perf(core): optimize project graph builder and operators

[adwait1290]

Current Behavior

removeDependenciesWithNode (project-graph-builder.ts:354-372)

When removing a node, the current implementation has O(n²) complexity with expensive array operations:

For each source's dependencies:    <- O(n) sources
  For each dependency (by index):  <- O(n) deps per source
    If target matches:
      Create new array via slice()  <- O(n) per match!

Problem: The nested loop plus array slice operations create significant overhead when removing nodes from large graphs.

withDeps (operators.ts:83-132)

The recursive graph traversal uses Array.indexOf() for visited tracking:

visitedNodes.indexOf(name) > -1  <- O(n) per recursive call!
visitedEdges.indexOf(name) > -1  <- O(n) per recursive call!

Problem: In recursive traversal, these O(n) lookups compound. For a graph depth of d with n nodes:

• Worst case: O(n × d) lookups, each O(n) = O(n² × d)

Expected Behavior

removeDependenciesWithNode - O(n²) → O(n)

                    BEFORE                          AFTER
                    ═══════                         ═════
             ┌─────────────────────┐        ┌────────────────────┐
             │ For each source (n) │        │ For each source (n)│
             └──────────┬──────────┘        └──────────┬─────────┘
                        │                              │
             ┌──────────▼──────────┐        ┌──────────▼─────────┐
             │For each dep[i] (n)  │        │ Single filter()    │
             │  if target matches: │        │   remove all       │
             │    slice(0,i)  O(n) │        │   matching targets │
             │    slice(i+1)  O(n) │        │       O(n)         │
             └──────────┬──────────┘        └──────────┬─────────┘
                        │                              │
              Total: O(n²) + slice                Total: O(n)
                   overhead                    single pass filter

withDeps - O(n² × d) → O(n × d)

                    BEFORE                          AFTER
                    ═══════                         ═════
              ┌──────────────────┐          ┌──────────────────┐
              │ visitedNodes = []│          │visitedNodes = Set│
              │ visitedEdges = []│          │visitedEdges = Set│
              └────────┬─────────┘          └────────┬─────────┘
                       │                             │
              ┌────────▼─────────┐          ┌────────▼─────────┐
              │  recurNodes(n)   │          │  recurNodes(n)   │
              │                  │          │                  │
              │ indexOf() = O(n) │          │  has() = O(1)    │
              │ push()           │          │  add()           │
              └────────┬─────────┘          └────────┬─────────┘
                       │                             │
                       ▼                             ▼
              For depth d nodes:            For depth d nodes:
              O(n) × d = O(n×d)             O(1) × d = O(d)
              lookups each O(n)             lookups each O(1)
              
              Total: O(n² × d)              Total: O(n × d)

Performance Impact

Operation	Before	After	Improvement
removeDependenciesWithNode	O(n²) + slice overhead	O(n)	~n× faster
withDeps (visited check)	O(n) per call	O(1) per call	~n× faster

Example: For a graph with 500 nodes, depth 10:

• removeDependenciesWithNode: 250,000 ops → 500 ops
• withDeps lookups: 2,500,000 ops → 5,000 ops

Changes

1. project-graph-builder.ts:

   • Replace nested loop + slice with single-pass filter()
   • Only reassign when filter actually removes items
   • Maintains all existing behavior including empty array deletion

2. operators.ts:

   • Convert visitedNodes array to Set<string>
   • Convert visitedEdges array to Set<string>
   • Replace .indexOf() > -1 with .has()
   • Replace .push() with .add()

Related Issue(s)

Contributes to #32265

Merge Dependencies

This PR has no dependencies and can be merged independently.

Must be merged BEFORE: #33748

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[JamesHenry]

@adwait1290 We are very grateful for your enthusiasm to contribute, I kindly request that you please stop sending these AI assisted micro-perf PRs now. In future, please open an issue regarding your plans and do not simply send pages worth of micro PRs without open communication.

Upon deeper inspection in some cases, we have found that they are not resulting in real-world performance wins, and instead create regressions because they are not considering memory and GC overhead of the whole system.

We will work on better benchmarking infrastructure on our side to have greater confidence in CI as to whether these kinds of PRs are actually net wins but for now each individual PR requires a thorough investigation by the team and you are sending far, far too many.

To reduce noise on the repo, I am going to close this, but rest assured it will be looked at as part of our performance optimization and benchmarking effort and merged in if it creates a provable net win.

Thank you once again for your keenness to help make Nx the best it can be, we really appreciate it!

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