rings: introduce PrefixToRange()

returning the beginning and end of a subnet

Signed-off-by: Alejandro Mery <amery@jpi.io>

pkg/rings/cidr.go

@@ -41,3 +41,37 @@ func AddrToU32(addr netip.Addr) (v uint32, ok bool) {
 
 	return 0, false
 }
+
+// PrefixToRange returns the beginning and end of a
+// [netip.Prefix] (aka CIDR or subnet).
+func PrefixToRange(subnet netip.Prefix) (from, to netip.Addr, ok bool) {
+	var u uint32
+
+	addr := subnet.Addr()
+	if u, ok = AddrToU32(addr); ok {
+		bits := subnet.Bits()
+
+		switch {
+		case bits > 32, bits < 0:
+			// bad
+		case bits == 32:
+			// single
+			from, to, ok = addr, addr, true
+		default:
+			// subnet
+			shift := 32 - bits
+
+			m1 := uint32((1 << shift) - 1)
+			m0 := uint32(0xffffffff) & ^m1
+
+			u0 := u & m0
+			u1 := u0 + m1
+
+			ok = true
+			from = AddrFromU32(u0)
+			to = AddrFromU32(u1)
+		}
+	}
+
+	return from, to, ok
+}

pkg/rings/cidr_test.go

@@ -90,3 +90,89 @@ func MustParseAddr(s string) netip.Addr {
 	}
 	return addr
 }
+
+func MustParsePrefix(s string) netip.Prefix {
+	subnet, err := netip.ParsePrefix(s)
+	if err != nil {
+		panic(err)
+	}
+	return subnet
+}
+
+func TestPrefixToRangeValid(t *testing.T) {
+	cases := []struct {
+		subnet netip.Prefix
+		from   netip.Addr
+		to     netip.Addr
+	}{
+		{
+			MustParsePrefix("127.0.0.1/32"),
+			MustParseAddr("127.0.0.1"),
+			MustParseAddr("127.0.0.1"),
+		},
+		{
+			MustParsePrefix("127.0.0.1/24"),
+			MustParseAddr("127.0.0.0"),
+			MustParseAddr("127.0.0.255"),
+		},
+		{
+			MustParsePrefix("127.0.1.2/16"),
+			MustParseAddr("127.0.0.0"),
+			MustParseAddr("127.0.255.255"),
+		},
+		{
+			MustParsePrefix("127.1.2.3/8"),
+			MustParseAddr("127.0.0.0"),
+			MustParseAddr("127.255.255.255"),
+		},
+		{
+			MustParsePrefix("10.20.30.40/12"),
+			MustParseAddr("10.16.0.0"),
+			MustParseAddr("10.31.255.255"),
+		},
+		{
+			MustParsePrefix("10.20.30.40/20"),
+			MustParseAddr("10.20.16.0"),
+			MustParseAddr("10.20.31.255"),
+		},
+		{
+			MustParsePrefix("10.0.0.0/12"),
+			MustParseAddr("10.0.0.0"),
+			MustParseAddr("10.15.255.255"),
+		},
+		{
+			MustParsePrefix("10.16.0.0/12"),
+			MustParseAddr("10.16.0.0"),
+			MustParseAddr("10.31.255.255"),
+		},
+		{
+			MustParsePrefix("10.32.0.0/12"),
+			MustParseAddr("10.32.0.0"),
+			MustParseAddr("10.47.255.255"),
+		},
+		{
+			MustParsePrefix("10.48.0.0/12"),
+			MustParseAddr("10.48.0.0"),
+			MustParseAddr("10.63.255.255"),
+		},
+	}
+
+	for i, tc := range cases {
+		from, to, ok := PrefixToRange(tc.subnet)
+		if ok && from.IsValid() && to.IsValid() &&
+			from.Compare(tc.from) == 0 &&
+			to.Compare(tc.to) == 0 {
+			//
+			t.Logf("[%v/%v]: %s → %s - %s",
+				i, len(cases),
+				tc.subnet,
+				from, to)
+		} else {
+			t.Errorf("ERROR: [%v/%v]: %q → %s - %s %v (expected %s - %s %v)",
+				i, len(cases),
+				tc.subnet,
+				from, to, ok,
+				tc.from, tc.to, true)
+		}
+	}
+}