Added topic utility functions to whisper

whisper/util.go

+package whisper
+
+import "github.com/ethereum/go-ethereum/crypto"
+
+func hashTopic(topic []byte) []byte {
+	return crypto.Sha3(topic)[:4]
+}
+
+// NOTE this isn't DRY, but I don't want to iterate twice.
+
+// Returns a formatted topics byte slice.
+// data: unformatted data (e.g., no hashes needed)
+func Topics(data [][]byte) [][]byte {
+	d := make([][]byte, len(data))
+	for i, byts := range data {
+		d[i] = hashTopic(byts)
+	}
+	return d
+}
+
+func TopicsFromString(data []string) [][]byte {
+	d := make([][]byte, len(data))
+	for i, str := range data {
+		d[i] = hashTopic([]byte(str))
+	}
+	return d
+}