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Merge pull request #209 from xexyl/1992-vern

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Fix 1992/vern/README.md
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Landon Curt Noll authored and GitHub committed 2023-03-29 13:50:28 -07:00
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make all make all
### To run ### To run:
./vern 3 # <-- default is 2 ./vern 3 # <-- default is 2
## Judges' comments ## Try:
You might start off by giving the following input:
You might start off by giving the following input:
63 43 63 43
76 55 76 55
71 52 71 52
67 57 (this may take a while) 67 57 (this may take a while)
The judges assume no responsibility for the obfuscated opening ## Judges' comments:
suggested above. :-)
For a real quick game try: The judges assume no responsibility for the obfuscated opening
suggested above. :-)
For a real quick game try:
./vern ./vern
65 55 65 55
66 46 66 46
NOTE: Move values must be restricted to the range 00 to 77. NOTE: Move values must be restricted to the range 00 to 77.
NOTE: Because some mailers have problems with the original winning source, NOTE: Because some mailers have problems with the original winning source,
a slightly modified version with shorter lines has been provided. a slightly modified version with shorter lines has been provided.
## Author's comments ## Author's comments:
This program plays chess. You play the white pieces and the program 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 black pieces. Moves are entered as a two-digit number specifying
the coordinates of the piece to be moved followed by another two-digit the coordinates of the piece to be moved followed by another two-digit
number specifying the coordinates of where to move it. Rows and number specifying the coordinates of where to move it. Rows and
columns are numbered starting with (0,0) for the upper-lefthand corner columns are numbered starting with (0,0) for the upper-lefthand corner
(black's queen's rook) and going to (7,7) for the lower-righthand (black's queen's rook) and going to (7,7) for the lower-righthand
corner (white's king's rook). For example, the PK4 opening for white corner (white's king's rook). For example, the PK4 opening for white
is indicated as "64 44". Moves are read using scanf() so care should is indicated as "64 44". Moves are read using scanf() so care should
be taken to not enter the wrong number of fields. be taken to not enter the wrong number of fields.
If the move you enter is illegal then you are just prompted again to 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 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 program starts computing its response. For every 64 board positions it
examines a "." is printed. Once it has chosen its move it simply examines a "." is printed. Once it has chosen its move it simply
displays the board updated to reflect the move. displays the board updated to reflect the move.
Your move is prompted for using a string like "1351 9 2 >". The first 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 number is how many board positions the program examined when computing
its previous response. The second number is its anticipated value for its previous response. The second number is its anticipated value for
the move it just made (with larger numbers meaning an ultimately better the move it just made (with larger numbers meaning an ultimately better
board position for it), and the third number is the evaluation it board position for it), and the third number is the evaluation it
assigns to the your current position (larger numbers meaning better assigns to the your current position (larger numbers meaning better
positions for you). A ">" indicates that you are not in check; for positions for you). A ">" indicates that you are not in check; for
check it is changed to "!". check it is changed to "!".
There are a few limitations on play: There are a few limitations on play:
o castling can only be done on the king's side; - castling can only be done on the king's side.
o prohibitions against castling out of or through check are - prohibitions against castling out of or through check are not enforced.
not enforced; - prohibitions against castling if the king or rook have moved are not enforced.
o prohibitions against castling if the king or rook have moved - pawns are always promoted to queens.
are not enforced; - en passant is not allowed.
o pawns are always promoted to queens; - the board-position-repeated-three-times and fifty-moves stalemates are not
o en passant is not allowed; recognized.
o 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 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 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 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 playing at a high "ply" - see the next paragraph). Stalemates are not
recognized per se, but when selecting its move the program assigns a 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 stalemate position a value of 0 so it will try for a stalemate if its
other options are worse. other options are worse.
The program takes one optional argument indicating the ply (lookahead) The program takes one optional argument indicating the ply (lookahead)
it should use. The default is 2, which is enough to keep it from 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 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 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 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 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 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 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 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 blunders but also immediately stupid moves altogether. A ply of 4
will find all mate-in-two's. will find all mate-in-two's.
### Obfuscation
This program is obfuscated in a number of ways. First, it abuses the This program is obfuscated in a number of ways. First, it abuses the
contest rule regarding ';', '{', and '}' in the source not counting contest rule regarding ';', '{', and '}' in the source not counting
against the non-whitespace character limit if followed by whitespace. against the non-whitespace character limit if followed by whitespace.
As can be seen from the build file, certain combinations of these As can be seen from the build file, certain combinations of these
characters followed by a particular whitespace character are expanded characters followed by a particular whitespace character are expanded
at build time into much-needed text. Without this hack the program 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 wouldn't pass the character limit. A nice side effect is that the
initial source is gloriously difficult to peruse. initial source is gloriously difficult to peruse.
Second, the rather ho-hum step has been taking of naming each variable 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 with a one or two letter identifier to render it meaningless (and save
space). This tactic is rather effective in a complex program. space). This tactic is rather effective in a complex program.
Third, the board is represented not as an array of pieces but rather as Third, the board is represented not as an array of pieces but rather as
an array of function pointers (with a separate array representing an array of function pointers (with a separate array representing
whether a piece is white or black). This obfuscates the program's whether a piece is white or black). This obfuscates the program's
central data structure. There is one function per type of piece; when 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 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 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 indicating the type of the piece (though the type can also be determined
by comparing the function pointer with the function itself). by comparing the function pointer with the function itself).
Fourth, there are no independent routines for generating potential Fourth, there are no independent routines for generating potential
moves. Instead, the program generates its moves by brute force: for 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 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 square on the board. Those moves that are legal it then explores to
see their effects (using alpha-beta search). This tactic somewhat see their effects (using alpha-beta search). This tactic somewhat
obfuscates the algorithm used by the program. obfuscates the algorithm used by the program.
Finally, there are three key constants that occur throughout the Finally, there are three key constants that occur throughout the
program: 64, 8, and 3 (lg 8). Rather than making these available at 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, 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 they are computed probabilistically at run-time. An instance of the
"Inspection Paradox" is used which happens to produce a value that on "Inspection Paradox" is used which happens to produce a value that on
average is close to .64. 10000 instances of this value are computed, average is close to .64. 10000 instances of this value are computed,
added up, and then divided by 100. Sometimes the value produced will 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), 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 so the result is then rounded to the nearest multiple of 4, and then
the other constants are derived from it. the other constants are derived from it.
## Copyright and CC BY-SA 4.0 License: ## Copyright and CC BY-SA 4.0 License: