a chess engine, built step by step

New to chess? Start here

Josh is teaching a computer to play chess. He isn't copying anyone else's chess program: he is building his own, called REMY, one small step at a time. Each step makes it a little stronger, and he keeps score. This page explains what that means if you know nothing about chess.

Chess in a minute

Chess is a board game for two players, White and Black. Each starts with an army of pieces, and each kind of piece moves in its own way. The players take turns moving one piece at a time. You win by trapping the other player's king so that it can't escape: that's called checkmate. A game can also end in a draw, when neither side can win.

How a computer plays chess

A program that plays chess is called a chess engine. It plays by imagining the future: if I move here, they might move there, and then I could do this. It tries a huge number of these possibilities, judges who would be ahead at the end of each one, and picks the move that turns out best. The hard parts are knowing which possibilities are worth a look, and judging a position well.

The best chess engines are far stronger than any human. The strongest of all is called Stockfish. Beating it is REMY's long-term goal.

How REMY grows

Josh adds one idea at a time. Each new version of REMY has to play a long match against the version before it, and it only takes over if it clearly wins. Here is every version so far.

  1. REMY 0. Knew only the rules. It picked any allowed move at random, like someone who has just learned how the pieces move.
  2. REMY 1. Learned that some pieces are worth more than others, and grabbed whatever it could take. REMY 1 played REMY 0 and won 991 games, drew 9 and lost 0.
  3. REMY 2. Learned to stop and think about what the other side could do back before grabbing something. REMY 2 played REMY 1 and won 997 games, drew 0 and lost 3.
  4. REMY 3. Learned to think one more move ahead. REMY 3 played REMY 2 and won 969 games, drew 11 and lost 20.
  5. REMY 4. Learned to use its thinking time: it keeps thinking further ahead until its time for the move is up. REMY 4 played REMY 3 and won 98 games, drew 2 and lost 6.
  6. REMY 5. Learned to think a trade of pieces all the way through before deciding who is ahead. REMY 5 played REMY 4 and won 96 games, drew 5 and lost 5.
  7. REMY 6. Learned to remember positions it has already thought about, so it doesn't waste time on them twice. REMY 6 played REMY 5 and won 99 games, drew 6 and lost 1.
  8. REMY 7. Learned, from its own games, which squares suit each piece best. REMY 7 played REMY 6 and won 88 games, drew 14 and lost 4.
  9. REMY 8. Learned to spend its thinking time better: it gives less attention to moves that look weak and more to dangerous ones, so it can see further ahead. REMY 8 played REMY 7 and won 80 games, drew 20 and lost 6.
  10. REMY 8.1. Kept the same way of looking ahead as the version before, and finished learning which squares suit each piece, after three rounds of studying its own games. REMY 8.1 played REMY 8 and won 61 games, drew 44 and lost 17.
  11. REMY 9. Learned two new habits in its thinking: it checks first whether a trade of pieces gains or loses before looking at it closely, and it focuses on the moves that fit what it expects. It also thinks the same thoughts faster, so it sees further in the same time. REMY 9 played REMY 8.1 and won 47 games, drew 51 and lost 24.

How good is it?

Chess players and chess programs get a rating: a number that goes up when you beat strong opponents. The higher the number, the stronger the player. To rate REMY, Josh has it play other chess programs whose ratings are already known, on the CCRL list, a well-known list of chess-program ratings.

The first version to be rated, REMY 5, came in at 883. REMY 9 is at 2213. These are estimates from REMY's own games, not an official CCRL rating.

REMY 5 883
REMY 6 1079
REMY 7 1609
REMY 8 1865
REMY 9 2213
Stockfish 3793

Stockfish is rated 3793 on the same list. That gap is the road ahead.

Its biggest upset so far: REMY 6 beat Shallow Blue 2.0.0, a program rated 499 points higher. The game lasted 114 moves. Watch it move by move.

The goal

The long-term goal is to beat Stockfish at quick games, where each player has two minutes for the whole game plus one second for every move, on an ordinary home computer. There are milestones on the way: 1500, 2000 and 2500.

1500: passed by REMY 7. 2000: passed by REMY 9. Next up: 2500.

Want to play against REMY?

Play REMY. You can play a game against REMY on Lichess, a free chess website. It takes a free Lichess account and one click.

Challenge REMY on Lichess How it works

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