Debug and refactor a legacy module
Overview
This question evaluates debugging, refactoring, naming and exception handling, input validation, code structure, and concurrency-safety design skills within a legacy codebase, requiring both hands-on code changes and higher-level reasoning about immutability, confinement, synchronization, and concurrent data structures.
Given a legacy module named DasherPicker and a failing test suite, systematically debug and refactor the code while following a provided review checklist. Identify and fix issues in naming conventions, exception handling, input validation, and code structure so that all tests pass. Then outline a plan to make the module safe for multi-threaded execution (e.g., immutability, confinement, synchronization, concurrent data structures) and describe how you would validate correctness under concurrency (tests, stress tools, and diagnostics).