[ruby-core:44858] [ruby-trunk - Feature #6361] Bitwise string operations

Issue #6361 has been updated by kernigh (George Koehler).


=begin
A new BinaryString (or Blob) class would entail several changes. For example, Array#pack and IO#read(n) would need to return BinaryString, not String. I prefer to keep String for binary strings.

The main reason, to perform bitwise operations on a String, is to use this String as an array of bits. The purpose of each bitwise operation is to clear or set some bits in this array. For example, a sieve of Erathosthenes might clear a bit in a String to show that a number is not prime.

String has no bitwise operations. So, the code to clear a bit is long, not simple.

 sieve = "\xAC(\x8A\xA2("
 # 25 is not prime. Clear bit 0x02 of byte 3.
 sieve.setbyte(3, sieve.getbyte(3) & ~0x02)

A simpler way, with current Ruby, is to unpack this String into an Array of Integers. The code can then perform bitwise operations with these Integers.

 sieve = "\xAC(\x8A\xA2(".unpack("C*")
 # 25 is not prime. Clear bit 0x02 of byte 3.
 sieve[3] &= ~0x02

The sieve in [1] uses an Array of 16-bit Fixnums. This works because Fixnum has bitwise operations. A sieve at [2] uses an Array of true and false. This also works, though it uses more memory.

[1] https://github.com/ruby/ruby/blob/trunk/lib/prime.rb
[2] http://rosettacode.org/wiki/Sieve_of_Eratosthenes#Ruby

If String would have bitwise operations, a sieve of Eratosthenes might use them. If String#& and String#~ would exist, then they can clear a bit.

 sieve = "\xAC(\x8A\xA2("
 sieve[3] &= ~"\x02"

With this design, String#& and String#~ never modify the original string. This design forgot that Strings are mutable. String#& must allocate a new string, before String#[]= modifies the original string. A better design might provide destructive methods that use bitwise operations to modify a slice of the original string.
=end

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Feature #6361: Bitwise string operations
https://bugs.ruby-lang.org/issues/6361#change-26433

Author: MartinBosslet (Martin Bosslet)
Status: Feedback
Priority: Normal
Assignee: 
Category: core
Target version: 


I know this has been discussed a lot in the past (and if there's still 
an open issue for this, I apologize, I couldn't find one), for example
in [1]. While it is generally no problem to implement this on the fly,
I still find that built-in support would be a real improvement. There
are quite some use cases in cryptography where this would come in very
handy, but I'm sure there are lots of other areas, too.

While of course I understand the reasons that were given in the previous
threads that ultimately lead to rejection, I still would like to reopen
the discussion as I felt that in every thread so far the consensus was
that having bitwise string operations would indeed be quite valuable. 

[1] http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/34586


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