From: Morton Goldberg <m_goldberg@...>
Date: 2007-02-16T07:58:15+09:00
Subject: Re: Converting Bytes to  a Negative Integer

On Feb 15, 2007, at 11:04 AM, James Edward Gray II wrote:

> On Feb 15, 2007, at 9:47 AM, Morton Goldberg wrote:
>
>> On Feb 14, 2007, at 6:37 PM, James Edward Gray II wrote:
>>
>>> On Feb 14, 2007, at 5:02 PM, Morton Goldberg wrote:
>>>
>>>> data = ""
>>>> [255, 255, 255, 236].each { |e| data << e }
>>>> data # => "\377\377\377\354"
>>>> data.unpack('l').first # => -20
>>>
>>> Watch what happens when I run the same code:
>>>
>>> >> data = ""
>>> => ""
>>> >> [255, 255, 255, 236].each { |e| data << e }
>>> => [255, 255, 255, 236]
>>> >> data
>>> => "\377\377\377\354"
>>> >> data.unpack('l').first
>>> => -318767105
>>
>> I see your point. Many new computers have 64-bit CPUs and we  
>> should remember that. Also, you want to be sure I, lowly 32-bit  
>> CPU owner, envy you for having one of these <just kidding>.
>
> I'll prop you back up then, this isn't a 32 to 64 bit (size)  
> issue.  It's an endian (order) issue:

Mea culpa -- I didn't see your point at all. Let's see if I've got it  
now.

1. Your result couldn't the result of conversion into a 64-bit  
integer with big-endian byte order, because if that were the case we  
would see either -20 or a large positive number (if sign extension  
didn't occur).

2. On my computer (iMac G5), I get

[-20].pack('i') # => "\377\377\377\354"
[-318767105].pack('i') # => "\354\377\377\377"

and

[-20].pack('N') # => "\377\377\377\354"
[-318767105].pack('N') # => "\354\377\377\377"

which shows it's big-endian because both unpack directives produce  
the same result.

3. On your computer (which I think is also a Macintosh), you would get

[-20].pack('i') # => "\354\377\377\377"
[-318767105].pack('i') # => "\377\377\377\354"

and

[-20].pack('N') # => "\377\377\377\354"
[-318767105].pack('N') # => "\354\377\377\377"

which would show it's little-endian and, presumably, some kind of  
Intel-based Mac.

Do I have it right now?

Regards, Morton

P.S. But I don't see how this props me back up :-)