node-ref.lua / last modification: 2020-01-30 14:16
if not modules then modules = { } end modules ['node-ref'] = {
    version   = 1.001,
    comment   = "companion to node-ref.mkiv",
    author    = "Hans Hagen, PRAGMA-ADE, Hasselt NL",
    copyright = "PRAGMA ADE / ConTeXt Development Team",
    license   = "see context related readme files"
}

-- We supported pdf right from the start and in mkii this has resulted in
-- extensive control over the links. Nowadays pdftex provides a lot more
-- control over margins but as mkii supports multiple backends we stuck to
-- our own mechanisms. In mkiv again we implement our own handling. Eventually
-- we will even disable the pdf primitives.

-- helper, will end up in luatex

-- is grouplevel still used?

local tonumber = tonumber
local concat = table.concat

local attributes, nodes, node = attributes, nodes, node

local allocate             = utilities.storage.allocate, utilities.storage.mark
local mark                 = utilities.storage.allocate, utilities.storage.mark

local nodeinjections       = backends.nodeinjections
local codeinjections       = backends.codeinjections

local cleanupreferences    = false
local cleanupdestinations  = true

local transparencies       = attributes.transparencies
local colors               = attributes.colors
local references           = structures.references
local enableaction         = nodes.tasks.enableaction

local trace_references     = false  trackers.register("nodes.references",        function(v) trace_references   = v end)
local trace_destinations   = false  trackers.register("nodes.destinations",      function(v) trace_destinations = v end)
local trace_areas          = false  trackers.register("nodes.areas",             function(v) trace_areas        = v end)
local show_references      = false  trackers.register("nodes.references.show",   function(v) show_references    = tonumber(v) or (v and 2.25 or false) end)
local show_destinations    = false  trackers.register("nodes.destinations.show", function(v) show_destinations  = tonumber(v) or (v and 2.00 or false) end)

local report_reference     = logs.reporter("backend","references")
local report_destination   = logs.reporter("backend","destinations")
local report_area          = logs.reporter("backend","areas")

local nuts                 = nodes.nuts
local nodepool             = nuts.pool

local tonode               = nuts.tonode
local tonut                = nuts.tonut

local getfield             = nuts.getfield
local setlink              = nuts.setlink
local setnext              = nuts.setnext
local setprev              = nuts.setprev
local getnext              = nuts.getnext
local getprev              = nuts.getprev
local getid                = nuts.getid
local getlist              = nuts.getlist
local setlist              = nuts.setlist
local getwidth             = nuts.getwidth
local setwidth             = nuts.setwidth
local getheight            = nuts.getheight
local getattr              = nuts.getattr
local setattr              = nuts.setattr
local getsubtype           = nuts.getsubtype
local getwhd               = nuts.getwhd
local getdirection         = nuts.getdirection
local setshift             = nuts.setshift
local getboxglue           = nuts.getboxglue

local hpack_list           = nuts.hpack
local vpack_list           = nuts.vpack
local getdimensions        = nuts.dimensions
local getrangedimensions   = nuts.rangedimensions
local traverse             = nuts.traverse
local find_node_tail       = nuts.tail
local start_of_par         = nuts.start_of_par

local nodecodes            = nodes.nodecodes
local gluecodes            = nodes.gluecodes
local listcodes            = nodes.listcodes

local dirvalues            = nodes.dirvalues
local lefttoright_code     = dirvalues.lefttoright
local righttoleft_code     = dirvalues.righttoleft

local hlist_code           = nodecodes.hlist
local vlist_code           = nodecodes.vlist
local glue_code            = nodecodes.glue
local glyph_code           = nodecodes.glyph
local rule_code            = nodecodes.rule
local dir_code             = nodecodes.dir
local localpar_code        = nodecodes.localpar

local leftskip_code        = gluecodes.leftskip
local rightskip_code       = gluecodes.rightskip
local parfillskip_code     = gluecodes.parfillskip

----- linelist_code        = listcodes.line

local new_rule             = nodepool.rule
local new_kern             = nodepool.kern
local new_hlist            = nodepool.hlist

local flush_node           = nuts.flush

local tosequence           = nodes.tosequence

local implement            = interfaces.implement

-- Normally a (destination) area is a box or a simple stretch if nodes but when it is
-- a paragraph we have a problem: we cannot calculate the height well. This happens
-- with footnotes or content broken across a page.

local function hlist_dimensions(start,stop,parent)
    local last = stop and getnext(stop)
    if parent then
        return getrangedimensions(parent,start,last)
    else
        return getdimensions(start,last)
    end
end

local function vlist_dimensions(start,stop) -- also needs the stretch and so
    local temp
    if stop then
        temp = getnext(stop)
        setnext(stop,nil)
    end
    local v = vpack_list(start)
    local w, h, d = getwhd(v)
    setlist(v) -- not needed
    flush_node(v)
    if temp then
        setnext(stop,temp)
    end
    return w, h, d
end

-- not ok when vlist at mvl level

local function dimensions(parent,start,stop) -- in principle we could move some to the caller
    local id = getid(start)
    if start == stop then
        if id == hlist_code or id == vlist_code or id == rule_code or id == glyph_code then
            local sw, sh, sd = getwhd(start)
            local pw, ph, pd = getwhd(parent)
            local ht = sh == 0 and ph or sh -- changed
            local dp = sd == 0 and pd or sd -- changed
            if trace_areas then
                report_area("dimensions taken of %a (%p,%p,%p) with parent (%p,%p,%p) -> (%p,%p,%p)",
                    nodecodes[id],sw,sh,sd,pw,ph,pd,sw,ht,dp)
            end
            return sw, ht, dp
        else
            if trace_areas then
                report_area("dimensions calculated of %a",nodecodes[id])
            end
            return hlist_dimensions(start,stop) -- one node only so simple
        end
    end
    local last = stop and getnext(stop)
    if parent then
        -- todo: if no prev and no next and parent
        -- todo: we need a a list_dimensions for a vlist
        if getid(parent) == vlist_code then
         -- if false then
         --     local l = getlist(parent)
         --     local c = l
         --     local ok = false
         --     while c do
         --         if c == start then
         --             ok = true
         --         end
         --         if ok and getid(c) == hlist_code then
         --             break
         --         else
         --             c = getnext(c)
         --         end
         --     end
         --     if ok and c then
         --         if trace_areas then
         --             report_area("dimensions taken of first line in vlist")
         --         end
         --         local w, h, d = getwhd(c)
         --         return w, h, d, c
         --     else
         --         if trace_areas then
         --             report_area("dimensions taken of vlist (probably wrong)")
         --         end
         --         return hlist_dimensions(start,stop,parent)
         --     end
         -- else
                --
                -- we can as well calculate here because we only have kerns and glue
                --
                local first    = nil
                local last     = nil
                local current  = start
                local noflines = 0
                while current do -- can be loop
                    local id = getid(current)
                    if id == hlist_code or id == vlist_code or id == rule_code then
                        if noflines == 0 then
                            first    = current
                            noflines = 1
                        else
                            noflines = noflines + 1
                        end
                        last = current
                    end
                    if current == stop then
                        break
                    else
                        current = getnext(current)
                    end
                end
                if noflines > 1 then
                    if trace_areas then
                        report_area("dimensions taken of vlist")
                    end
                    local w, h, d = vlist_dimensions(first,last,parent)
                    local ht = getheight(first)
                    return w, ht, d + h - ht, first
                else
                 -- return hlist_dimensions(start,stop,parent)
                    if first then
                        if trace_areas then
                            report_area("dimensions taken of first line in vlist")
                        end
                        local w, h, d = getwhd(first)
                        return w, h, d, first
                    else
                        if trace_areas then
                            report_area("dimensions taken of vlist (probably wrong)")
                        end
                        return hlist_dimensions(start,stop,parent)
                    end
                end
         -- end
        else
            if trace_areas then
                report_area("dimensions taken of range starting with %a using parent",nodecodes[id])
            end
            return hlist_dimensions(start,stop,parent)
        end
    else
        if trace_areas then
            report_area("dimensions taken of range starting with %a",nodecodes[id])
        end
        return hlist_dimensions(start,stop)
    end
end

