Sjaak Glebbeek

Sjaak, [1]

an open source engine under the GNU General Public Licence by Evert Glebbeek as younger sibling of Jazz written in C, but able to play chess variants on various board sizes compliant to the Chess Engine Communication Protocol. Sjaak was first released in January 2011, not to confused with Ronald de Man’s older private engine with the same name [2].

Variants

Amazon Chess, where the queen moves as an amazon (needs to be played as variant “fairy” in XBoard).
Berolina Chess, where pawns move one square diagonal and capture straight ahead.
Burmese Chess, working but not tested fully.
Capablanca Chess, a variant played on a 10x8 board with two extra pieces.
Capablanca Random Chess
Chess960 (Fischer Random Chess)
Chinese Chess
Courier Chess, a medieval variant played on a 12x8 board.
Crazyhouse
Gothic Chess, the same as Capablanca Chess but with a different starting position.
Grand Chess, on a 10x10 board.

Indian Grand Chess, or possibly Turkish Grand Chess. On a 10x10 with four extra pieces.

Knightmate, where the king moves as a knight and the knights move as a king.
Makruk, the Thai version of Chess.
Orthodox chess
Pocket Knight, like normal chess, but players have an extra knight they can drop on the board.
Seirawan Chess
Shatranj, a historic precursor of modern chess.
Shogi [4]
Spartan Chess, where black and white play with different armies and black has two kings
The Maharaja and the Sepoys, where white has only one piece (the Maharaja) (needs to be played as variant “fairy” in XBoard).

Description

Board Representation

Despite boards exceeding 64 squares, like 10x8, 12x8, and 10x10, Sjaak is a native bitboard engine, utilizing either GCC’s __int128 [5] extenstion on 64-bit platforms such as x86-64 or 128 bit vectors of integers on 32-bit platforms, the latter supporting SSE2 if available. Sjaak applies Kindergarten Bitboards to determine sliding piece attacks [6].

Search

Sjaak uses a fail-soft alpha-beta principal variation search with quiescence, null move pruning, late move reductions and check extensions inside an iterative deepening framework with transposition table [7]. Move ordering considers the killer, countermove and history heuristics. A modified version of a SEE routine from Jazz is used to evaluate captures .

Evaluation

Sjaak’s static evaluation takes material balance, static piece-square tables derived from heuristics based on how pieces move, mobility and king safety into account, the latter only in chess variants with one king per side [8].