defmodule Vela.Cache.Entry do
  @moduledoc """
  Represents a single item stored in the cache.
  """

  @type t :: %__MODULE__{
          key: term(),
          value: term(),
          inserted_at: integer(),
          expires_at: integer() | :infinity,
          version: non_neg_integer(),
          touched_at: integer(),
          node: node(),
          tags: [atom()]
        }

  defstruct [
    :key,
    :value,
    :inserted_at,
    :expires_at,
    :version,
    :touched_at,
    :node,
    tags: []
  ]

  @doc """
  Creates a new Entry. `ttl` is in milliseconds, or `:infinity`.
  """
  @spec new(term(), term(), keyword()) :: t()
  def new(key, value, opts \\ []) do
    now = System.monotonic_time(:millisecond)
    ttl = Keyword.get(opts, :ttl, :infinity)

    expires_at =
      case ttl do
        :infinity -> :infinity
        ms when is_integer(ms) -> now + ms
      end

    %__MODULE__{
      key: key,
      value: value,
      inserted_at: now,
      expires_at: expires_at,
      version: Keyword.get(opts, :version, 1),
      touched_at: now,
      node: node(),
      tags: Keyword.get(opts, :tags, [])
    }
  end

  @doc """
  Returns true if this entry has expired.
  """
  @spec expired?(t()) :: boolean()
  def expired?(%__MODULE__{expires_at: :infinity}), do: false

  def expired?(%__MODULE__{expires_at: expires_at}) do
    System.monotonic_time(:millisecond) >= expires_at
  end

  @doc """
  Returns remaining TTL in milliseconds, or :infinity.
  """
  @spec remaining_ttl(t()) :: non_neg_integer() | :infinity
  def remaining_ttl(%__MODULE__{expires_at: :infinity}), do: :infinity

  def remaining_ttl(%__MODULE__{expires_at: expires_at}) do
    remaining = expires_at - System.monotonic_time(:millisecond)
    max(0, remaining)
  end
end