根據UTF-8的編碼規則，可以知道：

1. 全形空格為12288，半形空格為32 

2. 其他字元半形(33-126)與全形(65281-65374)的對應關係是：均相差65248 

但是UTF-8不能位元組轉換位整型變數，因此需要一個函式，做一個轉換：

function utf8_to_num(raw_string)
    local result = {}
    pos = 1
    while pos <= #raw_string do
        local count_1_of_byte = get_continuous_1_count_of_byte(string.byte(raw_string, pos))
        local num = 0

        if count_1_of_byte < 1 then 
            num = string.byte(raw_string, pos)
            count_1_of_byte = 1
        else 
            boundary = 8
            i = count_1_of_byte + 1
            while i < boundary * count_1_of_byte do
                if 0 == i % boundary then
                    i = i + 2
                end
                if (1 << (boundary - i % boundary - 1)) & string.byte(raw_string, pos + math.floor(i / boundary)) ~= 0 then
                    --print(1)
                    num = (num << 1) | 1
                else
                    --print(0)
                    num = num << 1
                end
                i= i + 1
            end
        end
        pos = pos + count_1_of_byte
        table.insert(result, num)
    end

    return result
end
 

--獲取一個位元組中，從最高位開始連續的1的個數
function  get_continuous_1_count_of_byte(num)
    if nil == num then 
        return -1
    end
    local count = 0
    while num & 0x80 ~= 0 do
        count = count + 1
        num = num << 1
    end
    return count
end
 

function full_width_to_half_width(raw_string)
    local new_string = {}
    local pos = 1
    while pos <= #raw_string do
        local count_1_of_byte = get_continuous_1_count_of_byte(string.byte(raw_string, pos))
        if 3 == count_1_of_byte then
            char = string.sub(raw_string, pos, pos + 2)
            num_of_char = utf8_to_num(char)[1]
            if 12288 == num_of_char then
                num_of_char = 32
                table.insert(new_string, string.char(num_of_char))
            elseif 65281 <= num_of_char and num_of_char <= 65374 then 
                num_of_char = num_of_char - 65248
                table.insert(new_string, string.char(num_of_char))
            end
            pos = pos + count_1_of_byte 
        else
            table.insert(new_string, string.sub(raw_string, pos, pos))
            pos = pos + 1
        end
    end

    return table.concat(new_string)
end