submitted-code

# Submitted code Code that produces the right answer on your machine and fails on the grader's is the most expensive kind of wrong: the science was right and the score is zero. The asymmetry is simple. You developed with everything you installed, every file you had staged, your working directory, your thread count and your Python. Whoever re-runs your artifact has the base image, the inputs the contract names, a clean directory and a timer. Write for that second environment from the first line. ## The contract to extract first Before writing the entry point, read the specification and write down, as a checklist: - **Entry point**: exact path, file name, function or CLI signature, argument order and types, return type. A function that must return a plain list does not return a NumPy array; a CLI that must accept a path does not also require a flag you invented. - **Libraries you may rely on**: only what the specification or the base environment names. Everything else is unavailable at re-execution even if `pip install` succeeded for you during development. - **Inputs**: which paths are passed in, which are read-only, and what may differ from the examples you were given (size, layout, identifiers, parameter ranges, number of cases). - **Outputs**: exact paths, formats and whether extra files in that directory are tolerated. - **Limits**: wall clock and CPU per call or per case, memory, thread count, whether the network exists, and which user the process runs as. ## Rules 1. **One self-contained artifact.** The entry point imports the standard library, the declared third-party libraries and nothing else of yours. No sibling modules, no helper file next to it, no configuration or parameter file unless the contract lists it, no data file you wrote during development. If constants were fitted, embed them as literals in the file. 2. **Nothing you installed counts.** Treat every dependency you added during development as absent. If a method needs a package the environment does not have, either implement the part you need directly or choose a method the environment supports; note the choice in the write-up. 3. **No network, no side channels.** No downloads, no sockets, no subprocesses spawning compilers or fetching data, no `ctypes` loading of libraries you built, no reads outside the arguments you were given, no writes outside the declared output path. Sandboxes commonly block these outright, and a blocked call ends the run. 4. **Stateless and deterministic.** Each invocation stands alone: no cached globals from a previous call, no reliance on invocation order, no temporary files left behind. Seed every random draw from a seed the contract supplies or a fixed constant, and keep the computation reproducible across runs. 5. **Frugal with threads and memory.** Assume one or two cores and the stated memory, not your machine. Cap BLAS and OpenMP threads inside the artifact (`OMP_NUM_THREADS`, `OPENBLAS_NUM_THREADS`, `MKL_NUM_THREADS`) rather than relying on the caller, and stream large inputs instead of loading them whole. 6. **Fit the per-call budget with headroom.** Time the slowest realistic case and aim for a fraction of the cap; a pure-interpreter loop that is correct but two times over the limit scores the same as a wrong answer. Where the limit forces it, vectorize, precompute a table, or write the hot kernel in a compiled language the environment already has. When the requirement is *relative* — within some factor of a reference implementation, or better than a named baseline — time that reference yourself on the same machine and treat the ratio as part of correctness, not as polish. When the real inputs exist only at evaluation — data supplied through a callback, an oracle linked in by the grader, a hidden file — and an accuracy bar must be met on them, the headroom is not idle safety margin, it is the only time anyone ever computes on the real problem. Any setting you freeze during development (a resolution, a step count, an iteration cap) was chosen against cases you made up. Put the convergence check inside the artifact: solve at increasing settings, compare successive answers on the real inputs, stop when the change is well under the bar or when a clock the code keeps itself says the budget is nearly spent, and return the finest answer reached. 7. **Respect declared prohibitions literally.** A specification that bans a library, a module, a language feature or a class of tactic is often enforced by a scan that runs before your code does, so a single banned import fails the work outright however good the result. Collect the ban list with the rest of the contract and grep your final artifact for each item, including indirect imports pulled in by a helper. 8. **Fail informatively inside the contract.** Raise the exception type the specification names, or return the documented sentinel; never exit the process, never print to stdout when the contract expects a return value, and never swallow an error into a plausible wrong answer. ## Rehearsal before you finish Passing on the example inputs is necessary and never sufficient. Rehearse: 1. **Clean copy.** Copy only the artifact (and the files the contract names) into an empty temporary directory, and run it from there with the pinned interpreter. Anything that breaks here was a hidden dependency on your workspace. 2. **Mutated inputs.** Rerun on inputs you perturb yourself: renamed identifiers, shuffled row order, a different size or geometry, a case at the edge of the stated parameter range, an input with a legal but unseen structure. Generalizing beyond the visible case is the point of the exercise. 3. **Restricted process.** Run it as a subprocess with a wall-clock timeout, reduced threads, the network unavailable and, where the platform allows, an unprivileged user and a read-only input directory. Measure peak memory. 4. **Repeat.** Run it twice on the same input and compare the outputs byte for byte, or within the stated tolerance, to catch order and seeding bugs. Record what you rehearsed and the measured time and memory alongside the deliverable, so the reviewer sees the artifact was tested against its contract and not only against the example.