uniform random bits, credit cinap

Reference: /n/atom/patch/applied/mpnrand
Date: Fri Sep 18 06:31:05 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/include/mp.h	Fri Sep 18 06:30:51 2015
+++ /sys/include/mp.h	Fri Sep 18 06:30:52 2015
@@ -38,6 +38,8 @@
 
 /* random bits */
 mpint*	mprand(int bits, void (*gen)(uchar*, int), mpint *b);
+/* return uniform random [0..n-1] */
+mpint*	mpnrand(mpint *n, void (*gen)(uchar*, int), mpint *b);
 
 /* conversion */
 mpint*	strtomp(char*, char**, int, mpint*);	/* ascii */
--- /sys/src/libmp/port/mkfile	Fri Sep 18 06:30:53 2015
+++ /sys/src/libmp/port/mkfile	Fri Sep 18 06:30:55 2015
@@ -27,6 +27,7 @@
 	mpextendedgcd\
 	mpinvert\
 	mprand\
+	mpnrand\
 	crt\
 	mptoi\
 	mptoui\
--- /sys/src/libmp/port/mpnrand.c	Thu Jan  1 00:00:00 1970
+++ /sys/src/libmp/port/mpnrand.c	Fri Sep 18 06:30:55 2015
@@ -0,0 +1,34 @@
+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+#include "dat.h"
+
+/* return uniform random [0..n-1] */
+mpint*
+mpnrand(mpint *n, void (*gen)(uchar*, int), mpint *b)
+{
+	mpint *m;
+	int bits;
+
+	/* m = 2^bits - 1 */
+	bits = mpsignif(n);
+	m = mpnew(bits+1);
+	mpleft(mpone, bits, m);
+	mpsub(m, mpone, m);
+
+	if(b == nil)
+		b = mpnew(bits);
+
+	/* m = m - (m % n) */
+	mpmod(m, n, b);
+	mpsub(m, b, m);
+
+	do {
+		mprand(bits, gen, b);
+	} while(mpcmp(b, m) >= 0);
+
+	mpmod(b, n, b);
+	mpfree(m);
+
+	return b;
+}