class.lua

-- class.lua
-- defines C++ style classes using the same behavior as luaL_register
-- all classes are stored in the LUA_REGISTRYINDEX, support single
-- inheritance, metamethod inheritance, and implicit mixin support with
-- a user-defined table.merge.
-- distributed under the MIT license

local setmetatable = setmetatable
local package = package
local type = type
local error = error
local pcall = pcall
local unpack = unpack
local newproxy = newproxy
local gsub = string.gsub
local rawget = rawget
local ipairs = ipairs
local module = module
local _G = _G
local _R = debug.getregistry()

local function new (metatable)
  local object = {}
  setmetatable(object, metatable)
  return object
end

local function getbaseclass (metatable)
  local base = metatable.__base
  return package.loaded[base]
end

_G.getbaseclass = getbaseclass

local eventnames = {
  "__gc",
  "__add", "__sub", "__mul", "__div", "__mod",
  "__pow", "__unm", "__len", "__lt", "__le",
  "__concat", "__call",
  "__tostring"
}

local function metamethod (metatable, eventname)
  return function (...)
    local event = nil
    local base = getbaseclass(metatable)
    while base ~= nil do
      if base[eventname] then
        event = base[eventname]
        break
      end
      base = getbaseclass(base)
    end
    local type = type(event)
    if type ~= "function" then
      error("attempt to call metamethod '" .. eventname .. "' " ..
            "(a " .. type .. " value)", 2)
    end
    local returns = {pcall(event, ...)}
    if returns[1] ~= true then
      error(returns[2], 2)
    else
      return unpack(returns, 2)
    end
  end
end

local function setproxy (object)
  local __newproxy = newproxy(true)
  local metatable = getmetatable(__newproxy)
  metatable.__gc = function ()
    object:__gc()
  end
  object.__newproxy = __newproxy
end

_G.setproxy = setproxy

local function classinit (module)
  module.__index = module
  module.__type = gsub(module._NAME, module._PACKAGE, "")
  -- Override modinit from loadlib.c
  module._M = nil
  module._NAME = nil
  module._PACKAGE = nil
  -- Create a shortcut to name()
  setmetatable(module, {
    __call = function (self, ...)
      -- Create an instance of this object
      local object = new(self)
      -- Call its constructor (function name:name ( ... ) ... end) if it
      -- exists
      local constructor = rawget(self, self.__type)
      if constructor ~= nil then
        local type = type(constructor)
        if type ~= "function" then
          error("attempt to call constructor '" .. name .. "' " ..
                "(a " .. type .. " value)", 2)
        end
        constructor(object, ...)
      end
      -- Return the instance
      return object
    end
  })
end

local function inherit (base)
  return function (metatable)
    -- Set our base class
    metatable.__base = base
    -- Overwrite our existing __index value with a metamethod which checks
    -- table fields, metatable, and base class, in that order, per behavior
    -- via the Lua 5.1 manual's illustrative code for indexing access
    metatable.__index = function (table, key)
      local h
      -- if type(table) == "table" then
        local v = rawget(metatable, key)
        if v ~= nil then return v end
        local baseclass = getbaseclass(metatable)
        if baseclass == nil then
          error("attempt to index base class '" .. base .. "' " ..
                "(a nil value)", 2)
        end
        h = baseclass.__index
        if h == nil then return nil end
      -- else
      --   h = getmetatable(table).__index
      --   if h == nil then
      --     error(...)
      --   end
      -- end
      if type(h) == "function" then
        return (h(table, key))     -- call the handler
      else return h[key]           -- or repeat operation on it
      end
    end
    -- Create inheritable metamethods
    for _,event in ipairs(eventnames) do
      metatable[event] = metatable[event] or metamethod(metatable, event)
    end
  end
end

function class (modname)
  local function setmodule (modname)
    module(modname, classinit)
  end setmodule(modname)
  _R[modname] = package.loaded[modname]
  -- For syntactic sugar, return a function to set inheritance
  return function (base)
    local metatable = package.loaded[modname]
    inherit(base)(metatable)
  end
end