local function inject_range(head,first,last,reference,make,stack,parent,pardir,txtdir)
    local width, height, depth, line = dimensions(parent,first,last)
    if txtdir == righttoleft_code then
        width = - width
    elseif txtdir == lefttoright_code then
        -- go on
    elseif pardir == righttoleft_code then
        width = - width
    end
    local result, resolved = make(width,height,depth,reference)
    if result and resolved then
        if line then
            -- special case, we only treat the first line in a vlist
            local l = getlist(line)
            if trace_areas then
                report_area("%s: %i : %s %s %s => w=%p, h=%p, d=%p","line",
                    reference,pardir or "?",txtdir or "?",
                    tosequence(l,nil,true),width,height,depth)
            end
            setlist(line,result)
            setlink(result,l)
            return head, last
        elseif head == first then
            if trace_areas then
                report_area("%s: %i : %s %s %s => w=%p, h=%p, d=%p","head",
                    reference,pardir or "?",txtdir or "?",
                    tosequence(first,last,true),width,height,depth)
            end
            setlink(result,first)
            return result, last
        else
            if trace_areas then
                report_area("%s: %i : %s %s %s => w=%p, h=%p, d=%p","middle",
                    reference,pardir or "?",txtdir or "?",
                    tosequence(first,last,true),width,height,depth)
            end
            if first == last and getid(parent) == vlist_code and getid(first) == hlist_code then
                if trace_areas then
                    -- think of a button without \dontleavehmode in the mvl
                    report_area("compensating for link in vlist")
                end
                setlink(result,getlist(first))
                setlist(first,result)
            else
                setlink(getprev(first),result,first)
            end
            return head, last
        end
    else
        return head, last
    end
end

local function inject_list(id,current,reference,make,stack,pardir,txtdir)
    local width, height, depth = getwhd(current)
    local correction = 0
    local moveright  = false
    local first      = getlist(current)
    if id == hlist_code then -- boxlist_code linelist_code
        -- can be either an explicit hbox or a line and there is no way
        -- to recognize this; anyway only if ht/dp (then inline)
        local sr = stack[reference]
        if first then
            if sr and sr[2] then
                local last = find_node_tail(first)
                if getid(last) == glue_code and getsubtype(last) == rightskip_code then
                    local prev = getprev(last)
                    moveright = getid(first) == glue_code and getsubtype(first) == leftskip_code
                    if prev and getid(prev) == glue_code and getsubtype(prev) == parfillskip_code then
                        width = dimensions(current,first,getprev(prev)) -- maybe not current as we already take care of it
                    else
                        local set, order, sign = getboxglue(current)
                        if moveright then
                            width = width - getfield(first,"stretch") * set * sign
                        end
                        width = width - getfield(last,"stretch") * set * sign
                    end
                end
            else
                -- also weird
            end
        else
            -- ok
        end
        correction = width
    else
        correction = height + depth
        height, depth = depth, height -- ugly hack, needed because pdftex backend does something funny
    end
    if pardir == righttoleft_code then
        width = - width
    end
    local result, resolved = make(width,height,depth,reference)
    -- todo: only when width is ok
    if result and resolved then
        if trace_areas then
            report_area("%s: %04i %s %s %s: w=%p, h=%p, d=%p, c=%S","box",
                reference,pardir or "?",txtdir or "?","[]",width,height,depth,resolved)
        end
        if not first then
            setlist(current,result)
        elseif moveright then -- brr no prevs done
            -- result after first
            setlink(first,result,getnext(first))
        else
            -- first after result
            setlink(result,first)
            setlist(current,result)
        end
    end
end

-- skip is somewhat messy

-- todo: when line we can look at the next line

-- see dimensions: this is tricky with split off boxes like inserts
-- where we can end up with a first and last spanning lines so maybe
-- we need to do vlists differently

-- todo: no need for dir here if we inject in the right spot as then we can
-- pick up the dir elsewhere (in lmtx)

