Calculates the CIDR prefix from dot-decimal netmask

mask_to_cidr.lua

-- calculates binary equivalent of "n"
local function bin(n)
  local binary = {}
  n = tonumber(n)

  local function _bin(_n)
    if _n > 1 then _bin(_n // 2) end

    table.insert(binary, _n % 2)
  end

  _bin(n)

  return table.concat(binary)
end

-- fills string "str" with zeros on the left to match width "n"
local function zfill(str, n)
  str = tostring(str)

  if not n or #str >= n then return str end

  return string.rep('0', n - #str) .. str
end

-- removes trailing zeros from given string "str"
local function rzstrip(str)
  local str_tbl = {}

  str:gsub('%w', function(m) table.insert(str_tbl, m) end)

  local tbl_len = #str_tbl

  for i = tbl_len, 1, -1 do
    if tonumber(str_tbl[i]) > 0 then break end

    str_tbl[i] = nil
  end

  return table.concat(str_tbl)
end

-- calculates CIDR
local function mask_to_cidr(mask)
  mask = { mask:match('^(%d+).(%d+).(%d+).(%d+)$') }
  local binary_tbl = {}

  for oct = 1, #mask do
    table.insert(binary_tbl, zfill(bin(mask[oct]), 8))
  end

  return #rzstrip(table.concat(binary_tbl))
end

mask_to_cidr('255.255.255.0')