local oop = {}

oop.Visibility = {
    PUBLIC = "public",
    PROTECTED = "protected",
    PRIVATE = "private"
}

local tpack = table.pack or function(...)
    return { n = select("#", ...), ... }
end
local tunpack = table.unpack or unpack

local lookup_in_class
local lookup_with_owner
local lookup_visibility

local function super_call(self, method_name, ...)
    local class = getmetatable(self)
    if not class then
        error("super() called on non-instance")
    end
    local base = rawget(class, "__base")
    if not base then
        error("super() called but no base class")
    end
    local fn = base[method_name]
    if not fn then
        error("base class has no method '" .. tostring(method_name) .. "'")
    end
    return fn(self, ...)
end

local function_def_cache = setmetatable({}, { __mode = "k" })
oop._call_stack = {}

local function normalize_visibility(visibility)
    if visibility == nil then
        return oop.Visibility.PUBLIC
    end
    if visibility == oop.Visibility.PUBLIC or visibility == oop.Visibility.PROTECTED or visibility == oop.Visibility.PRIVATE then
        return visibility
    end
    error("visibility must be Visibility.PUBLIC, Visibility.PROTECTED, or Visibility.PRIVATE")
end

local function is_class_table(target)
    return type(target) == "table" and rawget(target, "__visibility") ~= nil
end

local function call_with_context(owner, fn, self, ...)
    local stack = oop._call_stack
    stack[#stack + 1] = owner
    local results = tpack(pcall(fn, self, ...))
    stack[#stack] = nil
    if not results[1] then
        error(results[2], 0)
    end
    return tunpack(results, 2, results.n)
end

local function current_caller()
    return oop._call_stack[#oop._call_stack]
end

local function can_access(owner, visibility, caller)
    if visibility == oop.Visibility.PUBLIC then
        return true
    end
    if not caller or not owner then
        return false
    end
    if visibility == oop.Visibility.PRIVATE then
        return caller == owner
    end
    if visibility == oop.Visibility.PROTECTED then
        return oop.issubclass(caller, owner)
    end
    return false
end

local function class_name(cls)
    if type(cls) ~= "table" then
        return tostring(cls)
    end
    return rawget(cls, "__name") or "Anonymous"
end

local function visibility_error(kind, member, visibility, owner, caller)
    local from = caller and class_name(caller) or "outside"
    local where = owner and class_name(owner) or "unknown"
    local msg = "access violation: " .. kind .. " '" .. tostring(member) .. "' is " .. visibility
        .. " in " .. where .. " (from " .. from .. ")"
    error(msg, 3)
end

function oop.setattr(target, key, value, visibility)
    target[key] = value
    if is_class_table(target) then
        local vis = normalize_visibility(visibility)
        target.__visibility[key] = vis
    end
end

function oop.new(class, ...)
    local actual_class = class
    if type(class) == "function" then
        actual_class = function_def_cache[class]
        if not actual_class then
            actual_class = oop.class(class)
            function_def_cache[class] = actual_class
        end
    end
    local obj = setmetatable({}, actual_class)
    rawset(obj, "__fields", {})
    local init_fn, init_owner = lookup_with_owner(actual_class, "__init")
    if init_fn then
        call_with_context(init_owner, init_fn, obj, ...)
    end
    return obj
end

function lookup_in_class(cls, key)
    local cur = cls
    while cur do
        local val = rawget(cur, key)
        if val ~= nil then
            return val
        end
        cur = rawget(cur, "__base")
    end
    return nil
end

function lookup_with_owner(cls, key)
    local cur = cls
    while cur do
        local val = rawget(cur, key)
        if val ~= nil then
            return val, cur
        end
        cur = rawget(cur, "__base")
    end
    return nil, nil
end

function lookup_visibility(cls, key)
    local cur = cls
    while cur do
        local vis_table = rawget(cur, "__visibility")
        if vis_table then
            local vis = vis_table[key]
            if vis then
                return vis, cur
            end
        end
        cur = rawget(cur, "__base")
    end
    return oop.Visibility.PUBLIC, nil
end