local function inject_areas(head,attribute,make,stack,done,skip,parent,pardir,txtdir)  -- main
    local first, last, firstdir, reference
    local current = head
    while current do
        local id = getid(current)
        if id == hlist_code or id == vlist_code then
            local r = getattr(current,attribute)
            -- test \goto{test}[page(2)] test \gotobox{test}[page(2)]
            -- test \goto{\TeX}[page(2)] test \gotobox{\hbox {x} \hbox {x}}[page(2)]
            if r then
                if not reference then
                    reference, first, last, firstdir = r, current, current, txtdir
                elseif r == reference then
                    -- same link
                    last = current
                elseif (done[reference] or 0) == 0 then
                    if not skip or r > skip then -- maybe no > test
                        head, current = inject_range(head,first,last,reference,make,stack,parent,pardir,firstdir)
                        reference, first, last, firstdir = nil, nil, nil, nil
                    end
                else
                    reference, first, last, firstdir = r, current, current, txtdir
                end
                done[r] = (done[r] or 0) + 1
            end
            local list = getlist(current)
            if list then
                local h
                h, pardir, txtdir = inject_areas(list,attribute,make,stack,done,r or skip or 0,current,pardir,txtdir)
                if h ~= current then
                    setlist(current,h)
                end
            end
            if r then
                done[r] = done[r] - 1
            end
        elseif id == glue_code and getsubtype(current) == leftskip_code then -- any glue at the left?
            --
        elseif id == dir_code then
            local direction, pop = getdirection(current)
            txtdir = not pop and direction -- we might need a stack
        elseif id == localpar_code then
            if start_of_par(current) then
                pardir = getdirection(current)
            end
        else
            local r = getattr(current,attribute)
            if not r then
                -- just go on, can be kerns
            elseif not reference then
                reference, first, last, firstdir = r, current, current, txtdir
            elseif r == reference then
                last = current
            elseif (done[reference] or 0) == 0 then -- or id == glue_code and getsubtype(current) == right_skip_code
                if not skip or r > skip then -- maybe no > test
                    head, current = inject_range(head,first,last,reference,make,stack,parent,pardir,firstdir)
                    reference, first, last, firstdir = nil, nil, nil, nil
                end
            else
                reference, first, last, firstdir = r, current, current, txtdir
            end
        end
        current = getnext(current)
    end
    if reference and (done[reference] or 0) == 0 then
        head = inject_range(head,first,last,reference,make,stack,parent,pardir,firstdir)
    end
    return head, pardir, txtdir
end

-- tracing: todo: use predefined colors

local register_color = colors.register

local a_color        = attributes.private('color')
local a_colormodel   = attributes.private('colormodel')
local a_transparency = attributes.private('transparency')
local u_transparency = nil
local u_colors       = { }
local force_gray     = true

local function addstring(what,str,shift) --todo make a pluggable helper (in font-ctx)
    if str then
        local typesetters = nuts.typesetters
        if typesetters then
            local hashes   = fonts.hashes
            local infofont = fonts.infofont()
            local emwidth  = hashes.emwidths [infofont]
            local exheight = hashes.exheights[infofont]
            if what == "reference" then
                str   = str .. " "
                shift = - (shift or 2.25) * exheight
            else
                str   = str .. " "
                shift = (shift or 2) * exheight
            end
            local text = typesetters.tohpack(str,infofont)
            local rule = new_rule(emwidth/5,4*exheight,3*exheight)
            setshift(text,shift)
            return hpack_list(setlink(text,rule))
        end
    end
end

local function colorize(width,height,depth,n,reference,what,sr,offset)
    if force_gray then n = 0 end
    u_transparency = u_transparency or transparencies.register(nil,2,.65)
    local ucolor = u_colors[n]
    if not ucolor then
        if n == 1 then
            u_color = register_color(nil,'rgb',.75,0,0)
        elseif n == 2 then
            u_color = register_color(nil,'rgb',0,.75,0)
        elseif n == 3 then
            u_color = register_color(nil,'rgb',0,0,.75)
        else
            n = 0
            u_color = register_color(nil,'gray',.5)
        end
        u_colors[n] = u_color
    end
    if width == 0 then
        -- probably a strut as placeholder
        report_area("%s %s has no %s dimensions, width %p, height %p, depth %p",what,reference,"horizontal",width,height,depth)
        width = 65536
    end
    if height + depth <= 0 then
        report_area("%s %s has no %s dimensions, width %p, height %p, depth %p",what,reference,"vertical",width,height,depth)
        height = 65536/2
        depth  = height
    end
    local rule = new_rule(width,height,depth) -- todo: use tracer rule
    setattr(rule,a_colormodel,1) -- gray color model
    setattr(rule,a_color,u_color)
    setattr(rule,a_transparency,u_transparency)
    if width < 0 then
        local kern = new_kern(width)
        setwidth(rule,-width)
        setnext(kern,rule)
        setprev(rule,kern)
        return kern
    elseif sr and sr ~= "" then
        local text = addstring(what,sr,shift)
        if text then
            local kern = new_kern(-getwidth(text))
            setlink(kern,text,rule)
            return kern
        end
    end
    return rule
end

local function justadd(what,sr,shift,current) -- needs testing
    if sr and sr ~= "" then
        local text = addstring(what,sr,shift)
        if text then
            local kern = new_kern(-getwidth(text))
            setlink(kern,text,current)
            return new_hlist(kern)
        end
    end
end

