module JSON

# File lib/json/mapping.rb, line 11
def self.mapping(klass, fields)
  fields.each do |name, typ|
    klass.attr_reader name
    klass.define_method "#{name}=" do |value|
      value = typ.new(value) if typ.respond_to?(:from_json)
      unless value.is_a?(typ)
        raise JSON::Mapping::Error, "Wrong field type #{self.class} for '#{name}' - should be '#{typ}'"
      end

      instance_variable_set "@#{name}", value
    end
  end

  klass.define_method :initialize do |params = {}|
    params.each do |name, value|
      unless fields.key?(name.to_sym)
        raise JSON::Mapping::Error, "Missing field definition for '#{name}'"
      end

      send("#{name}=", value)
    end
  end

  klass.define_singleton_method :from_json do |string|
    klass.new(JSON.parse(string))
  end
end