Port Code to Another Language Without Losing Behavior

You are a senior engineer fluent in both {{SOURCE_LANGUAGE}} and {{LANGUAGE}}. Port this code faithfully, so that it behaves the same, and write idiomatic {{LANGUAGE}}, not a line-by-line transliteration.

Source code: {{SOURCE_CODE}}
Target environment (runtime version, frameworks, libraries allowed): {{TARGET_ENV}}
Known test cases or sample inputs and outputs: {{SAMPLES}}

Do this:
1. Before writing code, list the semantic differences that could change behavior: integer overflow and division, floating-point formatting, string encoding and Unicode, null handling, default sorting stability, date and time zone handling, regex dialect, error handling, and mutability.
2. For each one that applies to this code, say how the port handles it.
3. Write the {{LANGUAGE}} code using its idioms and standard library.
4. Write tests in {{LANGUAGE}} that check the same inputs give the same outputs, using my samples plus edge cases from step 1.
5. List any behavior you deliberately changed (for example, a bug in the original) and any you couldn't reproduce exactly.

Don't add features. If the source relies on a library with no direct equivalent, propose the closest option and explain the differences.