berrypod/lib/berrypod_web/plugs/analytics.ex

defmodule BerrypodWeb.Plugs.Analytics do
  @moduledoc """
  Plug that records a pageview event on every shop HTTP request.

  This is Layer 1 of the progressive analytics pipeline — it fires on every
  GET request regardless of whether JS is enabled. Computes a privacy-friendly
  visitor hash, parses the user agent, extracts referrer and UTM params, and
  buffers the event for batch writing to SQLite.

  Also stores analytics data in the session so the LiveView hook (Layer 2)
  can read it for subsequent SPA navigations.
  """

  import Plug.Conn

  alias Berrypod.Analytics
  alias Berrypod.Analytics.{Salt, UAParser, Referrer}

  def init(opts), do: opts

  def call(%{method: "GET"} = conn, _opts) do
    ua = get_req_header(conn, "user-agent") |> List.first("")
    {browser, os} = UAParser.parse(ua)

    # Skip bots
    if browser == "Bot" do
      conn
    else
      # Skip if the logged-in admin is browsing
      if admin?(conn) do
        prepare_session(conn, ua, browser, os)
      else
        record_and_prepare(conn, ua, browser, os)
      end
    end
  end

  def call(conn, _opts), do: conn

  defp record_and_prepare(conn, ua, browser, os) do
    visitor_hash = Salt.hash_visitor(conn.remote_ip, ua)
    {referrer, referrer_source} = extract_referrer(conn)
    utm = extract_utms(conn)
    country_code = get_session(conn, "country_code")
    screen_size = get_session(conn, "analytics_screen_size")

    Analytics.track_pageview(%{
      pathname: conn.request_path,
      visitor_hash: visitor_hash,
      referrer: referrer,
      referrer_source: referrer_source,
      utm_source: utm.source,
      utm_medium: utm.medium,
      utm_campaign: utm.campaign,
      country_code: country_code,
      screen_size: screen_size,
      browser: browser,
      os: os
    })

    conn
    |> put_session("analytics_visitor_hash", visitor_hash)
    |> put_session("analytics_browser", browser)
    |> put_session("analytics_os", os)
    |> put_session("analytics_referrer", referrer)
    |> put_session("analytics_referrer_source", referrer_source)
    |> put_session("analytics_utm_source", utm.source)
    |> put_session("analytics_utm_medium", utm.medium)
    |> put_session("analytics_utm_campaign", utm.campaign)
  end

  # Store session data for the hook even when we skip recording for admins
  defp prepare_session(conn, ua, browser, os) do
    visitor_hash = Salt.hash_visitor(conn.remote_ip, ua)

    conn
    |> put_session("analytics_visitor_hash", visitor_hash)
    |> put_session("analytics_browser", browser)
    |> put_session("analytics_os", os)
  end

  defp admin?(conn) do
    case conn.assigns[:current_scope] do
      %{user: %{}} -> true
      _ -> false
    end
  end

  defp extract_referrer(conn) do
    referrer_url = get_req_header(conn, "referer") |> List.first()
    {referrer, source} = Referrer.parse(referrer_url)

    # Don't count self-referrals
    host = conn.host

    if referrer && referrer == host do
      {nil, nil}
    else
      {referrer, source}
    end
  end

  defp extract_utms(conn) do
    params = conn.query_params || %{}

    %{
      source: Map.get(params, "utm_source"),
      medium: Map.get(params, "utm_medium"),
      campaign: Map.get(params, "utm_campaign")
    }
  end
end