Re: flags in a bitmap

[Bill Allombert <Bill.Allombert@math.u-bordeaux.fr>]

On Thu, Jul 03, 2025 at 06:06:14PM +0200, Ruud H.G. van Tol wrote:
> 
> Bill, many thanks for the pointers, I'll explore.
> 
> 
> As an alternative, maybe add a Vecbits() or Vecflags() or such.
> No idea if the first bit should have offset 0 or 1.
> Only with bittest(), maybe 0?
> 
> my(B=Vecbits([0], 987654321));
> Vecbits_put(~B, 7654, v=1);  \\set the bit at offset 7654, by default to 1
> Vecbits_put(~B, 7654, [1,0,0,1,1]); \\set bits 7654..7658 to [1,0,0,1,1]
> my(b=Vecbits_get(B,7654)); \\get bit-7654
> my(b=bittest(B,7653)); \\...

What I can do is to allow to set/clean a bit in place inside an integer.
However this would only work if the integer is large enough.

I can do a git branch if you want to test:

? install(bitset,vWL)
? x=2^(2^20);
? forprime(p=2,2^20,bitset(~x,p))
  ***   last result computed in 8 ms.
? my(S=0);for(i=0,2^20-1,S+=bittest(x,i));S
%16 = 82025
  ***   last result computed in 159 ms.

Cheers,
Bill.