class Surrogate
  ::RSpec::Matchers.define :substitute_for do |original_class, options={}|

    comparison  = nil
    subset_only = options[:subset]
    types       = options.fetch :types, true

    def comparing_fields(comparison, subset_only, types)
      fields = {}
      fields[:instance_not_on_actual   ] = comparison[:instance][:not_on_actual]
      fields[:class_not_on_actual      ] = comparison[:class   ][:not_on_actual]
      fields[:instance_not_on_surrogate] = comparison[:instance][:not_on_surrogate] unless subset_only
      fields[:class_not_on_surrogate   ] = comparison[:class   ][:not_on_surrogate] unless subset_only
      fields[:instance_types           ] = comparison[:instance][:types]            if types
      fields[:class_types              ] = comparison[:class   ][:types]            if types
      fields
    end

    match do |mocked_class|
      comparison = ApiComparer.new(mocked_class, original_class).compare
      comparing_fields(comparison, subset_only, types).values.inject(:+).empty?
    end

    failure_message_for_should do
      extra_instance_methods   = comparison[:instance][:not_on_actual   ].to_a
      extra_class_methods      = comparison[:class   ][:not_on_actual   ].to_a
      missing_instance_methods = comparison[:instance][:not_on_surrogate].to_a
      missing_class_methods    = comparison[:class   ][:not_on_surrogate].to_a
      instance_type_mismatch   = comparison[:instance][:types           ]
      class_type_mismatch      = comparison[:class   ][:types           ]


      differences = []
      differences << "has extra instance methods: #{extra_instance_methods.inspect}" if extra_instance_methods.any?
      differences << "has extra class methods: #{extra_class_methods.inspect}"       if extra_class_methods.any?
      differences << "is missing instance methods: #{missing_instance_methods}"      if !subset_only && missing_instance_methods.any?
      differences << "is missing class methods: #{missing_class_methods}"            if !subset_only && missing_class_methods.any?

      if types # this conditional is not tested, nor are these error messages
        instance_type_mismatch.each { |name, types| differences << "##{name} had types #{types.inspect}" }
        class_type_mismatch.each    { |name, types| differences << ".#{name} had types #{types.inspect}" }
      end
      "Was not substitutable because surrogate " << differences.join("\n")
    end

    failure_message_for_should_not do
      "Should not have been substitute, but was"
    end
  end
end