Card 59 — Rust kernel matches the answer key to the last binary digit S3 DONE · BIT-EXACT

The fast engine version of "how long until price comes back?" was built and proven — and this time the match isn't just within a hair's width, it's PERFECT. Feature 1 (Hoeffding) matched its reference to within ~0.0000000000001 — spectacular, but with a sliver of rounding daylight from summing 40,000 tiny numbers. Feature 2 involves only comparisons ("is this close at-or-above that one?") and one division at the end. Comparisons have no rounding at all — so the Rust engine reproduced all 19,602 frozen reference answers on Bitcoin and Ethereum to the last binary digit. Zero difference. Not "close enough" — identical.

The engine version is also 200× more efficient than the reference. The reference checks every bar against every later bar (up to ~20,000 comparisons per window). The engine uses a classic trick — a "monotonic stack": walk the window once, keeping a short list of bars still waiting for their level to be reclaimed; each new bar instantly resolves everyone it out-prices. Every bar enters and leaves the list exactly once — one pass, same integers. To make sure the clever version can't hide a bookkeeping slip, the dumb-but-obvious version lives on inside the test suite and both must agree bit-for-bit at four different window sizes, forever.

One structural difference from feature 1: this feature reads only closing prices, so it plugged into the standard 200-bar window machinery every earlier feature shares — that machinery's record simply grew from 12 features to 13. Twelve existing plumbing points and the causality/determinism test drivers were widened by one slot, and — a genuinely obscure corner — Rust refuses to auto-print records wider than 12 fields, so two error-message sites needed a small reshaping. The compiler caught every single spot: this is why the record is a strict tuple rather than something loosely typed.

The watchdog is red again — on purpose, again. The schema-contract test noticed the engine now computes a 14th bar-close measurement that the database blueprint doesn't yet list. That wiring is the next iteration's job, and the watchdog stays red until it's complete. Everything else: 42 of 43 suites green.