defmodule Funx.Validator.In do
  @moduledoc """
  Validates that a value is a member of a given collection using an `Eq`
  comparator.

  `In` enforces a membership constraint of the form:
  "value must be one of these".

  Membership is defined by an `Eq` instance, not by structural equality or
  Elixir's `in` operator.

  Options

  - `:values` (required)
    The list of allowed values to compare against.

  - `:eq` (optional)
    An equality comparator. Defaults to `Funx.Eq.Protocol`.

  - `:message` (optional)
    A custom error message callback `(value -> String.t())`.

  Semantics

  - Succeeds if the value equals any element in `:values` under `Eq`,
    or if the value is a struct whose module is listed in `:values`.
  - Fails otherwise.
  - `Nothing` passes through.
  - `Just` is unwrapped before comparison.
  """

  @behaviour Funx.Validate.Behaviour

  alias Funx.Monad.Either
  alias Funx.Monad.Maybe.{Just, Nothing}
  alias Funx.Predicate
  alias Funx.Validator

  def validate(value) do
    validate(value, [])
  end

  def validate(value, opts) when is_list(opts) do
    validate(value, opts, %{})
  end

  @impl true
  def validate(value, opts, _env)

  def validate(%Nothing{} = value, _opts, _env) do
    Either.right(value)
  end

  def validate(%Just{value: v}, opts, _env) do
    validate_value(v, opts)
  end

  def validate(value, opts, _env) do
    validate_value(value, opts)
  end

  defp validate_value(value, opts) do
    values = Keyword.fetch!(opts, :values)
    predicate = Predicate.In.pred(opts)

    Either.lift_predicate(
      value,
      predicate,
      fn v ->
        rendered = Enum.map_join(values, ", ", &inspect/1)
        Validator.validation_error(opts, v, "must be one of: #{rendered}")
      end
    )
  end
end