~50% speedup by fast-pathing Eth2Digest merkle hashing

beacon_chain/spec/beaconstate.nim

@@ -258,8 +258,8 @@ func is_valid_genesis_state*(state: BeaconState): bool =
     return false
   return true
 
-# TODO candidate for spec?
-# https://github.com/ethereum/eth2.0-specs/blob/0.5.1/specs/core/0_beacon-chain.md#on-genesis
+# TODO this is now a non-spec helper function, and it's not really accurate
+# so only usable/used in research/ and tests/
 func get_initial_beacon_block*(state: BeaconState): BeaconBlock =
   BeaconBlock(
     slot: GENESIS_SLOT,

beacon_chain/ssz.nim

@@ -139,7 +139,7 @@ func writeFixedSized(c: var WriteCursor, x: auto) =
       c.append x
     else:
       for elem in x:
-        trs "WRITING FIXED SIZE ARRAY ELEMENENT"
+        trs "WRITING FIXED SIZE ARRAY ELEMENT"
         c.writeFixedSized toSszType(elem)
   elif x is tuple|object:
     enumInstanceSerializedFields(x, fieldName, field):
@@ -508,7 +508,12 @@ func bitlistHashTreeRoot(merkelizer: SszChunksMerkelizer, x: BitSeq): Eth2Digest
   mixInLength contentsHash, x.len
 
 func hashTreeRootImpl[T](x: T): Eth2Digest =
-  when (T is BasicType) or (when T is array: ElemType(T) is BasicType else: false):
+  when (when T is array: ElemType(T) is byte and
+      sizeof(T) == sizeof(Eth2Digest) else: false):
+    # TODO is this sizeof comparison guranteed? it's whole structure vs field
+    trs "ETH2DIGEST; IDENTITY MAPPING"
+    Eth2Digest(data: x)
+  elif (T is BasicType) or (when T is array: ElemType(T) is BasicType else: false):
     trs "FIXED TYPE; USE CHUNK STREAM"
     merkelizeSerializedChunks x
   elif T is string or (when T is (seq|openarray): ElemType(T) is BasicType else: false):