assert(#arg == 2, "\n" ..
  "[ERROR] usage: pandoc lua bavbavhaus.lua <index file> <output file>")
local index_file, output_file = table.unpack(arg)

local read = pandoc.read

NOTE_PATTERN = "^((.+)%.md)(#?.*)$"
PAGE_REPL = "public_html/%2.html"
PART_REPL = "public_html/%2.html.part"

local all = "all"
local makefile = { [all] = {} }
local function collect (fp)
  local f = assert(io.open(fp), "\n" ..
    "[ERROR] could not open " .. fp .. " for reading.")
  local data = f:read("a")
  f:close()

  local page = fp:gsub(NOTE_PATTERN, PAGE_REPL)
  table.insert(makefile[all], page)
  makefile[page] = {}

  read(data):walk({ Link = function (link)
    local fp, _slug, _anchor = link.target:match(NOTE_PATTERN)
    if not fp then return end
    local f = io.open(fp)
    if f == nil then return else f:close() end
    local target_page = link.target:gsub(NOTE_PATTERN, PAGE_REPL)
    if makefile[target_page] then return end
    local target_part = link.target:gsub(NOTE_PATTERN, PART_REPL)
    table.insert(makefile[page], target_part)
    collect(fp)
  end })
end
collect(index_file)

local lines = {}
for k, v in pairs(makefile) do
  local parts = table.concat(v, " ")
  table.insert(lines, ("%s: %s"):format(k, parts))
end

local f = assert(io.open(output_file, "w"), "\n" ..
  "[ERROR] could not open " .. output_file .. " for writing.")
f:write(table.concat(lines, "\n"))
f:close()