+/*
+ * Fast checksum update for possibly oddly-aligned UDP byte, from the
+ * code example in the draft.
+ */
+static void fast_csum(unsigned char *csum,
+ const unsigned char *optr,
+ const unsigned char *nptr,
+ int odd)
+{
+ long x, old, new;
+
+ x = csum[0] * 256 + csum[1];
+
+ x =~ x & 0xFFFF;
+
+ if (odd) old = optr[0] * 256;
+ else old = optr[0];
+
+ x -= old & 0xFFFF;
+ if (x <= 0) {
+ x--;
+ x &= 0xFFFF;
+ }
+
+ if (odd) new = nptr[0] * 256;
+ else new = nptr[0];
+
+ x += new & 0xFFFF;
+ if (x & 0x10000) {
+ x++;
+ x &= 0xFFFF;
+ }
+
+ x =~ x & 0xFFFF;
+ csum[0] = x / 256;
+ csum[1] = x & 0xFF;
+}
+
+/*
+ * Mangle IP address.
+ * - begin points to the start of the snmp messgae
+ * - addr points to the start of the address
+ */
+static inline void mangle_address(unsigned char *begin,
+ unsigned char *addr,
+ const struct oct1_map *map,
+ u_int16_t *check)
+{
+ if (map->from == NOCT1(*addr)) {
+ u_int32_t old;
+
+ if (debug)
+ memcpy(&old, (unsigned char *)addr, sizeof(old));
+
+ *addr = map->to;
+
+ /* Update UDP checksum if being used */
+ if (*check) {
+ unsigned char odd = !((addr - begin) % 2);
+
+ fast_csum((unsigned char *)check,
+ &map->from, &map->to, odd);
+
+ }
+
+ if (debug)
+ printk(KERN_DEBUG "bsalg: mapped %u.%u.%u.%u to "
+ "%u.%u.%u.%u\n", NIPQUAD(old), NIPQUAD(*addr));
+ }
+}
+