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Chessops supports 8 different chess variants, each with unique rules and gameplay mechanics. All variants extend the base Position class and provide specialized implementations for their specific rules.

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Supported Variants

Crazyhouse

Drop captured pieces back on the board

Atomic

Captures cause explosions that destroy surrounding pieces

Antichess

Lose all your pieces to win, captures are forced

King of the Hill

Get your king to the center to win

Three-check

Give check three times to win

Horde

White has 36 pawns vs Black’s normal army

Racing Kings

Race your king to the 8th rank, no checks allowed

Standard Chess

Classic chess rules (Chess class)

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Common API

All variant classes share a common interface:

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Creating Positions

import { Crazyhouse, Atomic, KingOfTheHill } from 'chessops/variant';

// Create default starting position
const crazyhouse = Crazyhouse.default();
const atomic = Atomic.default();
const koth = KingOfTheHill.default();

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Creating from Setup

import { Atomic } from 'chessops/variant';
import { parseFen } from 'chessops/fen';

const setup = parseFen('rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1').unwrap();
const result = Atomic.fromSetup(setup);

if (result.isOk) {
  const pos = result.value;
  // Use position
} else {
  console.error('Invalid setup:', result.error);
}

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Playing Moves

const pos = Crazyhouse.default();

// Make a move
pos.play({ from: 12, to: 28 }); // e2-e4

// Clone before playing (immutable pattern)
const newPos = pos.clone();
newPos.play({ from: 52, to: 36 }); // e7-e5

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Checking Game State

// Check if variant-specific win condition is met
if (pos.isVariantEnd()) {
  const outcome = pos.variantOutcome();
  if (outcome?.winner) {
    console.log(`${outcome.winner} wins!`);
  } else {
    console.log('Draw');
  }
}

// Standard checks work too
if (pos.isCheckmate()) {
  console.log('Checkmate!');
}

if (pos.isStalemate()) {
  console.log('Stalemate!');
}

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Variant Selection

Use the defaultPosition helper to create positions by variant name: 
import { defaultPosition } from 'chessops/variant';
import { Rules } from 'chessops/types';

const variant: Rules = 'atomic';
const pos = defaultPosition(variant);

// Works for all variants
const variants: Rules[] = [
  'chess',
  'crazyhouse',
  'atomic',
  'antichess',
  'kingofthehill',
  '3check',
  'horde',
  'racingkings'
];

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Validation

Each variant has custom validation rules: 
import { Atomic } from 'chessops/variant';
import { Result } from '@badrap/result';

const result = Atomic.fromSetup(setup);

if (result.isErr) {
  // Handle PositionError
  const error = result.error;
  console.log(error.message);
}
Variant-specific rules are enforced during setup validation and move generation.

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Next Steps

Crazyhouse

Learn about pockets and drop moves

Atomic Chess

Understand explosion mechanics

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