Re: digits(n,-4)

["Ruud H.G. van Tol" <rvtol@isolution.nl>]

On 2023-10-14 20:34, Bill Allombert wrote:
> On Sat, Oct 14, 2023 at 07:24:35PM +0200, Ruud H.G. van Tol wrote:
> > ? fromdigits( [1,3,0], -4 )
> > % 4
> Note that we do need to know that the set of digits is here.
>
> > ? my(n=4, v=[]); while( n=divrem(n,-4), v=concat(n[2],v); n=n[1] ); v
> > % [1, 3, 0]
> >
> > Is there a better way to do this?
> I would start with use digits(n,16) and split the digits.

Thanks, am still chewing on that.


Intermediate result:

? digits2( x, b=10 ) = {
  if( b >= 0, return(digits(x,b)) );
  if( -2 == b, return( digits( bitxor( x + 0xAAAAAAAA, 0xAAAAAAAA ), 2 ) )
  ,   -4 == b, return( digits( bitxor( x + 0xCCCCCCCC, 0xCCCCCCCC ), 4 ) )
  );

  my( v = [] );
  while( x = divrem( x, -4 )
  , v = concat( x[2], v );
    x = x[1]
  );
  v;
}


> > Should digits() be changed to support a negative base?
> There are too many variants...

ACK.

-- Ruud

https://en.wikipedia.org/wiki/Negative_base
(refers to Schroeppel)