Update EIP-6110: Spec parse_deposit_data

EIPS/eip-6110.md

@@ -76,11 +76,51 @@ Beginning with the `FORK_BLOCK`, each deposit accumulated in the block **MUST**
 in the order they appear in the logs. To illustrate:
 
 ```python
-def parse_deposit_data(deposit_event_data) -> bytes[]:
-  """
-  Parses deposit data from DepositContract.DepositEvent data
-  """
-  pass
+def parse_deposit_data(deposit_event_data):
+    """
+    Parses deposit data from DepositContract.DepositEvent data.
+    """
+
+    # TODO: verify byte length of deposit_event_data? Should be 576. Should this be part of the spec? (Yes?)
+
+    """ 
+        The deposit event is encoded as:
+            event DepositEvent(
+                bytes pubkey,
+                bytes withdrawal_credentials,
+                bytes amount,
+                bytes signature,
+                bytes index
+            );
+    """
+
+    # Start offset of data reading skips over the first 5 32-byte values
+    # which represents the dynamic size of the encoded bytes event data
+    # The size itself is encoded in a 32-byte, so skip over this as well 
+    # to arrive at the first actual data offset to be read (pubkey)
+    currentOffset = 32 * 5 + 32
+
+    # Read the pubkey data
+    pubkey = deposit_event_data[currentOffset:currentOffset + 48]
+    # The ABI data is encoded in 32-byte chunks, so the 64 bytes are reserved for the data
+    # The next 32 bytes are reserved for the size of the next item (withdrawals) so skip over this as well
+    currentOffset += 48 + 16 + 32
+
+    withdrawal_credentials = deposit_event_data[currentOffset:currentOffset + 32]
+    # This data fits in one 32-byte chunk. Also skip over the size 32-bytes of the next item (amount)
+    currentOffset += 32 + 32
+
+    amount = deposit_event_data[currentOffset:currentOffset + 8]
+    # This data fits in a 32 byte chunk. Also skip over the size 32-bytes of the next item (signature)
+    currentOffset += 8 + 24 + 32
+
+    signature = deposit_event_data[currentOffset:currentOffset + 96]
+    # This data fits in three 32 byte chunks. Also skip over the size 32-bytes of the next item (index)
+    currentOffset += 96 + 32
+
+    index = deposit_event_data[currentOffset:currentOffset + 8]
+
+    return [pubkey, withdrawal_credentials, amount, signature, index]
 
 def event_data_to_deposit_request(deposit_event_data) -> bytes:
     deposit_data = parse_deposit_data(deposit_event_data)