hierarchy: provide option to list the used keywords in a spec

To increase a user's control on how they validate a directory or
tar archive with a specification, it is helpful to know which
keywords are actually used in the spec provided. This way, the user
can see what keywords to use or not use with the '-k' or '-K' flags.

Signed-off-by: Stephen Chung <schung@redhat.com>

hierarchy.go

@@ -25,3 +25,27 @@ func (dh DirectoryHierarchy) WriteTo(w io.Writer) (n int64, err error) {
 	}
 	return sum, nil
 }
+
+// CollectUsedKeywords collects and returns all the keywords used in a
+// a DirectoryHierarchy
+func CollectUsedKeywords(dh *DirectoryHierarchy) []string {
+	if dh != nil {
+		usedkeywords := []string{}
+		for _, e := range dh.Entries {
+			switch e.Type {
+			case FullType, RelativeType, SpecialType:
+				if e.Type != SpecialType || e.Name == "/set" {
+					kvs := e.Keywords
+					for _, kv := range kvs {
+						kw := KeyVal(kv).Keyword()
+						if !inSlice(kw, usedkeywords) {
+							usedkeywords = append(usedkeywords, kw)
+						}
+					}
+				}
+			}
+		}
+		return usedkeywords
+	}
+	return nil
+}