From eb1e8311c3ceda5b71fc11b73cc497177ca2cd60 Mon Sep 17 00:00:00 2001
From: Vitalik Buterin <v@buterin.com>
Date: Mon, 5 Nov 2018 07:37:07 -0500
Subject: [PATCH] Implemented withdrawal rate limit in place of wihdrawal delay

---
 specs/beacon-chain.md | 36 ++++++++++++++++++++++++------------
 1 file changed, 24 insertions(+), 12 deletions(-)

diff --git a/specs/beacon-chain.md b/specs/beacon-chain.md
index 220759ef3..8396effe3 100644
--- a/specs/beacon-chain.md
+++ b/specs/beacon-chain.md
@@ -42,7 +42,9 @@ The primary source of load on the beacon chain are "attestations". Attestations
 | `MIN_VALIDATOR_SET_CHANGE_INTERVAL` | 2**8 (= 256) | slots | ~1.1 hours |
 | `RANDAO_SLOTS_PER_LAYER` | 2**12 (= 4096) | slots | ~18 hours |
 | `SQRT_E_DROP_TIME` | 2**16 (= 65,536) | slots | ~12 days |
-| `WITHDRAWAL_PERIOD` | 2**19 (= 524,288) | slots | ~97 days |
+| `MIN_WITHDRAWAL_PERIOD` | 2**12 (= 4096) | slots | ~18 hours |
+| `WITHDRAWALS_PER_CYCLE` | 8 | - | 4.3m ETH in ~6 months |
+| `COLLECTIVE_PENALTY_CALCULATION_PERIOD` | 2**19 (= 524,288) | slots | ~3 months |
 | `BASE_REWARD_QUOTIENT` | 2**15 (= 32,768) | — |
 | `MAX_VALIDATOR_CHURN_QUOTIENT` | 2**5 (= 32) | — | 
 | `LOGOUT_MESSAGE` | `"LOGOUT"` | — | 
@@ -210,6 +212,8 @@ The `CrystallizedState` has the following fields:
     'deposits_penalized_in_period': ['uint32'],
     # Hash chain of validator set changes (for light clients to easily track deltas)
     'validator_set_delta_hash_chain': 'hash32'
+    # Current sequence number for withdrawals
+    'current_exit_seq': 'uint64',
     # Parameters relevant to hard forks / versioning.
     # Should be updated only by hard forks.
     'pre_fork_version': 'uint32',
@@ -238,6 +242,8 @@ A `ValidatorRecord` has the following fields:
     'status': 'uint8',
     # Slot when validator exited (or 0)
     'exit_slot': 'uint64'
+    # Sequence number when validator exited (or 0)
+    'exit_seq': 'uint64'
 }
 ```
 
@@ -584,7 +590,8 @@ def add_validator(validators: List[ValidatorRecord],
         randao_last_change=current_slot,
         balance=DEPOSIT_SIZE * GWEI_PER_ETH,
         status=status,
-        exit_slot=0
+        exit_slot=0,
+        exit_seq=0
     )
     index = min_empty_validator(validators)
     if index is None:
@@ -601,9 +608,11 @@ def add_validator(validators: List[ValidatorRecord],
 def exit_validator(index, crystallized_state, penalize, current_slot):
     validator = crystallized_state.validators[index]
     validator.exit_slot = current_slot
+    validator.exit_seq = crystallized_state.current_exit_seq
+    crystallized_state.current_exit_seq += 1
     if penalize:
         validator.status = PENALIZED
-        crystallized_state.deposits_penalized_in_period[current_slot // WITHDRAWAL_PERIOD] += validator.balance
+        crystallized_state.deposits_penalized_in_period[current_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD] += validator.balance
     else:
         validator.status = PENDING_EXIT
     add_validator_set_change_record(crystallized_state, index, validator.pubkey, EXIT)
@@ -783,7 +792,7 @@ def change_validators(validators: List[ValidatorRecord]) -> None:
             break
 
     # Calculate the total ETH that has been penalized in the last ~2-3 withdrawal periods
-    period_index = current_slot // WITHDRAWAL_PERIOD
+    period_index = current_slot // COLLECTIVE_PENALTY_CALCULATION_PERIOD
     total_penalties = (
         (crystallized_state.deposits_penalized_in_period[period_index]) +
         (crystallized_state.deposits_penalized_in_period[period_index - 1] if period_index >= 1 else 0) +
@@ -791,15 +800,18 @@ def change_validators(validators: List[ValidatorRecord]) -> None:
     )
     # Separate loop to withdraw validators that have been logged out for long enough, and
     # calculate their penalties if they were slashed
-    for i in range(len(validators)):
-        if validators[i].status in (PENDING_WITHDRAW, PENALIZED) and current_slot >= validators[i].exit_slot + WITHDRAWAL_PERIOD:
-            if validators[i].status == PENALIZED:
-                validators[i].balance -= validators[i].balance * min(total_penalties * 3, total_balance) // total_balance
-            validators[i].status = WITHDRAWN
 
-            withdraw_amount = validators[i].balance
-            ...
-            # STUB: withdraw to shard chain
+    def withdrawable(v):
+        return v.status in (PENDING_WITHDRAW, PENALIZED) and current_slot >= v.exit_slot + MIN_WITHDRAWAL_PERIOD
+
+    withdrawable_validators = sorted(filter(withdrawable, validators), key=lambda v: v.exit_seq)
+    for v in withdrawable_validators[:WITHDRAWALS_PER_CYCLE]:
+        if v.status == PENALIZED:
+            v.balance -= v.balance * min(total_penalties * 3, total_balance) // total_balance
+        v.status = WITHDRAWN
+        withdraw_amount = v.balance
+        ...
+        # STUB: withdraw to shard chain
 ```
 
 Finally: