Chess Engine + AI

Overview

A ground-up chess engine and AI opponent in modern C++17. The engine handles every rule — castling, en passant, pawn promotion, check/checkmate/stalemate — with an efficient board representation and legal move generation system.

The AI uses minimax search with alpha-beta pruning to explore game trees, extended by quiescence search to resolve capture sequences and avoid tactical blunders. A killer move heuristic speeds up pruning by remembering moves that caused cutoffs. Positional evaluation includes king safety, pawn structure analysis, bishop pair bonuses, and piece-square tables.

The SDL2 graphical interface features click-to-move interaction, move highlighting, an undo button, a promotion dialog, a live scoreboard tracking wins/losses/draws across games, and a status panel showing real-time game messages. The entire application is compiled to WebAssembly via Emscripten, making it playable directly in the browser with no install required.

Key Features

Challenges & Solutions

• Horizon Effect: The AI would misjudge positions mid-capture-sequence, hanging pieces. Solved with quiescence search that continues evaluating captures past the depth limit until the position is "quiet."
• Search Performance: Depth 5 search with naive ordering was too slow. Implemented killer move heuristic and MVV-LVA capture ordering to maximize alpha-beta cutoffs, making deeper searches feasible.
• Evaluation Balance: Pure material counting led to passive play. Added king safety scoring (pawn shield, attacker counting), pawn structure penalties (doubled/isolated pawns), and passed pawn bonuses for more strategic play.
• State Restoration: The undo system must perfectly restore all game state — not just piece positions, but castling rights, en passant target, and half-move clock. Used a state snapshot stack.
• GUI Responsiveness: The AI blocks the main thread while searching. Rendered a "CPU is thinking..." frame before the search to keep the UI informative during computation.
• WebAssembly Port: SDL2's blocking main loop doesn't work in the browser. Refactored into a single-frame callback using emscripten_set_main_loop_arg, bundled assets with --preload-file, and replaced system fonts with embedded TTF files.

What I Learned

• Practical understanding of game tree search algorithms — minimax, alpha-beta pruning, and quiescence search, and how move ordering dramatically affects performance.
• Designing heuristic evaluation functions that balance multiple competing factors (material, position, king safety, pawn structure) into a single score.
• Building a real-time GUI application with SDL2 — event-driven rendering, texture management, and managing blocking computation in a render loop.
• Deep understanding of OOP design and efficient data structure design for board representation and move encoding.
• Building a multi-target CMake project with proper header organization, compilation units, and external library integration via vcpkg.
• Porting a native C++ application to WebAssembly with Emscripten — adapting the main loop, bundling assets for a virtual filesystem, and handling platform differences between desktop and browser.

Tech Stack