chore(protocol): simplify transition logic by merging InboxOptimized1 into Inbox

[ggonzalez94]

This PR aims to substantially simplify how transitions are handled in the protocol, both in storage and their aggregation logic.

1. Merge InboxOptimized1 into Inbox. This avoids some of the indirection of thinking how transitions are stored and aggregated in each use case. This removes the need for some of the functions to be virtual and allows to use a single storage variable(ring buffer with branching capacity - - check TransitionRecordSlot).
2. Since the inbox got more convoluted move the aggregation logic to a new library LibTransitionAggregation. This made the Inbox even simpler than the original version and allows us to make this library function external if we ever wanted to in order to reduce code size(probably not a good tradeoff at the moment).
3. Remove all unnecessary hashing functions _hashTransition, _hashTransitionRecord, etc. and use in place hashing instead

[claude]

Claude finished @ggonzalez94's task —— View job

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PR Review: Simplify Transition Logic

Completed analysis of the architectural refactoring that merges InboxOptimized1 into Inbox and introduces LibTransitionAggregation.

Todo List:

• Set up development environment
• Analyze core architectural changes
• Review new LibTransitionAggregation library
• Check gas optimization impacts
• Evaluate security implications
• Review test coverage changes
• Provide comprehensive feedback

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🎯 Overall Assessment: Strong Refactoring

This PR successfully simplifies the transition logic architecture while maintaining functionality. The merge of InboxOptimized1 into the base Inbox eliminates unnecessary abstraction layers and the new LibTransitionAggregation library provides clean separation of concerns.

✅ Strengths

1. Architectural Improvements

• Unified Storage: The new TransitionRecordSlot structure (packages/protocol/contracts/layer1/shasta/impl/Inbox.sol:91-101) elegantly handles both single transitions and branches with optimized storage patterns
• Clean Separation: Moving aggregation logic to LibTransitionAggregation improves modularity and reduces contract complexity
• Reduced Indirection: Eliminating InboxOptimized1 removes unnecessary virtual function calls and simplifies the inheritance hierarchy

2. Gas Optimizations

From the snapshots, the refactoring shows meaningful gas improvements:

• Prove operations: ~2-3% reduction in gas usage across consecutive proof scenarios
• Assembly optimization: LibTransitionAggregation.sol:122-124 uses inline assembly to resize arrays efficiently
• Storage optimization: The partial parent hash matching (Inbox.sol:847-856) reduces storage reads by ~50% for common cases

3. Code Quality

• Inline hashing: Eliminating separate hash functions reduces function call overhead
• Clear documentation: Excellent natspec throughout, especially in the new library
• Consistent patterns: The aggregation logic follows established Solidity best practices

🚨 Issues & Concerns

1. Security Concerns

Medium: Partial Hash Collision Risk (Inbox.sol:847-856)

function _isPartialParentTransitionHashMatch(
    bytes26 _partialParentTransitionHash,
    bytes32 _parentTransitionHash
) private pure returns (bool) {
    return _partialParentTransitionHash == bytes26(_parentTransitionHash);
}

• Using only 26 bytes increases collision probability to 2^-208
• While mathematically unlikely, consider documenting the risk assessment
• Recommendation: Add collision detection or increase to 30 bytes if storage allows

Low: Missing Validation in TODO Comment (Inbox.sol:466)

// TODO: verify I'm not missing anything here
// Same propoasal id and same parent transition hash, do nothing
return;

This TODO in production code suggests incomplete validation logic.

2. Code Quality Issues

Minor: Typo (Inbox.sol:467)

// Same propoasal id and same parent transition hash, do nothing

Should be "proposal"

Low: Assembly Usage Documentation (LibTransitionAggregation.sol:122)
The assembly block for array resizing is correct but could benefit from additional safety comments explaining why it's safe.

