implement a function to find the highest supported version

protocol/version.go

@@ -3,6 +3,7 @@ package protocol
 import (
 	"bytes"
 	"encoding/binary"
+	"sort"
 	"strconv"
 )
 
@@ -18,6 +19,7 @@ const (
 )
 
 // SupportedVersions lists the versions that the server supports
+// must be in sorted order
 var SupportedVersions = []VersionNumber{
 	Version34, Version35, Version36,
 }
@@ -49,6 +51,33 @@ func IsSupportedVersion(v VersionNumber) bool {
 	return false
 }
 
+type byVersionNumber []VersionNumber
+
+func (a byVersionNumber) Len() int           { return len(a) }
+func (a byVersionNumber) Swap(i, j int)      { a[i], a[j] = a[j], a[i] }
+func (a byVersionNumber) Less(i, j int) bool { return a[i] < a[j] }
+
+// HighestSupportedVersion finds the highest version number that is both present in other and in SupportedVersions
+// the versions in other do not need to be ordered
+// it returns true and the version number, if there is one, otherwise false
+func HighestSupportedVersion(other []VersionNumber) (bool, VersionNumber) {
+	sort.Sort(byVersionNumber(other))
+
+	i := len(other) - 1             // index of other
+	j := len(SupportedVersions) - 1 // index of SupportedVersions
+	for i >= 0 && j >= 0 {
+		if other[i] == SupportedVersions[j] {
+			return true, SupportedVersions[j]
+		}
+		if other[i] > SupportedVersions[j] {
+			i--
+		} else {
+			j--
+		}
+	}
+	return false, 0
+}
+
 func init() {
 	var b bytes.Buffer
 	for _, v := range SupportedVersions {