Chess Engine in C

Build a clean, fast chess engine in C that starts from a FEN string and produces fully legal moves, verified via PERFT for correctness and speed.

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Approach

• Correctness-first engine: FEN → board → legal movegen → PERFT

FEN (Forsyth–Edwards Notation)

FEN is a single line string that describes a chess position. Format:

<pieces> <side> <castling> <en-passant> <halfmove> <fullmove>

• pieces: 8 ranks (8→1) separated by /; letters for pieces (PNBRQK/pnbrqk), digits for empty squares.
• side: w (White) or b (Black) to move.
• castling: any of KQkq or - if none.
• en-passant: target square after a two-step pawn move, or -.
• halfmove: moves since last pawn move/capture (50-move rule).
• fullmove: move number (starts at 1, increments after Black’s move).

Example (starting position)

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

• pieces: starting layout
• side: w (White to move)
• castling: KQkq (both sides can castle both ways)
• en-passant: -
• halfmove: 0
• fullmove: 1

Perft Testing A Chess Engine

Perft (“performance test”) walks the move tree without move evaluation and counts nodes at certain depths in the tree. It’s a standard way to verify an engine's move generation + make/unmake (or make-on-copy) are correct. It's important to understand that the node counts have been generated and verified by the chess programming community, meaning that these counts are a reliable reference point for a chess engine to match against.

What perft checks

• Legality filtering: no moves that leave your own king in check.
• Special rules: castling path/rights, en passant (incl. pinned-EP edge case), promotions.
• State integrity: side-to-move flips; occupancy stays consistent.

Start-position reference counts

rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1

• d1 = 20
• d2 = 400
• d3 = 8,902
• d4 = 197,281
• d5 = 4,865,609
• d6 = 119,060,324

Other Positions

The chess programming wiki provides several other starting positions to test. They can be found here.

PERFT Results & test positions: https://www.chessprogramming.org/Perft_Results

Future Work

• Incorporate algorithms and evaluation methods for chess positions to enable an AI player.

• Build an API to facilitate chess games with the AI player over the web.

  • Starting with a commandline client.
  • Later building a web based client.

Chess Engine Programming & Information

• Chess Programming Wiki (CPW): https://www.chessprogramming.org/
• PERFT (definition & methodology): https://www.chessprogramming.org/Perft
• PERFT Results & test positions: https://www.chessprogramming.org/Perft_Results
• FEN reference: https://www.chessprogramming.org/Forsyth-Edwards_Notation
• Bitboards https://www.chessprogramming.org/Bitboards