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  • lua tips: convert between bytes and int

    converting from bytes to int (taking care of endianness at byte level, and signedness):

    --------------------------------------------------------------------------------------------
    require 'struct'
    -- convert character codes to a Lua string - this may come from your source
    local str = string.char(0x00, 0x1d, 0xff, 0x23, 0x44, 0x32)
    -- format string: < = little endian, In = unsigned int (n bytes)
    local u16, u32 = struct.unpack('<I2I4', str)
    print(u16, u32) --> 7424    843326463
    ---------------------------------------------------------------------------------------------

    function bytes_to_int(str,endian,signed) -- use length of string to determine 8,16,32,64 bits
        local t={str:byte(1,-1)}
        if endian=="big" then --reverse bytes
            local tt={}
            for k=1,#t do
                tt[#t-k+1]=t[k]
            end
            t=tt
        end
        local n=0
        for k=1,#t do
            n=n+t[k]*2^((k-1)*8)
        end
        if signed then
            n = (n > 2^(#t-1) -1) and (n - 2^#t) or n -- if last bit set, negative.
        end
        return n
    end

    function int_to_bytes(num,endian,signed)
        if num<0 and not signed then num=-num print"warning, dropping sign from number converting to unsigned" end
        local res={}
        local n = math.ceil(select(2,math.frexp(num))/8) -- number of bytes to be used.
        if signed and num < 0 then
            num = num + 2^n
        end
        for k=n,1,-1 do -- 256 = 2^8 bits per char.
            local mul=2^(8*(k-1))
            res[k]=math.floor(num/mul)
            num=num-res[k]*mul
        end
        assert(num==0)
        if endian == "big" then
            local t={}
            for k=1,n do
                t[k]=res[n-k+1]
            end
            res=t
        end
        return string.char(unpack(res))
    end
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  • 原文地址:https://www.cnblogs.com/wishing/p/3337980.html
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