--[[ Prototype Delegation and Cloning with Mixins Module
     H. Conrad Cunningham, Professor
     Computer and Information Science
     University of Mississippi

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2016-10-31: Completed first version
2016-11-02: Refined comments

A prototype-based language does not natively have the concept of 
class. It just has objects. Instead of using a class to instantiate
a new object, a program "copies" (or clones) an existing object
--the prototype--and modifies the copy to have the needed attributes
(variables) and operations (functions or methods).

Each prototype consists of a collection of slots. Each slot is filled
with either a data attribute or an operation

In Lua, the "copying" can occur by either physically copying
a variable or function closure reference into the new object's slot
or by delegating unchanged slots back to the prototype object itself.
Lua can use metaables and the __index metamethod to set up the
delegation.

This module provides two constructor functions, set up to use 
the method syntax. Prototype:new creates a new object with 
delegation of unchanged slots. Prototype:clone creates a new 
object that is a shallow copy of the prototype.

--]]

local Prototype = {}
Prototype.__index = Prototype

-- Constructor method "new" creates a new object with its fields
-- and methods copied from the table "mixin" but with other
-- references delegated to object "self". A call of "Prototype:new"
-- creates a basic object that delegates to object Prototype. If
-- "obj" has been created by this "new", then a call of "obj:new"
-- creates a new object that delegates to "obj".

function Prototype:new(mixin)
   mixin = mixin or {}
   local obj = { __index = self }
   for k, v in pairs(mixin) do
      if k ~= "__index" then
         obj[k] = v
      end
   end
   return setmetatable(obj,obj)
end

-- Constructor method "clone" creates a new object with the fields
-- and methods copied from both object "self" and "mixin".
-- It preserves the prototype delegation from "self".

function Prototype:clone(mixin)
   mixin = mixin or {}
   local obj = {}
   for k, v in pairs(self) do
      obj[k] = v
   end
   for k, v in pairs(mixin) do
      if k ~= "__index" then
         obj[k] = v
      end
   end
   return setmetatable(obj,obj)
end

return Prototype