feat(offline): local game engine (Phase B1)

The offline vs_ai game engine — a faithful TS port of backend/internal/engine that
drives a whole local game with no backend. Composes with the move generator (#188) and
robot strategy (#189) from Phase A; not yet wired into the UI (Phase B2/B3).

- ui/src/lib/localgame/ruleset.ts: static per-variant tile values / bag counts / blank
  count, mirrored from rules.go (offline scoring is self-contained; online uses the
  server alphabet). Pinned by ruleset.parity.test.ts against a Go fixture.
- bag.ts: the tile bag (fill from counts/blanks, draw-from-end, return+reshuffle) on a
  deterministic in-house PRNG — a game replays from its seed, not bit-identical to a
  server game (per plan).
- board.ts: the mutable board, satisfying the validator/generator read view + set().
- engine.ts: LocalGame — deal / play (reusing validate.ts) / pass / exchange / resign,
  scoreless(6) & out-of-tiles end detection, end-of-game rack penalties, winner; mirrors
  game.go. The end-game math is exported as pure functions, pinned against the Go engine
  (engine.parity.test.ts, 9 constructed positions).
- engine.test.ts: a full-loop smoke — two robots play a whole vs_ai game to completion
  via generateMoves + decide, and it is reproducible from the seed.
- backend: movegen now dumps the per-variant rulesets; a new in-package engine emitter
  (endfixture_test.go, env-gated) produces the end-game golden.

Pure additive library code; no runtime behavior change (unused at runtime, bundle unchanged).

backend/cmd/movegen/main.go

@@ -121,6 +121,8 @@ func main() {
 		return
 	}
 
+	emitRulesets()
+
 	buildSample(*out, "en", rules.English(), sampleWordsEN, []genCase{
 		emptyCase("empty-cared", englishRack("caredts", 0), scrabble.Both, false),
 		emptyCase("empty-dogs", englishRack("dogsent", 0), scrabble.Both, false),
@@ -363,6 +365,37 @@ func russianRack(letters string, blanks int) genRack {
 	return genRack{Letters: idx, Blanks: blanks}
 }
 
+// emitRulesets writes the per-variant static ruleset data (tile values, bag counts, blanks,
+// bingo, rack size) the offline engine mirrors in ui/src/lib/localgame/ruleset.ts, so a TS
+// parity test can pin that hand-copied table to the Go rulesets (scrabble-solver/rules).
+func emitRulesets() {
+	type rsFix struct {
+		Size     int   `json:"size"`
+		RackSize int   `json:"rackSize"`
+		Bingo    int   `json:"bingo"`
+		Blanks   int   `json:"blanks"`
+		Values   []int `json:"values"`
+		Counts   []int `json:"counts"`
+	}
+	out := map[string]rsFix{}
+	for _, v := range []struct {
+		name string
+		rs   *rules.Ruleset
+	}{
+		{"scrabble_en", rules.English()},
+		{"scrabble_ru", rules.RussianScrabble()},
+		{"erudit_ru", rules.Erudit()},
+	} {
+		out[v.name] = rsFix{Size: v.rs.Size(), RackSize: v.rs.RackSize, Bingo: v.rs.Bingo, Blanks: v.rs.Blanks, Values: v.rs.Values, Counts: v.rs.Counts}
+	}
+	dir := filepath.Join("ui", "src", "lib", "localgame", "testdata")
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		log.Fatalf("movegen: mkdir %s: %v", dir, err)
+	}
+	writeJSON(filepath.Join(dir, "rulesets.json"), out)
+	log.Printf("movegen: wrote %s (3 variants)", filepath.Join(dir, "rulesets.json"))
+}
+
 func writeJSON(path string, v any) {
 	data, err := json.MarshalIndent(v, "", "  ")
 	if err != nil {

