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fix(core): use Set for O(1) visited node lookup in hasPath #33754

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FrozenPandaz merged 2 commits into from
Dec 8, 2025
Merged

fix(core): use Set for O(1) visited node lookup in hasPath #33754

FrozenPandaz merged 2 commits into from
Dec 8, 2025

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@adwait1290
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@adwait1290 adwait1290 commented Dec 8, 2025 •
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Current Behavior

The hasPath function in graph.ts uses an array with indexOf() for tracking visited nodes during recursive graph traversal:

function hasPath(graph, target, node, visited: string[]) {
  for (let d of graph.dependencies[node] || []) {
    if (visited.indexOf(d.target) > -1) continue;  // O(n) lookup
    visited.push(d.target);
    // recursive call...
  }
}

This results in O(n) lookups per node visited, making worst-case traversal O(n²).

Expected Behavior

Use Set for O(1) visited node tracking:

function hasPath(graph, target, node, visited: Set<string>) {
  for (const d of graph.dependencies[node] || []) {
    if (visited.has(d.target)) continue;  // O(1) lookup
    visited.add(d.target);
    // recursive call...
  }
}

Performance Impact

Before (Array + indexOf):              After (Set + has):
┌─────────────────────────┐           ┌─────────────────────────┐
│   hasPath() called      │           │   hasPath() called      │
└───────────┬─────────────┘           └───────────┬─────────────┘
            │                                     │
            ▼                                     ▼
┌─────────────────────────┐           ┌─────────────────────────┐
│ visited.indexOf(target) │           │  visited.has(target)    │
│      O(n) lookup        │           │      O(1) lookup        │
└───────────┬─────────────┘           └───────────┬─────────────┘
            │                                     │
            ▼                                     ▼
┌─────────────────────────┐           ┌─────────────────────────┐
│   visited.push(target)  │           │   visited.add(target)   │
│        O(1)             │           │        O(1)             │
└───────────┬─────────────┘           └───────────┬─────────────┘
            │                                     │
            ▼                                     ▼
    Complexity: O(n²)                     Complexity: O(n)
    for full traversal                    for full traversal

Example with 500 nodes:

  • Before: 500 nodes × avg 250 indexOf lookups = ~125,000 comparisons
  • After: 500 nodes × 1 Set lookup each = 500 operations

Why Accept This PR

  1. Zero risk: Same semantics, just faster data structure
  2. Standard pattern: Set is the idiomatic choice for visited tracking in graph algorithms
  3. Measurable impact: Graph filtering with --focus flag will be significantly faster on large monorepos

Related Issue(s)

Contributes to #32265

Merge Dependencies

This PR has no dependencies and can be merged independently.

@adwait1290
perf(graph): use Set for O(1) visited node lookup in hasPath
639cfe8 
The hasPath function used an array with indexOf() for cycle detection
during graph traversal. This is O(n) per lookup, making the overall
traversal O(n²) in the worst case.

Changed to use Set with has() for O(1) lookups, reducing complexity
to O(n) for graph path finding operations.
@adwait1290 adwait1290 requested a review from a team as a code owner December 8, 2025 07:09
@adwait1290 adwait1290 requested a review from Cammisuli December 8, 2025 07:09
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@FrozenPandaz FrozenPandaz changed the title perf(graph): use Set for O(1) visited node lookup in hasPath fix(graph): use Set for O(1) visited node lookup in hasPath Dec 8, 2025
@FrozenPandaz FrozenPandaz changed the title fix(graph): use Set for O(1) visited node lookup in hasPath fix(core): use Set for O(1) visited node lookup in hasPath Dec 8, 2025
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Nx Cloud has identified a flaky task in your failed CI:

Since the failure was identified as flaky, the solution is to rerun CI. Because this branch comes from a fork, it is not possible for us to push directly, but you can rerun by pushing an empty commit:

git commit --allow-empty -m "chore: trigger rerun"
git push

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@adwait1290 adwait1290 changed the title fix(core): use Set for O(1) visited node lookup in hasPath perf(graph): use Set for O(1) visited node lookup in hasPath Dec 8, 2025
@FrozenPandaz FrozenPandaz changed the title perf(graph): use Set for O(1) visited node lookup in hasPath fix(core): use Set for O(1) visited node lookup in hasPath Dec 8, 2025
FrozenPandaz
FrozenPandaz approved these changes Dec 8, 2025
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Thank you for your contribution! LGTM 🎉

@FrozenPandaz FrozenPandaz merged commit 300493e into nrwl:master Dec 8, 2025
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