Addition of mp_radix_size_overestimate (behaviour like the function mpz_sizeinbase in GMP); changed mp_radix_size to use mp_ilogb

Author: czurnieden

bn_mp_fwrite.c

@@ -27,6 +27,7 @@ mp_err mp_fwrite(const mp_int *a, int radix, FILE *stream)
       err = MP_ERR;
       goto LBL_ERR;
    }
+
    err = MP_OKAY;
 
 LBL_ERR:

bn_mp_radix_size.c

@@ -3,13 +3,12 @@
 /* LibTomMath, multiple-precision integer library -- Tom St Denis */
 /* SPDX-License-Identifier: Unlicense */
 
-/* returns size of ASCII reprensentation */
+/* returns exact size of ASCII reprensentation */
 mp_err mp_radix_size(const mp_int *a, int radix, int *size)
 {
-   mp_err  err;
-   int     digs;
-   mp_int   t;
-   mp_digit d;
+   mp_err err;
+   mp_int a_, b;
+   int rem, bit_count, ln2;
 
    *size = 0;
 
@@ -23,42 +22,66 @@ mp_err mp_radix_size(const mp_int *a, int radix, int *size)
       return MP_OKAY;
    }
 
-   /* special case for binary */
-   if (radix == 2) {
-      *size = mp_count_bits(a) + ((a->sign == MP_NEG) ? 1 : 0) + 1;
+   /* A small shortcut for powers of two. */
+   switch (radix) {
+   case 4:
+      ln2 = 2;
+      break;
+   case 8:
+      ln2 = 3;
+      break;
+   case 16:
+      ln2 = 4;
+      break;
+   case 32:
+      ln2 = 5;
+      break;
+   case 64:
+      ln2 = 6;
+      break;
+   default:
+      ln2 = 0;
+      break;
+   }
+   if (ln2 != 0) {
+      bit_count = mp_count_bits(a);
+      *size = bit_count/ln2;
+      rem = bit_count - ((*size) * ln2);
+      /* Add 1 for the remainder if any and 1 for "\0". */
+      *size += (rem == 0) ? 1 : 2;
+      /* And one extra character for the minus sign */
+      if (a->sign == MP_NEG) {
+         (*size)++;
+      }
       return MP_OKAY;
    }
 
-   /* digs is the digit count */
-   digs = 0;
 
-   /* if it's negative add one for the sign */
-   if (a->sign == MP_NEG) {
-      ++digs;
+   if ((err = mp_init(&b)) != MP_OKAY) {
+      goto LBL_ERR;
    }
 
-   /* init a copy of the input */
-   if ((err = mp_init_copy(&t, a)) != MP_OKAY) {
-      return err;
+   a_ = *a;
+   a_.sign = MP_ZPOS;
+   if ((err = mp_ilogb(&a_, (uint32_t)radix, &b)) != MP_OKAY) {
+      goto LBL_ERR;
    }
 
-   /* force temp to positive */
-   t.sign = MP_ZPOS;
+   *size = (int)mp_get_l(&b);
 
-   /* fetch out all of the digits */
-   while (!MP_IS_ZERO(&t)) {
-      if ((err = mp_div_d(&t, (mp_digit)radix, &t, &d)) != MP_OKAY) {
-         goto LBL_ERR;
-      }
-      ++digs;
+   /* mp_ilogb truncates to zero, hence we need one extra put on top. */
+   (*size)++;
+
+   /* The sign needs to have a place to live, too*/
+   if (a->sign == MP_NEG) {
+      (*size)++;
    }
 
-   /* return digs + 1, the 1 is for the NULL byte that would be required. */
-   *size = digs + 1;
-   err = MP_OKAY;
+   /* return digs + 1, the 1 is for the NUL byte that would be required. */
+   (*size)++;
 
 LBL_ERR:
-   mp_clear(&t);
+   mp_clear(&b);
    return err;
 }