bc: shrink zbc_num_ulong()

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

1 files changed, 11 insertions, 10 deletions

diff --git a/miscutils/bc.c b/miscutils/bc.c
index 7be2d0b9b..876244b34 100644
--- a/miscutils/bc.c
+++ b/miscutils/bc.c
@@ -1442,20 +1442,21 @@ static void bc_num_copy(BcNum *d, BcNum *s)
 static BC_STATUS zbc_num_ulong(BcNum *n, unsigned long *result_p)
 {
         size_t i;
-        unsigned long pow, result;
+        unsigned long result;
 
         if (n->neg) RETURN_STATUS(bc_error("negative number"));
 
-        for (result = 0, pow = 1, i = n->rdx; i < n->len; ++i) {
-                unsigned long prev = result, powprev = pow;
-
-                result += ((unsigned long) n->num[i]) * pow;
-                pow *= 10;
-
-                if (result < prev || pow < powprev)
+        result = 0;
+        i = n->len;
+        while (i > n->rdx) {
+                unsigned long prev = result;
+                result = result * 10 + n->num[--i];
+                // Even overflowed N*10 can still satisfy N*10>=N. For example,
+                //    0x1ff00000 * 10 is 0x13f600000,
+                // or 0x3f600000 truncated to 32 bits. Which is larger.
+                // However, (N*10)/8 < N check is always correct.
+                if ((result / 8) < prev)
                         RETURN_STATUS(bc_error("overflow"));
-                prev = result;
-                powprev = pow;
         }
         *result_p = result;