D4 flow audit

The strongest reliable player here is a four-move-deep search, and it is largely a black box: it produces a column and a score. This program opens it up and records everything it saw and valued, one line per move.

The intuition

Suppose the reference search averages roughly 90 moves per game and you want to know why it dies. A mean score cannot tell you. What you need is the shape of the last twenty moves: was the board getting taller, were covered discs piling up out of reach, did the search's own opinion of the position collapse suddenly or drift down for thirty moves?

That means recording, at every move, three different things side by side:

• the position — the board itself, the next disc, how many drops remain before the rise, how many cells are occupied, how many are still covered, and how tall the tallest column is;
• what the hand-written evaluator thinks of it — its overall value plus the individual features it is built from: immediate chain potential, latent chain potential, exposure of cracked and solid grays, clog patterns such as adjacent 1s and triple 2s, dead low numbers, quiet build options, how ready a trigger is now and how ready it will be after the rise, projected occupancy debt, cover-altitude debt, peak-height risk, and low-cap load;
• what the search decided — the column chosen, the column a three-ply search would have chosen instead, the search's value for the chosen column, and the immediate points it expected.

Then the move is played and its outcome is appended: points scored, discs cleared, discs revealed, number of chain waves, whether the level advanced, whether the game ended.

Line up a few hundred of those and questions such as "does the depth-4 search disagree with depth 3 more often just before it dies?" become arithmetic.

How it works

1. Refuse to start unless the seed is one of four already-consumed development games (0x3d6e4000–0x3d6e4003). This is instrumentation on data whose status has already been spent; it must not open a fresh cohort.
2. Play the game with the unmodified depth-4 reference, and abort if the search ever fails to complete all four plies or returns an illegal column — an incomplete search would make the trace a record of a different policy.
3. Write one JSON object per move to a file, with the fields above.
4. Print a running summary to the terminal every 25 moves: score so far, clears per move, reveals per move.

It defines no policy of its own and it changes nothing about the search. It is labelled a diagnostic rather than public because it exists to describe a policy, not to be one.

What happened

No retained result. The experiment index lists this source as support-only and repository-verified; audit 04 §B.2 records it as "diagnostic only; no retained output". No trace file from it is kept in this repository, and no conclusion in the research documents cites one.

The related question it was built to answer has since been answered elsewhere, by different code: the depth-4 reference's flow deficit is measured in finding 01 as 1.973 clears and 1.090 reveals per move against the 2.400 and 1.400 the board demands, over 64 paired games at the exploratory development tier.

The technical record

Status: support-only — repository-verified. No ledger entry, no task record, no retained artifact.

d4-flow-audit.cpp includes the reference implementation approaches/fair-expectimax/reference/fair-only-depth4.cpp directly with its main disabled, so the audited policy is byte-for-byte the reference and not a copy. Options: --seed (restricted to 0x3d6e4000–0x3d6e4003), --max-moves (1–1,000, default 500), --output (default a path under /tmp). Features come from extractFairFeatures and the shared phase feature struct; the leaf value is fairLeaf.

The same four seeds 0x3d6e4000–0x3d6e4003 are the fitting quartet used by the structural terminal veto, which is why they are already consumed.

Source: d4-flow-audit.cpp.

What this taught us, and what is still open

• A per-move trace is the right instrument for asking why a search dies, and this one is already wired to the exact reference policy rather than an imitation of it.
• Four games is the entire licensed range. Anything statistical would need a fresh, declared development lease.
• Nothing produced by it is on record. If the traces were ever read, what they showed was not written down.