lua truncate multi-byte string

gistfile1.txt

local function truncate(s, n, tail)
    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 c > n then
            add_tail = true
            break
        end
        if string.byte(i) > 127 then
            c = c + 2
            c2 = c2 + 3
        else
            c = c + 1
            c2 = c2 + 1
        end
    end

    r = string.sub(s, 0, c2)
    if add_tail then
        if tail == nil then
            r = r .. "..."
        else
            r = r .. tail
        end
    end
    return r
  end