function oop.class(def, base)
    local cls = {}
    cls.__name = "Anonymous"
    cls.__base = base
    cls.__visibility = {}
    cls.visibility = oop.Visibility
    cls.__index = function(self, key)
        local fields = rawget(self, "__fields")
        if fields then
            local field_val = fields[key]
            if field_val ~= nil then
                local vis, owner = lookup_visibility(cls, key)
                if not can_access(owner, vis, current_caller()) then
                    visibility_error("field", key, vis, owner, current_caller())
                end
                return field_val
            end
        end

        local val, owner = lookup_with_owner(cls, key)
        if val == nil then
            return nil
        end
        local vis, vis_owner = lookup_visibility(cls, key)
        local access_owner = vis_owner or owner
        if not can_access(access_owner, vis, current_caller()) then
            visibility_error("member", key, vis, access_owner, current_caller())
        end
        if type(val) == "function" then
            return function(...)
                return call_with_context(owner, val, self, ...)
            end
        end
        return val
    end
    cls.__newindex = function(self, key, value)
        if key == "__fields" then
            rawset(self, key, value)
            return
        end
        local vis, owner = lookup_visibility(cls, key)
        if owner and not can_access(owner, vis, current_caller()) then
            visibility_error("field", key, vis, owner, current_caller())
        end
        local fields = rawget(self, "__fields")
        if not fields then
            fields = {}
            rawset(self, "__fields", fields)
        end
        fields[key] = value
    end

    function cls:super(method_name, ...)
        return super_call(self, method_name, ...)
    end
    function cls.setattr(target, key, value, visibility)
        return oop.setattr(target, key, value, visibility)
    end
    function cls.method(name, fn, visibility)
        if type(name) ~= "string" then
            error("method name must be a string")
        end
        if type(fn) ~= "function" then
            error("method must be a function")
        end
        cls[name] = fn
        cls.__visibility[name] = normalize_visibility(visibility)
    end
    function cls.inherit(parent)
        local actual_parent = parent
        if type(parent) == "function" then
            actual_parent = function_def_cache[parent]
            if not actual_parent then
                actual_parent = oop.class(parent)
                function_def_cache[parent] = actual_parent
            end
        end
        cls.__base = actual_parent
        cls.super = setmetatable({ __class = actual_parent }, {
            __index = function(_, key)
                local val = actual_parent[key]
                if type(val) == "function" then
                    return function(this, ...)
                        return call_with_context(actual_parent, val, this, ...)
                    end
                end
                return val
            end
        })
        local mt = getmetatable(cls) or {}
        mt.__index = actual_parent
        mt.__call = mt.__call or function(c, ...)
            return oop.new(c, ...)
        end
        setmetatable(cls, mt)
    end

    setmetatable(cls, {
        __index = base,
        __call = function(c, ...)
            return oop.new(c, ...)
        end
    })

    if type(def) == "string" then
        cls.__name = def
    elseif type(def) == "function" then
        def(cls)
    elseif def ~= nil then
        error("class definition must be a function or name string")
    end
    if base then
        cls.inherit(base)
    end

    return cls
end

function oop.isinstance(obj, class)
    local mt = getmetatable(obj)
    while mt do
        if mt == class then
            return true
        end
        mt = rawget(mt, "__base")
    end
    return false
end

function oop.issubclass(class, base)
    local mt = class
    while mt do
        if mt == base then
            return true
        end
        mt = rawget(mt, "__base")
    end
    return false
end

function oop.install(env)
    local target = env or _G
    target.class = oop.class
    target.new = oop.new
    target.setattr = oop.setattr
    target.isinstance = oop.isinstance
    target.issubclass = oop.issubclass
    target.Visibility = oop.Visibility
end

oop.install()

return oop