Simple number formatter in Lua

formatNumber.lua

-- Function formatNumber - Simple number formatter
-- Copyright (C) 2025 Matous Jan Fialka, <https://mjf.cz/>
-- Released under the terms of the "MIT" license

local function formatNumber(format, ...)
  local args = { ... }
  local argind = 1
  local res = ''
  local i = 1

  while i <= #format do
    local chr = format:sub(i, i)

    if chr == '%' then
      i = i + 1

      local padchr = '0'
      local padcnt = 0

      if string.match(format:sub(i, i), '[^%dN]') then
        padchr = format:sub(i, i)
        i = i + 1
      end

      local cntstr = ''

      while string.match(format:sub(i, i), '%d') do
        cntstr = cntstr .. format:sub(i, i)
        i = i + 1
      end

      if cntstr ~= '' then
        padcnt = tonumber(cntstr)
      end

      if format:sub(i, i) == 'N' then
        if argind <= #args then
          local num = tostring(args[argind])
          local neg = false

          if string.sub(num, 1, 1) == '-' then
            neg = true
            num = string.sub(num, 2)
          end

          if padcnt == 0 and padchr ~= '0' then
            padcnt = 2
          end

          local pad = string.rep(padchr,
		    math.max(0, padcnt - #num - (neg and 1 or 0)))

          if neg and string.match(padchr, '%d') then
            res = res .. '-' .. pad .. num
          else
            res = res .. pad .. (neg and '-' or '') .. num
          end

          argind = argind + 1
          i = i + 1
        else
          error('Not enough arguments!')
        end
      else
        res = res .. '%' .. format:sub(i, i)
        i = i + 1
      end
    else
      res = res .. chr
      i = i + 1
    end
  end

  if argind <= #args then
    error('Too many arguments!')
  end

  return res
end

-- TESTS

-- print(formatNumber('%N', 123456))
-- print(formatNumber('%N', -123456))
-- print(formatNumber('%xN', 3))
-- print(formatNumber('%xN', -3))
-- print(formatNumber('%x2N', 3))
-- print(formatNumber('%x2N', -3))
-- print(formatNumber('%16N', 123456))
-- print(formatNumber('%16N', -123456))
-- print(formatNumber('%016N', 123456))
-- print(formatNumber('%016N', -123456))
-- print(formatNumber('% 16N', 123456))
-- print(formatNumber('% 16N', -123456))
-- print(formatNumber('%.16N', 123456))
-- print(formatNumber('%+16N', -123456))

-- vi:ft=lua