Initialize SyncDag with genesis root/slot to avoid downloading.

Author: cheatfate

beacon_chain/sync/sync_dag.nim

@@ -88,13 +88,6 @@ func init*[T](
     columnsMap: Opt.some(columns)
   )
 
-proc init*(
-    t: typedesc[SyncDag],
-    A: typedesc,
-    B: typedesc
-): SyncDag[A, B] =
-  SyncDag[A, B]()
-
 iterator parents*(entry: SyncDagEntryRef): SyncDagEntryRef =
   doAssert(not(isNil(entry)), "Entry should not be nil")
   var currentEntry = entry
@@ -283,3 +276,21 @@ proc prune*[A, B](
     if sdag.roots.pop(item, entry):
       entry.parent = nil
       entry = nil
+
+proc init*(
+    t: typedesc[SyncDag],
+    A: typedesc,
+    B: typedesc
+): SyncDag[A, B] =
+  var res = SyncDag[A, B]()
+  let
+    genesisRoot =
+      Eth2Digest()
+    genesisBid =
+      BlockId(root: genesisRoot, slot: GENESIS_SLOT)
+    genesisEntry =
+      SyncDagEntryRef(blockId: genesisBid, flags: {DagEntryFlag.Finalized})
+  res.roots[genesisRoot] = genesisEntry
+  res.slots[GENESIS_SLOT] = toHashSet(@[genesisRoot])
+  res.lastSlot = GENESIS_SLOT
+  res