--- name: verify-algorithms description: Verify probabilistic, distributional, or random behaviour empirically before changing search-engine code. Loads when reading or editing src/houndarr/engine/. Use when a user, code review, or another AI surfaces a claim about bias, ordering, page selection, randomness, or "we are searching the same things over and over". paths: - "src/houndarr/engine/**" --- # Verifying claims about algorithms Before modifying search-engine logic, scheduling, randomisation, ordering, distribution, or any code where probability or stateful iteration governs behaviour, verify the claim empirically and analytically first. Most reported "bugs" in this class turn out to be sample noise, observation bias, or misreadings of timing-dependent state, and shipping a fix for a non-bug introduces real risk for no real gain. ## When this rule fires Apply this workflow whenever a user, a code review, or another AI surfaces a claim along the lines of: - "X picks the wrong page / item / branch" - "Y is biased / unfair / skewed toward Z" - "Random does not feel random" - "The cycle order is broken" - "We are searching the same things over and over" It does not apply to clear logic bugs, typos, or behaviour-change requests. The trigger is specifically: claims about probabilistic or distribution-shaped behaviour where the right answer is a measured histogram, not a code reading. ## Required workflow 1. Reproduce the algorithm in isolation against `tests/mock_arr/`, not against the live test instances or short-window log dumps. The live test *arrs hold tens of records, which is far below the sample size needed to distinguish bias from variance, and live state (cooldowns, hourly caps, *arr-side sort orders) confounds the measurement. 2. Derive analytically what each page, item, or branch's probability should be under the current code. Read the loop, write the math down, and predict the distribution shape before running anything. "I think it should be uniform" is not a prediction; "uniform with chi-square below 16.92 at df=9" is. 3. Run hundreds of cycles through `tests/mock_arr/probe_distribution.py` or a similar probe modelled on it. Compute chi-square, max/min ratio, and per-bucket standard deviation. Compare against the analytical prediction and against the 5% chi-square critical value at `df = N - 1`. 4. Decide on evidence. If the empirical result agrees with the prediction and the chi-square lands below the critical value, the claim is wrong. Document the finding, reference the probe output, and close the investigation. If the result confirms real bias, scope the fix to the smallest change that closes the measured gap, then re-run the probe to prove the gap is gone. ## Tooling to use - `just mock-arr port=PORT items=N seed=S` launches the seeded multi-app mock server with configurable item counts and a deterministic seed; identical seeds produce byte-identical responses. - `.venv/bin/python -m tests.mock_arr.probe_distribution` boots the mock in-process, drives the production `run_instance_search` for many cycles across a sweep of library sizes, and reports per-cycle start-page distributions plus full visit histograms. Use it as the template for any new programmatic probe. - The mock exposes `GET /__page_log__/{app}` and `GET /__commands__/{app}` for ground-truth request and dispatch records, plus `POST /__reset__/{app}` to clear them between configurations. - For statistical-power-bound questions (100k+ trials), a short pure-Python simulation of just the algorithm beats running through HTTP. Use it when the measurement is about the math, not the integration. ## What not to do - Do not treat a short-window dev-DB histogram (a few hours, dozens of cycles, a handful of items) as evidence of algorithmic bias. Cooldown phase, *arr-side sort order, and small-sample variance dominate that signal. The math you owe is a many-cycle distribution against a predicted shape. - Do not adopt an external diagnostic write-up without re-deriving the math yourself. Direction (page 1 vs page N) and magnitude (1.5x vs 5x) routinely invert in second-hand summaries of probabilistic algorithms, and shipping a fix for an inverted claim ships a regression. - Do not start coding because the claim is plausible. Plausibility is not evidence. The bar is a reproducible measurement that disagrees with the predicted distribution by more than chance. ## Closing the loop When measurement contradicts the claim, the writeup is the engineering contribution. Reference the probe output, state the measured statistics, explain what the original observation was actually picking up (cooldown saturation, recency effects, sort-order interaction, sample noise), and close the discussion. A correct "no change required" is a successful task, not a non-result. ## Known emergent behaviours (already measured) These are real but minor effects that have been verified by probe and deliberately left alone. Do not re-investigate them unless the operating point changes or a user reports a concrete regression. - Partial-last-page over-selection on missing/cutoff under random search order. When the engine's `page_size` does not divide `totalRecords` evenly, items on the (short) last page are drained every visit because the engine dispatches up to `batch_size` items per page. Measured at most 2x attention skew for the 1-9 items on the last page at default settings (batch=1, pageSize=10) and 4x in contrived configurations (batch=5, pageSize=20). Affects a small slice of the backlog; the only clean fix is a virtual flat-index draw which is a substantial redesign of `_run_search_pass`. Probe: `tests/mock_arr/probe_cooldown.py`. - Sonarr / Whisparr v2 windowed-rotation coverage time. The upgrade pass visits 5 series per cycle; full-library coverage takes approximately `ceil(eligible_episodes * H / batch)` cycles where H is the harmonic-coverage factor. Measured 91% theoretical and 85-89% empirical coverage at 60 cycles with batch=5 on 50 series. This is the intentional trade-off versus hammering one series with a single huge *arr fetch. Probe: `tests/mock_arr/probe_upgrade_coverage.py`.