I don't know whether or not this produces the correct results because I
don't have any test data, but...
SH =: 16bffffffff AND 34 b.
mix =: verb define
for_i. _11 2 _8 16 _10 4 _8 9 do.
'a b c d e f g h' =. y
y =. 1 |. (a XOR b SH i), (b+c), c, (d+a), e, f, g, h
end.
)
mix i.8 NB. 4294967351 51 8589934561 2058 4294957073 262170 4160749608
4294967324
On Fri, Sep 4, 2015 at 9:55 PM, Dan Bron <[email protected]> wrote:
> Periodically I review the list of open J tasks on RosettaCode [1]. Today I
> came across an obscure but (if you believe the task description)
> meritorious cipher, called ISAAC ("Indirection, Shift, Accumulate, Add, and
> Count”).
>
> I understand the basic concepts of cryptography, but am generally unversed
> in the practical details of implementation. In other words: at the moment,
> I’m blindly transliterating the C implementation (marked as canonical) to
> J, hewing as closely to the source material as possible, loops and all.
>
> Which, of course, results in some damn ugly J. Take, for example, this C
> preprocessor macro, which I’ve dutifully transliterated as in the
> post-script.
>
> #define mix(a,b,c,d,e,f,g,h) \
> { \
> a^=b<<11; d+=a; b+=c; \
> b^=c>>2; e+=b; c+=d; \
> c^=d<<8; f+=c; d+=e; \
> d^=e>>16; g+=d; e+=f; \
> e^=f<<10; h+=e; f+=g; \
> f^=g>>4; a+=f; g+=h; \
> g^=h<<8; b+=g; h+=a; \
> h^=a>>9; c+=h; a+=b; \
> }
>
> Can you write this function in idiomatic, perhaps even elegant, J? Extra
> brownie points for something along the lines of f/ a,b,c,d,e,f,g,h ,
>
> If it gives you a jumpstart, here is the pattern of variable names, as
> well as the bitshift direction and degree:
>
> abdabc < 11
> bcebcd > 2
> cdfcde < 8
> degdef > 16
> efhefg < 10
> fgafgh > 4
> ghbgha < 8
> hachab > 9
>
> Expressed as left-hand arguments to |.!.0, the bitshifts are 11 _2 8 _16
> 10 _4 8 _9 .
>
> And the variable name matrix, expressed as indices into ‘abcdefgh’ is:
>
> 0 1 3 0 1 2
> 1 2 4 1 2 3
> 2 3 5 2 3 4
> 3 4 6 3 4 5
> 4 5 7 4 5 6
> 5 6 0 5 6 7
> 6 7 1 6 7 0
> 7 0 2 7 0 1
>
> Which pattern can be expressed as 0 1 3 0 1 2 (|."0 1) i.8 if you
> conceive of it column-wise, or 0 1 3 0 1 2 (8 | +/~) i.8 if row-wise.
> Either way, the salient structure appears to be 0 1 3 0 1 2 .
>
>
> -Dan
>
> [1] Tasks on RosettaCode with no J implementation:
> http://rosettacode.org/wiki/Reports:Tasks_not_implemented_in_J <
> http://rosettacode.org/wiki/Reports:Tasks_not_implemented_in_J>
>
> [2] The “wash shuffle” (which Wikipedia calls the “Corgi shuffle”) is
> that basic, no-frills move you see at the cheap tables in Atlantic City:
> the dealer simply spreads all the cards face down on the table and slides
> them all around and over each other. Not pretty, but effective, and less
> susceptible to sleight-of-hand cheating.
> https://en.wikipedia.org/wiki/Shuffling#Corgi_shuffle <
> https://en.wikipedia.org/wiki/Shuffling#Corgi_shuffle>
>
> PS: Here is a direct transliteration, but be WARNED: I haven’t gotten the
> overall ISAAC cypher to produce the correct outputs yet, so this is
> untested and may be incorrect.
>
>
> NB. ISAAC operates on 32-bit ints, meaning b. will break it on 64-bit
> systems
> bww =: (32#2)&#:
> hlfUnd =: 2 : 'v^:_1@:(u v)’
>
> bwXor =: ~:&.bww
> bwShift =: |.!.0 hlfUnd bww
>
> mix =: verb define
> 'a b c d e f g h'=.y
>
> NB. a^=b<<11; d+=a; b+=c;
> a =. a bwXor 11 bwShift b
> d =. d + a
> b =. b + c
>
> NB. b^=c>>2; e+=b; c+=d;
> b =. b bwXor _2 bwShift c
> e =. e + b
> c =. c + d
>
> NB. c^=d<<8; f+=c; d+=e;
> c =. c bwXor 8 bwShift d
> f =. f + c
> d =. d + e
>
> NB. d^=e>>16; g+=d; e+=f;
> d =. d bwXor _16 bwShift e
> g =. g + d
> e =. e + f
>
> NB. e^=f<<10; h+=e; f+=g;
> e =. e bwXor 10 bwShift f
> h =. h + e
> f =. f + g
>
> NB. f^=g>>4; a+=f; g+=h;
> f =. f bwXor _4 bwShift g
> a =. a + f
> g =. g + h
>
> NB. g^=h<<8; b+=g; h+=a;
> g =. g bwXor 8 bwShift h
> b =. b + g
> h =. h + a
>
> NB. h^=a>>9; c+=h; a+=b;
> h =. h bwXor _9 bwShift a
> c =. c + h
> a =. a + b
>
> ".&> ;: 'a b c d e f g h'
> )
>
>
>
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