Rewrite of https://github.com/rails/rails/blob/9f141a423d551f7f421f54d1372e65ef6ed1f0be/railties/lib/rails/initializable.rb for fun

initializable.rb

require "tsort"

module Rails
  module Initializable
    extend ActiveSupport::Concern

    def run_initializers(group = :default, *arguments)
      @ran ||= true.tap do
        Initializer.tsort(initializers).each { instance_exec(*arguments, &_1.block) if _1.in?(group) }
      end
    end

    def initializers
      self.class.ancestors.reverse.flat_map { _1.initializers if _1.respond_to?(:initializers) }
    end

    class_methods do
      def initializer(name, group: :default, before: nil, after: nil, &block)
        raise ArgumentError, "A block must be passed when defining an initializer" unless block

        after ||= initializers.last&.name unless initializers.any? { _1.name == before }
        initializers << Initializer.new(name:, block:, group:, before:, after:)
      end

      def initializers = @initializers ||= []
    end

    class Initializer < Struct.new(:name, :block, :group, :before, :after, keyword_init: true)
      def initialize(name:, block:, group: :default, before: nil, after: nil) = super

      def self.tsort(initializers)
        TSort.tsort initializers.to_enum,
          ->(initializer, &block) { initializer.pluck(initializers).each(&block) }
      end

      def pluck(initializers)
        initializers.select { _1.before == name || after == _1.name }
      end

      # Alternative to pluck above to try to name, but the original inner select makes no sense to me.
      def around?(other) = self < other || self > other
      def >(other) = other.before == name
      def <(other) = after == other.name

      def in?(group)
        @group == group || @group == :all
      end
    end
  end
end