limited_map_set

defmodule LimitedMapSet do
  @moduledoc """
  A bounded, processless version of `MapSet` that keeps insertion order (FIFO)
  and evicts the oldest elements when reaching the given limit.

  Combines `MapSet` (for fast membership) with `:queue` (for insertion order).
  """

  defstruct set: MapSet.new(),
            queue: :queue.new(),
            limit: 200,
            size: 0

  @type t() :: %__MODULE__{
          set: term(),
          queue: term(),
          limit: pos_integer(),
          size: non_neg_integer()
        }

  @doc """
  Creates an empty `LimitedMapSet` with a specified limit.

  ## Examples

      iex> LimitedMapSet.new(100)
      %LimitedMapSet{limit: 100, size: 0}
  """
  @spec new(pos_integer()) :: t()
  def new(limit) when is_integer(limit) and limit > 0 do
    %__MODULE__{limit: limit}
  end

  @doc """
  Creates a `LimitedMapSet` from a list of values with a specified limit.

  If the list exceeds the limit, the oldest items are trimmed.

  ## Examples

      iex> LimitedMapSet.new([1, 2, 3, 4], 3) |> LimitedMapSet.to_list()
      [2, 3, 4]
  """
  @spec new(list(), pos_integer()) :: t()
  def new(list, limit) when is_list(list) and is_integer(limit) and limit > 0 do
    trimmed =
      if length(list) > limit do
        Enum.take(Enum.reverse(list), limit) |> Enum.reverse()
      else
        list
      end

    queue =
      Enum.reduce(trimmed, :queue.new(), fn v, q ->
        :queue.in(v, q)
      end)

    set = MapSet.new(trimmed)
    %__MODULE__{set: set, queue: queue, limit: limit, size: length(trimmed)}
  end

  @doc "Checks if the given value is in the set."
  @spec member?(t(), any()) :: boolean()
  def member?(%__MODULE__{set: set}, value), do: MapSet.member?(set, value)

  @doc "Returns the number of elements in the set."
  @spec size(t()) :: non_neg_integer()
  def size(%__MODULE__{size: s}), do: s

  @doc "Returns all elements as a list in insertion order (oldest → newest)."
  @spec to_list(t()) :: [any()]
  def to_list(%__MODULE__{queue: queue}), do: :queue.to_list(queue)

  @doc """
  Adds a new element to the set.

  - If it already exists, returns the set unchanged.
  - If full, evicts the oldest element (FIFO).
  """
  @spec put(t(), any()) :: t()
  def put(%__MODULE__{set: set, queue: queue, size: size, limit: limit} = s, value) do
    cond do
      MapSet.member?(set, value) ->
        s

      size < limit ->
        %{
          s
          | set: MapSet.put(set, value),
            queue: :queue.in(value, queue),
            size: size + 1
        }

      true ->
        {{:value, oldest}, q2} = :queue.out(queue)
        q2 = :queue.in(value, q2)
        set = set |> MapSet.delete(oldest) |> MapSet.put(value)
        %{s | set: set, queue: q2}
    end
  end

  @doc "Removes a value if it exists."
  @spec delete(t(), any()) :: t()
  def delete(%__MODULE__{set: set, queue: queue, size: size} = s, value) do
    if MapSet.member?(set, value) do
      new_queue = :queue.filter(&(&1 != value), queue)
      %{s | set: MapSet.delete(set, value), queue: new_queue, size: size - 1}
    else
      s
    end
  end

  @doc "Clears all elements."
  @spec clear(t()) :: t()
  def clear(%__MODULE__{limit: limit}), do: new(limit)

  defimpl Enumerable do
    def count(lset), do: {:ok, LimitedMapSet.size(lset)}
    def member?(lset, value), do: {:ok, LimitedMapSet.member?(lset, value)}
    def slice(_), do: {:error, __MODULE__}

    def reduce(lset, acc, fun) do
      Enumerable.List.reduce(LimitedMapSet.to_list(lset), acc, fun)
    end
  end

  defimpl Inspect do
    import Inspect.Algebra

    def inspect(%LimitedMapSet{limit: limit, size: size}, opts) do
      concat(["#LimitedMapSet<limit: ", to_doc(limit, opts), ", size: ", to_doc(size, opts), ">"])
    end
  end
end