class.lua

-- class.lua
-- Compatible with Lua 5.1 (not 5.0).
function class(base, init)
    local c = {}    -- a new class instance
    if not init and type(base) == 'function' then
        init = base
        base = nil
    elseif type(base) == 'table' then
     -- our new class is a shallow copy of the base class!
        for i,v in pairs(base) do
            c[i] = v
        end
        c._base = base
    end
    -- the class will be the metatable for all its objects,
    -- and they will look up their methods in it.
    c.__index = c

    -- expose a constructor which can be called by <classname>(<args>)
    local mt = {}
    mt.__call = function(class_tbl, ...)
        local obj = {}
        setmetatable(obj,c)
        if init then
            init(obj,...)
        else
            -- make sure that any stuff from the base class is initialized!
            if base and base.init then
                base.init(obj, ...)
            end
        end
        return obj
    end
    c.init = init
    c.is_a = function(self, klass)
        local m = getmetatable(self)
        while m do
            if m == klass then return true end
            m = m._base
        end
        return false
    end
    setmetatable(c, mt)
    return c
end