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libmp: avoid temporary buffer allocation in mprand()

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

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

@@ -6,10 +6,7 @@

 mprand(int bits, void (*gen)(uchar*, int), mpint *b)

 {

 	mpdigit mask;

-	int n, m;

-	uchar *p;

-	n = DIGITS(bits);

 	if(b == nil){

 		b = mpnew(bits);

 		setmalloctag(b, getcallerpc(&bits));

@@ -16,21 +13,13 @@

 	}else

 		mpbits(b, bits);

-	p = malloc(n*Dbytes);

-	if(p == nil)

-		sysfatal("mprand: %r");

-	(*gen)(p, n*Dbytes);

-	betomp(p, n*Dbytes, b);

-	free(p);

+	b->sign = 1;

+	b->top = DIGITS(bits);

+	(*gen)((uchar*)b->p, b->top*Dbytes);

-	// make sure we don't give too many bits

-	m = bits%Dbits;

-	if(m == 0)

-		return b;

+	mask = ((mpdigit)1 << (bits%Dbits))-1;

+	if(mask != 0)

+		b->p[b->top-1] &= mask;

-	mask = 1;

-	mask <<= m;

-	mask--;

-	b->p[n-1] &= mask;

 	return mpnorm(b);

 }