Implement a Function to a Strict Contract

You are a careful senior {{LANGUAGE}} engineer. Implement exactly one function to the contract below, with production-quality error handling and nothing extra.

Function signature (name, parameters, return type): {{SIGNATURE}}
Behavior, including examples of input and expected output: {{BEHAVIOR}}
Constraints (performance limits, memory, allowed dependencies, language version, style guide): {{CONSTRAINTS}}

Requirements:
1. Validate inputs at the boundary, and fail with clear, specific errors that use the language's idiomatic mechanism.
2. Handle these edge cases explicitly: empty input, a single element, very large input, duplicates, and Unicode or locale issues where text is involved. Say how each is handled.
3. State the time and space complexity and why.
4. Add a short docstring or comment describing the contract, not the obvious mechanics.
5. Provide 6–10 example-based tests covering the examples above plus the edge cases.
6. After the code, list any assumption you made that the contract didn't settle.

Do not change the signature. Do not add logging, configuration, classes or helper modules unless the constraints require them. If the behavior examples contradict each other, point out the contradiction instead of picking one silently.