Added a comment about the zlib compressed content and meta data in the blocks.

tilemapper/blockdecoder.go

@@ -30,6 +30,12 @@ type DecodedBlock struct {
 	IndexMap   map[int]int
 }
 
+// The content of the map and the meta data are compressed with zlib.
+// Unfortunately the byte length of this two structures are not stored
+// explicitly in the block data. To access the informations behind
+// them (e.g. the node id mappings) we have to count the bytes consumed
+// by the zlib reader and continue our extraction process behind this
+// offset. PosBuf implements such a counting reader source.
 type PosBuf struct {
 	Data []byte
 	Pos  int