backend/internal/engine/endfixture_test.go

@@ -0,0 +1,123 @@
+package engine
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"gitea.iliadenisov.ru/developer/scrabble-solver/rules"
+)
+
+// The offline engine (ui/src/lib/localgame) reproduces the end-of-game rack settlement and the
+// winner rule so a local game finishes with the same scores as the server. These golden fixtures
+// pin the ported pure functions (applyEndAdjustment / winner / rackValue) to the real Go engine.
+// Being in-package, this emitter constructs Game values directly and drives the unexported
+// end-game math on chosen positions.
+
+type endCaseIn struct {
+	Name     string  `json:"name"`
+	Variant  string  `json:"variant"`
+	Reason   string  `json:"reason"`
+	Hands    [][]int `json:"hands"`
+	Scores   []int   `json:"scores"`
+	Resigned []bool  `json:"resigned"`
+	ToMove   int     `json:"toMove"`
+}
+
+type endCaseOut struct {
+	endCaseIn
+	ScoresAfter []int `json:"scoresAfter"`
+	Winner      int   `json:"winner"`
+}
+
+func rulesetFor(variant string) *rules.Ruleset {
+	switch variant {
+	case "scrabble_ru":
+		return rules.RussianScrabble()
+	case "erudit_ru":
+		return rules.Erudit()
+	default:
+		return rules.English()
+	}
+}
+
+func reasonFor(s string) EndReason {
+	switch s {
+	case "out_of_tiles":
+		return EndOutOfTiles
+	case "scoreless":
+		return EndScoreless
+	case "resign":
+		return EndResign
+	case "aborted":
+		return EndAborted
+	}
+	return EndNotOver
+}
+
+func handsBytes(hands [][]int) [][]byte {
+	out := make([][]byte, len(hands))
+	for i, h := range hands {
+		b := make([]byte, len(h))
+		for j, x := range h {
+			b[j] = byte(x)
+		}
+		out[i] = b
+	}
+	return out
+}
+
+// TestEmitEndgameFixtures regenerates ui/src/lib/localgame/testdata/endgame.json. Gated by
+// EMIT_ENGINE_FIXTURES. Regenerate with:
+//
+//	EMIT_ENGINE_FIXTURES=1 go test ./backend/internal/engine -run TestEmitEndgameFixtures
+func TestEmitEndgameFixtures(t *testing.T) {
+	if os.Getenv("EMIT_ENGINE_FIXTURES") == "" {
+		t.Skip("set EMIT_ENGINE_FIXTURES=1 to regenerate ui/src/lib/localgame/testdata/endgame.json")
+	}
+
+	cases := []endCaseIn{
+		{"out-basic", "scrabble_en", "out_of_tiles", [][]int{{}, {0, 1, 2}}, []int{50, 40}, []bool{false, false}, 0},
+		{"out-blank", "scrabble_en", "out_of_tiles", [][]int{{}, {255, 0}}, []int{30, 30}, []bool{false, false}, 0},
+		{"out-erudit-yo", "erudit_ru", "out_of_tiles", [][]int{{}, {6, 32}}, []int{10, 10}, []bool{false, false}, 0},
+		{"out-tie", "scrabble_en", "out_of_tiles", [][]int{{}, {}}, []int{30, 30}, []bool{false, false}, 0},
+		{"out-3p", "scrabble_ru", "out_of_tiles", [][]int{{}, {0, 1}, {2}}, []int{10, 10, 10}, []bool{false, false, false}, 0},
+		{"scoreless", "scrabble_en", "scoreless", [][]int{{0, 1}, {2, 3}}, []int{20, 20}, []bool{false, false}, 0},
+		{"resign-2p", "scrabble_en", "resign", [][]int{{}, {0}}, []int{100, 10}, []bool{true, false}, 1},
+		{"resign-3p", "scrabble_en", "resign", [][]int{{}, {}, {}}, []int{50, 60, 40}, []bool{false, true, false}, 0},
+		{"aborted", "scrabble_en", "aborted", [][]int{{0}, {1}}, []int{40, 30}, []bool{false, false}, 0},
+	}
+
+	out := make([]endCaseOut, 0, len(cases))
+	for _, c := range cases {
+		rs := rulesetFor(c.Variant)
+		scores := append([]int(nil), c.Scores...)
+		g := &Game{
+			rules:    rs,
+			hands:    handsBytes(c.Hands),
+			scores:   scores,
+			resigned: c.Resigned,
+			toMove:   c.ToMove,
+		}
+		reason := reasonFor(c.Reason)
+		g.over = true
+		g.reason = reason
+		g.applyEndAdjustment(reason)
+		out = append(out, endCaseOut{endCaseIn: c, ScoresAfter: g.scores, Winner: g.winner()})
+	}
+
+	dir := filepath.Join("..", "..", "..", "ui", "src", "lib", "localgame", "testdata")
+	if err := os.MkdirAll(dir, 0o755); err != nil {
+		t.Fatalf("mkdir %s: %v", dir, err)
+	}
+	data, err := json.MarshalIndent(map[string]any{"cases": out}, "", "  ")
+	if err != nil {
+		t.Fatalf("marshal: %v", err)
+	}
+	path := filepath.Join(dir, "endgame.json")
+	if err := os.WriteFile(path, append(data, '\n'), 0o644); err != nil {
+		t.Fatalf("write %s: %v", path, err)
+	}
+	t.Logf("wrote %s (%d cases)", path, len(out))
+}

ui/src/lib/localgame/bag.test.ts

@@ -0,0 +1,50 @@
+import { describe, it, expect } from 'vitest';
+import { Bag } from './bag';
+import { RULESETS } from './ruleset';
+import { BLANK_INDEX } from '../alphabet';
+
+describe('offline bag', () => {
+  it('holds exactly the variant tile distribution', () => {
+    const rs = RULESETS.scrabble_en;
+    const total = rs.counts.reduce((a, b) => a + b, 0) + rs.blanks;
+    const bag = new Bag('scrabble_en', 42);
+    expect(bag.length).toBe(total);
+
+    const drawn = bag.draw(total);
+    expect(drawn.length).toBe(total);
+    expect(bag.length).toBe(0);
+
+    const tally = new Map<number, number>();
+    for (const t of drawn) tally.set(t, (tally.get(t) ?? 0) + 1);
+    for (let i = 0; i < rs.counts.length; i++) expect(tally.get(i) ?? 0).toBe(rs.counts[i]);
+    expect(tally.get(BLANK_INDEX) ?? 0).toBe(rs.blanks);
+  });
+
+  it('draws beyond the remaining, returning all and emptying', () => {
+    const bag = new Bag('scrabble_ru', 7);
+    const total = bag.length;
+    const all = bag.draw(total + 5);
+    expect(all.length).toBe(total);
+    expect(bag.length).toBe(0);
+    expect(bag.draw(3)).toEqual([]);
+  });
+
+  it('returns tiles back to the bag', () => {
+    const bag = new Bag('erudit_ru', 1);
+    const before = bag.length;
+    const seven = bag.draw(7);
+    expect(bag.length).toBe(before - 7);
+    bag.return(seven);
+    expect(bag.length).toBe(before);
+  });
+
+  it('is deterministic for a given seed and operation sequence', () => {
+    const a = new Bag('scrabble_en', 123);
+    const b = new Bag('scrabble_en', 123);
+    expect(a.draw(30)).toEqual(b.draw(30));
+    // A return reshuffles both identically, so subsequent draws still agree.
+    a.return([0, 1, 2]);
+    b.return([0, 1, 2]);
+    expect(a.draw(10)).toEqual(b.draw(10));
+  });
+});

