fertig-classic-games/public/src/games/backgammon/BackgammonLogic.js

// Pure game logic — no Phaser dependency.
// Convention: point indices 0–23 (0 = point 1, White's home ace-point).
// White moves high→low (23→0), bears off past index 0.
// Black moves low→high (0→23), bears off past index 23.
// White home board: indices 0–5. Black home board: indices 18–23.

export function createInitialState() {
  const points = Array.from({ length: 24 }, () => ({ color: null, count: 0 }));
  points[23] = { color: 'white', count: 2 };
  points[12] = { color: 'white', count: 5 };
  points[7]  = { color: 'white', count: 3 };
  points[5]  = { color: 'white', count: 5 };
  points[0]  = { color: 'black', count: 2 };
  points[11] = { color: 'black', count: 5 };
  points[16] = { color: 'black', count: 3 };
  points[18] = { color: 'black', count: 5 };
  return {
    points,
    bar: { white: 0, black: 0 },
    borneOff: { white: 0, black: 0 },
    dice: null,
    movesLeft: [],
    currentPlayer: 'white',
    phase: 'roll',
    winner: null,
  };
}

export function cloneState(state) {
  return JSON.parse(JSON.stringify(state));
}

export function rollDice(state) {
  const d1 = Math.ceil(Math.random() * 6);
  const d2 = Math.ceil(Math.random() * 6);
  return rollSpecificDice(state, d1, d2);
}

export function rollSpecificDice(state, d1, d2) {
  const s = cloneState(state);
  s.dice = [d1, d2];
  s.movesLeft = d1 === d2 ? [d1, d1, d1, d1] : [d1, d2];
  s.phase = 'move';
  return s;
}

export function endTurn(state) {
  const s = cloneState(state);
  s.currentPlayer = s.currentPlayer === 'white' ? 'black' : 'white';
  s.dice = null;
  s.movesLeft = [];
  s.phase = 'roll';
  return s;
}

export function allCheckersInHome(state, player) {
  if (state.bar[player] > 0) return false;
  const [homeMin, homeMax] = player === 'white' ? [0, 5] : [18, 23];
  for (let i = 0; i < 24; i++) {
    if (i < homeMin || i > homeMax) {
      if (state.points[i].color === player && state.points[i].count > 0) return false;
    }
  }
  return true;
}

// Furthest checker from the bear-off edge (by index):
// White: highest index in 0–5 (index 5 = 6 pips away)
// Black: lowest index in 18–23 (index 18 = 6 pips away)
function furthestHomeChecker(state, player) {
  const { points } = state;
  if (player === 'white') {
    for (let i = 5; i >= 0; i--) {
      if (points[i].color === 'white' && points[i].count > 0) return i;
    }
  } else {
    for (let i = 18; i <= 23; i++) {
      if (points[i].color === 'black' && points[i].count > 0) return i;
    }
  }
  return -1;
}

export function getValidMoves(state) {
  const { currentPlayer: player, movesLeft, points, bar } = state;
  const opp = player === 'white' ? 'black' : 'white';
  const uniqueDice = [...new Set(movesLeft)];
  const inBearOff = allCheckersInHome(state, player);
  const moves = [];

  const isBlocked = (idx) => points[idx]?.color === opp && points[idx].count >= 2;
  const isHit     = (idx) => points[idx]?.color === opp && points[idx].count === 1;

  // Bar entry takes priority — only these moves if bar has checkers
  if (bar[player] > 0) {
    for (const die of uniqueDice) {
      // White enters opposite side: die 1 → index 23, die 6 → index 18
      // Black enters near side:     die 1 → index 0,  die 6 → index 5
      const entryIdx = player === 'white' ? (24 - die) : (die - 1);
      if (!isBlocked(entryIdx)) {
        moves.push({ from: 'bar', to: entryIdx, dieUsed: die, hit: isHit(entryIdx) });
      }
    }
    return dedup(moves);
  }

  for (const die of uniqueDice) {
    for (let i = 0; i < 24; i++) {
      if (points[i].color !== player || points[i].count === 0) continue;

      if (player === 'white') {
        const dest = i - die;
        if (dest >= 0) {
          if (!isBlocked(dest)) moves.push({ from: i, to: dest, dieUsed: die, hit: isHit(dest) });
        } else if (inBearOff) {
          if (dest === -1) {
            // Exact bear-off (die == i + 1)
            moves.push({ from: i, to: 'off', dieUsed: die, hit: false });
          } else {
            // Overshoot — only the furthest checker may be borne off
            if (furthestHomeChecker(state, 'white') === i) {
              moves.push({ from: i, to: 'off', dieUsed: die, hit: false });
            }
          }
        }
      } else {
        const dest = i + die;
        if (dest <= 23) {
          if (!isBlocked(dest)) moves.push({ from: i, to: dest, dieUsed: die, hit: isHit(dest) });
        } else if (inBearOff) {
          if (dest === 24) {
            // Exact bear-off (die == 24 - i)
            moves.push({ from: i, to: 'off', dieUsed: die, hit: false });
          } else {
            // Overshoot
            if (furthestHomeChecker(state, 'black') === i) {
              moves.push({ from: i, to: 'off', dieUsed: die, hit: false });
            }
          }
        }
      }
    }
  }

  return dedup(moves);
}

function dedup(moves) {
  const seen = new Set();
  return moves.filter((m) => {
    const key = `${m.from}|${m.to}|${m.dieUsed}`;
    if (seen.has(key)) return false;
    seen.add(key);
    return true;
  });
}

export function applyMove(state, move) {
  let s = cloneState(state);
  const player = s.currentPlayer;
  const opp = player === 'white' ? 'black' : 'white';

  // Remove from source
  if (move.from === 'bar') {
    s.bar[player] = Math.max(0, s.bar[player] - 1);
  } else {
    s.points[move.from].count--;
    if (s.points[move.from].count === 0) s.points[move.from].color = null;
  }

  // Handle hit — send opponent blot to bar
  if (move.hit && move.to !== 'off') {
    s.points[move.to].count = 0;
    s.points[move.to].color = null;
    s.bar[opp]++;
  }

  // Place on destination
  if (move.to === 'off') {
    s.borneOff[player]++;
  } else {
    s.points[move.to].color = player;
    s.points[move.to].count++;
  }

  // Consume the die
  const dieIdx = s.movesLeft.indexOf(move.dieUsed);
  if (dieIdx !== -1) s.movesLeft.splice(dieIdx, 1);

  // Win check
  if (s.borneOff[player] >= 15) {
    s.winner = player;
    s.phase = 'game_over';
    return s;
  }

  // Auto-end turn if no dice remain or no legal moves remain
  if (s.movesLeft.length === 0 || getValidMoves(s).length === 0) {
    s = endTurn(s);
  }

  return s;
}

export function hasAnyMove(state) {
  return getValidMoves(state).length > 0;
}

export function computePipCount(state, player) {
  let pips = 0;
  for (let i = 0; i < 24; i++) {
    const pt = state.points[i];
    if (pt.color !== player || pt.count === 0) continue;
    const dist = player === 'white' ? (i + 1) : (24 - i);
    pips += dist * pt.count;
  }
  pips += state.bar[player] * 25;
  return pips;
}