perf: change zone positions to phmap::flat_hash_set

[dudantas]

Description

This PR replaces std::unordered_set with phmap::flat_hash_set in the Zone class to improve performance by reducing hash collisions and optimizing memory usage.

Motivation

The use of std::unordered_set for the positions attribute caused performance issues due to hash collisions, especially when managing large datasets or heavily clustered data. Collisions result in increased lookup, insertion, and deletion times as the hash table degrades into a linked list traversal for conflicting buckets.

Switching to phmap::flat_hash_set provides several benefits:

1. Reduced hash collisions: The hashing strategy used by phmap::flat_hash_set significantly reduces the likelihood of collisions, improving lookup and insertion performance.
2. Improved memory locality: Unlike std::unordered_set, which uses separate allocations for each bucket, flat_hash_set uses a contiguous memory layout, enhancing cache efficiency and reducing overhead.
3. Faster operations: Benchmarks show that flat_hash_set outperforms std::unordered_set in scenarios with frequent insertions, lookups, and deletions due to its optimized design.

---

Behaviour

Actual

• High collision rates with std::unordered_set degrade performance, especially for large datasets or when hash functions produce similar values for certain inputs.

Expected

• Switching to phmap::flat_hash_set improves performance by reducing collisions, enhancing memory locality, and speeding up operations.

---

Type of change

• Performance improvement (non-breaking change which improves efficiency)

---

How Has This Been Tested

• Benchmarked lookup, insertion, and deletion operations with datasets of varying sizes to compare performance before and after the change.
• Verified correctness and consistency of data handling after the change.

Tests performed:

• Insertion of large position sets.
• Lookups for positions at various scales.
• Removal of positions and validation of the internal state.

---

Checklist

• My code follows the style guidelines of this project.
• I have performed a self-review of my own code.
• I checked the PR checks reports.
• I have commented my code, particularly in hard-to-understand areas.
• My changes generate no new warnings.
• I have added tests that prove my fix is effective or that my feature works.

[murilo09]

Maybe fixes #2568

[dudantas]

Maybe fixes #2568

This pr is only for better performance. The cause from issue is probably caused by reload or bad config.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud