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libmp: mpnrand(), what was i *THINKING*

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the prior implementation was unneccesarily complicated for
no good reason due to me misunderstanding how libc's nrand()
works. in contrast to libc, we already generate the *closest*
power-of-2 random number with mprand() in the sampling loop.
This commit is contained in:
cinap_lenrek 2016-08-29 00:45:16 +02:00
parent b80684a1d2
commit 33862ff793
1 changed files with 3 additions and 15 deletions
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18
sys/src/libmp/port/mpnrand.c
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@ -6,30 +6,18 @@
mpint* mpint*
mpnrand(mpint *n, void (*gen)(uchar*, int), mpint *b) mpnrand(mpint *n, void (*gen)(uchar*, int), mpint *b)
{ {
mpint *m;
int bits; int bits;
/* m = 2^bits - 1 */
bits = mpsignif(n); bits = mpsignif(n);
m = mpnew(bits+1); if(bits == 0)
mpleft(mpone, bits, m); abort();
mpsub(m, mpone, m);
if(b == nil){ if(b == nil){
b = mpnew(bits); b = mpnew(bits);
setmalloctag(b, getcallerpc(&n)); setmalloctag(b, getcallerpc(&n));
} }
/* m = m - (m % n) */
mpmod(m, n, b);
mpsub(m, b, m);
do { do {
mprand(bits, gen, b); mprand(bits, gen, b);
} while(mpcmp(b, m) >= 0); } while(mpmagcmp(b, n) >= 0);
mpmod(b, n, b);
mpfree(m);
return b; return b;
} }