Since s2 is quadratic, it overflows quickly in J32. However
ieee floating points have 52 bits mantissa, so it can still
give correct result for medium size data albeit overflowed.
9!:11[16
(3!:0;]) a32 1e6{.a.
Вс, 24 авг 2014, Raul Miller написал(а):
> Hmm... actually there might be a floating point overflow problem for
> gigabyte length arguments. I do not have the patience, however, to
> find what (for example) adler32 1e9#{:a. is.
>
> Thanks,
>
> --
> Raul
>
>
>
> On Sun, Aug 24, 2014 at 4:37 AM, Raul Miller <[email protected]> wrote:
> > I believe this is equivalent, and a bit faster:
> >
> > a32=: 65536#. 65521| _1 0+ [:(+/ , {.) 65521| [:+/\. 1,~ a.i. |.
> >
> > a32 'Wikipedia'
> > 300286872
> > a32 'The quick brown fox jumps over the lazy dog'
> > 1541148634
> >
> > Thanks,
> >
> > --
> > Raul
> >
> >
> > On Sun, Aug 24, 2014 at 4:05 AM, bill lam <[email protected]> wrote:
> >> Does anyone have a J-ish solution to adler32 checksum
> >> and avoid overflow to floating points ?
> >>
> >> Adler-32 is composed of two sums accumulated per byte: s1 is
> >> the sum of all bytes, s2 is the sum of all s1 values. Both sums
> >> are done modulo 65521. s1 is initialized to 1, s2 to zero. The
> >> Adler-32 checksum is stored as s2*65536 + s1 in most-
> >> significant-byte first (network) order.
> >>
> >> * https://www.ietf.org/rfc/rfc1950.txt
> >> * http://en.wikipedia.org/wiki/Adler-32
> >>
> >> direct translation to J
> >>
> >> adler32=: 3 : 0
> >> 's1 s2'=. 1 0
> >> for_a. a.i.y do.
> >> s1=. 65521 | s1 + a
> >> s2=. 65521 | s2 + s1
> >> end.
> >> s1 + 16 (33 b.) s2
> >> )
> >>
> >> test data:
> >>
> >> adler32 'Wikipedia'
> >> 300286872
> >>
> >> adler32 'The quick brown fox jumps over the lazy dog'
> >> 1541148634
> >>
> >> --
> >> regards,
> >> ====================================================
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> >> gpg --keyserver subkeys.pgp.net --armor --export 4434BAB3
> >> ----------------------------------------------------------------------
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> ----------------------------------------------------------------------
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