net: Fix CIDR notation in ToString()

Only use CIDR notation if the netmask can be represented as such.

src/netbase.cpp

@@ -1311,20 +1311,58 @@ bool CSubNet::Match(const CNetAddr &addr) const
     return true;
 }
 
+static inline int NetmaskBits(uint8_t x)
+{
+    switch(x) {
+    case 0x00: return 0; break;
+    case 0x80: return 1; break;
+    case 0xc0: return 2; break;
+    case 0xe0: return 3; break;
+    case 0xf0: return 4; break;
+    case 0xf8: return 5; break;
+    case 0xfc: return 6; break;
+    case 0xfe: return 7; break;
+    case 0xff: return 8; break;
+    default: return -1; break;
+    }
+}
+
 std::string CSubNet::ToString() const
 {
-    std::string strNetmask;
+    /* Parse binary 1{n}0{N-n} to see if mask can be represented as /n */
     int cidr = 0;
-    for (int n = network.IsIPv4() ? 12 : 0 ; n < 16; ++n)
-    {
-        uint8_t netmaskpart = netmask[n];
-        while (netmaskpart)
-        {
-            cidr += ( netmaskpart & 0x01 );
-            netmaskpart >>= 1;
+    bool valid_cidr = true;
+    int n = network.IsIPv4() ? 12 : 0;
+    for (; n < 16 && netmask[n] == 0xff; ++n)
+        cidr += 8;
+    if (n < 16) {
+        int bits = NetmaskBits(netmask[n]);
+        if (bits < 0)
+            valid_cidr = false;
+        else
+            cidr += bits;
+        ++n;
     }
+    for (; n < 16 && valid_cidr; ++n)
+        if (netmask[n] != 0x00)
+            valid_cidr = false;
+
+    /* Format output */
+    std::string strNetmask;
+    if (valid_cidr) {
+        strNetmask = strprintf("%u", cidr);
+    } else {
+        if (network.IsIPv4())
+            strNetmask = strprintf("%u.%u.%u.%u", netmask[12], netmask[13], netmask[14], netmask[15]);
+        else
+            strNetmask = strprintf("%x:%x:%x:%x:%x:%x:%x:%x",
+                             netmask[0] << 8 | netmask[1], netmask[2] << 8 | netmask[3],
+                             netmask[4] << 8 | netmask[5], netmask[6] << 8 | netmask[7],
+                             netmask[8] << 8 | netmask[9], netmask[10] << 8 | netmask[11],
+                             netmask[12] << 8 | netmask[13], netmask[14] << 8 | netmask[15]);
     }
-    return network.ToString() + strprintf("/%u", cidr);
+
+    return network.ToString() + "/" + strNetmask;
 }
 
 bool CSubNet::IsValid() const

src/test/netbase_tests.cpp

@@ -229,6 +229,12 @@ BOOST_AUTO_TEST_CASE(subnet_test)
     BOOST_CHECK_EQUAL(subnet.ToString(), "1::/16");
     subnet = CSubNet("1:2:3:4:5:6:7:8/0000:0000:0000:0000:0000:0000:0000:0000");
     BOOST_CHECK_EQUAL(subnet.ToString(), "::/0");
+    subnet = CSubNet("1.2.3.4/255.255.232.0");
+    BOOST_CHECK_EQUAL(subnet.ToString(), "1.2.0.0/255.255.232.0");
+    subnet = CSubNet("1:2:3:4:5:6:7:8/ffff:ffff:ffff:fffe:ffff:ffff:ffff:ff0f");
+    BOOST_CHECK_EQUAL(subnet.ToString(), "1:2:3:4:5:6:7:8/ffff:ffff:ffff:fffe:ffff:ffff:ffff:ff0f");
+    subnet = CSubNet("1:2:3:4:5:6:7:8/fff:ffff:ffff:ffff:ffff:ffff:ffff:fff0");
+    BOOST_CHECK_EQUAL(subnet.ToString(), "1:2:3:4:5:6:7:0/fff:ffff:ffff:ffff:ffff:ffff:ffff:fff0");
 }
 
 BOOST_AUTO_TEST_CASE(netbase_getgroup)