joke-app/e2e/chess960-plan.ts

/**
 * Chess960 E2E helper: for a start number, a short legal line in which white clears its back rank and castles
 * (whichever side is ready first) while black only shuffles a knight between its start square and the 6th rank.
 * Black never attacks white's back rank that way, and white only develops to ranks 2–3 without capturing, so the
 * line exists for every one of the 960 positions (checked for all of them when this was written).
 */
import { Position, chess960Fen, type Move } from '../packages/games/src/chess/movegen';

const PAWN = 1;
const KNIGHT = 2;

export interface Ply {
  /** UCI as the server records it (Chess960 castling: king square + rook square). */
  uci: string;
  san: string;
  castle: boolean;
}

/** Squares (0x88) that must be empty, except for the king and rook themselves, for white castling on one side. */
function needEmpty(p: Position, i: 0 | 1): number[] {
  const k = p.king[0]!;
  const r = p.castle[i]!;
  if (r < 0) return [];
  const out = new Set<number>();
  for (const [a, z] of [
    [k, i === 0 ? 6 : 2],
    [r, i === 0 ? 5 : 3],
  ] as const) {
    for (let s = Math.min(a, z); s <= Math.max(a, z); s++) if (s !== k && s !== r) out.add(s);
  }
  return [...out];
}

function blockers(p: Position, i: 0 | 1): number {
  return needEmpty(p, i).filter((s) => p.b[s]).length;
}

/** Legal moves of the side *not* to move (null move), for "can this blocker leave next turn?". */
function nextTurnMoves(p: Position): Move[] {
  const ep = p.ep;
  p.ep = -1;
  p.side ^= 1;
  const out = p.moves();
  p.side ^= 1;
  p.ep = ep;
  return out;
}

export function plan960(index: number, maxPlies = 40): Ply[] {
  const p = Position.fromFen(chess960Fen(index));
  const plies: Ply[] = [];
  const push = (m: Move, legal: Move[]) => {
    const done = p.play(m, legal);
    plies.push({ uci: done.uci, san: done.san, castle: !!done.castle });
  };
  let knightHome = -1;
  while (plies.length < maxPlies) {
    const legal = p.moves();
    if (p.side === 1) {
      // Black: one knight out to the 6th rank and back.
      const out = knightHome < 0;
      const m = out
        ? legal.find((x) => x.piece === KNIGHT && x.from >> 4 === 7 && x.to >> 4 === 5 && !x.captured)!
        : legal.find((x) => x.piece === KNIGHT && x.to === knightHome)!;
      knightHome = out ? m.from : -1;
      push(m, legal);
      continue;
    }
    const castle = legal.find((x) => x.rook >= 0);
    if (castle) {
      push(castle, legal);
      return plies;
    }
    // White: the quiet move that best frees either castling side (fewest blockers left, then most blockers able to
    // leave). Only pawns step forward (to rank 3) and blockers step out of the way (to ranks 1–3, never into either
    // side's squares), so nothing can repeat and the line always makes progress.
    const zones = ([0, 1] as const).filter((i) => p.castle[i]! >= 0).map((i) => ({ i, zone: new Set(needEmpty(p, i)) }));
    const anyZone = (sq: number) => zones.some((z) => z.zone.has(sq));
    let best: Move | null = null;
    let bestScore = Infinity;
    for (const m of legal) {
      if (m.captured || m.to >> 4 > 2 || anyZone(m.to) || !(anyZone(m.from) || m.piece === PAWN)) continue;
      p.make(m);
      const next = nextTurnMoves(p);
      for (const { i, zone } of zones) {
        if (p.castle[i]! < 0) continue;
        const leavers = new Set(next.filter((x) => zone.has(x.from) && !anyZone(x.to) && x.to >> 4 <= 2 && !x.captured).map((x) => x.from)).size;
        const score = blockers(p, i) * 1000 - leavers * 10 + (m.to >> 4) + m.to / 1000;
        if (score < bestScore) {
          bestScore = score;
          best = m;
        }
      }
      p.unmake();
    }
    if (!best) throw new Error(`960 #${index}: no developing move after ${plies.map((x) => x.san).join(' ')}`);
    push(best, legal);
  }
  throw new Error(`960 #${index}: no castling within ${maxPlies} plies`);
}