convert hash_tree_roots_prefix(...) to an iterator

beacon_chain/spec/beaconstate.nim

@@ -229,11 +229,12 @@ func initialize_beacon_state_from_eth1*(
   # Process deposits
   let
     leaves = deposits.mapIt(it.data)
-    prefix_roots =
-      hash_tree_roots_prefix(leaves, 2'i64^DEPOSIT_CONTRACT_TREE_DEPTH)
-  for i, deposit in deposits:
-    state.eth1_data.deposit_root = prefix_roots[i]
-    discard process_deposit(state, deposit, flags)
+  var i = 0
+  for prefix_root in hash_tree_roots_prefix(
+      leaves, 2'i64^DEPOSIT_CONTRACT_TREE_DEPTH):
+    state.eth1_data.deposit_root = prefix_root
+    discard process_deposit(state, deposits[i], flags)
+    i += 1
 
   # Process activations
   for validator_index in 0 ..< state.validators.len:

beacon_chain/ssz.nim

@@ -573,8 +573,8 @@ func hash_tree_root*(x: auto): Eth2Digest =
 
   trs "HASH TREE ROOT FOR ", name(type x), " = ", "0x", $result
 
-func hash_tree_roots_prefix*[T](lst: openarray[T], limit: auto):
-    seq[Eth2Digest] =
+iterator hash_tree_roots_prefix*[T](lst: openarray[T], limit: auto):
+    Eth2Digest =
   # This is a particular type's instantiation of a general fold, reduce,
   # accumulation, prefix sums, etc family of operations. As long as that
   # Eth1 deposit case is the only notable example -- the usual uses of a
@@ -583,7 +583,7 @@ func hash_tree_roots_prefix*[T](lst: openarray[T], limit: auto):
   var merkelizer = SszChunksMerkelizer(limit: uint64(limit))
   for i, elem in lst:
     merkelizer.addChunk(hash_tree_root(elem).data)
-    result.add mixInLength(merkelizer.getFinalHash(), i + 1)
+    yield mixInLength(merkelizer.getFinalHash(), i + 1)
 
 func lastFieldName(RecordType: type): string {.compileTime.} =
   enumAllSerializedFields(RecordType):