Reuse indexes with full sweep

specs/_features/reuse_indexes/beacon-chain.md

@@ -0,0 +1,45 @@
+# Reuse indexes -- The Beacon Chain
+
+## Table of contents
+
+<!-- TOC -->
+<!-- START doctoc generated TOC please keep comment here to allow auto update -->
+<!-- DON'T EDIT THIS SECTION, INSTEAD RE-RUN doctoc TO UPDATE -->
+
+- [Introduction](#introduction)
+- [Preset](#preset)
+  - [Time parameters](#time-parameters)
+- [Beacon chain state transition function](#beacon-chain-state-transition-function)
+  - [Block processing](#block-processing)
+    - [Modified `assign_index_to_deposit`](#modified-assign_index_to_deposit)
+
+<!-- END doctoc generated TOC please keep comment here to allow auto update -->
+<!-- /TOC -->
+
+## Introduction
+
+This is the beacon chain specification to assign new deposits to existing validator records that have withdrawn long ago.
+
+*Note:* This specification is built upon [Capella](../../capella/beacon_chain.md) and is under active development.
+
+## Preset
+
+### Time parameters
+
+| Name | Value | Unit | Duration |
+| - | - | - |
+| `REUSE_VALIDATOR_INDEX_DELAY` | `uint64(2**16)` (= 65,536) | epochs | ~1 year |
+
+## Beacon chain state transition function
+
+### Block processing
+
+#### Modified `assign_index_to_deposit`
+
+```python
+def assign_index_to_deposit(state: BeaconState) -> uint64:
+    for index, validator in enumerate(state.validators):
+        if validator.withdrawable_epoch < get_current_epoch(state) - REUSE_VALIDATOR_INDEX_DELAY:
+            return index
+    return len(state.validators)
+```

specs/altair/beacon-chain.md

@@ -511,16 +511,28 @@ def apply_deposit(state: BeaconState,
         signing_root = compute_signing_root(deposit_message, domain)
         # Initialize validator if the deposit signature is valid
         if bls.Verify(pubkey, signing_root, signature):
-            state.validators.append(get_validator_from_deposit(pubkey, withdrawal_credentials, amount))
-            state.balances.append(amount)
+            index = assign_index_to_deposit(state)
+            update_list(state.validators, index, get_validator_from_deposit(pubkey, withdrawal_credentials, amount))
+            update_list(state.balances, index, amount)
             # [New in Altair]
-            state.previous_epoch_participation.append(ParticipationFlags(0b0000_0000))
-            state.current_epoch_participation.append(ParticipationFlags(0b0000_0000))
-            state.inactivity_scores.append(uint64(0))
+            update_list(state.previous_epoch_participation, index, ParticipationFlags(0b0000_0000))
+            update_list(state.current_epoch_participation, index, ParticipationFlags(0b0000_0000))
+            update_list(state.inactivity_scores, index, uint64(0))
     else:
         # Increase balance by deposit amount
         index = ValidatorIndex(validator_pubkeys.index(pubkey))
         increase_balance(state, index, amount)
+
+
+def assign_index_to_deposit(state: BeaconState) -> ValidatorIndex:
+    return len(state.validators)
+
+
+def update_list<T>(list: List<T>, index: uint64, value: T):
+        if index == len(list):
+            list.append(value)
+        else:
+            list[index] = value
 ```
 
 #### Sync aggregate processing