defmodule CodeNameRaven.Channel.Notification do
  @moduledoc """
  The payload delivered to a channel when an alert fires.

  Produced by the Handler when a monitor's status changes to a condition
  that matches a configured alert rule. Passed to `Channel.deliver/2`.

  Never crosses node or version boundaries — created and consumed within a
  single handler process — so it carries no `vsn`/upcast machinery.
  """

  @type status :: :up | :degraded | :down | :unknown

  @type t :: %__MODULE__{
          monitor_id: String.t(),
          monitor_name: String.t() | nil,
          status: status(),
          previous_status: status() | nil,
          occurred_at: DateTime.t(),
          node: node() | nil,
          instance_id: String.t() | nil,
          alert_started_at: DateTime.t() | nil
        }

  defstruct [
    :monitor_id,
    :monitor_name,
    :status,
    :previous_status,
    :occurred_at,
    :node,
    :instance_id,
    :alert_started_at
  ]

  @doc """
  Seconds between `alert_started_at` and `occurred_at`, or nil when the
  alert's start is unknown. Shared by channel formatters so "was down
  N minutes" is computed one way everywhere.
  """
  @spec duration_down(t()) :: non_neg_integer() | nil
  def duration_down(%__MODULE__{alert_started_at: nil}), do: nil
  def duration_down(%__MODULE__{occurred_at: nil}), do: nil

  def duration_down(%__MODULE__{alert_started_at: started, occurred_at: occurred}) do
    max(DateTime.diff(occurred, started, :second), 0)
  end

  @doc """
  Human-readable duration-down ("14m", "3h 12m", "2d 5h"), or nil when the
  alert's start is unknown. Sub-minute durations render as "<1m".
  """
  @spec format_duration_down(t()) :: String.t() | nil
  def format_duration_down(%__MODULE__{} = n) do
    case duration_down(n) do
      nil -> nil
      s when s < 60 -> "<1m"
      s when s < 3600 -> "#{div(s, 60)}m"
      s when s < 86_400 -> "#{div(s, 3600)}h #{rem(s, 3600) |> div(60)}m"
      s -> "#{div(s, 86_400)}d #{rem(s, 86_400) |> div(3600)}h"
    end
  end
end