accelerate getShufflingRef

AllTests-mainnet.md

@@ -438,6 +438,11 @@ OK: 1/1 Fail: 0/1 Skip: 0/1
 + RestErrorMessage writer tests                                                              OK
 ```
 OK: 2/2 Fail: 0/2 Skip: 0/2
+## Shufflings
+```diff
++ Accelerated shuffling computation                                                          OK
+```
+OK: 1/1 Fail: 0/1 Skip: 0/1
 ## Slashing Interchange tests  [Preset: mainnet]
 ```diff
 + Slashing test: duplicate_pubkey_not_slashable.json                                         OK
@@ -669,4 +674,4 @@ OK: 2/2 Fail: 0/2 Skip: 0/2
 OK: 9/9 Fail: 0/9 Skip: 0/9
 
 ---TOTAL---
-OK: 382/387 Fail: 0/387 Skip: 5/387
+OK: 383/388 Fail: 0/388 Skip: 5/388

beacon_chain/consensus_object_pools/block_dag.nim

@@ -149,6 +149,30 @@ func get_ancestor*(blck: BlockRef, slot: Slot,
 
     blck = blck.parent
 
+func commonAncestor*(a, b: BlockRef, lowSlot: Slot): Opt[BlockRef] =
+  ## Return the common ancestor with highest slot of two non-nil `BlockRef`,
+  ## limited by `lowSlot` (`err` if exceeded).
+  doAssert a != nil
+  doAssert b != nil
+  if a.slot < lowSlot or b.slot < lowSlot:
+    return err()
+
+  var
+    aa = a
+    bb = b
+  while aa != bb:
+    if aa.slot >= bb.slot:
+      aa = aa.parent
+      doAssert aa != nil, "All `BlockRef` lead to `finalizedHead`"
+      if aa.slot < lowSlot:
+        return err()
+    else:
+      bb = bb.parent
+      doAssert bb != nil, "All `BlockRef` lead to `finalizedHead`"
+      if bb.slot < lowSlot:
+        return err()
+  ok aa
+
 func atSlot*(blck: BlockRef, slot: Slot): BlockSlot =
   ## Return a BlockSlot at a given slot, with the block set to the closest block
   ## available. If slot comes from before the block, a suitable block ancestor