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mpdiv: fix sign bug

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

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

@@ -9,7 +9,7 @@

 void

 mpdiv(mpint *dividend, mpint *divisor, mpint *quotient, mpint *remainder)

 {

-	int j, s, vn, sign;

+	int j, s, vn, sign, qsign, rsign;

 	mpdigit qd, *up, *vp, *qp;

 	mpint *u, *v, *t;

@@ -49,6 +49,13 @@

 			mpassign(mpzero, quotient);

 		return;

 	}

+	

+	qsign = divisor->sign * dividend->sign;

+	rsign = dividend->sign;

+	if(quotient != nil)

+		quotient->sign = qsign;

+	if(remainder != nil)

+		remainder->sign = rsign;

 	// D1: shift until divisor, v, has hi bit set (needed to make trial

 	//     divisor accurate)

@@ -121,14 +128,11 @@

 	if(qp != nil){

 		assert((quotient->flags & MPtimesafe) == 0);

 		mpnorm(quotient);

-		if(dividend->sign != divisor->sign)

-			quotient->sign = -1;

 	}

 	if(remainder != nil){

 		assert((remainder->flags & MPtimesafe) == 0);

 		mpright(u, s, remainder);	// u is the remainder shifted

-		remainder->sign = dividend->sign;

 	}

 	mpfree(t);