in-addr-util: introduce in_addr_prefix_nth

src/basic/in-addr-util.c

@@ -226,6 +226,92 @@ int in_addr_prefix_next(int family, union in_addr_union *u, unsigned prefixlen)
         return -EAFNOSUPPORT;
 }
 
+/*
+ * Calculates the nth prefix of size prefixlen starting from the address denoted by u.
+ *
+ * On success 1 will be returned and the calculated prefix will be available in
+ * u. In the case nth == 0 the input will be left unchanged and 1 will be returned.
+ * In case the calculation cannot be performed (invalid prefix length,
+ * overflows would occur) -ERANGE is returned. If the address family given isn't
+ * supported -EAFNOSUPPORT will be returned.
+ *
+ *
+ * Examples:
+ *   - in_addr_prefix_nth(AF_INET, 192.168.0.0, 24, 2), returns 1, writes 192.168.2.0 to u
+ *   - in_addr_prefix_nth(AF_INET, 192.168.0.0, 24, 0), returns 1, no data written
+ *   - in_addr_prefix_nth(AF_INET, 255.255.255.0, 24, 1), returns -ERANGE, no data written
+ *   - in_addr_prefix_nth(AF_INET, 255.255.255.0, 0, 1), returns -ERANGE, no data written
+ *   - in_addr_prefix_nth(AF_INET6, 2001:db8, 64, 0xff00) returns 1, writes 2001:0db8:0000:ff00:: to u
+ */
+int in_addr_prefix_nth(int family, union in_addr_union *u, unsigned prefixlen, uint64_t nth) {
+        assert(u);
+
+        if (prefixlen <= 0)
+                return -ERANGE;
+
+        if (nth == 0)
+                return 1;
+
+        if (family == AF_INET) {
+                uint32_t c, n, t;
+                if (prefixlen > 32)
+                        prefixlen = 32;
+
+                c = be32toh(u->in.s_addr);
+
+                t = nth << (32 - prefixlen);
+
+                /* Check for wrap */
+                if (c > UINT32_MAX - t)
+                        return -ERANGE;
+
+                n = c + t;
+
+                n &= UINT32_C(0xFFFFFFFF) << (32 - prefixlen);
+                u->in.s_addr = htobe32(n);
+                return 1;
+        }
+
+        if (family == AF_INET6) {
+                struct in6_addr result = {};
+                uint8_t overflow = 0;
+                uint64_t delta;  /* this assumes that we only ever have to up to 1<<64 subnets */
+                unsigned start_byte = (prefixlen - 1) / 8;
+
+                if (prefixlen > 128)
+                        prefixlen = 128;
+
+                /* First calculate what we have to add */
+                delta = nth << ((128 - prefixlen) % 8);
+
+                for (unsigned i = 16; i > 0; i--) {
+                        unsigned j = i - 1;
+                        unsigned d = 0;
+
+                        if (j <= start_byte) {
+                                int16_t t;
+
+                                d = delta & 0xFF;
+                                delta >>= 8;
+
+                                t = u->in6.s6_addr[j] + d + overflow;
+                                overflow = t > UINT8_MAX ? t - UINT8_MAX : 0;
+
+                                result.s6_addr[j] = (uint8_t)t;
+                        } else
+                                result.s6_addr[j] = u->in6.s6_addr[j];
+                }
+
+                if (overflow || delta != 0)
+                        return -ERANGE;
+
+                u->in6 = result;
+                return 1;
+        }
+
+        return -EAFNOSUPPORT;
+}
+
 int in_addr_random_prefix(
                 int family,
                 union in_addr_union *u,

src/basic/in-addr-util.h

@@ -36,6 +36,7 @@ bool in4_addr_equal(const struct in_addr *a, const struct in_addr *b);
 int in_addr_equal(int family, const union in_addr_union *a, const union in_addr_union *b);
 int in_addr_prefix_intersect(int family, const union in_addr_union *a, unsigned aprefixlen, const union in_addr_union *b, unsigned bprefixlen);
 int in_addr_prefix_next(int family, union in_addr_union *u, unsigned prefixlen);
+int in_addr_prefix_nth(int family, union in_addr_union *u, unsigned prefixlen, uint64_t nth);
 int in_addr_random_prefix(int family, union in_addr_union *u, unsigned prefixlen_fixed_part, unsigned prefixlen);
 int in_addr_to_string(int family, const union in_addr_union *u, char **ret);
 int in_addr_prefix_to_string(int family, const union in_addr_union *u, unsigned prefixlen, char **ret);