--- /n/sources/plan9/sys/include/mp.h	Thu Jun 27 01:38:28 2024
+++ /sys/include/mp.h	Thu Jun 27 02:02:05 2024
@@ -81,6 +81,9 @@
 /* return neg, 0, pos as b1-b2 is neg, 0, pos */
 int	mpcmp(mpint *b1, mpint *b2);
 
+/* res = s != 0 ? b1 : b2 */
+void	mpsel(int s, mpint *b1, mpint *b2, mpint *res);
+
 /* extended gcd return d, x, and y, s.t. d = gcd(a,b) and ax+by = d */
 void	mpextendedgcd(mpint *a, mpint *b, mpint *d, mpint *x, mpint *y);
 
--- /n/sources/plan9/sys/include/ape/mp.h	Thu Jun 27 01:38:28 2024
+++ /sys/include/ape/mp.h	Thu Jun 27 02:04:07 2024
@@ -90,6 +90,9 @@
 /* return neg, 0, pos as b1-b2 is neg, 0, pos */
 int	mpcmp(mpint *b1, mpint *b2);
 
+/* res = s != 0 ? b1 : b2 */
+void	mpsel(int s, mpint *b1, mpint *b2, mpint *res);
+
 /* extended gcd return d, x, and y, s.t. d = gcd(a,b) and ax+by = d */
 void	mpextendedgcd(mpint *a, mpint *b, mpint *d, mpint *x, mpint *y);
 
--- /n/sources/plan9/sys/man/2/mp	Thu Jun 27 01:38:28 2024
+++ /sys/man/2/mp	Thu Jun 27 02:02:06 2024
@@ -1,6 +1,6 @@
 .TH MP 2
 .SH NAME
-mpsetminbits, mpnew, mpfree, mpbits, mpnorm, mpcopy, mpassign, mprand, mpnrand, strtomp, mpfmt,mptoa, betomp, mptobe, mptober, letomp, mptole, mptolel, mptoui, uitomp, mptoi, itomp, uvtomp, mptouv, vtomp, mptov, mpdigdiv, mpadd, mpsub, mpleft, mpright, mpmul, mpexp, mpmod, mpdiv, mpcmp, mpextendedgcd, mpinvert, mpsignif, mplowbits0, mpvecdigmuladd, mpvecdigmulsub, mpvecadd, mpvecsub, mpveccmp, mpvecmul, mpmagcmp, mpmagadd, mpmagsub, crtpre, crtin, crtout, crtprefree, crtresfree \- extended precision arithmetic
+mpsetminbits, mpnew, mpfree, mpbits, mpnorm, mpcopy, mpassign, mprand, mpnrand, strtomp, mpfmt,mptoa, betomp, mptobe, mptober, letomp, mptole, mptolel, mptoui, uitomp, mptoi, itomp, uvtomp, mptouv, vtomp, mptov, mpdigdiv, mpadd, mpsub, mpleft, mpright, mpmul, mpexp, mpmod, mpdiv, mpcmp¸ mpsel, mpextendedgcd, mpinvert, mpsignif, mplowbits0, mpvecdigmuladd, mpvecdigmulsub, mpvecadd, mpvecsub, mpveccmp, mpvecmul, mpmagcmp, mpmagadd, mpmagsub, crtpre, crtin, crtout, crtprefree, crtresfree \- extended precision arithmetic
 .SH SYNOPSIS
 .B #include <u.h>
 .br
@@ -127,6 +127,9 @@
 int	mpmagcmp(mpint *b1, mpint *b2)
 .PP
 .B
+void	mpsel(int s, mpint *b1, mpint *b2, mpint *res)
+.PP
+.B
 void	mpextendedgcd(mpint *a, mpint *b, mpint *d, mpint *x,
 .br
 .B
@@ -510,6 +513,18 @@
 the same as
 .I mpcmp
 but ignores the sign and just compares magnitudes.
+.TP
+.I mpsel
+assigns
+.I b1
+to
+.I res
+when
+.I s
+is not zero, otherwise
+.I b2
+is assigned to
+.IR res .
 .PD
 .PP
 .I Mpextendedgcd
diff -Nru /n/sources/plan9/sys/src/ape/lib/mp/port/mkfile /sys/src/ape/lib/mp/port/mkfile
--- /n/sources/plan9/sys/src/ape/lib/mp/port/mkfile	Thu Jun 27 00:35:51 2024
+++ /sys/src/ape/lib/mp/port/mkfile	Thu Jun 27 02:04:34 2024
@@ -16,6 +16,7 @@
 	mpadd\
 	mpsub\
 	mpcmp\
+	mpsel\
 	mpfactorial\
 	mpmul\
 	mpleft\
diff -Nru /n/sources/plan9/sys/src/libmp/port/mkfile /sys/src/libmp/port/mkfile
--- /n/sources/plan9/sys/src/libmp/port/mkfile	Thu Jun 27 00:16:50 2024
+++ /sys/src/libmp/port/mkfile	Thu Jun 27 02:02:06 2024
@@ -14,6 +14,7 @@
 	mpadd\
 	mpsub\
 	mpcmp\
+	mpsel\
 	mpfactorial\
 	mpmul\
 	mpleft\
diff -Nru /n/sources/plan9/sys/src/libmp/port/mpsel.c /sys/src/libmp/port/mpsel.c
--- /n/sources/plan9/sys/src/libmp/port/mpsel.c	Wed Dec 31 19:00:00 1969
+++ /sys/src/libmp/port/mpsel.c	Thu Jun 27 02:02:09 2024
@@ -0,0 +1,42 @@
+#include "os.h"
+#include <mp.h>
+#include "dat.h"
+
+// res = s != 0 ? b1 : b2
+void
+mpsel(int s, mpint *b1, mpint *b2, mpint *res)
+{
+	mpdigit d;
+	int n, m, i;
+
+	res->flags |= (b1->flags | b2->flags) & MPtimesafe;
+	if((res->flags & MPtimesafe) == 0){
+		mpassign(s ? b1 : b2, res);
+		return;
+	}
+	res->flags &= ~MPnorm;
+
+	n = b1->top;
+	m = b2->top;
+	mpbits(res, Dbits*(n >= m ? n : m));
+	res->top = n >= m ? n : m;
+
+	s = (-s^s|s)>>(sizeof(s)*8-1);
+	res->sign = (b1->sign & s) | (b2->sign & ~s);
+
+	d = -((mpdigit)s & 1);
+
+	i = 0;
+	while(i < n && i < m){
+		res->p[i] = (b1->p[i] & d) | (b2->p[i] & ~d);
+		i++;
+	}
+	while(i < n){
+		res->p[i] = b1->p[i] & d;
+		i++;
+	}
+	while(i < m){
+		res->p[i] = b2->p[i] & ~d;
+		i++;
+	}
+}