eth2.0-specs/specs/_features/reuse_indexes/beacon-chain.md

# Reuse indexes -- The Beacon Chain

## Table of contents

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- [Introduction](#introduction)
- [Preset](#preset)
  - [Time parameters](#time-parameters)
- [Helpers](#helpers)
  - [Predicates](#predicates)
    - [`is_reusable_validator`](#is_reusable_validator)
- [Beacon chain state transition function](#beacon-chain-state-transition-function)
  - [Block processing](#block-processing)
    - [Modified `get_index_for_new_validator`](#modified-get_index_for_new_validator)

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## 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This is the beacon chain specification to assign new deposits to existing validator records that have withdrawn long ago.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 |

## Helper functions

### Predicates

#### `is_reusable_validator`

```python
def is_reusable_validator(validator: Validator, balance: Gwei, epoch: Epoch) -> bool:
    """
    Check if ``validator`` index can be re-assigned to a new deposit.
    """
    return (
      validator.withdrawable_epoch < epoch - REUSE_VALIDATOR_INDEX_DELAY
      and balance == 0
    )
```

## Beacon chain state transition function

### Block processing

#### Modified `get_index_for_new_validator`

```python
def get_index_for_new_validator(state: BeaconState) -> int:
    for index, validator in enumerate(state.validators):
        if is_reusable_validator(validator, state.balances[index], get_current_epoch(state)):
            return index
    return len(state.validators)
```