proper hex editing in neovim

hex_editor.lua

local xxd_dump_cmd = 'xxd -g 1 -u'
local xxd_cur_pos = nil

local function is_binary_file()
  local filename = vim.fn.expand('%:t')
  -- local basename = string.match(filename, "^[a-z]*$")
  local binary_ext = { 'png', 'jpg', 'jpeg', 'out' }
  local ext = string.match(filename, "%.([^%.]+)$")

  if ext == nil and string.match(filename, '[a-z]+') then return true end
  if vim.tbl_contains(binary_ext, ext) then return true end

  return false
end

local function drop_undo_history()
  local undolevels = vim.o.undolevels
  vim.o.undolevels = -1
  vim.cmd [[exe "normal a \<BS>\<Esc>"]]
  vim.o.undolevels = undolevels
end

local function buf_read_pre()
  if not is_binary_file() then return end
  vim.bo.bin = true
end

local function buf_read_post()
  if not is_binary_file() then return end
  if vim.bo.bin then
    vim.cmd([[%! ]]..xxd_dump_cmd)
    vim.bo.ft = 'xxd'
    drop_undo_history()
  end
end

local function buf_write_pre()
  if not is_binary_file() then return end
  if vim.bo.bin then
    xxd_cur_pos = vim.fn.getcurpos()
    vim.cmd [[%! xxd -r]]
  end
end

local function buf_write_post()
  if not is_binary_file() then return end
  if vim.bo.bin then
    vim.cmd([[%! ]]..xxd_dump_cmd)
    vim.fn.setpos('.', xxd_cur_pos)
    vim.bo.mod = true
  end
end

local augroup_hex_editor = vim.api.nvim_create_augroup('hex_editor', { clear = true })

vim.api.nvim_create_autocmd({'BufReadPre'},   { group = augroup_hex_editor, callback = buf_read_pre })
vim.api.nvim_create_autocmd({'BufReadPost'},  { group = augroup_hex_editor, callback = buf_read_post })
vim.api.nvim_create_autocmd({'BufWritePre'},  { group = augroup_hex_editor, callback = buf_write_pre })
vim.api.nvim_create_autocmd({'BufWritePost'}, { group = augroup_hex_editor, callback = buf_write_post })