Lua | Number Formatting

--[[
    turn numbers like 1216112.5161 into 1,216,112.5161
]]

function number_format(num)
    local neg = num < 0
    num = math.abs(num)
    local size = math.floor(math.log10(num)) + 1
    local offset = size % 3
    local str = ""

    local s, e = tostring(num):find("%.")
    for i = #tostring(num), 1, -1 do
        if math.abs(i % 3 - offset) == 0 and i ~= size and ((s ~= nil and i < s) or s == nil) then
            str = "," .. str
        end
        str = tostring(num):sub(i,i) .. str
    end

    return ((neg and "-") or "") .. str
end