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

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.

--- a/sys/src/libmp/port/mpnrand.c

+++ b/sys/src/libmp/port/mpnrand.c

@@ -6,30 +6,18 @@

 mpint*

 mpnrand(mpint *n, void (*gen)(uchar*, int), mpint *b)

 {

-	mpint *m;

 	int bits;

-	/* m = 2^bits - 1 */

 	bits = mpsignif(n);

-	m = mpnew(bits+1);

-	mpleft(mpone, bits, m);

-	mpsub(m, mpone, m);

-

+	if(bits == 0)

+		abort();

 	if(b == nil){

 		b = mpnew(bits);

 		setmalloctag(b, getcallerpc(&n));

 	}

-

-	/* m = m - (m % n) */

-	mpmod(m, n, b);

-	mpsub(m, b, m);

-

 	do {

 		mprand(bits, gen, b);

-	} while(mpcmp(b, m) >= 0);

-

-	mpmod(b, n, b);

-	mpfree(m);

+	} while(mpmagcmp(b, n) >= 0);

 	return b;

 }