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Marlin uses a command-line interface similar to UCI (Universal Chess Interface) protocol. The engine reads commands from stdin and responds to stdout.

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Command Reference

CommandDescriptionExample
position [moves]Set up position by playing movesposition 4433
displayShow the boarddisplay
goFind the best movego
helpShow commandshelp
quitExitquit

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position

Sets up a board position by playing a sequence of moves from an empty board. Syntax: 
position [moves]
Parameters:
  • moves (optional): A string of digits 1-7 representing column numbers. If omitted, resets to an empty board.
Examples: 
> position
Position reset to empty board
How it works (source/src/main.cpp:71-84):
  1. Resets the board to empty
  2. Parses each character as a column number (1-7)
  3. Validates the move is legal
  4. Applies moves sequentially, alternating players

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display

Shows the current board state with pieces and column numbers. Syntax: 
display
Alias: d Example output: 
> position 4433
Played 4 moves

> display
| . . . . . . . |
| . . . . . . . |
| . . . . . . . |
| . . . . . . . |
| . . O O . . . |
| . . X X . . . |
+---------------+
  1 2 3 4 5 6 7
Legend:
  • X = Current player’s pieces
  • O = Opponent’s pieces
  • . = Empty cell
  • Numbers at bottom = Column numbers (1-7)

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go

Analyzes the current position and finds the best move using the Negamax solver with alpha-beta pruning. Syntax: 
go
Example output: 
> position 4433
Played 4 moves

> go
Analyzing...
  Column 1: score -18
  Column 2: score 16
  Column 3: score 18
  Column 4: score -20
  Column 5: score 16
  Column 6: score -18
  Column 7: score -18
bestmove 3 score 18 (WIN)
Nodes analyzed: 12847
Output format:
  • Analysis lines: Shows the score for each legal move
  • bestmove: The recommended column to play (1-7)
  • score: The game-theoretic value (see Understanding Scores)
  • result: WIN, LOSE, or DRAW
  • Nodes analyzed: Number of positions evaluated
Special cases: 
> go
bestmove 4 score WIN (immediate)
The solver checks for immediate winning moves first (source/src/main.cpp:93-98), providing instant results without deep analysis.

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help

Displays a summary of available commands. Syntax: 
help
Output: 
> help
Commands:
  position [moves]  - Set position (e.g., 'position 4433')
  display           - Show the board
  go                - Find best move
  quit              - Exit

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quit

Exits the Marlin engine. Syntax: 
quit
Alias: exit Example: 
> quit
Goodbye!

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Error Handling

Marlin provides clear error messages for invalid input: 
> analyze
Unknown command: analyze (type 'help' for commands)

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Implementation Details

The command loop is implemented in source/src/main.cpp:145-199:
  1. Reads lines from stdin
  2. Parses command and arguments
  3. Dispatches to handler functions
  4. Continues until quit or EOF
Empty lines are ignored (source/src/main.cpp:190-192).

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