defmodule Plausible.Site.GateKeeper do
  @type policy() :: :allow | :not_found | :block | :throttle | :payment_required
  @policy_for_non_existing_sites :not_found

  @type t() :: {:allow, Plausible.Site.t()} | {:deny, policy()}

  @moduledoc """
  Thin wrapper around `Plausible.RateLimit` for gate keeping domain-specific events
  during the ingestion phase. When the site is allowed, gate keeping
  check returns `:allow`, otherwise a `:deny` tagged tuple is returned
  with one of the following policy markers:
    * `:not_found` (indicates site not found in cache)
    * `:block` (indicates disabled sites)
    * `:throttle` (indicates rate limiting)

  Rate Limiting buckets are configured per site (externally via the CRM).
  See: `Plausible.Site`

  To look up each site's configuration, the RateLimiter fetches
  a Site by domain using `Plausible.Cache` interface.

  The module defines two policies outside the regular bucket inspection:
    * when the site is not found in cache: #{@policy_for_non_existing_sites}
    * when the underlying rate limiting mechanism returns
      an internal error: :allow
  """
  alias Plausible.{Site, RateLimit}
  alias Plausible.Site.Cache

  @spec check(String.t(), Keyword.t()) :: t()
  def check(domain, opts \\ []) when is_binary(domain) do
    case policy(domain, opts) do
      {:allow, site} -> {:allow, site}
      other -> {:deny, other}
    end
  end

  @spec key(String.t()) :: String.t()
  def key(domain) do
    "ingest:site:#{domain}"
  end

  defp policy(domain, opts) when is_binary(domain) do
    with %Site{team: %{accept_traffic_until: accept_traffic_until}} = site <-
           Cache.get(domain, Keyword.get(opts, :cache_opts, [])),
         true <- Plausible.Sites.regular?(site) do
      if not is_nil(accept_traffic_until) and
           Date.after?(Date.utc_today(), accept_traffic_until) do
        :payment_required
      else
        check_rate_limit(site, opts)
      end
    else
      _ ->
        @policy_for_non_existing_sites
    end
  end

  defp check_rate_limit(%Site{ingest_rate_limit_threshold: nil} = site, _opts) do
    {:allow, site}
  end

  defp check_rate_limit(%Site{ingest_rate_limit_threshold: 0}, _opts) do
    :block
  end

  defp check_rate_limit(%Site{ingest_rate_limit_threshold: threshold} = site, opts)
       when is_integer(threshold) do
    key = Keyword.get(opts, :key, key(site.domain))
    scale_ms = site.ingest_rate_limit_scale_seconds * 1_000

    case RateLimit.check_rate(key, scale_ms, threshold) do
      {:deny, _} -> :throttle
      {:allow, _} -> {:allow, site}
    end
  end
end