fix: Not all IPRanges with a size of power 2 can convert to CIDR

Signed-off-by: iiiceoo <iiiceoo@foxmail.com>

.github/workflows/code-coverage.yml

@@ -4,7 +4,7 @@ jobs:
   unit-test:
     strategy:
       matrix:
-        go-version: [1.20.x, 1.21.x, 1.22.x]
+        go-version: [1.20.x]
         os: [ubuntu-latest]
     runs-on: ${{ matrix.os }}
     steps:

example_test.go

@@ -140,18 +140,18 @@ func ExampleIPRanges_Union() {
 }
 
 func ExampleIPRanges_Diff() {
-	ranges1, err := iprange.Parse("172.18.0.20-30", "172.18.0.1-25")
+	ranges1, err := iprange.Parse("172.18.0.20-30", "172.18.0.0-25")
 	if err != nil {
 		log.Fatalf("error parsing IP ranges: %v", err)
 	}
-	ranges2, err := iprange.Parse("172.18.0.5-25")
+	ranges2, err := iprange.Parse("172.18.0.4-26")
 	if err != nil {
 		log.Fatalf("error parsing IP ranges: %v", err)
 	}
 
 	fmt.Println(ranges1.Diff(ranges2))
 	// Output:
-	// [172.18.0.1/30 172.18.0.26-172.18.0.30]
+	// [172.18.0.0/30 172.18.0.27-172.18.0.30]
 }
 
 func ExampleIPRanges_Intersect() {
@@ -170,14 +170,14 @@ func ExampleIPRanges_Intersect() {
 }
 
 func ExampleIPRanges_Slice() {
-	ranges, err := iprange.Parse("172.18.0.1-5", "172.18.0.10-14")
+	ranges, err := iprange.Parse("172.18.0.0-3", "172.18.0.10-14")
 	if err != nil {
 		log.Fatalf("error parsing IP ranges: %v", err)
 	}
 
 	fmt.Println(ranges.Slice(big.NewInt(2), big.NewInt(-2)))
 	// Output:
-	// [172.18.0.3-172.18.0.5 172.18.0.10/30]
+	// [172.18.0.2/31 172.18.0.10-172.18.0.13]
 }
 
 func ExampleIPRanges_IsOverlap() {
@@ -230,7 +230,7 @@ func ExampleIPRanges_IPIterator() {
 }
 
 func ExampleIPRanges_BlockIterator() {
-	ranges, err := iprange.Parse("172.18.0.1-5")
+	ranges, err := iprange.Parse("172.18.0.0-4")
 	if err != nil {
 		log.Fatalf("error parsing IP ranges: %v", err)
 	}
@@ -255,10 +255,10 @@ func ExampleIPRanges_BlockIterator() {
 	}
 
 	// Output:
-	// 172.18.0.1/31
-	// 172.18.0.3/31
-	// 172.18.0.5
-	// 172.18.0.5
+	// 172.18.0.0/31
+	// 172.18.0.2/31
+	// 172.18.0.4
+	// 172.18.0.4
 }
 
 func ExampleIPRanges_CIDRIterator() {

range.go

@@ -139,18 +139,22 @@ func (r *ipRange) String() string {
 		return r.start.String()
 	}
 
-	if inc.And(inc, dv).Sign() != 0 {
-		return r.start.String() + "-" + r.end.String()
-	}
+	if inc.And(inc, dv).Sign() == 0 {
+		bits := 32
+		if r.start.version() == IPv6 {
+			bits = 128
+		}
 
-	bits := 32
-	if r.start.version() == IPv6 {
-		bits = 128
-	}
-	ipNet := &net.IPNet{
-		IP:   r.start.IP,
-		Mask: net.CIDRMask(bits-bl, bits),
+		ip := r.start.IP
+		mask := net.CIDRMask(bits-bl, bits)
+		if ip.Mask(mask).Equal(ip) {
+			ipNet := net.IPNet{
+				IP:   ip,
+				Mask: mask,
+			}
+			return ipNet.String()
+		}
 	}
 
-	return ipNet.String()
+	return r.start.String() + "-" + r.end.String()
 }

ranges.go

@@ -459,10 +459,6 @@ func (rr *IPRanges) String() string {
 
 // Strings returns a slice of the string representations of the IPRanges rr.
 func (rr *IPRanges) Strings() []string {
-	if len(rr.ranges) == 0 {
-		return nil
-	}
-
 	ss := make([]string, 0, len(rr.ranges))
 	for _, r := range rr.ranges {
 		ss = append(ss, r.String())

ranges_test.go

@@ -1637,7 +1637,7 @@ var ipRangesStringsTests = []struct {
 	{
 		name:   "zero",
 		ranges: &IPRanges{},
-		want:   nil,
+		want:   []string{},
 	},
 }