defmodule Store.APIUsage do
  @moduledoc """
  Module for recording and checking API usage data. Cross OS process
  coordination is handled with regard to file reads/writes. It is up to the
  caller to ensure that requests are internally ordered for consistency.
  """

  @path "usage.json"
  @tag "[usage]"
  # OpenAI can return fractional reset windows (e.g. "1.043s"), so accept floats.
  @re_reset ~r/^(\d+(?:\.\d+)?)(ms|s)$/
  @debug_env_var "FNORD_DEBUG_API_USAGE"

  @type model_usage :: %{
          updated_at: non_neg_integer,
          requests_max: non_neg_integer,
          requests_left: non_neg_integer,
          requests_reset: non_neg_integer,
          tokens_max: non_neg_integer,
          tokens_left: non_neg_integer,
          tokens_reset: non_neg_integer
        }

  @type usage :: %{
          optional(binary) => model_usage
        }

  @typep http_response ::
           {:ok, HTTPoison.Response.t()}
           | {:error, HTTPoison.Error.t()}

  @doc """
  Returns the path to the usage store file.
  """
  @spec store_path() :: binary
  def store_path do
    Path.join(Store.store_home(), @path)
  end

  @doc """
  Records API usage for a provided model or automatically identifies the model
  from the response headers or body. For 2xx responses, attempts to collect usage
  data and update the store. If model can't be determined or on non-2xx
  responses, logs a warning and returns the original http_result.
  """
  @spec record_for_model(binary | nil, http_response) :: http_response
  def record_for_model(model, http_result) do
    case http_result do
      {:ok, %HTTPoison.Response{headers: headers, body: body} = response} ->
        ensure_store_file()
        headers_map = Enum.into(headers, %{})

        model_result =
          if model do
            {:ok, model}
          else
            identify_model(headers_map, body)
          end

        case response.status_code do
          code when code >= 200 and code < 300 ->
            case model_result do
              {:ok, actual_model} ->
                with {:ok, usage} <- collect_usage_data(headers_map),
                     :ok <- update_file(actual_model, usage) do
                  case System.get_env(@debug_env_var, nil) do
                    "" -> false
                    "0" -> false
                    nil -> false
                    _ -> UI.debug(@tag, "#{actual_model}: #{inspect(usage, pretty: true)}")
                  end
                else
                  reason ->
                    UI.warn(
                      @tag,
                      "Failed to record API usage data\n\nReason:\n#{inspect(reason, pretty: true)}\n\nResponse:\n#{inspect(response, pretty: true)}"
                    )
                end

              {:error, reason} ->
                UI.warn(
                  @tag,
                  "Failed to record API usage data\n\nReason:\n#{inspect(reason, pretty: true)}\n\nResponse:\n#{inspect(response, pretty: true)}"
                )
            end

          _other ->
            :ok
        end

        http_result

      _other ->
        http_result
    end
  end

  @doc """
  Checks if a request can be made for the given model based on the last
  recorded usage data for the model. If a request can be made, returns `:ok`.
  Otherwise, returns `{:wait, milliseconds}` indicating how long to wait before
  attempting another request.
  """
  @spec check(binary) ::
          :ok
          | {:wait, non_neg_integer}
          | {:error, term}
  def check(model) do
    ensure_store_file()

    with :ok <- can_request?(model) do
      :ok
    end
  end

  # ----------------------------------------------------------------------------
  # Internals
  # ----------------------------------------------------------------------------
  @spec can_request?(binary) ::
          :ok
          | {:wait, non_neg_integer}
          | {:error, term}
  defp can_request?(model) do
    FileLock.with_lock(store_path(), fn ->
      with {:ok, data} <- read_file() do
        usage = Map.get(data, model, %{})
        updated_at = Map.get(usage, :updated_at, 0)
        elapsed_time = now() - updated_at
        requests_reset = Map.get(usage, :requests_reset, 0)

        if requests_reset <= elapsed_time do
          :ok
        else
          {:wait, requests_reset - elapsed_time}
        end
      end
    end)
    |> case do
      {:ok, result} -> result
      {:callback_error, exc} -> {:error, exc}
      {:error, reason} -> {:error, reason}
    end
  end

  @spec update_file(binary, model_usage) :: :ok | {:error, term}
  defp update_file(model, usage) do
    FileLock.with_lock(store_path(), fn ->
      with {:ok, usage_data} <- read_file() do
        usage = Map.put(usage, :updated_at, now())
        payload = Map.put(usage_data, model, usage)
        write_file(payload)
      end
    end)
    |> case do
      {:ok, result} -> result
      {:callback_error, exc} -> {:error, exc}
      {:error, reason} -> {:error, reason}
    end
  end