3. Performance Considerations

Medium: Memory Allocation Strategy (LibTransitionAggregation.sol:55)

records_ = new AggregatedRecord[](_proposals.length);

Worst-case allocation followed by assembly resize works but could be more memory efficient with dynamic growth. Consider this for large batches.

📊 Test Coverage Analysis

Strengths:

• Comprehensive removal: All InboxOptimized1 test files properly deleted
• Updated documentation: Test suite markdown correctly reflects the merge
• Maintained coverage: No functionality gaps in test coverage

Concerns:

• Gas benchmarks: Consider adding specific benchmarks for the new aggregation logic
• Edge cases: Test coverage for the partial hash matching optimization could be enhanced

🔧 Recommendations

High Priority:

1. Resolve TODO comment: Complete validation logic or remove the TODO if intentional
2. Fix typo: Correct "propoasal" → "proposal"
3. Security review: Document collision risk analysis for partial hash matching

Medium Priority:

1. Enhanced testing: Add specific tests for partial hash collision scenarios
2. Gas benchmarks: Add performance tests for aggregation with varying batch sizes
3. Assembly documentation: Add safety comments for the array resize operation

Low Priority:

1. Memory optimization: Consider dynamic growth for very large aggregation batches
2. Monitoring: Consider adding metrics for aggregation efficiency in production

🏁 Verdict: Approve with Minor Fixes

This is a well-executed refactoring that achieves its goals of simplification and optimization. The architectural improvements outweigh the minor issues identified. The TODO comment should be resolved before merging, but otherwise this represents solid engineering.

Estimated Impact:

• ✅ Maintainability: Significantly improved
• ✅ Gas Efficiency: Measurably better
• ✅ Code Clarity: Much cleaner
• ⚠️ Security: Minor concerns addressed above

[dantaik]

A few thoughts:

• The layered approach (core contract + sub-contracts) helps keep tests modular and makes code reviews easier. Core functions can be tested in a minimal setup, while sub-contracts add their own logic and get separate test suites. This separation reduces unnecessary setup and makes coverage easier to manage.

• I’m not convinced merging sub-contracts back into the parent would reduce code size. I do like the idea of introducing two extra libraries to simplify sub-contracts. The trade-off is that switching some functions from internal to public may shrink bytecode but adds extra gas per call, so we’d need to evaluate whether the savings are worth it.

[ggonzalez94]

The layered approach (core contract + sub-contracts) helps keep tests modular and makes code reviews easier. Core functions can be tested in a minimal setup, while sub-contracts add their own logic and get separate test suites. This separation reduces unnecessary setup and makes coverage easier to manage.

Let me try to convince you why I still think this approach is more readable :)

1. It reduced over 200LOC more than it is adding, so there's less to maintain
2. Allows us to have one single storage for transitions. No more need to keep two separate mental models as to where transitions are stored. I think we should do this regardless if we merge the two contracts or not
3. While the simpler inbox contract was a bit easier to reason about, I think this design embraces complexity better. Instead of having a separate implementation that treats transitions differently, we now have a single source of truth. The reality is that we will always use the aggregation version, so some of the code of the inbox would always go unused
4. Both functions require the same inputs and outputs as far as I recall, so testing should remain same as before. If we need to test internal functions we can always expose those in the concrete implementation for tests and test them independently

I guess my TL;DR: is I think we had code in the original inbox that would always go unused(our deployment would always use the optimized version). We are better off embracing that complexity, and managing it appropriately than treating it as an extension(since only one version will get deployed).
Maybe we can have a sync later today to discuss the best design?

I do like the idea of introducing two extra libraries to simplify sub-contracts. The trade-off is that switching some functions from internal to public may shrink bytecode but adds extra gas per call, so we’d need to evaluate whether the savings are worth it.

Agree the tradeoff is not worth it. I was just highlighting it gives us the flexibility if we ever need that in the future.

[ggonzalez94]

@dantaik can you double check me on this? before we were just overwriting with the same value, which should be equivalent to just returning