bugfix in correction-loop of nroot

demo/test.c

@@ -1930,6 +1930,25 @@ static int test_mp_root_n(void)
          EXPECT(mp_cmp(&r, &c) == MP_EQ);
       }
    }
+   /* 0^(1/x) = 0 with x != 0 is allowed, test */
+   mp_set(&a, 0);
+   DO(mp_root_n(&a, 2, &c));
+   EXPECT(mp_cmp_d(&c, 0) == MP_EQ);
+
+   /* Not allowed: division by zero */
+   mp_set(&a, 2);
+   EXPECT(mp_root_n(&a, 0, &c) == MP_VAL);
+
+   /* root^base == input with small input and base */
+   mp_set(&a, 4);
+   DO(mp_root_n(&a, 2, &c));
+   EXPECT(mp_cmp_d(&c, 2) == MP_EQ);
+
+   /* (root^base)^(1/(base + 1)) with small root */
+   DO(mp_2expt(&a, 48));
+   DO(mp_root_n(&a, 49, &c));
+   EXPECT(mp_cmp_d(&c, 1) == MP_EQ);
+
    mp_clear_multi(&a, &c, &r, NULL);
    return EXIT_SUCCESS;
 LBL_ERR:

mp_root_n.c

@@ -18,6 +18,22 @@ mp_err mp_root_n(const mp_int *a, int b, mp_int *c)
    int    ilog2;
    mp_err err;
 
+
+   if (b == 0) {
+      mp_set(c, 0);
+      return MP_VAL;
+   }
+
+   /* 0^(1/x) = 0 with x != 0 is allowed */
+   if (mp_iszero(a)) {
+      mp_set(c, 0);
+      if (b != 0) {
+         return MP_OKAY;
+      } else {
+         return MP_VAL;
+      }
+   }
+
    if (b < 0 || (unsigned)b > (unsigned)MP_DIGIT_MAX) {
       return MP_VAL;
    }
@@ -109,7 +125,8 @@ mp_err mp_root_n(const mp_int *a, int b, mp_int *c)
       cmp = mp_cmp(&t2, &a_);
       if (cmp == MP_EQ) {
          err = MP_OKAY;
-         goto LBL_ERR;
+         /* On point, skip overshoot correction */
+         goto LBL_SET;
       }
       if (cmp == MP_LT) {
          if ((err = mp_add_d(&t1, 1uL, &t1)) != MP_OKAY)          goto LBL_ERR;
@@ -127,6 +144,7 @@ mp_err mp_root_n(const mp_int *a, int b, mp_int *c)
       }
    }
 
+LBL_SET:
    /* set the result */
    mp_exch(&t1, c);