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libmp: with mpmod() m = 2^a - c ensure that digits(c) < digits(m)

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

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

@@ -13,7 +13,7 @@

 	sign = x->sign;

 	assert(n->flags & MPnorm);

-	if(n->top < 2 || n->top > nelem(q) || (x->top-n->top) > nelem(q))

+	if(n->top <= 2 || n->top > nelem(q) || (x->top-n->top) > nelem(q))

 		goto hard;

 	/*

@@ -42,11 +42,15 @@

 			c = mpnew(0);

 			p = mpnew(0);

 		}

-		mpassign(n, p);

-

-		mpleft(n, s, m);

+		mpleft(p, s, m);

 		mpleft(mpone, k*Dbits, c);

 		mpsub(c, m, c);

+		if(c->top >= k){

+			mpassign(mpzero, p);

+			busy = 0;

+			goto hard;

+		}

+		mpassign(n, p);

 	}

 	mpleft(x, s, r);