Add subnet_mask and tests

This was actually 95% done months ago, but I'm just committing it now.

Author: nuew

src/lib.rs

@@ -266,6 +266,18 @@ impl MaskedIpAddr {
         self.cidr()
     }
 
+    /// Returns the subnet mask, calculated from the CIDR.
+    pub fn subnet_mask(&self) -> u128 {
+        fn cidr2mask(full_cidr: u8, cidr: u8, max: u128) -> u128 {
+            (!((1 << full_cidr - cidr) - 1)) & max
+        }
+
+        match self.addr {
+            IpAddr::V4(_) => cidr2mask(IPV4_CIDR_FULL, self.cidr, std::u32::MAX as u128),
+            IpAddr::V6(_) => cidr2mask(IPV6_CIDR_FULL, self.cidr, std::u128::MAX),
+        }
+    }
+
     /// Consumes the `MaskedIpAddr`, returning the IP address and netmask.
     ///
     /// # Examples

src/tests.rs

@@ -24,7 +24,8 @@ fn _db() -> Connection {
     conn.execute(
         "CREATE TEMPORARY TABLE foo (id SERIAL PRIMARY KEY, cidr CIDR, inet INET)",
         &[],
-    ).unwrap();
+    )
+    .unwrap();
     conn
 }
 
@@ -51,7 +52,7 @@ fn _new_masked_ipv4() -> (MaskedIpAddr, IpAddr, u8) {
 
 fn _new_masked_ipv6() -> (MaskedIpAddr, IpAddr, u8) {
     let expected = IpAddr::V6(Ipv6Addr::new(0x2001, 0xDB8, 0, 0, 0, 0, 0, 0));
-    let cidr = 32;
+    let cidr = 48;
 
     (MaskedIpAddr::new(expected, cidr), expected, cidr)
 }
@@ -242,6 +243,18 @@ fn netmask_ipv6() {
     assert_eq!(mip.netmask(), cidr);
 }
 
+#[test]
+fn subnet_mask_v4() {
+    let (mip, _, _) = _new_masked_ipv4();
+    assert_eq!(mip.subnet_mask(), 0xffffff00);
+}
+
+#[test]
+fn subnet_mask_v6() {
+    let (mip, _, _) = _new_masked_ipv6();
+    assert_eq!(mip.subnet_mask(), 0xffffffffffff00000000000000000000);
+}
+
 #[test]
 fn into_inner_ipv4() {
     let (mip, expected, cidr) = _new_full_ipv4();