ui/src/lib/localgame/bag.ts

@@ -0,0 +1,76 @@
+// The offline tile bag — the shuffled draw pile for one local game. Structurally a port of
+// backend/internal/engine/bag.go (fill from the variant's counts + blanks, draw from the end,
+// return-and-reshuffle for an exchange), but shuffled with a small DETERMINISTIC in-house PRNG
+// rather than Go's math/rand: a local game only needs to be reproducible from its own seed and
+// sequence of operations (for replay from the stored journal), not bit-identical to a server
+// game (docs plan). Blanks ride as BLANK_INDEX, matching lib/alphabet.ts (and the engine's
+// blankTile = 0xff = 255).
+
+import { RULESETS } from './ruleset';
+import { BLANK_INDEX } from '../alphabet';
+import type { Variant } from '../model';
+
+// mulberry32 is a compact deterministic PRNG returning a float in [0, 1). Seeded from the game
+// seed so the shuffle sequence — and thus the draws — replay identically for the same seed and
+// the same sequence of returns.
+function mulberry32(seed: number): () => number {
+  let a = seed >>> 0;
+  return () => {
+    a = (a + 0x6d2b79f5) | 0;
+    let t = Math.imul(a ^ (a >>> 15), 1 | a);
+    t = (t + Math.imul(t ^ (t >>> 7), 61 | t)) ^ t;
+    return ((t ^ (t >>> 14)) >>> 0) / 4294967296;
+  };
+}
+
+/**
+ * Bag is one local game's draw pile. Construct it with the variant and a numeric seed; draw()
+ * and return() mutate it in place. It is reproducible: the same seed and the same sequence of
+ * operations yield the same draws.
+ */
+export class Bag {
+  private tiles: number[];
+  private readonly rand: () => number;
+
+  constructor(variant: Variant, seed: number) {
+    const rs = RULESETS[variant];
+    const tiles: number[] = [];
+    for (let i = 0; i < rs.counts.length; i++) {
+      for (let n = 0; n < rs.counts[i]; n++) tiles.push(i);
+    }
+    for (let n = 0; n < rs.blanks; n++) tiles.push(BLANK_INDEX);
+    this.tiles = tiles;
+    this.rand = mulberry32(seed);
+    this.shuffle();
+  }
+
+  /** length is the number of tiles left in the bag. */
+  get length(): number {
+    return this.tiles.length;
+  }
+
+  /**
+   * draw removes up to n tiles from the end of the bag and returns them. Drawing more than
+   * remain returns all of them; drawing from an empty bag returns an empty array.
+   */
+  draw(n: number): number[] {
+    const take = Math.min(n, this.tiles.length);
+    return this.tiles.splice(this.tiles.length - take, take);
+  }
+
+  /** return puts tiles back into the bag and reshuffles, as when a player exchanges tiles. */
+  return(tiles: readonly number[]): void {
+    for (const t of tiles) this.tiles.push(t);
+    this.shuffle();
+  }
+
+  // shuffle randomises the remaining tiles in place with the bag's own PRNG (Fisher–Yates).
+  private shuffle(): void {
+    for (let i = this.tiles.length - 1; i > 0; i--) {
+      const j = Math.floor(this.rand() * (i + 1));
+      const tmp = this.tiles[i];
+      this.tiles[i] = this.tiles[j];
+      this.tiles[j] = tmp;
+    }
+  }
+}

ui/src/lib/localgame/board.ts

@@ -0,0 +1,44 @@
+// The mutable local-game board: a 15x15 row-major grid of placed tiles (alphabet-index letter
+// plus a blank flag). It satisfies the read view the validator and generator need (GenBoard,
+// which extends validate.ts's Board) and adds set() so the engine can apply a play. The board is
+// alphabet-agnostic — a cell's letter is a variant index, meaningful with the variant's ruleset.
+
+import { BOARD_SIZE } from '../premiums';
+import type { Cell } from '../dict/validate';
+import type { GenBoard } from '../dict/generate';
+
+/** LocalBoard is the engine's in-memory board; it reads as a GenBoard and applies plays via set. */
+export class LocalBoard implements GenBoard {
+  readonly rows = BOARD_SIZE;
+  readonly cols = BOARD_SIZE;
+  private readonly grid: (Cell | null)[];
+  private count = 0;
+
+  constructor() {
+    this.grid = new Array<Cell | null>(BOARD_SIZE * BOARD_SIZE).fill(null);
+  }
+
+  inBounds(row: number, col: number): boolean {
+    return row >= 0 && row < this.rows && col >= 0 && col < this.cols;
+  }
+
+  filled(row: number, col: number): boolean {
+    return this.inBounds(row, col) && this.grid[row * this.cols + col] !== null;
+  }
+
+  cellAt(row: number, col: number): Cell {
+    return this.grid[row * this.cols + col]!;
+  }
+
+  isEmpty(): boolean {
+    return this.count === 0;
+  }
+
+  /** set places a tile at (row, col). It assumes the square was empty (a play only lays tiles
+   *  on empty squares), keeping the filled count exact for isEmpty. */
+  set(row: number, col: number, letter: number, blank: boolean): void {
+    const i = row * this.cols + col;
+    if (this.grid[i] === null) this.count++;
+    this.grid[i] = { letter, blank };
+  }
+}

ui/src/lib/localgame/engine.parity.test.ts

@@ -0,0 +1,35 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { applyEndAdjustment, winner, type EndReason } from './engine';
+import { RULESETS } from './ruleset';
+import type { Variant } from '../model';
+
+// The offline engine must settle unplayed racks and decide the winner exactly as the Go engine
+// does, so a local game finishes with the same scores. Golden from the in-package emitter
+// (backend/internal/engine/endfixture_test.go).
+interface EndCase {
+  name: string;
+  variant: Variant;
+  reason: EndReason;
+  hands: number[][];
+  scores: number[];
+  resigned: boolean[];
+  toMove: number;
+  scoresAfter: number[];
+  winner: number;
+}
+
+const fx = JSON.parse(
+  readFileSync(new URL('./testdata/endgame.json', import.meta.url), 'utf8'),
+) as { cases: EndCase[] };
+
+describe('offline engine end-game parity vs Go', () => {
+  for (const c of fx.cases) {
+    it(c.name, () => {
+      const values = RULESETS[c.variant].values;
+      const after = applyEndAdjustment(c.reason, c.hands, c.scores, c.toMove, values);
+      expect(after).toEqual(c.scoresAfter);
+      expect(winner(true, c.reason, after, c.resigned)).toBe(c.winner);
+    });
+  }
+});

