module ViewModel::Controller::MigrationVersions

def migration_versions
  @migration_versions ||=
    begin
      specified_migration_versions.reject do |viewmodel_class, required_version|
        viewmodel_class.schema_version == required_version
      end.freeze
    end
end

def specified_migration_versions
  @specified_migration_versions ||=
    begin
      version_spec =
        if params.include?(:versions)
          params[:versions]
        elsif request.headers.include?(MIGRATION_VERSION_HEADER)
          begin
            JSON.parse(request.headers[MIGRATION_VERSION_HEADER])
          rescue JSON::ParserError
            raise ViewModel::Error.new(status: 400, detail: "Invalid JSON in #{MIGRATION_VERSION_HEADER}")
          end
        else
          {}
        end
      versions =
        IknowParams::Parser.parse_value(
          version_spec,
          with: IknowParams::Serializer::HashOf.new(
            IknowParams::Serializer::String, IknowParams::Serializer::Integer))
      migration_versions = {}
      versions.each do |view_name, required_version|
        viewmodel_class = ViewModel::Registry.for_view_name(view_name)
        migration_versions[viewmodel_class] = required_version
      rescue ViewModel::DeserializationError::UnknownView
        # Ignore requests to migrate types that no longer exist
        next
      end
      migration_versions.freeze
    end
end