parser: extract height from coinbase transaction

parser/block.go

@@ -24,6 +24,28 @@ func (b *block) GetTxCount() int {
 	return len(b.vtx)
 }
 
+// GetHeight() extracts the block height from the coinbase transaction. See
+// BIP34. Returns block height on success, or -1 on error.
+func (b *block) GetHeight() int {
+	coinbaseScript := bytestring.String(b.vtx[0].transparentInputs[0].ScriptSig)
+	var heightByte byte
+	if ok := coinbaseScript.ReadByte(&heightByte); !ok {
+		return -1
+	}
+	heightLen := int(heightByte)
+	var heightBytes = make([]byte, heightLen)
+	if ok := coinbaseScript.ReadBytes(&heightBytes, heightLen); !ok {
+		return -1
+	}
+	// uint32 should last us a while (Nov 2018)
+	var blockHeight uint32
+	for i := heightLen - 1; i >= 0; i-- {
+		blockHeight <<= 8
+		blockHeight = blockHeight | uint32(heightBytes[i])
+	}
+	return int(blockHeight)
+}
+
 func (b *block) ParseFromSlice(data []byte) (rest []byte, err error) {
 	hdr := NewBlockHeader()
 	data, err = hdr.ParseFromSlice(data)