ui/src/lib/localgame/engine.test.ts

@@ -0,0 +1,78 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { Dawg } from '../dict/dawg';
+import { RACK_SIZE } from '../premiums';
+import { LocalGame } from './engine';
+import { decide } from '../robot/strategy';
+import type { Move } from '../dict/validate';
+
+// A full-loop smoke: two robots play a whole local vs_ai game to completion over the committed
+// sample dictionary, exercising deal / play / refill / exchange / pass / end detection / winner.
+// (The rich full-dictionary robot behaviour is pinned elsewhere by the generator + strategy parity
+// suites; this test proves the engine drives to a valid finish.)
+const dawg = new Dawg(new Uint8Array(readFileSync(new URL('../dict/testdata/sample_en.dawg', import.meta.url))));
+
+function bestOpponentScore(game: LocalGame, seat: number): number {
+  let best = 0;
+  for (let i = 0; i < game.playerCount; i++) if (i !== seat) best = Math.max(best, game.scoreOf(i));
+  return best;
+}
+
+// robotTurn plays the seat to move: it picks the robot's action from the ranked legal plays and
+// dispatches it. The sample dict has a one-letter word ("a") the generator emits but the validator
+// rejects (len < 2) — real dictionaries have none, so filtering to main length >= 2 is a no-op in
+// production; an exchange the bag is too small to satisfy falls back to a pass.
+function robotTurn(game: LocalGame, seed: bigint): void {
+  const seat = game.currentPlayer;
+  const cands: Move[] = game.generateMoves().filter((m) => m.main.letters.length >= 2);
+  const dec = decide(seed, game.moveCount, cands, game.scoreOf(seat), bestOpponentScore(game, seat), game.handOf(seat), game.bagLength);
+  if (dec.kind === 'play') {
+    game.play(dec.move.dir, dec.move.tiles);
+  } else if (dec.kind === 'exchange' && game.bagLength >= RACK_SIZE) {
+    game.exchange(dec.exchange);
+  } else {
+    game.pass();
+  }
+}
+
+describe('offline engine full-game smoke', () => {
+  it('plays a whole 2-player vs_ai game to completion', () => {
+    const game = new LocalGame({
+      variant: 'scrabble_en',
+      version: 'sample',
+      seed: 123456789n,
+      players: 2,
+      dawg,
+      multipleWords: true,
+    });
+
+    let turns = 0;
+    while (!game.isOver && turns < 2000) {
+      robotTurn(game, game.seed);
+      turns++;
+    }
+
+    expect(game.isOver).toBe(true);
+    expect(turns).toBeLessThan(2000);
+    expect(['out_of_tiles', 'scoreless', 'resign']).toContain(game.endReason);
+
+    const w = game.winnerIndex;
+    expect(w === -1 || (w >= 0 && w < game.playerCount)).toBe(true);
+    for (let i = 0; i < game.playerCount; i++) expect(Number.isInteger(game.scoreOf(i))).toBe(true);
+    // The move log recorded every turn.
+    expect(game.history.length).toBe(turns);
+  });
+
+  it('is reproducible from the seed', () => {
+    const play = (): { reason: string; scores: number[]; turns: number } => {
+      const g = new LocalGame({ variant: 'scrabble_en', version: 'sample', seed: 42n, players: 2, dawg, multipleWords: true });
+      let t = 0;
+      while (!g.isOver && t < 2000) {
+        robotTurn(g, g.seed);
+        t++;
+      }
+      return { reason: g.endReason, scores: [g.scoreOf(0), g.scoreOf(1)], turns: t };
+    };
+    expect(play()).toEqual(play());
+  });
+});

