classes.lua

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local classes = {}

-- Baseclass of all objects.
classes.Object = {}
classes.Object.class = classes.Object

--- Nullary constructor.
function classes.Object:init(...)
end

--- Base alloc method.
function classes.Object.alloc(mastertable)
    return setmetatable({}, {__index = classes.Object, __newindex = mastertable})
end

--- Base new method.
function classes.Object.new(...)
    return classes.Object.alloc({}):init(...)
end

--- Checks if this object is an instance of class.
-- @param class The class object to check.
-- @return Returns true if this object is an instance of class, false otherwise.
function classes.Object:instanceOf(class)
    -- Recurse up the supertypes until class is found, or until the supertype is not part of the inheritance tree.
    if self.class == class then
        return true
    end
    if self.super then
        return self.super.__base:instanceOf(class)
    end
    return false
end

--- Creates a new class.
-- @param baseclass The Baseclass of this class, or nil.
-- @return A new class reference.
function classes.class(baseclass)
    -- Create the class definition and metatable.
    local classdef = {}

    -- Find the super class, either Object or user-defined.
    baseclass = baseclass or classes.Object

    -- If this class definition does not know of a function, it will 'look up' to the Baseclass via the __index of the metatable.
    setmetatable(classdef, {__index = baseclass})

    -- All class instances have a reference to the class object.
    classdef.class = classdef

    -- Looks up a key 'k' by traversing up the inheritance tree until a match is found.
    local function lookup(t, k, rootInstance, fromSuper)
        -- Quick value lookup, usually anything that is not a function will be set on the root instance.
        local val = rawget(rootInstance, k)
        if val ~= nil then return val end

        -- When calling via the 'super' reference, we start traversing from that superclass.
        -- Otherwise always start at the root of the inheritance tree.
        local current
        if fromSuper then
            current = rawget(t, '__base')
        else
            current = rootInstance
        end
        repeat
            val = rawget(current.class, k)
            if val ~= nil then
                if type(val) == 'function' and val ~= classes.Object.instanceOf then
                    -- Instead of returning the found function (which would be run with the original instance as self),
                    -- we pack it in another function which calls it with the 'current' level of the inheritance tree.
                    return function(tt, ...)
                        return val(current, ...)
                    end
                else -- When calling instanceOf (which requires the root instance to work) or anything that's not function.
                    return val
                end
            end

            -- We did not find the function in the class definition so we repeat the lookup on super.
            if current.super then
                current = rawget(current.super, '__base')
            else
                current = nil
            end
        until val ~= nil or current == nil
    end

    --- Recursively allocates the inheritance tree of the instance.
    -- @param rootInstance The root of the inheritance tree.
    -- @return Returns the instance with the allocated inheritance tree.
    function classdef.alloc(rootInstance)
        local instance = {
            class = classdef,
            super = { __base = baseclass.alloc(rootInstance) }
        }

        setmetatable(instance.super, {__index = function(t, k) return lookup(t, k, rootInstance, true) end, __newindex = rootInstance})
        setmetatable(instance, {__index = function(t, k) return lookup(t, k, rootInstance, false) end, __newindex = rootInstance})

        return instance
    end

    --- Constructs a new instance from this class definition.
    -- @param arg Arguments to this class' constructor
    -- @return Returns a new instance of this class.
    function classdef.new(...)
        -- Create the empty object.
        local instance = {}
        instance.class = classdef

        -- Store the reference to the superclass instance in a separate table with its own metatable.
        -- This way we can tell accesses via self.super apart (e.g. self:function() vs. self.super:function()).
        instance.super = { __base = baseclass.alloc(instance) }

        -- Accesses/reads (__index) get redirected to a lookup function which will traverse up the inheritance tree.
        -- Assignment/writes (__newindex) get directed to the root instance itself.
        setmetatable(instance.super, { __index = function(t, k) return lookup(t, k, instance, true) end })
        setmetatable(instance, {__index = function(t, k) return lookup(t, k, instance, false) end})

        -- Finally, init the object, it is up to the programmer to choose to call the super init method.
        instance:init(...)

        return instance
    end

    -- Finally, return the class we created.
    return classdef
end

return classes

-- CREDITS: based on Paul Moore's classes library

--[[
    The classes library enables simple OOP constructs using prototypes and meta-tables.
   
    @author Paul Moore
   
    Copyright (C) 2011 by Strange Ideas Software
   
    Permission is hereby granted, free of charge, to any person obtaining a copy
    of this software and associated documentation files (the "Software"), to deal
    in the Software without restriction, including without limitation the rights
    to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
    copies of the Software, and to permit persons to whom the Software is
    furnished to do so, subject to the following conditions:
   
    The above copyright notice and this permission notice shall be included in
    all copies or substantial portions of the Software.
   
    THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
    IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
    FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
    AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
    LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
    OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
    THE SOFTWARE.
]]