Tunneling

Dig a channel down beside the tall side of the board so that the dangerous gray discs high up can actually be reached and opened.

The intuition

A covered gray disc only opens when a numbered disc clears right next to it. A gray disc sitting high on a tall stack, walled in by other discs, therefore has nothing that can reach it — and every rise pushes it higher and adds seven more underneath it. Human players describe the answer as tunneling: keep one column low, so its wall of neighbours is exposed and you have somewhere to attack from.

The lab turns that picture into two claims that a program can measure:

• A gray disc's danger grows quadratically with its altitude, and a gray disc on the left or right edge is worse still, because it has one fewer horizontal direction from which it can be attacked.
• A low column earns credit only for the covered cells that can actually be reached from it. A jagged wall of numbered discs earns nothing — the source comment is explicit that generic board roughness is not the point.

The same file adds a small credit for stacking repeated 5s, 6s and 7s in a still-live column, on the theory that each addition advances all of them toward the same vertical trigger.

How it works, step by step

1. Read the public position — visible board, visible next disc, rise clock.
2. Score the board's tunneling shape. Six features: covered-disc altitude risk, edge-covered altitude risk, cliff access to covered cells, edge cliff access, and two high-number cohesion terms.
3. Score the move itself. Five transition features that pay for what the move did: committing a high number, extending a trench, cracking a covered disc high up, revealing one, and damaging a cover on the edge.
4. Add the result to the ordinary hand evaluator as a residual, scaled by a fixed constant, and use that sum as the value inside a short rollout search over sampled futures.
5. Play the best-scoring column.

What happened, in plain English

Only half the idea survived its own testing, and the code records which half. The transition bonus — paying for damage actually done to high or edge covers — was supported by paired validation and ships switched on at a quarter weight. The state residual — scoring the standing shape of the board — was not supported, and ships at weight zero: available for ablations, off by default.

As a whole policy it worked in the sense that it beat the comparator it was built against, and did not work in the sense that mattered: it came nowhere near the million-point target, and it never displaced the depth-4 reference search.

The technical record

The experiment index records this lab as completed, task-record only: "a 64-game comparison improved the open-loop baseline, but did not approach qualification." A task-record-only label means the outcome is reported in a research conversation and has never been promoted into the experiment history; no artifact, cohort listing, or per-game output is retained here. No score, move count, or margin for that 64-game comparison exists in any retained record in this repository, so none is quoted on this page. The comparator was the open-loop beam, which the index itself classifies as support-only.

Repository-verified from the source: src/core/typescript/tunneling-heuristic.ts defines six state features with weights −45 (covered altitude risk), −65 (edge covered altitude risk), +90 (cliff access), +110 (edge cliff access), +240 and +180 (high-number cohesion), and five action features with weights +160, +200, +100, +180 and +120. DEFAULT_TUNNELING_STATE_SCALE is 0 and DEFAULT_TUNNELING_ACTION_SCALE is 0.25, with the comment: "Paired validation supported the transition bonus, but not the state residual." The lab main.ts compares the combined and tunneling profiles through the shared rollout solver, and the evaluator is also reachable from the open-loop, gray-throughput and sparse-expectimax labs.

The evaluator reads only the visible board, next disc and rise clock. It is a public-information policy.

What this taught us, and what is still open

• Reachability is a real feature and standing shape is not. The one durable output of this lab is the asymmetry between its two halves: paying for cover damage that a move actually achieved held up, while scoring how tunnel-shaped a board merely looks did not. That is the same lesson the strategy catalog draws from static board potential in general — similar looking boards can have very different futures.
• The human hypothesis behind it is still untested. "Attack the high covers, and watch the edge columns" comes from an experienced player's strategy notes, which the catalog labels explicitly anecdotal. Testing it properly is a separate lab, edge priority, which has no located result at all.
• What was ruled out is nothing. This lab produced a positive comparison against a weak comparator with no retained numbers. What would make it evidence is a paired, whole-game comparison against the unchanged depth-4 reference on seeds disjoint from anything used to pick the 0.25 scale.