Connect Four
July 20, 2026 · View on GitHub
A Rust port of John Tromp's Fhourstones Connect Four solver + a terminal UI for interactive play.
Connect-four [4] is a two player game, and the TUI allows the players to be drawn from:
- Human
- Perfect (the solver) [2]
- Minimax [5]
- Mcts (Monte Carlo Tree Search) [6]
Verification
Checked against the original C (v3.2) and Java (v3.1) reference implementations, which agree with each other exactly:
| Position (moves played) | Score | Nodes searched |
|---|---|---|
| 45461667 | Win | 51,596 |
| 35333571 | Loss | 8,716,732 |
| 13333111 | Draw | 169,704,432 |
| (empty board, full 42-ply solve) | Win | 1,479,113,766 |
Run the exact-node-count regression tests with:
cargo test --release --lib
Usage: solve (benchmark / analysis)
Reads one position per line from stdin, as a string of column digits (1–7, left to right); other characters are ignored, and an empty line solves the starting position. This matches the original Fhourstones CLI input format, so existing test/benchmark input files work unchanged.
echo "4453" | cargo run --release --bin solve
For each position it reports the game-theoretic score, log2 of positions stored in the transposition table ("work"), total nodes searched, search speed, and a histogram of stored transposition-table entries by score.
Usage: play (interactive)
The play terminal app allows two players to play interactively.
The flags --player1 and --player2 configures who plays each side.
Choices are human, perfect (the solver), minimax (alpha-beta negamax [5]) and mcts (Monte Carlo Tree Search [6]).
cargo run --release --bin play # Human (red) vs perfect-play AI (yellow)
cargo run --release --bin play -- --player1 human --player2 human # two humans
cargo run --release --bin play -- --player1 perfect --player2 perfect # watch the perfect solver play itself
cargo run --release --bin play -- --player1 minimax --depth 6 --player2 mcts --mcts-millis 1000
cargo run --release --bin play -- --moves 4453 --player2 perfect # start from a given position