ui/src/lib/localgame/engine.ts

@@ -0,0 +1,395 @@
+// The offline game engine — a faithful port of backend/internal/engine/game.go. It owns the
+// board, the bag, each seat's hand, the scores, whose turn it is and the move log, applies plays
+// (reusing the already-ported validator lib/dict/validate.ts and generator lib/dict/generate.ts),
+// and detects the end of a game (out-of-tiles / scoreless / resign) with the standard end-of-game
+// rack adjustment. It is pure in-memory logic with no I/O; a local vs_ai game is driven by feeding
+// it the robot's choice (lib/robot/strategy.ts) each turn. The bag RNG is our own deterministic
+// PRNG (see bag.ts), so a game replays from its seed but is not bit-identical to a server game.
+//
+// End-of-game scoring, the winner rule and the rack value are exported as pure functions so they
+// can be pinned against the Go engine (engine.parity.test.ts) on constructed positions.
+
+import { LocalBoard } from './board';
+import { Bag } from './bag';
+import { RULESETS } from './ruleset';
+import { generateMoves, GenRack, Both } from '../dict/generate';
+import { validatePlay, type Direction, type Placement, type Ruleset, type Move } from '../dict/validate';
+import { premiumGrid, centre, BOARD_SIZE, RACK_SIZE, BINGO, type Premium } from '../premiums';
+import { BLANK_INDEX } from '../alphabet';
+import type { Dawg } from '../dict/dawg';
+import type { Variant } from '../model';
+
+/** scorelessLimit is the number of consecutive scoreless turns (passes and exchanges) that ends
+ *  a game, mirroring engine.scorelessLimit. */
+export const scorelessLimit = 6;
+
+/** EndReason explains why a game finished, using the engine's stable labels. */
+export type EndReason = 'not_over' | 'out_of_tiles' | 'scoreless' | 'resign' | 'aborted';
+
+/** Disposition of a dropped-out (resigned/timed-out) seat's tiles in a 3-4 player game. */
+export type DropoutTiles = 'remove' | 'return';
+
+/** LocalMove is one recorded turn. tiles/dir are set on a play, count on an exchange. */
+export interface LocalMove {
+  player: number;
+  action: 'play' | 'pass' | 'exchange' | 'resign';
+  dir?: Direction;
+  tiles?: Placement[];
+  count?: number;
+  score: number;
+  total: number;
+}
+
+/** GameError carries a stable code for the engine's rejection reasons. */
+export class GameError extends Error {
+  constructor(public readonly code: string) {
+    super(code);
+    this.name = 'GameError';
+  }
+}
+
+function letterMultOf(p: Premium): number {
+  return p === 'DL' ? 2 : p === 'TL' ? 3 : 1;
+}
+function wordMultOf(p: Premium): number {
+  return p === 'DW' ? 2 : p === 'TW' ? 3 : 1;
+}
+
+/**
+ * buildRuleset assembles the validator/generator ruleset for a variant from the static offline
+ * tile values (ruleset.ts) and the board geometry (premiums.ts). multipleWords false selects the
+ * single-word-per-turn rule (perpendicular cross-words ignored). Mirrors the server engine's
+ * ruleset for the same variant; online scoring instead reads the server-sent alphabet values.
+ */
+export function buildRuleset(variant: Variant, multipleWords: boolean): Ruleset {
+  const prem = premiumGrid(variant);
+  const ctr = centre(variant);
+  return {
+    cols: BOARD_SIZE,
+    center: ctr.row * BOARD_SIZE + ctr.col,
+    rackSize: RACK_SIZE,
+    bingo: BINGO[variant],
+    values: RULESETS[variant].values,
+    letterMult: (r, c) => letterMultOf(prem[r][c]),
+    wordMult: (r, c) => wordMultOf(prem[r][c]),
+    ignoreCrossWords: !multipleWords,
+  };
+}
+
+/** rackValue sums the tile values left on a hand; blanks (BLANK_INDEX) count zero. Mirrors
+ *  engine (*Game).rackValue. */
+export function rackValue(hand: readonly number[], values: readonly number[]): number {
+  let v = 0;
+  for (const t of hand) if (t !== BLANK_INDEX) v += values[t];
+  return v;
+}
+
+/**
+ * applyEndAdjustment settles the unplayed racks and returns the adjusted scores. Out-of-tiles: the
+ * player who went out (toMove) gains the sum of every opponent's rack value and each opponent loses
+ * their own. Scoreless: everyone loses their own rack value. Resign/aborted: no adjustment.
+ * Mirrors engine (*Game).applyEndAdjustment.
+ */
+export function applyEndAdjustment(
+  reason: EndReason,
+  hands: readonly (readonly number[])[],
+  scores: readonly number[],
+  toMove: number,
+  values: readonly number[],
+): number[] {
+  const out = [...scores];
+  if (reason === 'out_of_tiles') {
+    let bonus = 0;
+    for (let i = 0; i < hands.length; i++) {
+      if (i === toMove) continue;
+      const v = rackValue(hands[i], values);
+      out[i] -= v;
+      bonus += v;
+    }
+    out[toMove] += bonus;
+  } else if (reason === 'scoreless') {
+    for (let i = 0; i < hands.length; i++) out[i] -= rackValue(hands[i], values);
+  }
+  return out;
+}
+
+/**
+ * winner returns the index of the single highest-scoring seat, or -1 on a tie for the lead, an
+ * unfinished game or an aborted (draw) game. Resigned seats are excluded, so a two-player game
+ * returns the remaining player even when the resigner led. Mirrors engine (*Game).winner.
+ */
+export function winner(over: boolean, reason: EndReason, scores: readonly number[], resigned: readonly boolean[]): number {
+  if (!over || reason === 'aborted') return -1;
+  let best = -1;
+  let tie = false;
+  for (let i = 0; i < scores.length; i++) {
+    if (resigned[i]) continue;
+    if (best === -1 || scores[i] > scores[best]) {
+      best = i;
+      tie = false;
+    } else if (scores[i] === scores[best]) {
+      tie = true;
+    }
+  }
+  return tie ? -1 : best;
+}
+
+/** Options for a new local game. */
+export interface LocalGameOptions {
+  variant: Variant;
+  version: string;
+  /** Seeds the bag; the whole game replays from it. Also the strategy seed the caller uses. */
+  seed: bigint;
+  players: number;
+  dawg: Dawg;
+  multipleWords: boolean;
+  dropoutTiles?: DropoutTiles;
+}
+
+// placementTiles maps placements to the tiles they consume (BLANK_INDEX for a blank).
+function placementTiles(tiles: readonly Placement[]): number[] {
+  return tiles.map((p) => (p.blank ? BLANK_INDEX : p.letter));
+}
+
+/**
+ * LocalGame is the in-memory state of one local match and the rules engine over it. Construct it
+ * with a loaded dictionary; drive it with play/pass/exchange/resign. It performs no I/O.
+ */
+export class LocalGame {
+  readonly variant: Variant;
+  readonly version: string;
+  readonly seed: bigint;
+  private readonly vrs: Ruleset;
+  private readonly values: readonly number[];
+  private readonly rackSize: number;
+  private readonly dawg: Dawg;
+  private readonly dropoutTiles: DropoutTiles;
+
+  private readonly board: LocalBoard;
+  private readonly bag: Bag;
+  private readonly hands: number[][];
+  private readonly scores: number[];
+  private readonly resigned: boolean[];
+  private toMove = 0;
+  private scorelessRun = 0;
+  private over = false;
+  private reason: EndReason = 'not_over';
+  private readonly log: LocalMove[] = [];
+
+  constructor(opts: LocalGameOptions) {
+    if (opts.players < 2 || opts.players > 4) {
+      throw new GameError('players_out_of_range');
+    }
+    this.variant = opts.variant;
+    this.version = opts.version;
+    this.seed = opts.seed;
+    this.dawg = opts.dawg;
+    this.dropoutTiles = opts.dropoutTiles ?? 'remove';
+    this.vrs = buildRuleset(opts.variant, opts.multipleWords);
