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libmp: allow passing nil to v,x,y results of mpextendedgcd(), simplify mpinvert()

--- a/libmp/mpextendedgcd.c

+++ b/libmp/mpextendedgcd.c

@@ -15,8 +15,16 @@

 	mpint *u, *A, *B, *C, *D;

 	int g;

+	if(v == nil){

+		v = mpnew(0);

+		mpextendedgcd(a, b, v, x, y);

+		mpfree(v);

+		return;

+	}

+	assert(x == nil || (x->flags & MPtimesafe) == 0);

+	assert(y == nil || (y->flags & MPtimesafe) == 0);

 	assert((a->flags&b->flags) & MPnorm);

-	assert(((a->flags|b->flags|v->flags|x->flags|y->flags) & MPtimesafe) == 0);

+	assert(((a->flags|b->flags|v->flags) & MPtimesafe) == 0);

 	if(a->sign < 0 || b->sign < 0){

 		mpassign(mpzero, v);

@@ -104,6 +112,4 @@

 	mpfree(u);

 	mpfree(a);

 	mpfree(b);

-

-	return;

 }

--- a/libmp/mpinvert.c

+++ b/libmp/mpinvert.c

@@ -1,21 +1,17 @@

 #include "os.h"

 #include <mp.h>

-#define iseven(a)	(((a)->p[0] & 1) == 0)

-

 // use extended gcd to find the multiplicative inverse

 // res = b**-1 mod m

 void

 mpinvert(mpint *b, mpint *m, mpint *res)

 {

-	mpint *dc1, *dc2;	// don't care

+	mpint *v;

-	dc1 = mpnew(0);

-	dc2 = mpnew(0);

-	mpextendedgcd(b, m, dc1, res, dc2);

-	if(mpcmp(dc1, mpone) != 0)

+	v = mpnew(0);

+	mpextendedgcd(b, m, v, res, nil);

+	if(mpcmp(v, mpone) != 0)

 		abort();

+	mpfree(v);

 	mpmod(res, m, res);

-	mpfree(dc1);

-	mpfree(dc2);

 }