Re: [PATCH 1/4] Disallow use of DES encryption functions in new programs.

[Joseph Myers <joseph at codesourcery dot com>]

On Tue, 22 May 2018, Rical Jasan wrote:

> > -@code{strfry} creates a pseudorandom anagram of a string, replacing the
> > -input with the anagram in place.  For each position in the string,
> > -@code{strfry} swaps it with a position in the string selected at random
> > -(from a uniform distribution).  The two positions may be the same.
> > +@code{strfry} performs an in-place shuffle on @var{string}.  Each
> > +character is swapped to a position selected at random, within the
> > +portion of the string starting with the character's original position.
> > +(This is the Fisher-Yates algorithm for unbiased shuffling.)
> 
> I like the "pseudorandom anagram" analogy, personally, as it's
> rearranging a string.  What about combining the two?  I'm not sure we
> even need to go into this much detail about how it happens, though I do
> think it's good to mention that it happens "in-place" (which both
> versions do).  What about:
> 
> @code{strfry} creates a pseudorandom anagram of @var{string}, replacing
> it in-place using the Fisher-Yates algorithm for unbiased shuffling.

Note that there's the pre-existing condition that, contrary to the 
documentation, the distribution is not actually uniform (especially for 
very long strings), because it uses a random nonnegative 32-bit integer 
modulo the remaining length of the string (see bug 4403).  (As a 
pre-existing condition, this is not a problem with any variant of this 
patch.)

-- 
Joseph S. Myers
joseph@codesourcery.com