implement Iter::chunk_by

[juliano]

As requested here

[peter-jerry-ye-code-review]

Initial array capacity could be more memory efficient

Category
Performance
Code Snippet
let mut buffer : Array[T] = []
Recommendation
Consider using a smaller initial capacity like 4 or 8 instead of the default capacity of 16 that's set later
Reasoning
The current implementation creates an empty array first, then later replaces it with a new array of capacity 16. For small chunks (which are common in chunking operations), this may be wasteful. Starting with a smaller capacity would be more memory efficient while still allowing growth for larger chunks.

Nested callback makes control flow harder to follow

Category
Maintainability
Code Snippet
fn emit_current_chunk() -> IterResult {
if buffer.is_empty() {
return IterContinue
}
...
}
Recommendation
Consider extracting the chunk emission logic to a separate helper function outside the iterator callback
Reasoning
Having nested function definitions makes the code harder to follow and maintain. The chunk emission logic is a distinct operation that could be separated for better readability and potential reuse.

Missing documentation for edge cases

Category
Correctness
Code Snippet
/// Groups elements of an iterator according to a discriminator function.
Recommendation
Add documentation about behavior with empty iterators and when the discriminator function returns equal keys for non-consecutive elements
Reasoning
While the test cases cover these scenarios, the documentation should explicitly state how the function behaves in edge cases to help users understand the API contract.

[coveralls]

Pull Request Test Coverage Report for Build 6557

Details

• 9 of 11 (81.82%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 92.494%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/iter.mbt	9	11	81.82%

Totals
Change from base Build 6554:	-0.02%
Covered Lines:	6124
Relevant Lines:	6621

---

💛 - Coveralls

[peter-jerry-ye]

I'm still not quite sure if the signature should be -> Iter[(K, Iter[V])] or -> Iter[(K, Array[V])]. Maybe we need some advices from the others here