+    this.values = RULESETS[opts.variant].values;
+    this.rackSize = RULESETS[opts.variant].rackSize;
+
+    this.board = new LocalBoard();
+    this.bag = new Bag(opts.variant, Number(BigInt.asUintN(32, opts.seed)));
+    this.hands = [];
+    this.scores = [];
+    this.resigned = [];
+    for (let i = 0; i < opts.players; i++) {
+      this.hands.push(this.bag.draw(this.rackSize));
+      this.scores.push(0);
+      this.resigned.push(false);
+    }
+  }
+
+  // --- queries ---------------------------------------------------------------
+  get playerCount(): number {
+    return this.hands.length;
+  }
+  get currentPlayer(): number {
+    return this.toMove;
+  }
+  get isOver(): boolean {
+    return this.over;
+  }
+  get endReason(): EndReason {
+    return this.reason;
+  }
+  get bagLength(): number {
+    return this.bag.length;
+  }
+  get moveCount(): number {
+    return this.log.length;
+  }
+  scoreOf(player: number): number {
+    return this.scores[player];
+  }
+  /** rackOf returns a copy of a seat's hand (alphabet-index bytes, BLANK_INDEX for blanks). */
+  handOf(player: number): number[] {
+    return [...this.hands[player]];
+  }
+  /** winnerIndex is the finished game's winner, or -1 (tie / in progress / aborted). */
+  get winnerIndex(): number {
+    return winner(this.over, this.reason, this.scores, this.resigned);
+  }
+  /** history returns a copy of the move log. */
+  get history(): LocalMove[] {
+    return this.log.map((m) => ({ ...m }));
+  }
+
+  /** generateMoves returns every legal play for the current player, ranked by descending score. */
+  generateMoves(): Move[] {
+    return generateMoves(this.dawg, this.board, this.rackOf(this.toMove), this.vrs, Both);
+  }
+
+  // --- turns -----------------------------------------------------------------
+
+  /** play validates and applies the current player's placement, scores it, refills the rack and
+   *  advances the turn (or ends the game). Throws GameError on an illegal play. */
+  play(dir: Direction, tiles: Placement[]): LocalMove {
+    if (this.over) throw new GameError('game_over');
+    const player = this.toMove;
+    const used = placementTiles(tiles);
+    if (!this.holds(player, used)) throw new GameError('tiles_not_on_rack');
+
+    const res = validatePlay(this.board, this.vrs, this.dawg, dir, tiles);
+    if (!res.legal || !res.move) throw new GameError('illegal_play');
+    const move = res.move;
+
+    for (const t of tiles) this.board.set(t.row, t.col, t.letter, t.blank);
+    this.removeFromHand(player, used);
+    this.scores[player] += move.score;
+    this.refill(player);
+    this.scorelessRun = 0;
+
+    const rec: LocalMove = {
+      player,
+      action: 'play',
+      dir,
+      tiles: tiles.map((t) => ({ ...t })),
+      score: move.score,
+      total: this.scores[player],
+    };
+    this.log.push(rec);
+
+    if (this.hands[player].length === 0 && this.bag.length === 0) this.finish('out_of_tiles');
+    else this.advance();
+    return rec;
+  }
+
+  /** pass forfeits the current turn, extending the scoreless run (which may end the game). */
+  pass(): LocalMove {
+    if (this.over) throw new GameError('game_over');
+    const player = this.toMove;
+    this.scorelessRun++;
+    const rec: LocalMove = { player, action: 'pass', score: 0, total: this.scores[player] };
+    this.log.push(rec);
+    this.endTurnAfterScoreless();
+    return rec;
+  }
+
+  /** exchange swaps the given tiles (alphabet-index bytes, BLANK_INDEX for blanks) for fresh ones.
+   *  Legal only while the bag holds at least a full rack; the fresh tiles are drawn before the
+   *  swapped ones return, so a player cannot draw back their own. Extends the scoreless run. */
+  exchange(tiles: number[]): LocalMove {
+    if (this.over) throw new GameError('game_over');
+    if (tiles.length === 0) throw new GameError('nothing_to_exchange');
+    if (this.bag.length < this.rackSize) throw new GameError('not_enough_tiles_to_exchange');
+    const player = this.toMove;
+    if (!this.holds(player, tiles)) throw new GameError('tiles_not_on_rack');
+
+    this.removeFromHand(player, tiles);
+    const drawn = this.bag.draw(tiles.length);
+    for (const t of drawn) this.hands[player].push(t);
+    this.bag.return(tiles);
+    this.scorelessRun++;
+
+    const rec: LocalMove = { player, action: 'exchange', count: tiles.length, score: 0, total: this.scores[player] };
+    this.log.push(rec);
+    this.endTurnAfterScoreless();
+    return rec;
+  }
+
+  /** resign drops the current player out of the game (they forfeit the win, keep their score). */
+  resign(): LocalMove {
+    return this.resignSeat(this.toMove);
+  }
+
+  /** resignSeat resigns a specific seat regardless of whose turn it is. */
+  resignSeat(seat: number): LocalMove {
+    if (this.over) throw new GameError('game_over');
+    if (seat < 0 || seat >= this.hands.length || this.resigned[seat]) throw new GameError('game_over');
+    this.resigned[seat] = true;
+    this.disposeHand(seat);
+    const rec: LocalMove = { player: seat, action: 'resign', score: 0, total: this.scores[seat] };
+    this.log.push(rec);
+    if (this.activeCount() <= 1) this.finish('resign');
+    else if (seat === this.toMove) this.advance();
+    return rec;
+  }
+
+  // --- internals -------------------------------------------------------------
+
+  private rackOf(player: number): GenRack {
+    const letters: number[] = [];
+    let blanks = 0;
+    for (const t of this.hands[player]) {
+      if (t === BLANK_INDEX) blanks++;
+      else letters.push(t);
+    }
+    return GenRack.from(this.values.length, letters, blanks);
+  }
+
+  private finish(reason: EndReason): void {
+    this.over = true;
+    this.reason = reason;
+    const adjusted = applyEndAdjustment(reason, this.hands, this.scores, this.toMove, this.values);
+    for (let i = 0; i < adjusted.length; i++) this.scores[i] = adjusted[i];
+  }
+
+  private endTurnAfterScoreless(): void {
+    if (this.scorelessRun >= scorelessLimit) this.finish('scoreless');
+    else this.advance();
+  }
+
+  private advance(): void {
+    const n = this.hands.length;
+    for (let i = 1; i <= n; i++) {
+      const next = (this.toMove + i) % n;
+      if (!this.resigned[next]) {
+        this.toMove = next;
+        return;
+      }
+    }
+  }
+
+  private activeCount(): number {
+    return this.resigned.reduce((n, r) => (r ? n : n + 1), 0);
+  }
+
+  private disposeHand(player: number): void {
+    if (this.dropoutTiles === 'return') this.bag.return(this.hands[player]);
+    this.hands[player] = [];
+  }
+
+  private holds(player: number, want: readonly number[]): boolean {
+    const avail = new Map<number, number>();
+    for (const t of this.hands[player]) avail.set(t, (avail.get(t) ?? 0) + 1);
+    const need = new Map<number, number>();
+    for (const t of want) need.set(t, (need.get(t) ?? 0) + 1);
+    for (const [t, n] of need) if ((avail.get(t) ?? 0) < n) return false;
+    return true;
+  }
+
+  private removeFromHand(player: number, used: readonly number[]): void {
+    const hand = this.hands[player];
+    for (const t of used) {
+      const i = hand.indexOf(t);
+      if (i >= 0) hand.splice(i, 1);
+    }
+  }
+
+  private refill(player: number): void {
+    const need = this.rackSize - this.hands[player].length;
+    if (need > 0) for (const t of this.bag.draw(need)) this.hands[player].push(t);
+  }
+}

