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

local context, commands = context, commands

local insert, remove, find, gmatch = table.insert, table.remove, string.find, string.gmatch
local strip, format = string.strip, string.format

local trace_parallel  = false  trackers.register("buffers.parallel", function(v) trace_parallel = v end)

local report_parallel = logs.reporter("buffers","parallel")

local variables         = interfaces.variables

local parallel          = buffers.parallel or { }
buffers.parallel        = parallel

local settings_to_array = utilities.parsers.settings_to_array

local data              = { }

function parallel.define(category,tags)
    local tags = settings_to_array(tags)
    local entries = { }
    data[category] = {
        tags    = tags,
        entries = entries,
    }
    for i=1,#tags do
        entries[tags[i]] = {
            lines  = { },
            number = 0,
        }
    end
end

function parallel.reset(category,tags)
    if not tags or tags == "" or tags == variables.all then
        tags = table.keys(entries)
    else
        tags = settings_to_array(tags)
    end
    for i=1,#tags do
        entries[tags[i]] = {
            lines  = { },
            number = 0,
        }
    end
end

function parallel.next(category)
    local dc = data[category]
    local tags = dc.tags
    local entries = dc.entries
    for i=1,#tags do
        insert(entries[tags[i]].lines, { })
    end
end

function parallel.save(category,tag,content)
    local dc = data[category]
    if not dc then
        return
    end
    local entries = dc.entries[tag]
    if not entries then
        return
    end
    local lines = entries.lines
    if not lines then
        return
    end
    local line = lines[#lines]
    if not line then
        return
    end
    -- maybe no strip
    -- use lpeg
    if find(content,"%s*%[") then
        local done = false
        for label, content in gmatch(content,"%s*%[(.-)%]%s*([^%[]+)") do
            if done then
                line = { }
                insert(lines,line)
            else
                done = true
            end
            if trace_parallel and label ~= "" then
                report_parallel("reference found of category %a, tag %a, label %a",category,tag,label)
            end
            line.label   = label
            line.content = strip(content)
        end
    else
        line.content = strip(content)
        line.label = ""
    end
end

function parallel.hassomecontent(category,tags)
    local dc = data[category]
    if not dc then
        return false
    end
    local entries = dc.entries
    if not tags or tags == "" or tags == variables.all then
        tags = table.keys(entries)
    else
        tags = utilities.parsers.settings_to_array(tags)
    end
    for t=1,#tags do
        local tag = tags[t]
        local lines = entries[tag].lines
        for i=1,#lines do
            local content = lines[i].content
            if content and content ~= "" then
                return true
            end
        end
    end
    return false
end

local save = resolvers.savers.byscheme

function parallel.place(category,tags,options)
    local dc = data[category]
    if not dc then
        return
    end
    local entries = dc.entries
    local tags = utilities.parsers.settings_to_array(tags)
    local options = utilities.parsers.settings_to_hash(options)
    local start, n, criterium = options.start, options.n, options.criterium
    start, n = start and tonumber(start), n and tonumber(n)
    local max = 1
    if n then
        max = n
    elseif criterium == variables.all then
        max = 0
        for t=1,#tags do
            local tag = tags[t]
            local lines = entries[tag].lines
            if #lines > max then
                max = #lines
            end
        end
    end
    for i=1,max do
        for t=1,#tags do
            local tag = tags[t]
            local entry = entries[tag]
            if entry then
                local lines = entry.lines
                local number = entry.number + 1
                entry.number = number
                local line = remove(lines,1)
                if line and line.content then
                    local content = format("\\input{%s}",save("virtual","parallel",line.content))
                    context.doflushparallel(tag,1,number,line.label,content)
                else
                    context.doflushparallel(tag,0,number,"","")
                end
            end
        end
    end
end

-- interface

commands.defineparallel = parallel.define
commands.nextparallel   = parallel.next
commands.saveparallel   = parallel.save
commands.placeparallel  = parallel.place
commands.resetparallel  = parallel.reset

function commands.doifelseparallel(category,tags)
    commands.doifelse(parallel.hassomecontent(category,tags))
end