refactor: update the structure of state regen operation

[nazarhussain]

Motivation

Use smaller and pure functions for complex logic.

Description

• Split the operation into plan, fetch and apply stages

Steps to test or reproduce

Run all tests

[gemini-code-assist]

Summary of Changes

Hello @nazarhussain, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This PR refactors the state regeneration logic by breaking it down into three distinct phases: planning the regeneration steps, fetching the necessary data (snapshots and differentials), and applying these artifacts to construct the final state. This change improves code organization and readability by replacing a single large operation file with several smaller, focused modules and a new orchestration layer.

Highlights

• Intent: This pull request refactors the state regeneration operation to improve modularity and maintainability. The primary goal is to decompose the complex logic into smaller, pure functions, specifically by splitting the operation into distinct 'plan', 'fetch', and 'apply' stages.
• Key Changes: The core change involves replacing the monolithic differentialOperation.ts file with a new, structured approach across several new files:

• plan.ts: Introduces StateRegenPlan and buildStateRegenPlan to determine the necessary steps (snapshot slot, differential slots, and optional block replay range) required to regenerate a state for a given target slot.
• fetch.ts: Defines StateRegenArtifacts and fetchStateRegenArtifacts to retrieve the required BeaconStateSnapshot and BeaconStateDifferentials from the database based on the generated plan.
• apply.ts: Introduces StateRegenContext and applyStateRegenPlan to execute the regeneration. This function takes the plan and fetched artifacts, applies state differentials, and performs block replay if specified.
• execute.ts: Provides a high-level entry point, regenerateState, which orchestrates the plan, fetch, and apply stages to regenerate a BeaconState.

Accompanying these functional changes, the pull request also includes:

• Removal of the old differentialOperation.test.ts and addition of plan.test.ts to reflect the new structure.
• Minor non-functional updates to comments in replayBlocks.ts, reordering of exports in index.ts, and slight wording adjustments in test fixture descriptions in hierarchicalLayers.ts.

• Reviewer Notes: No specific reviewer activity or comments were provided in the context.

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[gemini-code-assist]

Code Review

This is a great refactoring that improves the structure and clarity of the state regeneration logic by splitting it into plan, fetch, and apply stages. I've found a critical issue and a few areas for improvement regarding performance and code clarity. Please see my comments below.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
⚠️ Please upload report for BASE (feature/differential-archive@01b3fce). Learn more about missing BASE report.

Additional details and impacted files

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##             feature/differential-archive    #8509   +/-   ##
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  Hits                                    ?    34142           
  Misses                                  ?    31080           
  Partials                                ?       69           

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[ensi321]

I think we can make an assumption that plan.diffSlots is ordered, and therefore artifacts.diffs is ordered. I think we can just use artifacts.diffs instead of orderedDiffs for any subsequent code from this point.

[ensi321]

What's the purpose of targetSlot? I don't see it being used anywhere. It can also easily be derived from diffSlots[-1] can't it?