SUNTHREAD

The weakness we measured in the WARDLIGHT engine was plain: at level 400, 100% of moves were forced and the player never made a decision. Most one-line puzzles either run on guessing or on dictation. SUNTHREAD’s first principle follows from that: solvability is the entry ticket; uniqueness and deduction are the product. Every level must have exactly one solution, reachable without guessing.

The generator produces candidates that must pass five hard gates: exactly one solution; no clue removable without breaking uniqueness; a human-style solver using only named deduction rules must finish the level; the forced-move ratio must sit in the 35–70% band; and a level is rejected if the solver’s rule multiset matches any of the last five.

The solver works in five tiers (dead ends, breeze propagation, chessboard parity, bed-entry counting, turn counting) and the difficulty score derives from them. The generator was written in Node first as the reference oracle; the C# port is diffed byte for byte against golden fixtures. The thread of light is a custom triangle strip with arc-length UVs rather than a LineRenderer, and feedback fires haptic first, then audio, then visual.

Six chapters × 20 curated levels plus 200 daily levels: all 320 pass the five-step verification report committed to the repo, with zero failures. The forced-move ratio runs from 0.44 to 0.79 with a 0.59 average; the same measurement on WARDLIGHT read 100%. 27 EditMode and 34 PlayMode tests are green, and the PlayMode run re-renders every review sheet. Music is generative, there are four season palettes and a high-contrast mode. Development continues.