whisper/whisperv6: implement pow/bloom exchange protocol (#15802)

This is the main feature of v6.

whisper/whisperv6/envelope.go

@@ -42,9 +42,11 @@ type Envelope struct {
 	Data   []byte
 	Nonce  uint64
 
-	pow  float64     // Message-specific PoW as described in the Whisper specification.
-	hash common.Hash // Cached hash of the envelope to avoid rehashing every time.
-	// Don't access hash directly, use Hash() function instead.
+	pow float64 // Message-specific PoW as described in the Whisper specification.
+
+	// the following variables should not be accessed directly, use the corresponding function instead: Hash(), Bloom()
+	hash  common.Hash // Cached hash of the envelope to avoid rehashing every time.
+	bloom []byte
 }
 
 // size returns the size of envelope as it is sent (i.e. public fields only)
@@ -227,3 +229,30 @@ func (e *Envelope) Open(watcher *Filter) (msg *ReceivedMessage) {
 	}
 	return msg
 }
+
+// Bloom maps 4-bytes Topic into 64-byte bloom filter with 3 bits set (at most).
+func (e *Envelope) Bloom() []byte {
+	if e.bloom == nil {
+		e.bloom = TopicToBloom(e.Topic)
+	}
+	return e.bloom
+}
+
+// TopicToBloom converts the topic (4 bytes) to the bloom filter (64 bytes)
+func TopicToBloom(topic TopicType) []byte {
+	b := make([]byte, bloomFilterSize)
+	var index [3]int
+	for j := 0; j < 3; j++ {
+		index[j] = int(topic[j])
+		if (topic[3] & (1 << uint(j))) != 0 {
+			index[j] += 256
+		}
+	}
+
+	for j := 0; j < 3; j++ {
+		byteIndex := index[j] / 8
+		bitIndex := index[j] % 8
+		b[byteIndex] = (1 << uint(bitIndex))
+	}
+	return b
+}