  @spec read_file() :: {:ok, usage} | {:error, term}
  defp read_file do
    with {:ok, json} <- File.read(store_path()),
         {:ok, data} <- Jason.decode(json) do
      data
      |> Enum.map(fn {model, usage} ->
        {model, Util.string_keys_to_atoms(usage)}
      end)
      |> Enum.into(%{})
      |> then(&{:ok, &1})
    end
  end

  @spec write_file(map) :: :ok | {:error, term}
  defp write_file(data) do
    with {:ok, json} <- Jason.encode(data, pretty: true) do
      File.write(store_path(), json)
    end
  end

  @spec collect_usage_data(map) :: {:ok, model_usage} | {:error, atom, binary}
  defp collect_usage_data(headers) do
    with {:ok, requests_max} <- collect_usage_metric(headers, "x-ratelimit-limit-requests"),
         {:ok, requests_left} <- collect_usage_metric(headers, "x-ratelimit-remaining-requests"),
         {:ok, requests_reset} <- collect_reset_metric(headers, "x-ratelimit-reset-requests"),
         {:ok, tokens_max} <- collect_usage_metric(headers, "x-ratelimit-limit-tokens"),
         {:ok, tokens_left} <- collect_usage_metric(headers, "x-ratelimit-remaining-tokens"),
         {:ok, tokens_reset} <- collect_reset_metric(headers, "x-ratelimit-reset-tokens") do
      {:ok,
       %{
         updated_at: now(),
         requests_max: requests_max,
         requests_left: requests_left,
         requests_reset: requests_reset,
         tokens_max: tokens_max,
         tokens_left: tokens_left,
         tokens_reset: tokens_reset
       }}
    end
  end

  @spec collect_usage_metric(map, binary) :: {:ok, non_neg_integer} | {:error, atom, binary}
  defp collect_usage_metric(headers, key) do
    headers
    |> Map.fetch(key)
    |> case do
      {:ok, value} ->
        value
        |> Integer.parse()
        |> case do
          {int_value, _} -> {:ok, int_value}
          :error -> {:error, :invalid_integer}
        end

      :error ->
        {:error, :not_found, key}
    end
  end

  @spec collect_reset_metric(map, binary) :: {:ok, non_neg_integer} | {:error, atom, binary}
  defp collect_reset_metric(headers, key) do
    headers
    |> Map.fetch(key)
    |> case do
      # In "2ms" or "1s" format
      {:ok, value} -> parse_reset_metric(value)
      :error -> {:error, :not_found, key}
    end
  end

  @spec parse_reset_metric(binary) :: {:ok, non_neg_integer} | {:error, atom, binary}
  defp parse_reset_metric(value) do
    with [amount, unit] <- Regex.run(@re_reset, value, capture: :all_but_first),
         {float_value, _rest} <- Float.parse(amount) do
      ms_value =
        case unit do
          "ms" -> float_value
          "s" -> float_value * 1000.0
        end
        # Be conservative: never under-wait.
        |> Float.ceil()
        |> trunc()
        |> max(0)

      {:ok, ms_value}
    else
      _ -> {:error, :invalid_reset_format, value}
    end
  end

  @spec identify_model(map, binary) :: {:ok, binary} | {:error, atom}
  defp identify_model(headers, body) do
    with :error <- identify_model_from_headers(headers),
         :error <- identify_model_from_body(body) do
      {:error, :model_not_found}
    end
  end

  @spec identify_model_from_headers(map) :: {:ok, binary} | :error
  defp identify_model_from_headers(headers) do
    headers
    |> Map.fetch("openai-model")
    |> case do
      {:ok, model} -> {:ok, model}
      :error -> :error
    end
  end

  @spec identify_model_from_body(binary) :: {:ok, binary} | :error
  defp identify_model_from_body(body) do
    body
    |> Jason.decode()
    |> case do
      {:ok, %{"model" => model}} -> {:ok, model}
      _ -> :error
    end
  end

  @spec now() :: non_neg_integer
  defp now do
    DateTime.utc_now()
    |> DateTime.to_unix(:millisecond)
  end

  @spec ensure_store_file() :: :ok | {:error, term}
  defp ensure_store_file() do
    path = store_path()

    unless File.exists?(path) do
      File.write(path, "{}")
    end

    :ok
  end
end