Copenhagan Hnefatafl

An engine similar to the Go Text Protocol.

Differences from the Go Text Protocol

• The character set is UTF-8.
• clear_board -> reset_board
• genmove -> generate_move
• generate_move returns COLOR VERTEX_FROM VERTEX_TO
• We keep track of whose turn it is.
• play takes play COLOR VERTEX_FROM VERTEX_TO or play COLOR resigns and returns = CAPTURES, where CAPTURES has the format a2 c2 .... The color is whose turn it is.
• showboard -> show_board
• time_settings none | time_settings fischer MINUTES ADD_SECONDS_AFTER_EACH_MOVE
• final_status_list -> final_status = black_wins | draw | ongoing | white_wins

Rules

From Copenhagen Hnefatafl with minor changes.

The Copenhagen rules were outlined 2012 by Aage Nielsen (Denmark), Adam Bartley (Norway) and Tim Millar (UK). English text and diagrams: Adam Bartley (Norway)

Based on the version that was last updated 21.12.2024.

Copyright © 1998-2025 Aage Nielsen, All Rights Reserved;

X: attacker
x: captured attacker
K: king
k: captured king
O: defender
o: captured defender
R: restricted space

1. Starting Board Position

  ┌───────────┐
11│R..XXXXX..R│
10│.....X.....│
 9│...........│
 8│X....O....X│
 7│X...OOO...X│
 6│XX.OOKOO.XX│
 5│X...OOO...X│
 4│X....O....X│
 3│...........│
 2│.....X.....│
 1│R..XXXXX..R│
  └───────────┘
   ABCDEFGHJKL

2. First Turn

The attackers (black) move first.

3. Movement

You can move to the edge of the board or another piece orthogonally:

  ┌───────────┐
11│R.........R│
10│...........│
 9│...........│
 8│...........│
 7│...........│
 6│.....R.....│
 5│.....↑.....│
 4│....←O→....│
 3│.....↓.....│
 2│...........│
 1│R.........R│
  └───────────┘
   ABCDEFGHJKL

4. Capture

All pieces except the king are captured if sandwiched between two enemy pieces, or between an enemy piece and a restricted square. A piece is only captured if the trap is closed by the aggressor's move, it is therefore permitted to move in between two enemy pieces. The king may take part in captures.

Captures

  ┌───────────┐ ┌───────────┐ ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│ │R.........R│ │Ro........R│
10│...........│ │...........│ │...........│ │..↑........│
 9│...........│ │...........│ │...........│ │..X........│
 8│...........│ │...........│ │...........│ │...........│
 7│...........│ │...........│ │...........│ │...........│
 6│.....R.....│ │...X.R.....│ │..K..R.....│ │.....R.....│
 5│.....o.....│ │...o.......│ │..↓........│ │...........│
 4│.X→→→......│ │.Xo.oX.....│ │...........│ │...........│
 3│...........│ │...↑.......│ │..x........│ │...........│
 2│...........│ │...X.......│ │..O........│ │...........│
 1│R.........R│ │R.........R│ │R.........R│ │R.........R│
  └───────────┘ └───────────┘ └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL   ABCDEFGHJKL   ABCDEFGHJKL

Doesn't Capture

  ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│
10│...........│ │...........│
 9│...........│ │...........│
 8│...........│ │...........│
 7│...........│ │...........│
 6│.....K.....│ │.....R.....│
 5│.....O.....│ │.O.O.......│
 4│.X→→→......│ │..↑........│
 3│...........│ │..X........│
 2│...........│ │...........│
 1│R.........R│ │R.........R│
  └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL

Shield Wall

A row of two or more taflmen along the board edge may be captured together, by bracketing the whole group at both ends, as long as every member of the row has an enemy taflman directly in front of him.

A corner square may stand in for one of the bracketing pieces at one end of the row. The king may take part in the capture, either as part of the shield wall or as a bracketing piece. If the king plus one or more defenders are attacked with a shield wall, the attack will capture the defenders but not the king.

  ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│
10│...........│ │...........│
 9│...........│ │...........│
 8│...........│ │...........│
 7│...........│ │...........│
 6│.....R.....│ │.....R.....│
 5│...........│ │...........│
 4│...........│ │...........│
 3│..O........│ │...........│
 2│..↓OOO.....│ │........XX.│
 1│R..xxxO...R│ │R....X→.KoR│
  └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL

5. Restricted Squares

Restricted squares may only be occupied by the king. The central restricted square is called the throne. It is allowed for the king to re-enter the throne, and all pieces may pass through the throne when it is empty.

Restricted squares are hostile, which means they can replace one of the two pieces taking part in a capture. The throne is always hostile to the attackers, but only hostile to the defenders when it is empty.

The four corner squares are also restricted and hostile, just like the throne. The board edge is NOT hostile.

  ┌───────────┐
11│R.........R│
10│...........│
 9│...........│
 8│...........│
 7│...........│
 6│.....R.....│
 5│...........│
 4│...........│
 3│...........│
 2│...........│
 1│R.........R│
  └───────────┘
   ABCDEFGHJKL

6. King's Side Win (Defenders)

If the king reaches any corner square, the king has escaped and his side wins.

  ┌───────────┐
11│R.........R│
10│...........│
 9│...........│
 8│...........│
 7│...........│
 6│.....R.....│
 5│...........│
 4│...........│
 3│...........│
 2│...........│
 1│K.........R│
  └───────────┘
   ABCDEFGHJKL

Exit Forts

The defenders also win if the king has contact with the board edge, is able to move, and it is impossible for the attackers to break the fort.

  ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│
10│...........│ │...........│
 9│...........│ │...........│
 8│...........│ │...........│
 7│...........│ │...........│
 6│.....R.....│ │.....R.....│
 5│...........│ │...........│
 4│...........│ │...........│
 3│...........│ │....OO.....│
 2│....OO.....│ │....O.O....│
 1│R..OK.O...R│ │R...OKO...R│
  └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL

7. Attackers Win

The attackers win if they can capture the king.

The king is captured when the attackers surround him on all four cardinal points, except when he is next to the throne.

If on a square next to the throne, the attackers must occupy the three remaining squares around him.

The king cannot be captured on the board edge.

The King is Captured

  ┌───────────┐ ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│ │R.........R│
10│...........│ │...........│ │...........│
 9│...........│ │...........│ │...........│
 8│...........│ │...........│ │...........│
 7│.....X.....│ │...........│ │...........│
 6│....XkX....│ │.....R.....│ │.....R.....│
 5│.....X.....│ │....XkX....│ │...........│
 4│...........│ │.....X.....│ │....X......│
 3│...........│ │...........│ │...XkX.....│
 2│...........│ │...........│ │....X......│
 1│R.........R│ │R.........R│ │R.........R│
  └───────────┘ └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL   ABCDEFGHJKL

If the attackers surround the king and ALL remaining defenders with an unbroken ring, then they win, as they have prevented the king from escaping.

  ┌───────────┐
11│R.........R│
10│...........│
 9│.....XXX...│
 8│....X...XX.│
 7│...X.o....X│
 6│..X..R....X│
 5│.X.o...o..X│
 4│.X..ok...X.│
 3│..X...o.X..│
 2│...XXX.X...│
 1│R.....X...R│
  └───────────┘
   ABCDEFGHJKL

The King is Not Captured

  ┌───────────┐ ┌───────────┐
11│R.........R│ │R.........R│
10│...........│ │...........│
 9│...........│ │...........│
 8│...........│ │...........│
 7│...........│ │...........│
 6│.....R.....│ │.....R.....│
 5│...........│ │...........│
 4│...........│ │...........│
 3│...........│ │...........│
 2│....X......│ │.X.........│
 1│R..XKX....R│ │RKX.......R│
  └───────────┘ └───────────┘
   ABCDEFGHJKL   ABCDEFGHJKL

8. Perpetual Repetitions

Perpetual repetitions are forbidden. Any perpetual repetition results in a loss for white.

Added Rule

If a move would repeat a board position, it is not allowed.

9. Automatic Loss

If a player cannot move, he loses the game.

NOT IMPLEMENTED YET BELOW THIS LINE

10. Draw (not possible?)

If it is not possible to end the game, for example because both sides have too few pieces left, it is a draw.