-- 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 = arg[1]
local filepaths_arr = {}
local filepaths_map = {}
for i = 2, #arg do
  table.insert(filepaths_arr, arg[i])
  filepaths_map[arg[i]] = {}
end

for _,filepath in ipairs(filepaths_arr) 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 filepaths_map[link.target] then return end
      if visited_map[link.target] then return end
      table.insert(filepaths_map[filepath], link.target)
      visited_map[link.target] = true
    end
  })
end

local data = json.encode(filepaths_map)
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