-- Class: utilities for setting up inheritace and polymorphism.


-- Create a class, possibly with a base class.
-- The metatable and the __index are setup accordingly.
-- The constructor is accessed by the function call operator.
-- A default constructor is defined with no parameters,
-- and it can be overrided by defining an _init method.
-- Returns the class.
-- Usage example:
--
-- MyClass = Class()
--
-- function MyClass:_init(x, y)
--   if x < 0 then
--     return nil
--   end
--
--   self.x = x
--   self.y = y
--   return self
-- end
--
-- function MyClass:method()
--   print(self.x, self.y)
-- end
--
--
-- Derived = Class(MyClass)
--
-- function Derived:_init(x)
--   if not MyClass._init(self, x, x) then
--     return nil
--   end
--
--   return self
-- end
--
-- function Derived:method()
--   print(self.x)
-- end
--
--
-- obj = Derived(5)
-- obj:method() -- prints "5"
function Class(base)
  local class = setmetatable(
    {},
    {
      __index = base,
      __call = function (class, ...)
        local self = setmetatable({}, class)

        return type(self._init) == "function"  -- check for a user defined constructor.
          and self:_init(...)
          or self
      end
    }
  )

  class.__index = class

  return class
end