## Help for the Game of Tripod

Welcome to the network Tripod server. The challenge command is described here. Other commands are the same as for all pbmserv games.

tripod challenge [-size=number] userid1 userid2

starts a new game between two players.

The -size parameter sets the board size (default is 11 cells per side).

### Introduction

Tripod is a pure connection game for two players, a bit like Y played across two boards.

### Rules

The board consists of two triangular halves. The board is initially empty.

Players take turns placing a piece of their colour on an empty cell. Any piece played on an edge cell is duplicated on the corresponding cell of the other half-board, hence connections can cross between the two halves.

A player wins by surrounding all three corners with a single group of their pieces.

The second player may elect to swap colours after the first player's opening move. There can be no ties.

### Example

The following example shows a game won by X:

`             .                          .                       . .                        . .              o o .                      o . .          . . o o                    . o o o         x . . . .                  x . . . .        . x . . . .                . x x . . .       . . x . . x x              . . . x x x x      . . . x x x . .            . . . x . . . .        o o . . x . . . o          o . . x . . . . o    . . o . x . . . o .        . o . . x . . . o .      . . o . x . . . . o .      . . o . x . . . . o .   `

Note that O has surrounded each corner in isolation, but that this is not enough to win - all three corners must be surrounded by the same group.

### Notes

A group surrounds a corner if (considered in isolation) it blocks all possible paths to any other corner. A piece played in a corner will therefore surround that corner.

Surrounding two corners with a single group will automatically surround the third corner and win the game.

Superficially, Tripod looks like Y played on a double board. Similar principles apply, however forming a Y connection in either half in isolation will not necessarily help. For example, the following position is a win for O despite X forming a Y on the left half:

`             .                          .                       . .                        . .              . . .                      . . .             . . . .                    . . . .            x . . . x                  x x x x x        . x . . x .                . . . . . .       . . x x x . .              . . . . . . .         . . . x . . . .            . . o o o o . .        . . . . x . . . .          . . o . . . o . .       . . . . x . . . . .        . . o . . . . o . .      o o o . . x . . o o o      o o o . . x . . o o o`

The closer a loop is to a corner the smaller it needs to be, however the further it will be from loops around the other corners. There is a balance between loop size and loop distance:

`             o                          o  <-------- size=1,  dist=8+8, total=17            . .                        . .              o o o                      o o o  <------ size=4,  dist=6+6, total=16          . . . .                    . . . .            o o o o o                  o o o o o  <---- size=8,  dist=4+4, total=16        . . . . . .                . . . . . .           o o o o o o o              o o o o o o o  <-- size=12, dist=2+2, total=16      . . . . . . . .            . . . . . . . .        . . . . . . . . .          . . . . . . . . .       o . . . . . . . . o        o . . . . . . . . o   . o . . . . . . . o .      . o . . . . . . . o .   `

The two board halves are essentially the front and back of a single spherical board, with the duplicated edge pieces representing pieces played on the equator between the two hemispheres.

Note that each corner cell has two unique neighbours, both of which have five unique neighbours. All other cells have six unique neighbours.

Edge cells appear to be stronger as they lie on a direct line between two corners.

Note that while it is possible to enclose a corner with a single piece (such as the O piece in the example below) it is also possible to virtually enclose a corner with two pieces (such as the two X pieces in the example below). Although the bottom left corner is not technically enclosed yet, O cannot stop X from eventually enclosing it.

`             o                          o            . .                        . .              . . .                      . . .             . . . .                    . . . .            . x . . .                  . x . . .        . . . . . .                . . . . . .`

### Syntax

The move syntax is:

tripod move board# userid password a7       (place a piece at a7)
tripod move board# userid password swap
(swap sides - second move only)

Edge pieces duplictaed on both halves are marked '+' in the move list.

### History

Tripod rules copyright (c) Cameron Browne, March 2007.

The Tripod board is similar to an Antipod board with half the corners sliced off, as suggested by Bill Taylor. The name “Tripod” refers to this Antipod lineage, as well as the fact that the board has three bases.

Implementation and Help file by Cameron Browne, March 2007.