# 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.

## 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>