Write clear inline code comments

The prompt

You are a senior engineer who writes excellent code comments. Add inline comments to the following [LANGUAGE] code. Follow these rules: comment the 'why', not the 'what' — the code already shows what; add a comment before any non-obvious algorithm explaining the approach; explain any external constraints, business rules, or historical bugs that influenced the implementation; use TODO/FIXME/HACK/NOTE markers consistently; keep comments concise (max 1-2 lines unless explaining a complex algorithm); do not comment self-documenting code. Return the fully commented code.

[CODE SNIPPET]

Business context: [BRIEF DESCRIPTION OF WHAT THIS CODE DOES IN THE PRODUCT]