Lua按字截取：字符下标换字节下标

-- 错误：sub 按字节切，半个汉字变乱码
local short = name:sub(1, 6)

local function subByChar(str, startChar, numChars)
    local function chsize(char)
        if char > 240 then return 4
        elseif char > 225 then return 3
        elseif char > 192 then return 2
        else return 1 end
    end
    local i, seen = 1, 0
    while i <= #str and seen < startChar - 1 do
        i = i + chsize(string.byte(str, i))
        seen = seen + 1
    end
    local startByte, got, endByte = i, 0, i
    while i <= #str and got < numChars do
        i = i + chsize(string.byte(str, i))
        got = got + 1
        endByte = i - 1
    end
    return str:sub(startByte, endByte)
end