How to return Tx to mem pool from orphan block This question is centered around the implementation of how transactions on an orphan block return to the mem pool.

In my understanding, once a block becomes orphan all it's transactions (that aren't on other non-orphan blocks) need to return to the mem pool.

Do we start from genesis block until we reach the last block of the active chain checking every transaction along the way, or is it there a more efficient algorithm that's based on some constraint/property that allows the algorithm to start from a more recent block?

Proto code for starting from the genesis block:

Blockchain blockchain = ...
function Dictionary&lt;Key, Tx&gt; deleteOrphanTx (Dictionary&lt;Key, Tx&gt; orphanTx) {
    foreach (Block currBlock : blockchain.getBlocks())
        foreach (Tx tx : currBlock.getTx())
            Key txKey = tx.getKey();
            if(orphanTx.contains(txKey)
                orphanTx.remove(txKey);
    return orphanTx;
}
Proto code for returning transactions from orphan block:

foreach ( tx: orphanBlock )
  if ( tx.isValidAfterReorg ( ) )
    mempool.insert ( tx );