# The Merge -- Fork Choice

**Notice**: This document is a work-in-progress for researchers and implementers.

## Table of contents
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- [Introduction](#introduction)
- [Protocols](#protocols)
  - [`ExecutionEngine`](#executionengine)
    - [`notify_forkchoice_updated`](#notify_forkchoice_updated)
- [Helpers](#helpers)
  - [`PowBlock`](#powblock)
  - [`get_pow_block`](#get_pow_block)
  - [`is_valid_terminal_pow_block`](#is_valid_terminal_pow_block)
- [Updated fork-choice handlers](#updated-fork-choice-handlers)
  - [`on_block`](#on_block)

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## Introduction

This is the modification of the fork choice according to the executable beacon chain proposal.

*Note*: It introduces the process of transition from the last PoW block to the first PoS block.

## Protocols

### `ExecutionEngine`

*Note*: The `notify_forkchoice_updated` function is added to the `ExecutionEngine` protocol to signal the fork choice updates.

The body of this function is implementation dependent.
The Engine API may be used to implement it with an external execution engine.

#### `notify_forkchoice_updated`

This function performs two actions *atomically*:
* Re-organizes the execution payload chain and corresponding state to make `head_block_hash` the head.
* Applies finality to the execution state: it irreversibly persists the chain of all execution payloads
and corresponding state, up to and including `finalized_block_hash`.

```python
def notify_forkchoice_updated(self: ExecutionEngine, head_block_hash: Hash32, finalized_block_hash: Hash32) -> None:
    ...
```

*Note*: The call of the `notify_forkchoice_updated` function maps on the `POS_FORKCHOICE_UPDATED` event defined in the [EIP-3675](https://eips.ethereum.org/EIPS/eip-3675#definitions).
As per EIP-3675, before a post-transition block is finalized, `notify_forkchoice_updated` must be called with `finalized_block_hash = Hash32()`.

## Helpers

### `PowBlock`

```python
class PowBlock(Container):
    block_hash: Hash32
    parent_hash: Hash32
    total_difficulty: uint256
    difficulty: uint256
```

### `get_pow_block`

Let `get_pow_block(block_hash: Hash32) -> PowBlock` be the function that given the hash of the PoW block returns its data.

*Note*: The `eth_getBlockByHash` JSON-RPC method may be used to pull this information from an execution client.

### `is_valid_terminal_pow_block`

Used by fork-choice handler, `on_block`.

```python
def is_valid_terminal_pow_block(block: PowBlock, parent: PowBlock) -> bool:
    if block.block_hash == TERMINAL_BLOCK_HASH:
        return True

    is_total_difficulty_reached = block.total_difficulty >= TERMINAL_TOTAL_DIFFICULTY
    is_parent_total_difficulty_valid = parent.total_difficulty < TERMINAL_TOTAL_DIFFICULTY
    return is_total_difficulty_reached and is_parent_total_difficulty_valid
```

## Updated fork-choice handlers

### `on_block`

*Note*: The only modification is the addition of the verification of transition block conditions.

```python
def on_block(store: Store, signed_block: SignedBeaconBlock) -> None:
    block = signed_block.message
    # Parent block must be known
    assert block.parent_root in store.block_states
    # Make a copy of the state to avoid mutability issues
    pre_state = copy(store.block_states[block.parent_root])
    # Blocks cannot be in the future. If they are, their consideration must be delayed until the are in the past.
    assert get_current_slot(store) >= block.slot

    # Check that block is later than the finalized epoch slot (optimization to reduce calls to get_ancestor)
    finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
    assert block.slot > finalized_slot
    # Check block is a descendant of the finalized block at the checkpoint finalized slot
    assert get_ancestor(store, block.parent_root, finalized_slot) == store.finalized_checkpoint.root

    # Check the block is valid and compute the post-state
    state = pre_state.copy()
    state_transition(state, signed_block, True)

    # [New in Merge]
    if is_merge_block(pre_state, block.body):
        pow_block = get_pow_block(block.body.execution_payload.parent_hash)
        pow_parent = get_pow_block(pow_block.parent_hash)
        assert is_valid_terminal_pow_block(pow_block, pow_parent)

    # Add new block to the store
    store.blocks[hash_tree_root(block)] = block
    # Add new state for this block to the store
    store.block_states[hash_tree_root(block)] = state

    # Update justified checkpoint
    if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
        if state.current_justified_checkpoint.epoch > store.best_justified_checkpoint.epoch:
            store.best_justified_checkpoint = state.current_justified_checkpoint
        if should_update_justified_checkpoint(store, state.current_justified_checkpoint):
            store.justified_checkpoint = state.current_justified_checkpoint

    # Update finalized checkpoint
    if state.finalized_checkpoint.epoch > store.finalized_checkpoint.epoch:
        store.finalized_checkpoint = state.finalized_checkpoint

        # Potentially update justified if different from store
        if store.justified_checkpoint != state.current_justified_checkpoint:
            # Update justified if new justified is later than store justified
            if state.current_justified_checkpoint.epoch > store.justified_checkpoint.epoch:
                store.justified_checkpoint = state.current_justified_checkpoint
                return

            # Update justified if store justified is not in chain with finalized checkpoint
            finalized_slot = compute_start_slot_at_epoch(store.finalized_checkpoint.epoch)
            ancestor_at_finalized_slot = get_ancestor(store, store.justified_checkpoint.root, finalized_slot)
            if ancestor_at_finalized_slot != store.finalized_checkpoint.root:
                store.justified_checkpoint = state.current_justified_checkpoint
```