ui/src/lib/localgame/ruleset.parity.test.ts

@@ -0,0 +1,37 @@
+import { describe, it, expect } from 'vitest';
+import { readFileSync } from 'node:fs';
+import { RULESETS } from './ruleset';
+import type { Variant } from '../model';
+
+// The offline engine is self-contained: it carries each variant's tile values, bag counts and
+// blank count in ruleset.ts (these are not otherwise on the client — online scoring uses the
+// server-sent alphabet). This pins that hand-copied table to the Go rulesets
+// (scrabble-solver/rules), whose values the movegen tool dumps to testdata/rulesets.json.
+interface RulesetFix {
+  size: number;
+  rackSize: number;
+  bingo: number;
+  blanks: number;
+  values: number[];
+  counts: number[];
+}
+
+const fx = JSON.parse(
+  readFileSync(new URL('./testdata/rulesets.json', import.meta.url), 'utf8'),
+) as Record<Variant, RulesetFix>;
+
+describe('offline ruleset table parity vs Go rules', () => {
+  for (const variant of Object.keys(fx) as Variant[]) {
+    it(variant, () => {
+      const rs = RULESETS[variant];
+      expect({
+        size: rs.size,
+        rackSize: rs.rackSize,
+        bingo: rs.bingo,
+        blanks: rs.blanks,
+        values: [...rs.values],
+        counts: [...rs.counts],
+      }).toEqual(fx[variant]);
+    });
+  }
+});

