class GraphQL::StaticValidation::FragmentsAreUsed

def add_errors(context, used_fragments, defined_fragments)
  undefined_fragments = find_difference(used_fragments, defined_fragments.map(&:name))
  undefined_fragments.each do |fragment|
    context.errors << message("Fragment #{fragment.name} was used, but not defined", fragment.node, path: fragment.path)
  end
  unused_fragments = find_difference(defined_fragments, used_fragments.map(&:name))
  unused_fragments.each do |fragment|
    context.errors << message("Fragment #{fragment.name} was defined, but not used", fragment.node, path: fragment.path)
  end
end

def find_difference(fragments, allowed_fragment_names)
  fragments.select {|f| f.name && !allowed_fragment_names.include?(f.name) }
end

def validate(context)
  v = context.visitor
  used_fragments = []
  defined_fragments = []
  v[GraphQL::Language::Nodes::Document] << -> (node, parent) {
    defined_fragments = node.definitions
      .select { |defn| defn.is_a?(GraphQL::Language::Nodes::FragmentDefinition) }
      .map { |node| FragmentInstance.new(node: node, path: context.path) }
  }
  v[GraphQL::Language::Nodes::FragmentSpread] << -> (node, parent) {
    used_fragments << FragmentInstance.new(node: node, path: context.path)
    if defined_fragments.none? { |defn| defn.name == node.name }
      GraphQL::Language::Visitor::SKIP
    end
  }
  v[GraphQL::Language::Nodes::Document].leave << -> (node, parent) { add_errors(context, used_fragments, defined_fragments) }
end