Add the range.CIDRs functions

net/address/address.go

@@ -73,6 +73,40 @@ type CIDR struct {
 	PrefixLen int
 }
 
+// CIDRs returns a list of CIDR-aligned ranges which cover this range.
+func (r Range) CIDRs() []CIDR {
+	const (
+		fullMask         = ^Address(0)
+		cidrMaxPrefixLen = 32
+	)
+	var cidrs []CIDR
+
+	for start, end := r.Start, r.End-1; end >= start; {
+		mask, prefixLen := fullMask, cidrMaxPrefixLen
+		// Find the smallest mask which would cover some part of [start;end].
+		// Once we found such, apply it by OR'ing
+		for mask > 0 {
+			tmpMask := mask << 1
+			// Check whether mask neither too short nor too long
+			if (start&tmpMask) != start || (start|^tmpMask) > end {
+				break
+			}
+			mask = tmpMask
+			prefixLen--
+		}
+		cidrs = append(cidrs, CIDR{start, prefixLen})
+		// Apply mask
+		start |= ^mask
+		// Check for overflow
+		if start+1 < start {
+			break
+		}
+		start++
+	}
+
+	return cidrs
+}
+
 func ParseIP(s string) (Address, error) {
 	if ip := net.ParseIP(s); ip != nil {
 		return FromIP4(ip), nil

net/address/address_test.go

@@ -39,3 +39,25 @@ func TestBiggestPow2AlignedRange(t *testing.T) {
 	}
 	require.NoError(t, quick.Check(prop, &quick.Config{MaxCount: 1000000}))
 }
+
+func ip(s string) Address {
+	addr, _ := ParseIP(s)
+	return addr
+}
+
+func cidr(s string) CIDR {
+	c, err := ParseCIDR(s)
+	if err != nil {
+		panic(err)
+	}
+	return c
+}
+
+func TestCIDRs(t *testing.T) {
+	start := ip("10.0.0.1")
+	end := ip("10.0.0.9")
+	r := NewRange(start, Subtract(end, start))
+	require.Equal(t,
+		[]CIDR{cidr("10.0.0.1/32"), cidr("10.0.0.2/31"), cidr("10.0.0.4/30"), cidr("10.0.0.8/32")},
+		r.CIDRs())
+}