From: Joel VanderWerf <vjoel@...>
Date: 2008-01-11T03:19:46+09:00
Subject: Re: Question about Hex => Signed Int

Tim Conner wrote:
> Gary Wright wrote:
>> The tricky part is dealing with the twos compliment notation but
>> String#to_i will do the hex/decimal conversion:
>>
>>  >> "0xFF467A7B".to_i(16) - 0x100000000
>> => -12158341
>>
>>  >> "0x38CFEF".to_i(16)
>> => 3723247
>>
>> Gary Wright
> 
> Thanks for that.  Is there a ruby function which I can pass a hex number 
> and it works out if it is +/- and then returns the correct decimal 
> value?

Convert to unsigned first, like above, then do something like this:

     length = 32 # in bits; you have to choose this

     mid = 2**(length-1)
     max_unsigned = 2**length
     to_signed = proc {|n| (n>=mid) ? n - max_unsigned : n}

     p to_signed["0xFF467A7B".to_i(16)]
     p to_signed["0x38CFEF".to_i(16)]

See http://en.wikipedia.org/wiki/Twos_complement.

-- 
       vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407