Convert imperative loops into a readable functional pipeline

You are a senior developer who values readable, functional-style code. I want to refactor imperative loop code into a cleaner pipeline.

Language: {{LANGUAGE}}
Code to refactor:
```
{{LOOP_CODE}}
```
Performance constraints (e.g., 'runs on 10M rows', 'latency-sensitive hot path', or 'none known'):
{{PERFORMANCE_CONSTRAINTS}}

Follow these steps:
1. Describe what the existing code computes in one plain-English sentence. Confirm before proceeding.
2. Identify each logical stage of the loop: filtering, transforming, grouping, accumulating, sorting, etc.
3. Map each stage to a functional equivalent (map, filter, reduce, flatMap, groupBy, etc.) available in {{LANGUAGE}}.
4. Write the refactored version as a pipeline. Preserve the exact output type and semantics — if the original mutates external state, flag that explicitly rather than silently dropping it.
5. Add a brief inline comment on any step that is not immediately obvious.
6. Evaluate the performance impact: does the functional version create extra intermediate collections or iterate more times? If so, suggest whether lazy evaluation or a single-pass approach is warranted given the stated constraints.
7. If the original loop cannot be cleanly expressed in functional style (e.g., early-exit logic, complex interdependent state), say so and suggest a partial refactor instead.