Add Ta Yü

Rivers laid out from the centre of an 18x18 board on 1x3 tiles, each with
three mouths that must line up with whatever they touch. One side runs water
north and south, the other east and west, and a side's score is one edge
multiplied by the other — so reaching only one of your two edges is worth
nothing, which is the whole tension of the game.

Two things fell out of the geometry rather than needing to be tabulated. The
tile set is generated: three mouths chosen from the eight places one can sit
on a 1x3 tile gives 56 arrangements, turning a tile end for end pairs them
off with none fixed, so there are exactly 28 distinct tiles and the box's 84
is three of each. And because every tile joins the network by a mouth while
all three of its own mouths interconnect, the board is always one connected
river system — so scoring never traces connectivity, and a mouth at the edge
is an exit by construction. Both properties are asserted over whole games.

The placement check is one comparison doing two jobs: where a new tile abuts
an old one, both sides must agree. That refuses a river dead-ending against
stone and, read the other way, refuses a blank flank that would wall off a
river already down.

18x18 comes out under 20dp a cell on a phone, too fine to tap, so laying a
tile is a tap to line it up and a button to commit. Tapping the same cell
again cycles the other legal ways the tile could cover it, which is also how
it gets turned — a rotate control would have landed on illegal positions. A
tap on a cell the tile cannot cover is ignored rather than treated as a
deselection, so a fingertip one cell wide of the mark does not throw away a
placement.

Two AI settings, not three. A blocking-heavy variant was written and dropped
because it played the full setting dead even over 30 games; shipping it as a
step up would have been a claim that could not be backed. What ships is held
to measured numbers: full strength won 20 of 20 against random placement from
either axis, gentle beats random about three games in four and loses to full
strength about four in five, and random against random comes out even, which
is what makes the rest of those numbers mean anything.

The reconstruction's uncertainties stay called out in the rules doc rather
than being smoothed over now that the code exists: the 18x18 board reading,
the marked exits placed at 3, 8 and 13 by choice, and the three-player flood
variant left out because nothing found says how the flood scores.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_015pyhC1KyFkQk6hUnnX2Tof

README.md

@@ -5,12 +5,12 @@ or against other people over Wi-Fi (LAN) or Bluetooth.
 
 ## What works today
 
-The app shell is complete and eight games are playable end to end:
+The app shell is complete and nine games are playable end to end:
 
 | | |
 | --- | --- |
-| **Playable** | Euchre, Kaiser, President, Golf, Wizard, Crazy 8s, Sequence, Chess |
-| **Listed, not yet implemented** | Mastermind, Backgammon, Checkers, Ta Yü |
+| **Playable** | Euchre, Kaiser, President, Golf, Wizard, Crazy 8s, Sequence, Chess, Ta Yü |
+| **Listed, not yet implemented** | Mastermind, Backgammon, Checkers |
 
 Unimplemented games appear in the menu marked *Coming soon* **on dev builds
 only** — a production release lists just the games that can actually be played,
@@ -93,6 +93,11 @@ always square and sized to fit whatever is left after the hand, and card widths
 are a fraction of the smaller screen dimension. Rotation is handled inside the
 activity, so a hand is not lost when the device turns.
 
+The Ta Yü board is the tightest fit of the lot — 18 × 18 cells comes out under
+20dp a cell on a phone — which is why laying a tile there is a tap to line the
+placement up and a button to commit it, rather than a single tap on a target
+smaller than a fingertip.
+
 ## Building
 
 ```sh
@@ -170,9 +175,9 @@ game:
 - [Wizard](docs/RULES-wizard.md)
 - [Crazy 8s](docs/RULES-crazy8s.md)
 - [Chess](docs/RULES-chess.md)
-- [Ta Yü](docs/RULES-tayu.md) — **not implemented yet**: this one is the
-  specification the engine will be written against, and it says which parts of
-  the out-of-print rulebook had to be reconstructed
+- [Ta Yü](docs/RULES-tayu.md) — **note the caveats about the reconstruction**:
+  the game is out of print and the rulebook could not be obtained, so the
+  document says which parts are attested and which were derived
 
 ## Layout
 
@@ -188,6 +193,7 @@ game/          rules engines and AI — pure Kotlin, no Android, unit-tested
   wizard/      Wizard model, rules, AI
   crazyeights/ Crazy 8s model, rules, AI
   chess/       Chess model, FEN, move generation, search
+  tayu/        Ta Yü tile geometry, river rules, scoring, AI
 net/           wire protocol, LAN and Bluetooth transports, lobby, match driver
 settings/      DataStore-backed preferences
 update/        GitHub Releases OTA updater