⚡ Bolt: Optimize allocation in Tarjan's SCC algorithm

[bashandbone]

💡 What: Optimized tarjan_dfs inside crates/flow/src/incremental/invalidation.rs to stop constantly re-allocating v.to_path_buf() inside the graph traversal inner loops. Instead, it creates the owned copy exactly once during initialization and directly uses the borrowed reference v for all state.indices and state.lowlinks HashMap lookups.

🎯 Why: Graph traversals are performance-critical. Re-allocating the PathBuf for every lookup creates enormous amounts of memory churn and limits traversal speed, significantly increasing runtime on large module trees.

📊 Impact: Considerably reduces heap allocations by substituting O(E) object creations with zero-cost borrowed queries (&Path). Graph iteration and change-propagation phases should execute noticeably faster for deep project trees.

🔬 Measurement: Verify via cargo test -p thread-flow --test invalidation_tests and cargo test -p thread-flow --lib. The benchmarks running compute_invalidation_set on the large topological graphs in the suite should indicate reduced wall-time variance.

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PR created automatically by Jules for task 10362869827034348044 started by @bashandbone

Summary by Sourcery

Enhancements:

• Reduce PathBuf allocations in tarjan_dfs by reusing a single owned path per node and using borrowed lookups for indices and lowlinks.

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[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Refactors the Tarjan SCC DFS implementation in the invalidation logic to avoid repeated PathBuf allocations by creating a single owned PathBuf per traversal and reusing references for hashmap lookups and stack/sets, improving performance of graph traversals.

Class diagram for TarjanState and InvalidationDetector tarjan_dfs changes

classDiagram
    class InvalidationDetector {
        +tarjan_dfs(v_path, state, sccs)
    }

    class TarjanState {
        +indices
        +lowlinks
        +stack
        +on_stack
        +index_counter
    }

    InvalidationDetector --> TarjanState : uses

    class Path {
    }

    class PathBuf {
    }

    TarjanState "1" --> "*" PathBuf : keys_in_indices
    TarjanState "1" --> "*" PathBuf : keys_in_lowlinks
    TarjanState "1" --> "*" PathBuf : elements_in_stack
    TarjanState "1" --> "*" PathBuf : elements_in_on_stack

    InvalidationDetector ..> Path : borrows
    InvalidationDetector ..> PathBuf : allocates_once_per_call

    InvalidationDetector : +tarjan_dfs(v_path Path, state TarjanState, sccs Vec_PathBuf)
    InvalidationDetector : -create_v_buf_once
    InvalidationDetector : -reuse_v_in_hash_lookups

Loading

File-Level Changes

Change	Details	Files
Remove repeated PathBuf allocations for a node within Tarjan's DFS by creating one owned PathBuf up front and reusing it in state tracking structures while switching hashmap lookups to use the borrowed Path reference.	Introduce a single v_buf PathBuf created from v at the start of tarjan_dfs Use v_buf.clone() when inserting into indices, lowlinks, and stack to reuse the initial allocation rather than calling v.to_path_buf() repeatedly Insert v_buf (without cloning) into the on_stack set to track membership Replace lowlinks.get_mut(&v.to_path_buf()) calls with lowlinks.get_mut(v) to perform borrowed lookups Replace indices.get(&v.to_path_buf()) and lowlinks.get(&v.to_path_buf()) calls with borrowed lookups using v	crates/flow/src/incremental/invalidation.rs

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[sourcery-ai]

Hey - I've left some high level feedback:

• You can avoid one of the PathBuf clones by reordering initialization so that you move v_buf into one of the maps/sets and then clone from that (e.g., insert v_buf into indices, then clone from that binding for lowlinks and stack) rather than cloning v_buf multiple times in a row.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- You can avoid one of the `PathBuf` clones by reordering initialization so that you move `v_buf` into one of the maps/sets and then clone from that (e.g., insert `v_buf` into `indices`, then `clone` from that binding for `lowlinks` and `stack`) rather than cloning `v_buf` multiple times in a row.

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[Copilot]

Pull request overview

This PR optimizes the inner-loop behavior of Tarjan’s SCC traversal in the incremental invalidation logic by removing repeated PathBuf allocations during map lookups, using borrowed &Path queries instead.

Changes:

• Allocate v.to_path_buf() once per DFS entry and reuse it for initial state insertion.
• Replace repeated &v.to_path_buf() lookups with borrowed &Path lookups for indices/lowlinks access.

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