count length of multi-byte string

mb_len.lua

local function mb_len(s)
    local c = 0 -- 判斷字寬, 中文: 2, 西歐字母: 1
    local c2 = 0 -- 判斷字元 (中文大部分3byte, TODO)
    local add_tail = false
    local r = ""
    for i in s:gmatch("[\1-\127\194-\244][\128-\191]*") do
        if string.byte(i) > 127 then
            c = c + 2
            c2 = c2 + 3
        else
            c = c + 1
            c2 = c2 + 1
        end
    end
    return c, c2
end