BitBully — Connect-4 Engine

BitBully is a high-performance perfect-play Connect-4 solver written in modern C++. It reaches every win/loss/draw verdict for any position by combining a bitboard representation, a hand-tuned alpha-beta negamax, a transposition table and pre-computed opening books.

This documentation covers the C++ core of the project.

• For Python usage and tutorials see the main BitBully docs.
• For the source repository, visit the BitBully GitHub project.

BitBully

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Library at a glance

Component	Header	Purpose
BitBully::Board	Board.h	Bitboard position with legal-move generation
BitBully::BitBully	BitBully.h	Negamax + MTD(f) solver driver
BitBully::TranspositionTable	TranspositionTable.h	Direct-mapped score cache
BitBully::OpeningBook	OpeningBook.h	8-/12-ply opening database loader
BitBully::MoveList	MoveList.h	Tiny priority queue used for move ordering

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Quick example

#include "BitBully.h"
 
int main() {
  BitBully::Board b;          // empty starting position
  if (!b.play("4435")) {       // play a few moves (column indices)
    return 1;
  }
  std::cout << b.toString() << '\n';
 
  BitBully::BitBully solver;   // create the search engine
  const int score = solver.mtdf(b, /*firstGuess=*/0);
  std::cout << "score = " << score << '\n';
  return 0;
}

The score follows the convention introduced by Pascal Pons: positive means the side to move wins, negative means it loses, and 0 is a draw. Use BitBully::BitBully::scoreToMovesLeft to translate the compact score into the actual number of plies remaining.

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Where to go next

• Architecture — high-level overview of the engine
• Building and Testing the C++ Core — how to compile and test the C++ core
• References — papers, blog posts, and prior art that shaped the implementation