-- local l = require("pandoc.logging")
local read, json = pandoc.read, pandoc.json

assert(#arg > 1, "\n" ..
  "[ERROR] usage: pandoc lua links.lua <destination file> <input file>...")
local destination_filepath
local input_filepaths = {}
local input_filepaths_keys = {}
for _, option in ipairs(arg) do
  if destination_filepath == nil then
    destination_filepath = option
  else
    table.insert(input_filepaths, option)
    input_filepaths_keys[option] = { json.null }
  end
end

for _,filepath in ipairs(input_filepaths) do
  local f = assert(io.open(filepath), "\n" ..
      "[ERROR] could not open " .. filepath)
  local data = f:read("a")
  f:close()
  local visited_map = {}
  read(data):walk({
    Link = function (link)
      if not input_filepaths_keys[link.target] then return end
      if visited_map[link.target] then return end
      table.insert(input_filepaths_keys[filepath], link.target)
      table.insert(input_filepaths_keys[link.target], 1, filepath)
      visited_map[link.target] = true
    end
  })
end

local data = json.encode(input_filepaths_keys)
local f = io.open(destination_filepath)
if f == nil or f:read("a") ~= data then
  local f = assert(io.open(destination_filepath, "w"), "\n" ..
    "[ERROR] could not open " .. destination_filepath .. " for writing")
  f:write(data)
  f:close()
end