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mp: fix mpnot and add mpasr

--- a/sys/man/2/mp

+++ b/sys/man/2/mp

@@ -127,6 +127,9 @@

 void	mpxtend(mpint *b, int n, mpint *res)

 .PP

 .B

+void	mpasr(mpint *b, int n, mpint *res)

+.PP

+.B

 void	mpmul(mpint *b1, mpint *b2, mpint *prod)

 .PP

 .B

@@ -583,6 +586,12 @@

 .TP

 .I mpnot

 .BR "res = ~b1" .

+.TP

+.I mpasr

+.BR "res = b>>shift"

+(\fImpasr\fR, unlike

+.IR mpright ,

+uses two's complement).

 .TP

 .I mptrunc

 truncates

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

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

@@ -85,7 +85,8 @@

 mpnot(mpint *b, mpint *r)

 {

 	mpadd(b, mpone, r);

-	r->sign ^= -2;

+	if(r->top != 0)

+		r->sign ^= -2;

 }

 void

@@ -192,4 +193,16 @@

 			c = 0;

 	}

 	mpnorm(r);

+}

+

+void

+mpasr(mpint *b, int n, mpint *r)

+{

+	if(b->sign > 0 || n <= 0){

+		mpright(b, n, r);

+		return;

+	}

+	mpadd(b, mpone, r);

+	mpright(r, n, r);

+	mpsub(r, mpone, r);

 }