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

assert(#arg == 1, "\n" ..
  "[ERROR] usage: pandoc lua collect_links.lua <file>...")

local f = assert(io.open(arg[1]), "\n" ..
  "[ERROR] could not open " .. arg[1] .. " for reading.")

local data = f:read("a")
f:close()
local visited = {}
read(data):walk({ Link = function (link)
  if not link.target:find(".%.md$") then return end
  local f = io.open(link.target)
  if f == nil then return else f:close() end
  if visited[link.target] then return end
  visited[link.target] = true
  io.write(link.target .. "\n")
end })