if not modules then modules = { } end modules ['luat-cbk'] = {
    version   = 1.001,
    comment   = "companion to luat-lib.mkiv",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files"
}

local insert, remove, concat = table.insert, table.remove, table.concat
local find, format = string.find, string.format
local collectgarbage, type, next = collectgarbage, type, next
local round = math.round
local sortedhash, sortedkeys, tohash = table.sortedhash, table.sortedkeys, table.tohash

-- Callbacks are the real asset of LuaTeX. They permit you to hook your own code
-- into the TeX engine. Here we implement a few handy auxiliary functions. Watch
-- out, there are diferences between LuateX and LuaMetaTeX.

callbacks       = callbacks or { }
local callbacks = callbacks

-- When you (temporarily) want to install a callback function, and after a while
-- wants to revert to the original one, you can use the following two functions.
-- This only works for non-frozen ones.

local trace_callbacks   = false  trackers.register("system.callbacks", function(v) trace_callbacks = v end)
local trace_calls       = false  -- only used when analyzing performance and initializations
local trace_checking    = false  trackers.register("memory.checking", function(v) trace_checking = v end)

local report_system     = logs.reporter("system")
local report_callbacks  = logs.reporter("system","callbacks")
local report_memory     = logs.reporter("system","memory")

local register_callback = callback.register
local find_callback     = callback.find
local list_callbacks    = callback.list
local register_usercall = false
local original_register = register_callback

local frozen            = { }
local stack             = { }
local list              = callbacks.list
local permit_overloads  = false
local block_overloads   = false

-- By now most callbacks are frozen and most provide a way to plug in your own code.
-- For instance all node list handlers provide before/after namespaces and the file
-- handling code can be extended by adding schemes and if needed I can add more
-- hooks. So there is no real need to overload a core callback function. It might be
-- ok for quick and dirty testing but anyway you're on your own if you permanently
-- overload callback functions.

-- This might become a configuration file only option when it gets abused too much.

directives.register("system.callbacks.permitoverloads", function(v)
    if block_overloads or permit_overloads then
        -- once bad news, always bad news
    elseif v then
        permit_overloads = { }
        report_system()
        report_system("The callback system has been brought in an unprotected state. As a result of directly")
        report_system("setting of callbacks subsystems of ConTeXt can stop working. There is no support for")
        report_system("bugs resulting from this state. It's better to use the official extension mechanisms.")
        report_system()
    end
end)

sandbox.initializer {
    category = "functions",
    action   = function()
        block_overloads = true
    end
}

if not list then -- otherwise counters get reset

    list = utilities.storage.allocate(list_callbacks())

    local supported = { }

    for k in next, list do
        list[k]      = 0
        supported[k] = true
    end

    callbacks.list      = list
    callbacks.supported = supported

end

local delayed = tohash {
    "buildpage_filter",
}

if trace_calls then

    local functions = { }

    register_callback = function(name,func)
        if type(func) == "function" then
            if functions[name] then
                functions[name] = func
                return find_callback(name)
            else
                functions[name] = func
                local cnuf = function(...)
                    list[name] = list[name] + 1
                    return functions[name](...)
                end
                return original_register(name,cnuf)
            end
        else
            return original_register(name,func)
        end
    end

end

-- temporary, not public:

callbacks.functions = { }

-- till here

local reported = { }

local function register_usercall(what,name,func)
    if list[name] then
        if trace_callbacks or not reported[name] then
            report_system()
            report_system("disabling core code by %s user function into callback '%s' (reported only once)",what,name)
            report_system()
            reported[name] = true
        end
        permit_overloads[name] = true
        return original_register(name,function(...)
            if trace_callbacks then
                report_callbacks("calling user function from '%s'",name)
            end
            return func(...)
        end)
    else
        report_callbacks("not %s function into invalid callback '%s'",name)
        return nil, format("unknown callback '%s'",name)
    end
end

local function frozen_callback(name)
    report_callbacks("not %s frozen %a","registering",name)
    return nil, format("callback '%s' is frozen",name) -- no formatter yet
end

local function state(name)
    local f = find_callback(name)
    if f == false then
        return "disabled"
    elseif f then
        return "enabled"
    else
        return "undefined"
    end
end

function callbacks.known(name)
    return list[name]
end

function callbacks.report()
    local usage = list_callbacks()
    for name, _ in sortedhash(list) do
        local str = frozen[name]
        local sta = state(name)
        local use = usage[name] and "+" or "-"
        if str then
            report_callbacks("%-9s : %s : %-22s -> %s",sta,use,name,str)
        else
            report_callbacks("%-9s : %s : %-22s",      sta,use,name)
        end
    end
end

function callbacks.freeze(name,freeze)
    if not permit_overloads then
        freeze = type(freeze) == "string" and freeze
        if find(name,"*",1,true) then
            local pattern = name
            for name, _ in next, list do
                if find(name,pattern) then
                    frozen[name] = freeze or frozen[name] or "frozen"
                end
            end
        else
            frozen[name] = freeze or frozen[name] or "frozen"
        end
    end
end

function callbacks.register(name,func,freeze)
    if frozen[name] then
        if permit_overloads then
            return register_usercall("registering",name,func)
        else
            return frozen_callback(name)
        end
    elseif freeze then
        frozen[name] = type(freeze) == "string" and freeze or "registered"
    end
    if delayed[name] and environment.initex then
        return nil
    end
    return register_callback(name,func)
end

function callback.register(name,func) -- original
    if not frozen[name] then
        return register_callback(name,func)
    elseif permit_overloads then
        return register_usercall("registering",name,func)
    else
        return frozen_callback(name)
    end
end

function callbacks.push(name,func)
    if not frozen[name] or permit_overloads then
        local sn = stack[name]
        if not sn then
            sn = { }
            stack[name] = sn
        end
        insert(sn,find_callback(name))
        if permit_overloads then
            register_usercall("pushing",name,func)
        else
            register_callback(name,func)
        end
    else
        report_callbacks("not %s frozen %a","pushing",name)
    end
end

function callbacks.pop(name)
    if not frozen[name] or permit_overloads then
        local sn = stack[name]
        if not sn or #sn == 0 then
            -- some error
            register_callback(name,nil) -- ! really needed
        else
         -- this fails: register_callback(name, remove(stack[name]))
            local func = remove(sn)
            register_callback(name,func)
        end
    end
end

if trace_calls then
    statistics.register("callback details", function()
        local t = { } -- todo: pass function to register and quit at nil
        for name, n in sortedhash(list) do
            if n > 0 then
                t[#t+1] = format("%s -> %s",name,n)
            end
        end
        return concat(t," ")
    end)
end

statistics.register("callbacks overloaded by user", function()
    if permit_overloads then
        return concat(sortedkeys(permit_overloads)," ")
    end
end)

-- this will move to a module

commands = commands or { }

function commands.showcallbacks()
    local NC, NR, verbatim = context.NC, context.NR, context.type
    context.starttabulate { "|l|l|p|" }
    for name, _ in sortedhash(list) do
        NC() verbatim(name) NC() verbatim(state(name)) NC() context(frozen[name] or "") NC() NR()
    end
    context.stoptabulate()
end