Tiny Neovim buffer formatter in lua

nvim-fmt-buf.lua

local read_buf = function(bufnr)
  local bufin = table.concat(vim.api.nvim_buf_get_lines(bufnr, 0, -1, true), "\n")
  if vim.api.nvim_get_option_value("eol", { buf = bufnr }) then bufin = bufin .. "\n" end
  return bufin
end

local format_buf = function(fmt_cmd)
  -- exit early if buffer is not writeable
  if vim.bo.modifiable == false or vim.bo.readonly == true then return end
  -- save cursor position
  local save_cursor = vim.fn.getpos(".")
  local save_view = vim.fn.winsaveview()

  local bufin = read_buf(vim.fn.bufnr("%"))
  local bufout = vim.fn.system(fmt_cmd, bufin)

  if vim.v.shell_error ~= 0 then
    print(bufout)
    return
  end

  if bufout == bufin then
    return
  end

  local new_lines = vim.fn.split(bufout, "\n")
  vim.api.nvim_buf_set_lines(0, 0, -1, false, new_lines)
  -- restore cursor position
  vim.fn.setpos(".", save_cursor)
  vim.fn.winrestview(save_view)
end

-- example use case
autocmd("BufWritePre", {
  pattern = { "*.py", "*.lua" },
  callback = function()
    local fmt_cmd_table = {
      ["lua"] = "stylua --indent-width 2 -",
      ["python"] = "ruff format -",
    }
    format_buf(fmt_cmd_table[vim.bo.filetype])
  end,
})