ui/src/lib/localgame/ruleset.ts

@@ -0,0 +1,56 @@
+// Static per-variant ruleset data for the offline engine — tile point values, bag tile counts
+// and blank count, indexed by alphabet letter index. Mirrored from scrabble-solver/rules/rules.go
+// (English / RussianScrabble / Erudit) so a local game is fully self-contained: online scoring
+// reads the server-sent alphabet (lib/alphabet.ts), but offline there is no server, so the values
+// live here. Board geometry, the centre and RACK_SIZE/BINGO already live in lib/premiums.ts; this
+// table adds the values (offline copy), the bag distribution and the blank count. Pinned to the Go
+// rulesets by ruleset.parity.test.ts.
+
+import type { Variant } from '../model';
+
+/** VariantRuleset is the offline scoring + bag data for one variant. */
+export interface VariantRuleset {
+  /** Number of letters in the alphabet (bag distribution and values are indexed 0..size-1). */
+  size: number;
+  /** Tiles drawn to a full rack (7 for every variant). */
+  rackSize: number;
+  /** All-tiles (bingo) bonus. */
+  bingo: number;
+  /** Number of blank tiles in the bag. */
+  blanks: number;
+  /** Tile point value per letter index; a blank scores 0 (handled by the caller). */
+  values: readonly number[];
+  /** Bag tile count per letter index (how many of each letter the bag holds). */
+  counts: readonly number[];
+}
+
+/** RULESETS is the static ruleset table, one entry per variant, mirrored from rules.go. */
+export const RULESETS: Record<Variant, VariantRuleset> = {
+  scrabble_en: {
+    size: 26,
+    rackSize: 7,
+    bingo: 50,
+    blanks: 2,
+    //       a  b  c  d   e  f  g  h  i  j  k  l  m  n  o  p   q  r  s  t  u  v  w  x  y   z
+    values: [1, 3, 3, 2, 1, 4, 2, 4, 1, 8, 5, 1, 3, 1, 1, 3, 10, 1, 1, 1, 1, 4, 4, 8, 4, 10],
+    counts: [9, 2, 2, 4, 12, 2, 3, 2, 9, 1, 1, 4, 2, 6, 8, 2, 1, 6, 4, 6, 4, 2, 2, 1, 2, 1],
+  },
+  scrabble_ru: {
+    size: 33,
+    rackSize: 7,
+    bingo: 50,
+    blanks: 2,
+    //       а  б  в  г  д  е  ё  ж  з  и  й  к  л  м  н  о  п  р  с  т  у   ф  х  ц  ч  ш   щ   ъ  ы  ь  э  ю  я
+    values: [1, 3, 1, 3, 2, 1, 3, 5, 5, 1, 4, 2, 2, 2, 1, 1, 2, 1, 1, 1, 2, 10, 5, 5, 5, 8, 10, 10, 4, 3, 8, 8, 3],
+    counts: [8, 2, 4, 2, 4, 8, 1, 1, 2, 5, 1, 4, 4, 3, 5, 10, 4, 5, 5, 5, 4, 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 2],
+  },
+  erudit_ru: {
+    size: 33,
+    rackSize: 7,
+    bingo: 15,
+    blanks: 3,
+    //       а   б  в  г  д  е  ё  ж  з  и  й  к  л  м  н  о  п  р  с  т  у   ф  х   ц  ч   ш   щ   ъ   ы  ь   э   ю  я
+    values: [1, 3, 2, 3, 2, 1, 0, 5, 5, 1, 2, 2, 2, 2, 1, 1, 2, 2, 2, 2, 3, 10, 5, 10, 5, 10, 10, 10, 5, 5, 10, 10, 3],
+    counts: [10, 3, 5, 3, 5, 9, 0, 2, 2, 8, 4, 6, 4, 5, 8, 10, 6, 6, 6, 5, 3, 1, 2, 1, 2, 1, 1, 1, 2, 2, 1, 1, 3],
+  },
+};

ui/src/lib/localgame/testdata/endgame.json

@@ -0,0 +1,246 @@
+{
+  "cases": [
+    {
+      "name": "out-basic",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          0,
+          1,
+          2
+        ]
+      ],
+      "scores": [
+        50,
+        40
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        57,
+        33
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-blank",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          255,
+          0
+        ]
+      ],
+      "scores": [
+        30,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        31,
+        29
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-erudit-yo",
+      "variant": "erudit_ru",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          6,
+          32
+        ]
+      ],
+      "scores": [
+        10,
+        10
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        13,
+        7
+      ],
+      "winner": 0
+    },
+    {
+      "name": "out-tie",
+      "variant": "scrabble_en",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        []
+      ],
+      "scores": [
+        30,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        30,
+        30
+      ],
+      "winner": -1
+    },
+    {
+      "name": "out-3p",
+      "variant": "scrabble_ru",
+      "reason": "out_of_tiles",
+      "hands": [
+        [],
+        [
+          0,
+          1
+        ],
+        [
+          2
+        ]
+      ],
+      "scores": [
+        10,
+        10,
+        10
+      ],
+      "resigned": [
+        false,
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        15,
+        6,
+        9
+      ],
+      "winner": 0
+    },
+    {
+      "name": "scoreless",
+      "variant": "scrabble_en",
+      "reason": "scoreless",
+      "hands": [
+        [
+          0,
+          1
+        ],
+        [
+          2,
+          3
+        ]
+      ],
+      "scores": [
+        20,
+        20
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        16,
+        15
+      ],
+      "winner": 0
+    },
+    {
+      "name": "resign-2p",
+      "variant": "scrabble_en",
+      "reason": "resign",
+      "hands": [
+        [],
+        [
+          0
+        ]
+      ],
+      "scores": [
+        100,
+        10
+      ],
+      "resigned": [
+        true,
+        false
+      ],
+      "toMove": 1,
+      "scoresAfter": [
+        100,
+        10
+      ],
+      "winner": 1
+    },
+    {
+      "name": "resign-3p",
+      "variant": "scrabble_en",
+      "reason": "resign",
+      "hands": [
+        [],
+        [],
+        []
+      ],
+      "scores": [
+        50,
+        60,
+        40
+      ],
+      "resigned": [
+        false,
+        true,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        50,
+        60,
+        40
+      ],
+      "winner": 0
+    },
+    {
+      "name": "aborted",
+      "variant": "scrabble_en",
+      "reason": "aborted",
+      "hands": [
+        [
+          0
+        ],
+        [
+          1
+        ]
+      ],
+      "scores": [
+        40,
+        30
+      ],
+      "resigned": [
+        false,
+        false
+      ],
+      "toMove": 0,
+      "scoresAfter": [
+        40,
+        30
+      ],
+      "winner": -1
+    }
+  ]
+}

ui/src/lib/localgame/testdata/rulesets.json

@@ -0,0 +1,216 @@
+{
+  "erudit_ru": {
+    "size": 33,
+    "rackSize": 7,
+    "bingo": 15,
+    "blanks": 3,
+    "values": [
+      1,
+      3,
+      2,
+      3,
+      2,
+      1,
+      0,
+      5,
+      5,
+      1,
+      2,
+      2,
+      2,
+      2,
+      1,
+      1,
+      2,
+      2,
+      2,
+      2,
+      3,
+      10,
+      5,
+      10,
+      5,
+      10,
+      10,
+      10,
+      5,
+      5,
+      10,
+      10,
+      3
+    ],
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