# Ethereum 2.0 The Merge

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

## Table of contents

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- [Introduction](#introduction)
- [Prerequisites](#prerequisites)
- [Protocols](#protocols)
  - [`ExecutionEngine`](#executionengine)
    - [`assemble_block`](#assemble_block)
- [Beacon chain responsibilities](#beacon-chain-responsibilities)
  - [Block proposal](#block-proposal)
    - [Constructing the `BeaconBlockBody`](#constructing-the-beaconblockbody)
      - [Execution Payload](#execution-payload)

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## Introduction

This document represents the changes to be made in the code of an "honest validator" to implement executable beacon chain proposal.

## Prerequisites

This document is an extension of the [Altair -- Honest Validator](../altair/validator.md) guide.
All behaviors and definitions defined in this document, and documents it extends, carry over unless explicitly noted or overridden.

All terminology, constants, functions, and protocol mechanics defined in the updated Beacon Chain doc of [The Merge](./beacon-chain.md) are requisite for this document and used throughout.
Please see related Beacon Chain doc before continuing and use them as a reference throughout.

## Protocols

### `ExecutionEngine`

The following methods are added to the `ExecutionEngine` protocol for use as a validator:

#### `assemble_block`

Produces a new instance of an execution payload, with the specified `timestamp`,
on top of the execution payload chain tip identified by `block_hash`.

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

```python
def assemble_block(self: ExecutionEngine, block_hash: Hash32, timestamp: uint64, random: Bytes32) -> ExecutionPayload:
    ...
```

## Beacon chain responsibilities

All validator responsibilities remain unchanged other than those noted below. Namely, the transition block handling and the addition of `ExecutionPayload`.

### Block proposal

#### Constructing the `BeaconBlockBody`

##### Execution Payload

* Set `block.body.execution_payload = get_execution_payload(state, transition_store, execution_engine, pow_chain)` where:

```python
def get_pow_block_at_total_difficulty(total_difficulty: uint256, pow_chain: Sequence[PowBlock]) -> Optional[PowBlock]:
    # `pow_chain` abstractly represents all blocks in the PoW chain
    for block in pow_chain:
        parent = get_pow_block(block.parent_hash)
        if block.total_difficulty >= total_difficulty and parent.total_difficulty < total_difficulty:
            return block

    return None


def compute_randao_mix(state: BeaconState, randao_reveal: BLSSignature) -> Bytes32:
    epoch = get_current_epoch(state)
    return xor(get_randao_mix(state, epoch), hash(randao_reveal))


def produce_execution_payload(state: BeaconState,
                              parent_hash: Hash32,
                              randao_reveal: BLSSignature,
                              execution_engine: ExecutionEngine) -> ExecutionPayload:
    timestamp = compute_timestamp_at_slot(state, state.slot)
    randao_mix = compute_randao_mix(state, randao_reveal)
    return execution_engine.assemble_block(parent_hash, timestamp, randao_mix)


def get_execution_payload(state: BeaconState,
                          transition_store: TransitionStore,
                          randao_reveal: BLSSignature,
                          execution_engine: ExecutionEngine,
                          pow_chain: Sequence[PowBlock]) -> ExecutionPayload:
    if not is_merge_complete(state):
        terminal_pow_block = get_pow_block_at_total_difficulty(transition_store.transition_total_difficulty, pow_chain)
        if terminal_pow_block is None:
            # Pre-merge, empty payload
            return ExecutionPayload()
        else:
            # Signify merge via producing on top of the last PoW block
            return produce_execution_payload(state, terminal_pow_block.block_hash, randao_reveal, execution_engine)

    # Post-merge, normal payload
    parent_hash = state.latest_execution_payload_header.block_hash
    return produce_execution_payload(state, parent_hash, randao_reveal, execution_engine)
```