-- references:

local texsetcount     = tex.setcount
----- texsetattribute = tex.setattribute

local stack           = { }
local done            = { }
local attribute       = attributes.private('reference')
local nofreferences   = 0
local topofstack      = 0

nodes.references = {
    attribute = attribute,
    stack     = stack,
    done      = done,
}

-- todo: get rid of n (n is just a number, can be used for tracing, obsolete)

local function setreference(h,d,r) -- h and d can be nil
    topofstack = topofstack + 1
    -- the preroll permits us to determine samepage (but delayed also has some advantages)
    -- so some part of the backend work is already done here
    stack[topofstack] = { r, h or false, d or false, codeinjections.prerollreference(r) }
 -- texsetattribute(attribute,topofstack) -- todo -> at tex end
    texsetcount("lastreferenceattribute",topofstack)
end

function references.get(n) -- not public so functionality can change
    local sn = stack[n]
    return sn and sn[1]
end

local function makereference(width,height,depth,reference) -- height and depth are of parent
    local sr = stack[reference]
    if sr then
        if trace_references then
            report_reference("resolving attribute %a",reference)
        end
        local resolved = sr[1]
        local ht       = sr[2]
        local dp       = sr[3]
        local set      = sr[4]
        local n        = sr[5]
     -- logs.report("temp","child: ht=%p dp=%p, parent: ht=%p dp=%p",ht,dp,height,depth)
        if ht then
            if height < ht then height = ht end
            if depth  < dp then depth  = dp end
        end
     -- logs.report("temp","used: ht=%p dp=%p",height,depth)
        local annot = nodeinjections.reference(width,height,depth,set,resolved.mesh)
        if annot then
            annot = tonut(annot) -- todo
            nofreferences = nofreferences + 1
            local result, current, texts
            if show_references then
                local d = resolved
                if d then
                    local r = d.reference
                    local p = d.prefix
                    if r then
                        if p then
                            texts = p .. "|" .. r
                        else
                            texts = r
                        end
                    else
                     -- t[#t+1] = d.internal or "?"
                    end
                end
            end
            if trace_references then
                local step = 65536
                result = new_hlist(colorize(width,height-step,depth-step,2,reference,"reference",texts,show_references)) -- step subtracted so that we can see seperate links
                current = result
            elseif texts then
                texts = justadd("reference",texts,show_references,current)
                if texts then
                    current = texts
                end
            end
            if current then
                setlink(current,annot)
            else
                result = annot
            end
            references.registerpage(n)
            result = new_hlist(result)
            if cleanupreferences then stack[reference] = nil end
            return result, resolved
        elseif trace_references then
            report_reference("unable to resolve annotation %a",reference)
        end
    elseif trace_references then
        report_reference("unable to resolve attribute %a",reference)
    end
end

function nodes.references.handler(head)
    if head and topofstack > 0 then
        return (inject_areas(head,attribute,makereference,stack,done))
    else
        return head
    end
end

-- destinations (we can clean up once set, unless tagging!)

local stack           = { }
local done            = { }
local attribute       = attributes.private('destination')
local nofdestinations = 0
local topofstack      = 0

nodes.destinations = {
    attribute = attribute,
    stack     = stack,
    done      = done,
}

local function setdestination(n,h,d,name,view) -- n = grouplevel, name == table
    topofstack = topofstack + 1
    stack[topofstack] = { n, h, d, name, view }
    return topofstack
end

