Alessandro Donati worked on the valhalla/valhalla repository, focusing on improving the determinism of tile generation within the codebase. He implemented a deterministic approach by sorting tweeners based on their GraphId using a custom comparator, graphid_less, written in C++. This change addressed the challenge of nondeterministic tile creation, which previously complicated debugging, caching, and continuous integration processes. By leveraging his skills in data structures, graph algorithms, and software engineering, Alessandro ensured that tile generation would produce consistent results across multiple runs. The work demonstrated a targeted solution to reproducibility, though it was limited in scope to a single feature.