optimize network builder

old: 208 B/op   11 allocs/op
new:  80 B/op    5 allocs/op

Author: IncSW

common.go

@@ -4,10 +4,29 @@ import (
 	"encoding/binary"
 	"errors"
 	"math"
+	"net"
 	"strconv"
 	"unsafe"
 )
 
+func getNetworkString(ip net.IP, mask uint) string {
+	bitLen := 128
+	for i := 0; i < len(ip); i++ {
+		if ip[i] != 0 {
+			if i == 10 {
+				bitLen = 32
+			}
+			break
+		}
+	}
+	cidrMask := net.CIDRMask(int(mask), bitLen)
+	ipNet := net.IPNet{
+		IP:   ip.Mask(cidrMask),
+		Mask: cidrMask,
+	}
+	return ipNet.String()
+}
+
 func readControl(buffer []byte, offset uint) (byte, uint, uint, error) {
 	controlByte := buffer[offset]
 	offset++

geoip2_test.go

@@ -2,6 +2,7 @@ package geoip2
 
 import (
 	"net"
+	"runtime"
 	"testing"
 )
 
@@ -73,7 +74,7 @@ func TestReader(t *testing.T) {
 }
 
 func BenchmarkGeoIP2(b *testing.B) {
-	ip := net.ParseIP("81.2.69.142")
+	ip := net.ParseIP("1.128.0.0")
 	b.ReportAllocs()
 
 	b.Run("country", func(b *testing.B) {
@@ -159,13 +160,21 @@ func BenchmarkGeoIP2(b *testing.B) {
 		}
 		b.Run("sync", func(b *testing.B) {
 			for i := 0; i < b.N; i++ {
-				_, _ = reader.Lookup(ip)
+				asn, err := reader.Lookup(ip)
+				if err != nil {
+					b.Fatal(err)
+				}
+				runtime.KeepAlive(asn.Network)
 			}
 		})
 		b.Run("parallel", func(b *testing.B) {
 			b.RunParallel(func(pb *testing.PB) {
 				for pb.Next() {
-					_, _ = reader.Lookup(ip)
+					asn, err := reader.Lookup(ip)
+					if err != nil {
+						b.Fatal(err)
+					}
+					runtime.KeepAlive(asn.Network)
 				}
 			})
 		})

reader_asn.go

@@ -2,7 +2,6 @@ package geoip2
 
 import (
 	"errors"
-	"fmt"
 	"io/ioutil"
 	"net"
 	"strconv"
@@ -22,8 +21,7 @@ func (r *ASNReader) Lookup(ip net.IP) (*ASN, error) {
 		return nil, err
 	}
 	result := &ASN{}
-	_, network, err := net.ParseCIDR(fmt.Sprintf("%s/%s", ip.String(), strconv.Itoa(int(prefix))))
-	result.Network = network.String()
+	result.Network = getNetworkString(ip, prefix)
 	switch dataType {
 	case dataTypeMap:
 		_, err = readASNMap(result, r.decoderBuffer, size, offset)