Lua 截取中英混合的字符串

--返回截取的实际Index
function SubStringGetTrueIndex(str, index)
    local curIndex = 0
    local i = 1
    local lastCount = 1
    repeat
        lastCount = SubStringGetByteCount(str, i)
        i = i + lastCount
        curIndex = curIndex + 1
    until (curIndex >= index)
    return i - lastCount
end

--返回当前字符实际占用的字符数
function SubStringGetByteCount(str, index)
    local curByte = string.byte(str, index)
    local byteCount = 1
    if curByte == nil then
        byteCount = 0
    elseif curByte > 0 and curByte <= 127 then
        byteCount = 1
    elseif curByte >= 192 and curByte <= 223 then
        byteCount = 2
    elseif curByte >= 224 and curByte <= 239 then
        byteCount = 3
    elseif curByte >= 240 and curByte <= 247 then
        byteCount = 4
    end
    return byteCount
end

--截取中英混合的字符串
function SubString(str, startIndex, endIndex)
    if type(str) ~= "string" then
        return
    end
    if startIndex == nil or startIndex < 0 then
        return
    end

    if endIndex == nil or endIndex < 0 then
        return
    end

    return string.sub(str, SubStringGetTrueIndex(str, startIndex),
            SubStringGetTrueIndex(str, endIndex + 1) - 1)
end