# A small, complete MatterFirst integration.
#
# It shows the four things every integration has to get right:
#
#   1. authenticate with an API key and find out who it acts for;
#   2. read a list page by page, following `links.next`;
#   3. create something with an `Idempotency-Key`, so a retry cannot make two,
#      waiting for `Retry-After` on 429 and backing off on 5xx;
#   4. receive webhooks and verify each one's signature and timestamp before
#      trusting it.
#
# Run it against your firm with `run.exs` in this folder. MatterFirst's own test
# suite loads this same file and runs it against the real API on every build, so
# it cannot fall behind the API it describes.
#
# Only `Req` and `Jason` are needed.

defmodule MatterFirstSample do
  @moduledoc false

  @tolerance_seconds 300
  @max_attempts 4

  @doc """
  A client for one firm. `opts`:

    * `:base_url`: e.g. "https://matterfirst.com"
    * `:api_key`: an `mf_live_…` key
    * `:req_options`: extra `Req` options (the test suite passes its router here)
    * `:sleep`: how to wait between retries; defaults to `Process.sleep/1`
  """
  def client(opts) do
    req =
      Req.new(
        base_url: String.trim_trailing(Keyword.fetch!(opts, :base_url), "/") <> "/api/v1",
        auth: {:bearer, Keyword.fetch!(opts, :api_key)},
        headers: [{"accept", "application/json"}],
        # This client does its own retrying, so it can honour Retry-After and
        # keep the same Idempotency-Key across attempts.
        retry: false,
        receive_timeout: 30_000
      )
      |> Req.merge(Keyword.get(opts, :req_options, []))

    %{req: req, sleep: Keyword.get(opts, :sleep, &Process.sleep/1)}
  end

  @doc "Who the key acts for: `{:ok, %{\"email\" => ..., ...}}`."
  def whoami(client) do
    with {:ok, body} <- request(client, :get, "/me") do
      {:ok, get_in(body, ["data", "attributes"])}
    end
  end

  @doc "Every open matter, following `links.next` until there is none."
  def list_open_matters(client, page_size \\ 50) do
    collect(client, "/matters?filter[status]=open&page_size=#{page_size}", [])
  end

  defp collect(_client, nil, acc), do: {:ok, acc}

  defp collect(client, path, acc) do
    with {:ok, body} <- request(client, :get, path) do
      next = get_in(body, ["links", "next"])
      next_path = if next && next != path, do: relative(next), else: nil
      collect(client, next_path, acc ++ Map.get(body, "data", []))
    end
  end

  # `links.next` is a path under /api/v1 (or a full URL); the client's base URL
  # already ends in /api/v1.
  defp relative(link) do
    %URI{path: path, query: query} = URI.parse(link)
    path = String.replace_prefix(path || "", "/api/v1", "")
    if query, do: path <> "?" <> query, else: path
  end

  @doc """
  Creates a matter. The same `idempotency_key` always means the same matter:
  retrying after a timeout replays the first answer instead of creating a
  second one.
  """
  def create_matter(client, attributes, idempotency_key) do
    with {:ok, body} <-
           request(client, :post, "/matters",
             json: %{"data" => %{"attributes" => attributes}},
             idempotency_key: idempotency_key
           ) do
      {:ok, body["data"]}
    end
  end

  @doc """
  Registers an https address for the given events. Returns the endpoint and its
  signing secret, which MatterFirst shows only this once: store it.
  """
  def register_webhook(client, url, events, idempotency_key) do
    with {:ok, body} <-
           request(client, :post, "/webhooks",
             json: %{"data" => %{"attributes" => %{"url" => url, "events" => events}}},
             idempotency_key: idempotency_key
           ) do
      {:ok, %{endpoint: body["data"], secret: get_in(body, ["meta", "secret"])}}
    end
  end

  @doc """
  Verifies one delivery exactly as MatterFirst signs it, before anything in it
  is trusted. `headers` are the request's headers (lower-case names).

