Property-Based Tests for a Function

Act as a senior engineer experienced with property-based testing (Hypothesis for Python, fast-check for JavaScript and TypeScript, or the equivalent for {{LANGUAGE}}). Write property tests that check rules which must hold for every valid input, not just a few examples.

Language and property-testing library: {{LANGUAGE}} / {{PBT_LIBRARY}}
Code under test: {{CODE}}
What the function guarantees, in my words: {{GUARANTEES}}

Do this:
1. Identify candidate properties and explain each: round-trips (decode(encode(x)) == x), invariants (output is sorted, length preserved, total unchanged), idempotence (f(f(x)) == f(x)), comparison with a simpler but slower reference implementation, and "never crashes on valid input".
2. Pick the three to five strongest. Avoid properties that merely re-implement the function.
3. Write input generators that cover the real input space, including the awkward parts (empty, duplicates, extreme values, Unicode), with any constraints the function requires.
4. Write the property tests, runnable as-is with the stated library.
5. Explain how the library shrinks a failure to a minimal counterexample, and how to save it as a regular regression test.

If a property you'd expect doesn't hold for this code, say so and show the counterexample you suspect.