Starting on phase 1 misc beacon changes

specs/core/1_beacon_chain_misc.md

@@ -0,0 +1,75 @@
+# Phase 1 miscellaneous beacon chain changes
+
+## Table of contents
+
+<!-- TOC -->
+
+- [Helpers](#helpers)
+      - [pack_compact_validator](#pack_compact_validator)
+      - [unpack_compact_validator](#unpack_compact_validator)
+      - [committee_to_compact_committee](#committee_to_compact_committee)
+- [Changes](#changes)
+      - [Persistent committees](#persistent-committees)
+
+<!-- /TOC -->
+
+## Helpers
+
+#### `pack_compact_validator`
+
+```python
+def pack_compact_validator(index: uint64, slashed: bool, balance_in_increments: uint64) -> uint64:
+    """
+    Creates a compact validator object representing index, slashed status, and compressed balance.
+    Takes as input balance-in-increments (// EFFECTIVE_BALANCE_INCREMENT) to preserve symmetry with
+    the unpacking function.
+    """
+    return (index << 16) + (slashed << 15) + balance_in_increments
+```
+
+### `unpack_compact_validator`
+
+```python
+def unpack_compact_validator(compact_validator: uint64) -> Tuple[uint64, bool, uint64]:
+    """
+    Returns validator index, slashed, balance // EFFECTIVE_BALANCE_INCREMENT
+    """
+    return compact_validator >> 16, (compact_validator >> 15) % 2, compact_validator & (2**15 - 1)
+```
+
+#### `committee_to_compact_committee`
+
+```python
+def committee_to_compact_committee(state: BeaconState, committee: Sequence[ValidatorIndex]) -> CompactCommittee:
+    validators = [state.validators[i] for i in committee]
+    compact_validators = [
+        pack_compact_validator(i, v.slashed, v.effective_balance // EFFECTIVE_BALANCE_INCREMENT)
+        for i, v in zip(committee, validators)
+    ]
+    pubkeys = [v.pubkey for v in validators]
+    return CompactCommittee(pubkeys=pubkeys, compact_validators=compact_validators)
+```
+
+## Changes
+
+### Persistent committees
+
+Add to the beacon state the following fields:
+
+* `previous_persistent_committee_root: Hash`
+* `current_persistent_committee_root: Hash`
+* `next_persistent_committee_root: Hash`
+
+Process the following function before `process_final_updates`:
+
+```python
+def update_persistent_committee(state: BeaconState):
+    if (get_current_epoch(state) + 1) % EPOCHS_PER_SHARD_PERIOD == 0:
+        state.previous_persistent_committee_root = state.current_persistent_committee_root
+        state.current_persistent_committee_root = state.next_persistent_committee_root
+        committees = Vector[CompactCommittee, SHARD_COUNT]([
+            committee_to_compact_committee(state, get_period_committee(state, get_current_epoch(state) + 1, i))
+            for i in range(SHARD_COUNT)
+        ])
+        state.next_persistent_committee_root = hash_tree_root(committees)
+```