--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libtommath/bn_mp_to_radix.c	Thu Oct 15 19:55:15 2020 +0800
@@ -0,0 +1,84 @@
+#include "tommath_private.h"
+#ifdef BN_MP_TO_RADIX_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/* stores a bignum as a ASCII string in a given radix (2..64)
+ *
+ * Stores upto "size - 1" chars and always a NULL byte, puts the number of characters
+ * written, including the '\0', in "written".
+ */
+mp_err mp_to_radix(const mp_int *a, char *str, size_t maxlen, size_t *written, int radix)
+{
+   size_t  digs;
+   mp_err  err;
+   mp_int  t;
+   mp_digit d;
+   char   *_s = str;
+
+   /* check range of radix and size*/
+   if (maxlen < 2u) {
+      return MP_BUF;
+   }
+   if ((radix < 2) || (radix > 64)) {
+      return MP_VAL;
+   }
+
+   /* quick out if its zero */
+   if (MP_IS_ZERO(a)) {
+      *str++ = '0';
+      *str = '\0';
+      if (written != NULL) {
+         *written = 2u;
+      }
+      return MP_OKAY;
+   }
+
+   if ((err = mp_init_copy(&t, a)) != MP_OKAY) {
+      return err;
+   }
+
+   /* if it is negative output a - */
+   if (t.sign == MP_NEG) {
+      /* we have to reverse our digits later... but not the - sign!! */
+      ++_s;
+
+      /* store the flag and mark the number as positive */
+      *str++ = '-';
+      t.sign = MP_ZPOS;
+
+      /* subtract a char */
+      --maxlen;
+   }
+   digs = 0u;
+   while (!MP_IS_ZERO(&t)) {
+      if (--maxlen < 1u) {
+         /* no more room */
+         err = MP_BUF;
+         goto LBL_ERR;
+      }
+      if ((err = mp_div_d(&t, (mp_digit)radix, &t, &d)) != MP_OKAY) {
+         goto LBL_ERR;
+      }
+      *str++ = mp_s_rmap[d];
+      ++digs;
+   }
+   /* reverse the digits of the string.  In this case _s points
+    * to the first digit [exluding the sign] of the number
+    */
+   s_mp_reverse((unsigned char *)_s, digs);
+
+   /* append a NULL so the string is properly terminated */
+   *str = '\0';
+   digs++;
+
+   if (written != NULL) {
+      *written = (a->sign == MP_NEG) ? (digs + 1u): digs;
+   }
+
+LBL_ERR:
+   mp_clear(&t);
+   return err;
+}
+
+#endif