fix Delta.DifferentAt behavior via changes to Path.Contains

pkg/compare/delta_test.go

@@ -53,11 +53,14 @@ func TestDifferentAt(t *testing.T) {
 	d = compare.NewDelta()
 	d.Add("Bar", a.Bar, b.Bar)
 	require.True(d.DifferentAt("Bar"))
-	require.False(d.DifferentAt("Baz"))
+	require.False(d.DifferentAt("Baz")) // diff exists but was not added to Delta
 
 	d = compare.NewDelta()
 	d.Add("Baz.Y", a.Baz.Y, b.Baz.Y)
 	require.True(d.DifferentAt("Baz"))
-	require.True(d.DifferentAt("Y"))
-	require.False(d.DifferentAt("Bar"))
+	require.True(d.DifferentAt("Baz.Y"))
+	require.False(d.DifferentAt("Y")) // there is no diff for top-level field "Y"
+	require.False(d.DifferentAt("Bar")) // diff exists but it was not added to Delta
+	require.False(d.DifferentAt("Baz.Y.Z")) // subject length exceeds length of diff Path
+	require.False(d.DifferentAt("Baz.Z"))  // matches Path top-level field but not sub-field
 }

pkg/compare/path.go

@@ -51,14 +51,28 @@ func (p Path) Pop() {
 	}
 }
 
-// Contains returns true if the supplied string appears within the Path
+//	Contains returns true if the supplied string, delimited on ".", matches
+//	p.parts up to the length of the supplied string.
+//		e.g. if the Path p represents "A.B":
+//			subject "A" -> true
+//			subject "A.B" -> true
+//			subject "A.B.C" -> false
+//			subject "B" -> false
+//			subject "A.C" -> false
 func (p Path) Contains(subject string) bool {
-	for _, p := range p.parts {
-		if p == subject {
-			return true
+	subjectSplit := strings.Split(subject, ".")
+
+	if len(subjectSplit) > len(p.parts) {
+		return false
+	}
+
+	for i, s := range subjectSplit {
+		if p.parts[i] != s {
+			return false
 		}
 	}
-	return false
+
+	return true
 }
 
 // NewPath returns a new Path struct pointer from a dotted-notation string,