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+--- Provides a reuseable and convenient framework for creating classes in Lua.
+-- Two possible notations:
+--
+--    B = class(A)
+--    class.B(A)
+--
+-- The latter form creates a named class within the current environment. Note
+-- that this implicitly brings in `pl.utils` as a dependency.
+--
+-- See the Guide for further @{01-introduction.md.Simplifying_Object_Oriented_Programming_in_Lua|discussion}
+-- @module pl.class
+
+local error, getmetatable, io, pairs, rawget, rawset, setmetatable, tostring, type =
+    _G.error, _G.getmetatable, _G.io, _G.pairs, _G.rawget, _G.rawset, _G.setmetatable, _G.tostring, _G.type
+local compat
+
+-- this trickery is necessary to prevent the inheritance of 'super' and
+-- the resulting recursive call problems.
+local function call_ctor (c,obj,...)
+    -- nice alias for the base class ctor
+    local base = rawget(c,'_base')
+    if base then
+        local parent_ctor = rawget(base,'_init')
+        while not parent_ctor do
+            base = rawget(base,'_base')
+            if not base then break end
+            parent_ctor = rawget(base,'_init')
+        end
+        if parent_ctor then
+            rawset(obj,'super',function(obj,...)
+                call_ctor(base,obj,...)
+            end)
+        end
+    end
+    local res = c._init(obj,...)
+    rawset(obj,'super',nil)
+    return res
+end
+
+--- initializes an __instance__ upon creation.
+-- @function class:_init
+-- @param ... parameters passed to the constructor
+-- @usage local Cat = class()
+-- function Cat:_init(name)
+--   --self:super(name)   -- call the ancestor initializer if needed
+--   self.name = name
+-- end
+--
+-- local pussycat = Cat("pussycat")
+-- print(pussycat.name)  --> pussycat
+
+--- checks whether an __instance__ is derived from some class.
+-- Works the other way around as `class_of`.
+-- @function instance:is_a
+-- @param some_class class to check against
+-- @return `true` if `instance` is derived from `some_class`
+-- @usage local pussycat = Lion()  -- assuming Lion derives from Cat
+-- if pussycat:is_a(Cat) then
+--   -- it's true
+-- end
+local function is_a(self,klass)
+    local m = getmetatable(self)
+    if not m then return false end --*can't be an object!
+    while m do
+        if m == klass then return true end
+        m = rawget(m,'_base')
+    end
+    return false
+end
+
+--- checks whether an __instance__ is derived from some class.
+-- Works the other way around as `is_a`.
+-- @function some_class:class_of
+-- @param some_instance instance to check against
+-- @return `true` if `some_instance` is derived from `some_class`
+-- @usage local pussycat = Lion()  -- assuming Lion derives from Cat
+-- if Cat:class_of(pussycat) then
+--   -- it's true
+-- end
+local function class_of(klass,obj)
+    if type(klass) ~= 'table' or not rawget(klass,'is_a') then return false end
+    return klass.is_a(obj,klass)
+end
+
+--- cast an object to another class.
+-- It is not clever (or safe!) so use carefully.
+-- @param some_instance the object to be changed
+-- @function some_class:cast
+local function cast (klass, obj)
+    return setmetatable(obj,klass)
+end
+
+
+local function _class_tostring (obj)
+    local mt = obj._class
+    local name = rawget(mt,'_name')
+    setmetatable(obj,nil)
+    local str = tostring(obj)
+    setmetatable(obj,mt)
+    if name then str = name ..str:gsub('table','') end
+    return str
+end
+
+local function tupdate(td,ts,dont_override)
+    for k,v in pairs(ts) do
+        if not dont_override or td[k] == nil then
+            td[k] = v
+        end
+    end
+end
+
+local function _class(base,c_arg,c)
+    -- the class `c` will be the metatable for all its objects,
+    -- and they will look up their methods in it.
+    local mt = {}   -- a metatable for the class to support __call and _handler
+    -- can define class by passing it a plain table of methods
+    local plain = type(base) == 'table' and not getmetatable(base)
+    if plain then
+        c = base
+        base = c._base
+    else
+        c = c or {}
+    end
+   
+    if type(base) == 'table' then
+        -- our new class is a shallow copy of the base class!
+        -- but be careful not to wipe out any methods we have been given at this point!
+        tupdate(c,base,plain)
+        c._base = base
+        -- inherit the 'not found' handler, if present
+        if rawget(c,'_handler') then mt.__index = c._handler end
+    elseif base ~= nil then
+        error("must derive from a table type",3)
+    end
+
+    c.__index = c
+    setmetatable(c,mt)
+    if not plain then
+        c._init = nil
+    end
+
+    if base and rawget(base,'_class_init') then
+        base._class_init(c,c_arg)
+    end
+
+    -- expose a ctor which can be called by <classname>(<args>)
+    mt.__call = function(class_tbl,...)
+        local obj
+        if rawget(c,'_create') then obj = c._create(...) end
+        if not obj then obj = {} end
+        setmetatable(obj,c)
+
+        if rawget(c,'_init') then -- explicit constructor
+            local res = call_ctor(c,obj,...)
+            if res then -- _if_ a ctor returns a value, it becomes the object...
+                obj = res
+                setmetatable(obj,c)
+            end
+        elseif base and rawget(base,'_init') then -- default constructor
+            -- make sure that any stuff from the base class is initialized!
+            call_ctor(base,obj,...)
+        end
+
+        if base and rawget(base,'_post_init') then
+            base._post_init(obj)
+        end
+
+        if not rawget(c,'__tostring') then
+            c.__tostring = _class_tostring
+        end
+        return obj
+    end
+    -- Call Class.catch to set a handler for methods/properties not found in the class!
+    c.catch = function(self, handler)
+        if type(self) == "function" then
+            -- called using . instead of :
+            handler = self
+        end
+        c._handler = handler
+        mt.__index = handler
+    end
+    c.is_a = is_a
+    c.class_of = class_of
+    c.cast = cast
+    c._class = c
+
+    return c
+end
+
+--- create a new class, derived from a given base class.
+-- Supporting two class creation syntaxes:
+-- either `Name = class(base)` or `class.Name(base)`.
+-- The first form returns the class directly and does not set its `_name`.
+-- The second form creates a variable `Name` in the current environment set
+-- to the class, and also sets `_name`.
+-- @function class
+-- @param base optional base class
+-- @param c_arg optional parameter to class constructor
+-- @param c optional table to be used as class
+local class
+class = setmetatable({},{
+    __call = function(fun,...)
+        return _class(...)
+    end,
+    __index = function(tbl,key)
+        if key == 'class' then
+            io.stderr:write('require("pl.class").class is deprecated. Use require("pl.class")\n')
+            return class
+        end
+        compat = compat or require 'pl.compat'
+        local env = compat.getfenv(2)
+        return function(...)
+            local c = _class(...)
+            c._name = key
+            rawset(env,key,c)
+            return c
+        end
+    end
+})
+
+class.properties = class()
+
+function class.properties._class_init(klass)
+    klass.__index = function(t,key)
+        -- normal class lookup!
+        local v = klass[key]
+        if v then return v end
+        -- is it a getter?
+        v = rawget(klass,'get_'..key)
+        if v then
+            return v(t)
+        end
+        -- is it a field?
+        return rawget(t,'_'..key)
+    end
+    klass.__newindex = function (t,key,value)
+        -- if there's a setter, use that, otherwise directly set table
+        local p = 'set_'..key
+        local setter = klass[p]
+        if setter then
+            setter(t,value)
+        else
+            rawset(t,key,value)
+        end
+    end
+end
+
+
+return class
+