diff --git a/dagger/dataset.nim b/dagger/dataset.nim
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+## Nim-Dagger
+## Copyright (c) 2021 Status Research & Development GmbH
+## Licensed under either of
+##  * Apache License, version 2.0, ([LICENSE-APACHE](LICENSE-APACHE))
+##  * MIT license ([LICENSE-MIT](LICENSE-MIT))
+## at your option.
+## This file may not be copied, modified, or distributed except according to
+## those terms.
+
+{.push raises: [Defect].}
+
+import std/sequtils
+
+import pkg/libp2p
+import pkg/stew/byteutils
+import pkg/questionable
+import pkg/questionable/results
+
+import ./blockstream
+
+type
+  BlockSetRef* = ref object of BlockStreamRef
+    stream*: BlockStreamRef
+
+proc hashBytes*(mh: MultiHash): seq[byte] =
+  mh.data.buffer[mh.dpos..(mh.dpos + mh.size - 1)]
+
+proc hashBytes*(b: Block): seq[byte] =
+  without mh =? b.cid.mhash:
+    return
+
+  mh.hashBytes()
+
+method nextBlock*(d: BlockSetRef): ?!Block =
+  d.stream.nextBlock()
+
+proc treeHash*(d: BlockSetRef): ?!MultiHash =
+  var
+    stack: seq[seq[byte]]
+    codec: MultiCodec = InvalidMultiCodec
+
+  while true:
+    let (blk1, blk2) = (d.nextBlock().option, d.nextBlock().option)
+    if blk1.isNone and blk2.isNone and stack.len == 1:
+      let res = MultiHash.init($codec, stack[0])
+      if mh =? res:
+        return success mh
+
+      return failure($res.error)
+
+    if blk1.isSome: stack.add((!blk1).hashBytes())
+    if blk2.isSome: stack.add((!blk2).hashBytes())
+
+    codec = if codec == InvalidMultiCodec: (!blk1).cid.mcodec else: codec
+    while (stack.len mod 2) == 0:
+      let (b1, b2) = (stack.pop(), stack.pop())
+      let res = MultiHash.init($codec, b1 & b2)
+      if mh =? res:
+        stack.add(mh.hashBytes())
+      else:
+        return failure($res.error)
+
+proc new*(T: type BlockSetRef, blockStream: BlockStreamref) =
+  discard