From: Robert Evans <robert.evans@...>
Date: 2005-11-16T03:37:25+09:00
Subject: Re: Solution Comments on Re: Reading a signed byte in network byte order

Sweet. That's a very slick answer. Thanks.

Bob


On Nov 15, 2005, at 9:56 AM, Ara.T.Howard wrote:

> On Wed, 16 Nov 2005, Robert Evans wrote:
>
>> Hi,
>>
>> So, I came up with a solution, and would be grateful for Ruby  
>> style tips. It seemed a lot harder to do than it needed to be.
>>
>> Any help appreciated.
>>
>> Thanks,
>> Bob
>>
>> The Tests:
>>
>> class ByteReaderTest < Test::Unit::TestCase
>>
>>  def test_read_signed4_negative
>>    negative = "" << 0xF2 << 0x34 << 0x56 << 0x78
>>    assert_equal -231451016, signed4(StringIO.new(negative))
>>  end
>>
>>  def test_read_signed4_positive
>>    positive = "" << 0x02 << 0x34 << 0x56 << 0x78
>>    assert_equal 36984440, signed4(StringIO.new(positive))
>>  end
>>
>> end
>>
>> The Solution:
>>
>> def signed4(file)
>>  bits = (file.read(4).unpack('B32'))[0]
>>  if bits[0..0].eql? "0" # sign bit is positive
>>    bits.to_i(2)
>>  else # sign bit is negative
>>    compute_negative_number_from(bits)
>>  end
>> end
>>
>> def compute_negative_number_from(bits)
>>  twos_complement = flip(bits).join.to_i(2) + 1
>>  -1 * twos_complement
>> end
>>
>> def flip(bits)
>>  bits.scan(/\w/).collect { |bit| (bit.eql?("1")) ? "0" : "1" }
>> end
>
> harp:~ > cat a.rb
> require "test/unit"
> class ByteReaderTest < Test::Unit::TestCase
>   MAX_POS = 2 ** 31 -1
>   BIG_ENDIAN = [42].pack('N') == [42].pack('i')
>   def test_read_signed4_positive
>     positive = [0x02, 0x34, 0x56, 0x78].pack "c*"
>     assert_equal 36984440, signed4(positive)
>   end
>   def test_read_signed4_negative
>     negative = [0xF2, 0x34, 0x56, 0x78].pack "c*"
>     assert_equal -231451016, signed4(negative)
>   end
>   def signed4 buf
>     buf = buf.read 4 if buf.respond_to? "read"
>     raise RangeError unless buf.size == 4
>     n = buf.unpack('N').first
>     if n > MAX_POS
>       (BIG_ENDIAN ? buf : buf.reverse).unpack('l').first
>     else
>       n
>     end
>   end
> end
>
>
> harp:~ > ruby a.rb
> Loaded suite a
> Started
> ..
> Finished in 0.000583 seconds.
>
> 2 tests, 2 assertions, 0 failures, 0 errors
>
>
> -a
> -- 
> ====================================================================== 
> =========
> | ara [dot] t [dot] howard [at] gmail [dot] com
> | all happiness comes from the desire for others to be happy.  all  
> misery
> | comes from the desire for oneself to be happy.
> | -- bodhicaryavatara
> ====================================================================== 
> =========
>
>