Author:

• Name: Vern Paxson
  Location: US - United States of America (United States)

To build:

    make all

Bugs and (Mis)features:

The current status of this entry is:

STATUS: INABIAF - please DO NOT fix

For more detailed information see 1992/vern in bugs.html.

To use:

    ./vern 3    # <-- default is 2

Try:

You might start off by giving the following input:

    63 43
    76 55
    71 52
    67 57               # <-- this may take a while

Judges’ remarks:

The judges assume no responsibility for the obfuscated opening suggested above. :-)

For a real quick game try:

    ./vern

    65 55
    66 46

NOTE: Move values must be restricted to the range 00 to 77.

NOTE: Because some mailers have problems with the original winning source, a slightly modified version with shorter lines has been provided.

Author’s remarks:

This program plays chess. You play the white pieces and the program the black pieces. Moves are entered as a two-digit number specifying the coordinates of the piece to be moved followed by another two-digit number specifying the coordinates of where to move it. Rows and columns are numbered starting with (0,0) for the upper-left-hand corner (black’s queen’s rook) and going to (7,7) for the lower-right-hand corner (white’s king’s rook). For example, the PK4 opening for white is indicated as "64 44" (without the quotes). Moves are read using scanf() so care should be taken to not enter the wrong number of fields.

If the move you enter is illegal then you are just prompted again to move. If the move is legal then the new position is displayed and the program starts computing its response. For every 64 board positions it examines a “.” is printed. Once it has chosen its move it simply displays the board updated to reflect the move.

Your move is prompted for using a string like "1351 9 2 >". The first number is how many board positions the program examined when computing its previous response. The second number is its anticipated value for the move it just made (with larger numbers meaning an ultimately better board position for it), and the third number is the evaluation it assigns to the your current position (larger numbers meaning better positions for you). A > indicates that you are not in check; for check it is changed to !.

There are a few limitations on play:

• castling can only be done on the king’s side.
• prohibitions against castling out of or through check are not enforced.
• prohibitions against castling if the king or rook have moved are not enforced.
• pawns are always promoted to queens.
• en passant is not allowed.
• the board-position-repeated-three-times and fifty-moves stalemates are not recognized.

If you checkmate the computer it prints “Rats!” and exits. If your own checkmate is imminent it prints “Har har.” but doesn’t exit so it can rub your nose in defeat (which may be a move or two away if you’re playing at a high “ply” - see the next paragraph). Stalemates are not recognized per se, but when selecting its move the program assigns a stalemate position a value of 0 so it will try for a stalemate if its other options are worse.

The program takes one optional argument indicating the ply (lookahead) it should use. The default is 2, which is enough to keep it from making flaming blunders. The higher the ply, the better the play, and the longer you have to wait. For example, at a ply of 2, a response to the PK4 opening takes about 1.8 CPU seconds on a SPARC ELC when compiled -O with Sun cc. A ply of 3 takes 13 CPU seconds (and results in a different response to PK4) and a ply of 4 takes a little under 4 CPU minutes (and for PK4 comes up with the same response as a ply of 3). A ply of 3 is in general good enough to avoid not only flaming blunders but also immediately stupid moves altogether. A ply of 4 will find all mate-in-two’s.

NOTICE to those who wish for a greater challenge:

If you want a greater challenge, don’t read any further: just try to understand the program via the source.

If you get stuck, come back and read below for additional hints and information.

Obfuscation

This program is obfuscated in a number of ways. First, it abuses the contest rule regarding ;, {, and } in the source not counting against the non-whitespace character limit if followed by whitespace. As can be seen from the build file, certain combinations of these characters followed by a particular whitespace character are expanded at build time into much-needed text. Without this hack the program wouldn’t pass the character limit. A nice side effect is that the initial source is gloriously difficult to peruse. In particular, the Makefile does:

    sed <vern.c 's/{ /(/g;s/} /)/g;s/;       /#define /' |
        sed 's/}      /=/g;s/{        /i/g' >vern.tmp.c

and compiles vern.tmp.c.

Second, the rather ho-hum step has been taking of naming each variable with a one or two letter identifier to render it meaningless (and save space). This tactic is rather effective in a complex program.

Third, the board is represented not as an array of pieces but rather as an array of function pointers (with a separate array representing whether a piece is white or black). This obfuscates the program’s central data structure. There is one function per type of piece; when called they return 1 if the current move is illegal for that piece and 0 if it is legal. The functions also as a side effect set a global indicating the type of the piece (though the type can also be determined by comparing the function pointer with the function itself).

Fourth, there are no independent routines for generating potential moves. Instead, the program generates its moves by brute force: for every piece it controls on the board, it attempts to move it to every square on the board. Those moves that are legal it then explores to see their effects, using alpha-beta search. This tactic somewhat obfuscates the algorithm used by the program.

Finally, there are three key constants that occur throughout the program: 64, 8, and 3 (lg 8). Rather than making these available at compile-time, which provides a hint as to what the program is up to, they are computed probabilistically at run-time. An instance of the “Inspection Paradox” is used which happens to produce a value that on average is close to 64.

10000 instances of this value are computed, added up, and then divided by 100. Sometimes the value produced will be 63 or 65 instead of 64 (but I’ve never observed any other values), so the result is then rounded to the nearest multiple of 4, and then the other constants are derived from it.

Primary files

• vern.c - entry source code
• Makefile - entry Makefile
• vern.orig.c - original source code

Secondary files

• 1992_vern.tar.bz2 - download entry tarball
• README.md - markdown source for this web page
• .entry.json - entry summary and manifest in JSON
• .gitignore - list of files that should not be committed under git
• .path - directory path from top level directory
• index.html - this web page

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