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
+}