assert(#arg == 1, "\n" ..
  "[ERROR] usage: pandoc lua dependencies.lua <input-file>")
local input_file = arg[1]

local f = assert(io.open(input_file), "\n" ..
  "[ERROR] could not open " .. input_file .. " for reading.")
local data = f:read("a")
f:close()

local base, _ = pandoc.path.split_extension(input_file)
io.write(("public_html/%s.html:"):format(base))

local prerequisites = {}

pandoc.read(data):walk({ Link = function(link)
  -- early return for external links
  if link.target:find("^https?%:%/%/") then return end
  if link.target:find("^mailto%:") then return end

  local fp, anchor = link.target:match("^(.+%.md)#?(.*)$")
  -- early return for non markdown files
  if not fp then return end

  local f = io.open(fp)
  -- early return for broken internal links
  if f == nil then return else f:close() end

  -- early return for already collected
  if prerequisites[fp] then return end
  prerequisites[fp] = true

  local base, _ = pandoc.path.split_extension(fp)
  io.write(" ", ("public_html/%s.html.part"):format(base))
end })