), do: inspect(body)

  defp error_message(status, url, body) do
    %__MODULE__{status: status, url: url, body: error_message(body)}
  end

  defp event_error(%{"errors" => errors}) when is_list(errors), do: Enum.join(errors, ", ")
  defp event_error(%{"errors" => error}) when is_binary(error), do: error
  defp event_error(_), do: nil

  defp first_name(name), do: name |> String.split() |> List.first() || "Customer"

  defp last_name(name) do
    case String.split(name) do
      [_] -> "Customer"
      parts -> List.last(parts)
    end
  end

  defp normalize_phone("+" <> _ = phone), do: phone
  defp normalize_phone("0" <> rest), do: "+254#{rest}"
  defp normalize_phone("254" <> _ = phone), do: "+#{phone}"
  defp normalize_phone(phone), do: phone

  defp format_amount(amount) when is_float(amount),
    do: :erlang.float_to_binary(amount, decimals: 2)

  defp format_amount(amount), do: to_string(amount)

  defp secure_compare(left, right) when byte_size(left) == byte_size(right) do
    secure_compare(left, right, 0) == 0
  end

  defp secure_compare(_left, _right), do: false

  defp secure_compare(<<x, left::binary>>, <<y, right::binary>>, acc) do
    secure_compare(left, right, Bitwise.bor(acc, Bitwise.bxor(x, y)))
  end

  defp secure_compare(<<>>, <<>>, acc), do: acc

  defp base_url, do: Application.get_env(:kopokopo, :base_url, @default_base_url)
  defp client_id, do: Application.fetch_env!(:kopokopo, :client_id)
  defp client_secret, do: Application.fetch_env!(:kopokopo, :client_secret)
  defp api_key, do: Application.fetch_env!(:kopokopo, :api_key)
  defp till_number, do: Application.fetch_env!(:kopokopo, :till_number)
  defp site_url, do: Application.get_env(:kopokopo, :site_url, "https://example.com")

  defp callback_path do
    Application.get_env(:kopokopo, :callback_path, "/api/payments/kopokopo/callback")
  end

  defp user_agent, do: Application.get_env(:kopokopo, :user_agent, @default_user_agent)
end