SAN Strings

This simple script generates all 29,274 possible Standard Algebraic Notation (SAN) strings for chess moves, with logic to avoid listing SAN strings that can never actually occur for geometric reasons.

If someone notices missing strings, or strings which are generated but can never occur, please open an issue!*

* But after the last update, I truly think I got them all!

Note

Check (+) and checkmate (#) symbols are omitted in san_strings.txt but included in san_strings_with_symbols.txt. It is fairly easy to convince yourself that no special logic is required to determine which subset of all SAN moves could deliver check/mate: all moves can deliver either check or checkmate at least via a discovery. Therefore, san_strings_with_symbols.txt (29,274 lines) is exactly three times the length of san_strings.txt (9,758 lines) as it simply makes two additional copies of each SAN move, one appending + and one appending #. SANs in both files are sorted by the key (len(san), san).

Run it yourself

git clone https://github.com/jacksonthall22/SAN-strings.git && cd SAN-strings
pip install -r requirements.txt
python gen_san_strings.py

How it works

What are discriminators?

Generating every possible SAN move in chess would be simple if not for discriminators, which occasionally must be inserted into the "standard" SAN move to disambiguate between multiple legal options.

• Nbd2:

  

• R5e2:

  

• Qb6e3:

  

If we have a piece, a from_square, and a to_square, how do we decide which (if any) discriminators the SAN move might require? I'm glad you asked—this repo has the answer.

SAN generation algorithms

In a previous version of this code, special logic was used to manually handle all the edge cases for the different piece types. Unfortunately, this failed miserably, generating some syntactically-legal SAN moves that could never actually occur and failing to generate some that could. When I dug into the issues more deeply, I realized this problem is quite difficult to reason through.

Now, the code uses three much more logical and generalized algorithms that were designed to resemble how a human might decide whether a particular from_square -> to_square move by a certain piece type could require a rank, file, and/or full-square discriminator respectively, based on which other squares a piece of the same type could come from when moving to to_square. Read on for the pseudocode, as well as some more intuitive explanations.

Note that only knight, bishop, rook, and queen moves ever use discriminators. Pawns have their own special move syntax and there is never more than one king per color. The details about how SAN strings are generated for these piece types is omitted below because it is straightforward and comparatively uninteresting. However, you can find the implementations in gen_san_strings.py at get_pawn_sans() and get_king_sans().

Pseudocode

File discriminators

• Given piece (must be a N, B, R, or Q)
• For each to_square in the board:
  • Initialize an empty board b
  • Place a piece on b at to_square
  • Get the attacks bitboard for to_square, a 64-bit binary number representing squares the piece could move to
  • For each from_square in attacks:
  • Define a bitboard ray whose truthy bits start at to_square and extend toward from_square, continuing past it until reaching a board edge
  • Remove all truthy bits in ray from attacks
  • Remove all truthy bits on the same file as from_square from attacks
  • If any file in attacks has any truthy bit, this move can require a file discriminator

Rank discriminators

• Given piece (must be a N, B, R, or Q)
• For each to_square in the board:
  • Initialize an empty board b
  • Place a piece on b at to_square
  • Get the attacks bitboard for to_square, a 64-bit binary number representing squares the piece could move to
  • For each from_square in attacks:
    • Define a bitboard ray whose truthy bits start at to_square and move toward from_square, continuing past it until reaching a board edge
    • Remove all truthy bits in ray from attacks
    • Remove all truthy bits not on the same file as from_square from attacks
    • If any rank in attacks has any truthy bit, this move can require a rank discriminator

Full-square discriminators

• Given piece (must be a N, B, R, or Q)
• For each to_square in the board:
  • Initialize an empty board b
  • Place a piece on b at to_square
  • Get the attacks bitboard for to_square a 64-bit binary number representing squares the piece could move to
  • For each from_square in attacks:
  • Define a bitboard ray whose truthy bits start at to_square and move toward from_square, continuing past it until reaching a board edge
  • Remove all truthy bits in ray from attacks
  • If there is any truthy bit in from_square's rank and also in from_square's file, this move can require a full-square discriminator

Intuitive explanations

To really understand the code in this repo, we need to understand the algorithm a human uses to determine whether a move from a from_square to a to_square might require a file, rank, and/or full-square discriminator.

First we should consider that if moving from from_square to to_square is a legal move, then even if there is another piece of the same type and color on the ray which extends from to_square to from_square and continues on to an edge of the board, moving that piece to to_square would be illegal. If this piece falls between from_square and to_square, then the original move would not be legal, so we have a contradiction. If it is past from_square (on the extension of the ray between the squares that continues to the edge of the board), then it is not legal because it cannot jump over the piece at from_square to reach to_square.

This is important when considering disriminators because we are only interested in squares from which another piece can legally move to to_square, and those which might create a situation where a rank, file, or full-square discriminator is necessary.

File discriminators

With this in mind, the algorithm for determining whether we need a file discriminator is as follows:

• Take an empty board and place a piece on to_square, then get a bitboard attacks of all the squares it can move to. These may all be considered possible from_squares.
• Consider each from_square in attacks:
  • Assume the move from from_square to to_square is legal. Then we know that no other piece on the ray from to_square to from_square is relevant because its move to to_square would be illegal. Therefore, we can subtract the bitmask of that ray from attacks for the next step, creating a bitboard representing all the other possible locations of a piece that could legally move to to_square given that a piece can legally move from from_square to to_square.
  • We also know that any squares in this bitmask which fall on the same file as from_square are not relevant for determining whether a file discriminator might be required: if another piece were to occupy one of those squares, then its move to to_square would necessarily require a rank discriminator, not a file discriminator. Therefore we subtract the bitmask of all squares in from_square's file from the bitboard in the previous step as well.
  • We now have a bitboard of all squares from which a piece can legally move to to_square (given that the move piece from from_square to to_square is legal) such that, if a piece really were to occupy any one of those squares, it has potential to create a situation where a file discriminator is necessary. All that is left to do is check whether one or more files in this bitboard have any truthy bits. If so, then the from_square for this iteration can require a file discriminator.

Rank discriminators

The algorithm for determining whether we need a rank discriminator is similar, but has some differences which account for the fact that a file discriminator is preferred over a rank discriminator when both can disambiguate the move:

• Take an empty board and place a piece on to_square, then get a bitboard attacks of all the squares it can move to. These may all be considered possible from_squares.
• Consider each from_square in attacks:
  • Subtract the extended ray from to_square towards from_square from attacks by the same logic as above.
  • This time, we know that any squares that do not fall on the same file as from_square are not relevant for determining whether a rank discriminator might be required: if another piece were to occupy one of those squares, then its move to to_square would necessarily preference the file discriminator, not the rank discriminator. Therefore we use a logical AND between the bitboard from the previous step and the bitmask of all squares in from_square's file.
  • By the same logic as above, all that is left to do is check whether one or more ranks in this bitboard have any truthy bits. If so, then the from_square for this iteration can require a rank discriminator.

Full-square discriminator

Determining whether we need a full-square discriminator is actually the simplest:

• Take an empty board and place a piece on to_square, then get a bitboard attacks of all the squares it can move to. These may all be considered possible from_squares.
• Consider each from_square in attacks:
  • Subtract the extended ray from to_square towards from_square from attacks by the same logic as above.
  • A full-square discriminator is required when there is another piece on from_square's same file and another one on its same rank that can both move to to_square. Therefore, can use a logical AND between attacks and the bitmask of all squares in from_square's file, then do the same for its rank, and if both of these have truthy bits, then the from_square for this iteration can require a full-square discriminator.