You could incorporate the modulo operation in the summation directly:
As per Raul's solution, with the modulo addition:
a32_2=: 65536#. _1 0+ [:((65521|+)/ , {.) [: (65521|+)/\. 1,~ a.i. |.
NB. Test on 10M of random data
Randdata =: a.{~ ?10000000$256
timespacex 'r1=:a32_2 Randdata'
7.26671 2.68438e8
timespacex 'r1=:adler32 Randdata'
36.2094 2.01333e8
Jan-Pieter
2014-08-24 11:14 GMT+02:00 bill lam <[email protected]>:
> 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,
>> >> ====================================================
>> >> GPG key 1024D/4434BAB3 2008-08-24
>> >> gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3
>> >> gpg --keyserver subkeys.pgp.net --armor --export 4434BAB3
>> >> ----------------------------------------------------------------------
>> >> For information about J forums see http://www.jsoftware.com/forums.htm
>> ----------------------------------------------------------------------
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>
> --
> regards,
> ====================================================
> GPG key 1024D/4434BAB3 2008-08-24
> gpg --keyserver subkeys.pgp.net --recv-keys 4434BAB3
> gpg --keyserver subkeys.pgp.net --armor --export 4434BAB3
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
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