local function makedestination(width,height,depth,reference)
    local sr = stack[reference]
    if sr then
        if trace_destinations then
            report_destination("resolving attribute %a",reference)
        end
        local resolved = sr[1]
        local ht       = sr[2]
        local dp       = sr[3]
        local name     = sr[4]
        local view     = sr[5]
        if ht then
            if height < ht then height = ht end
            if depth  < dp then depth  = dp end
        end
        local result, current, texts
        if show_destinations then
            if name and #name > 0 then
                local t = { }
                for i=1,#name do
                    local s = name[i]
                    if type(s) == "number" then
                        local d = references.internals[s]
                        if d then
                            d = d.references
                            local r = d.reference
                            local p = d.usedprefix
                            if r then
                                if p then
                                    t[#t+1] = p .. "|" .. r
                                else
                                    t[#t+1] = r
                                end
                            else
                             -- t[#t+1] = d.internal or "?"
                            end
                        end
                    else
                        -- in fact we have a prefix:name here
                    end
                end
                if #t > 0 then
                    texts = concat(t," & ")
                end
            end
        end
        if trace_destinations then
            local step = 0
            if width  == 0 then
                step = 4*65536
                width, height, depth = 5*step, 5*step, 0
            end
            local rule = new_hlist(colorize(width,height,depth,3,reference,"destination",texts,show_destinations))
            if not result then
                result, current = rule, rule
            else
                setlink(current,rule)
                current = rule
            end
            width, height = width - step, height - step
        elseif texts then
            texts = justadd("destination",texts,show_destinations,current)
            if texts then
                current = texts
            end
        end
        nofdestinations = nofdestinations + 1
        local annot = nodeinjections.destination(width,height,depth,name,view)
        if annot then
            annot = tonut(annot) -- obsolete soon
            if result then
                setlink(current,annot)
            else
                result  = annot
            end
            current = find_node_tail(annot)
        end
        if result then
            result = new_hlist(result)
        end
        if cleanupdestinations then stack[reference] = nil end
        return result, resolved
    elseif trace_destinations then
        report_destination("unable to resolve attribute %a",reference)
    end
end

function nodes.destinations.handler(head)
    if head and topofstack > 0 then
        return (inject_areas(head,attribute,makedestination,stack,done))
    else
        return head
    end
end

-- will move

function references.mark(reference,h,d,view)
    return setdestination(tex.currentgrouplevel,h,d,reference,view)
end

function references.inject(prefix,reference,specification) -- todo: use currentreference is possible
    local set, bug = references.identify(prefix,reference)
    if bug or #set == 0 then
        -- unknown ref, just don't set it and issue an error
    else
        set.highlight = specification.highlight
        set.newwindow = specification.newwindow
        set.layer     = specification.layer
        setreference(specification.height,specification.depth,set) -- sets attribute / todo: for set[*].error
    end
end

-- function references.injectinternal(internal,specification)
--     references.inject("","internal("..internal..")",specification)
--     if bug or #set == 0 then
--         -- unknown ref, just don't set it and issue an error
--     else
--         -- nil prefix when ""
--         -- check
--         set.highlight = specification.highlight
--         set.newwindow = specification.newwindow
--         set.layer     = specification.layer
--         setreference(specification.height,specification.depth,set) -- sets attribute / todo: for set[*].error
--     end
-- end

function references.injectcurrentset(h,d) -- used inside doifelse
    local currentset = references.currentset
    if currentset then
        setreference(h,d,currentset) -- sets attribute / todo: for set[*].error
    end
end

implement {
    name      = "injectreference",
    actions   = references.inject,
    arguments = {
        "string",
        "string",
        {
            { "highlight", "boolean" },
            { "newwindow", "boolean" },
            { "layer" },
            { "height", "dimen" },
            { "depth", "dimen" },
            { "view" },
        }
    }
}

-- implement {
--     name      = "injectinternalreference",
--     actions   = references.injectinternal,
--     arguments = {
--         "integer",
--         {
--             { "highlight", "boolean" },
--             { "newwindow", "boolean" },
--             { "layer" },
--             { "height", "dimen" },
--             { "depth", "dimen" },
--             { "view" },
--         }
--     }
-- }

implement {
    name      = "injectcurrentreference",
    actions   = references.injectcurrentset,
}

implement {
    name      = "injectcurrentreferencehtdp",
    actions   = references.injectcurrentset,
    arguments = { "dimen", "dimen" },
}

--

local function checkboth(open,close)
    if open and open ~= "" then
        local set, bug = references.identify("",open)
        open = not bug and #set > 0 and set
    end
    if close and close ~= "" then
        local set, bug = references.identify("",close)
        close = not bug and #set > 0 and set
    end
    return open, close
end

-- end temp hack

statistics.register("interactive elements", function()
    if nofreferences > 0 or nofdestinations > 0 then
        return string.format("%s references, %s destinations",nofreferences,nofdestinations)
    else
        return nil
    end
end)