semver: fix Maven difference (google#191)

There is a bug when computing Maven difference: `1.0.0` is considered to
have a minor diff with `1.0.1` - this is because `1.0.0` is
canonicalized to `1` and returns `-1` for minor and patch numbers.

This PR fixes this issue by returning `0` as the number of an empty
element.

Author: cuixq

util/semver/maven.go

@@ -384,10 +384,10 @@ func compareMavenQualifier(a, b string) int {
 }
 
 // num returns the number of the i'th element. If the element
-// is not a number, it returns -1.
+// is empty or not a number, it returns 0.
 func (m *mavenExtension) num(i int) int64 {
 	if i >= len(m.elems) {
-		return -1
+		return 0
 	}
 	str := m.elems[i].str
 	// By the rules of building elements, we know that if the

util/semver/maven_test.go

@@ -232,11 +232,13 @@ func TestMavenDifference(t *testing.T) {
 		diff Diff
 	}{
 		{"1", "1", 0, Same},
+		{"1", "1.0.0", 0, Same},
 		{"1.2", "1.2", 0, Same},
 		{"1.2.3", "1.2.3", 0, Same},
 		{"2", "1", 1, DiffMajor},
 		{"1.3", "1.2", 1, DiffMinor},
 		{"1.2.4", "1.2.3", 1, DiffPatch},
+		{"1.0.1", "1.0.0", 1, DiffPatch},
 		{"1.a", "1.b", -1, DiffOther},
 	}
 	for _, test := range tests {