  Returns `{:ok, event}`, `{:error, :missing_headers}`, `{:error, :stale}` or
  `{:error, :bad_signature}`. `now` is unix seconds, for tests.
  """
  def verify_delivery(raw_body, headers, secret, now \\ System.os_time(:second)) do
    headers = Map.new(headers, fn {name, value} -> {String.downcase(name), value} end)

    with {:ok, timestamp} <- fetch_integer(headers, "x-matterfirst-timestamp"),
         {:ok, signature} <- fetch(headers, "x-matterfirst-signature"),
         :ok <- fresh(timestamp, now),
         :ok <- signed?(raw_body, secret, timestamp, signature) do
      {:ok, Jason.decode!(raw_body)}
    end
  end

  defp signed?(raw_body, secret, timestamp, "v1=" <> given) do
    expected =
      :crypto.mac(:hmac, :sha256, secret, "v1.#{timestamp}.#{raw_body}")
      |> Base.encode16(case: :lower)

    # Constant time, so a receiver does not leak how much of a guess was right.
    if byte_size(expected) == byte_size(given) and :crypto.hash_equals(expected, given),
      do: :ok,
      else: {:error, :bad_signature}
  end

  defp signed?(_raw_body, _secret, _timestamp, _other_scheme), do: {:error, :bad_signature}

  defp fetch(headers, name) do
    case Map.fetch(headers, name) do
      {:ok, value} -> {:ok, value}
      :error -> {:error, :missing_headers}
    end
  end

  defp fetch_integer(headers, name) do
    with {:ok, value} <- fetch(headers, name) do
      case Integer.parse(value) do
        {int, ""} -> {:ok, int}
        _other -> {:error, :missing_headers}
      end
    end
  end

  defp fresh(timestamp, now) do
    if abs(now - timestamp) <= @tolerance_seconds, do: :ok, else: {:error, :stale}
  end

  @doc """
  One request with safe retries: 429 waits for `Retry-After`; a 5xx or a
  dropped connection waits 1, 2 then 4 seconds. POSTs carry the same
  `Idempotency-Key` on every attempt. Any other 4xx is returned at once as
  `{:error, %{status:, code:, detail:}}`.
  """
  def request(client, method, path, opts \\ []) do
    {key, opts} = Keyword.pop(opts, :idempotency_key)
    headers = if key, do: [{"idempotency-key", key}], else: []
    attempt(client, [method: method, url: path, headers: headers] ++ opts, 1)
  end

  defp attempt(client, options, n) do
    case Req.request(client.req, options) do
      {:ok, %Req.Response{status: status, body: body}} when status in 200..299 ->
        {:ok, body}

      {:ok, %Req.Response{status: 429} = response} when n < @max_attempts ->
        client.sleep.(retry_after_ms(response))
        attempt(client, options, n + 1)

      {:ok, %Req.Response{status: status}} when status >= 500 and n < @max_attempts ->
        client.sleep.(backoff_ms(n))
        attempt(client, options, n + 1)

      {:ok, %Req.Response{status: status, body: body}} ->
        {:error, error(status, body)}

      {:error, _transport} when n < @max_attempts ->
        client.sleep.(backoff_ms(n))
        attempt(client, options, n + 1)

      {:error, exception} ->
        {:error, %{status: nil, code: "transport", detail: Exception.message(exception)}}
    end
  end

  defp retry_after_ms(response) do
    case Req.Response.get_header(response, "retry-after") do
      [seconds | _] ->
        case Integer.parse(seconds) do
          {s, _} when s >= 0 -> s * 1_000
          _other -> 1_000
        end

      [] ->
        1_000
    end
  end

  defp backoff_ms(n), do: 1_000 * Integer.pow(2, n - 1)

  defp error(status, %{"errors" => [first | _]}) do
    %{status: status, code: first["code"], detail: first["detail"]}
  end

  defp error(status, body), do: %{status: status, code: nil, detail: inspect(body)}

  @doc """
  The whole walk-through: who am I, what is open, and (with `:webhook_url`)
  register an address for new matters. Prints what it does.
  """
  def run(opts) do
    client = client(opts)

    with {:ok, me} <- whoami(client),
         {:ok, matters} <- list_open_matters(client) do
      IO.puts("This key acts for #{me["full_name"] || me["email"]}.")
      IO.puts("#{length(matters)} open matters.")

      case Keyword.get(opts, :webhook_url) do
        nil ->
          :ok

        url ->
          {:ok, %{secret: secret}} =
            register_webhook(client, url, ["matter.created"], "sample-webhook-#{url}")

          IO.puts("Registered #{url}. Store this signing secret now: #{secret}")
      end
    end
  end
end