This is a parser for the go/igo/weiqi/baduk fragment of the SGF format. Encodings latin-1, utf-8, and ascii are supported, and the parser strives to be robust to minor errors, especially those made by the most common SGF editors. There are plans to support other games and pretty-printing in future releases.

The Smart Games Format (SGF) is a rich way of storing records of games, along with metadata about the games. The format is arranged in a tree structure; to a first approximation, each node in the tree corresponds to a move in one of the games being recorded.

There are a few ways this first approximation needs to be corrected:

1. It is actually a non-empty forest: there may be several trees, each of which is totally independent. In particular, different trees may be recording not only different games, but even different game types: the file may contain records of both go and chess games, for example. Each root has games of only a single type, but there may be many actual games of that type. There is no restriction that different roots use different game types. (Luckily, most real-world files have have a single root and record a single game.)
2. A tree may encode variations within a single game. When there are many branches in a tree, there may or may not be an indication of which are separate games and which are variations.
3. Each node may additionally record metadata. This metadata may be about the game (for example, the names of the players), about the position (for example, that the whole board favors the black player), or about the move (for example, that the move is good for the black player).
4. Some nodes may record a modification of the game board rather than a move in the game. This is used especially for collections of problems, to record only parts of a game (i.e. to set up the initial position), and to handle other strange situations.

This module provides a way to parse files in the SGF format and traverse game forests. In addition to specifying a file format, the SGF specification talks about the behavior of applications that read and write SGF files. Wherever possible, these behaviors are enforced or encouraged via types. It is considered a bug if a required behavior is impossible, if an unrecommended behavior is easy, or if a prohibited behavior is allowed and the documentation does not have warnings in appropriate places.

The two basic starting points are collection and Collection.

The former is a Parsec-3 parser for consuming entire files in the SGF format. It additionally reports some warnings for the most common and easily-corrected errors in files, but these can generally be safely ignored.

The latter is a largish type which is intended to be inhabitable by all and only the valid SGF trees. There are a few places where we sacrifice this property in the name of convenience; wherever possible, the documentation states any restrictions on values that are not reflected by the type system.

In this section, we develop a short program to print, in human-readable form, the moves of the main line of a game of go. We will assume that we are given the contents of an SGF file on stdin, that the file only records one game on a standard size board, that the file doesn't do anything fancy (like change the move number in the middle of the game), and that any node without a valid move is the end of the game. (This saves us a lot of error-checking that would just obscure the idea.)

First, some boring stuff.

import Data.SGF
import Data.ByteString  (ByteString, getContents, unpack)
import Data.Tree
import Data.List hiding ((!!))
import Prelude   hiding ((!!), getContents)

(!!) = genericIndex

Now we'll extract the GameTree representing the game. As suggested in the overview section, we'll use the collection parser.

grabTree :: [Word8] -> TreeGo
grabTree s = case runParser collection () "stdin" s of
    Right ([Game { tree = TreeGo t }], _) -> t

We're already assuming huge swathes of things about the input; this is a horribly partial function. But let's continue. The next step is to extract the moves from the main line. The main line is specified to be the first child at each possible junction, so this is not too tricky: the levels function of Tree (together with a transpose) will flatten the game tree in just the right way that the first element will be the main line.

The one thing we will watch out for is that games of go often begin with one setup node to give the handicap stones, which we want to skip. Actually, for simplicity, we'll just skip every single setup node. Setup nodes are signaled by via the action field of GameNodes: if this field is a Left, then it is a setup node, and otherwise it is a move node.

grabMoves :: TreeGo -> [MoveGo]
grabMoves n = [move | Right Move { move = Just (color, move) } <- mainLine]
    where mainLine = map action . head . transpose . levels $ n

Let's wrap these functions up:

parse :: ByteString -> [MoveGo]
parse = grabMoves . grabTree . unpack

Now that we have the moves, we need a way to show them. In go, it's standard practice to give coordinates as a single upper-case letter followed by a number. To avoid confusion, the letter 'I' is skipped as a coordinate, since it looks much like a one in some fonts.

coordinates :: [Char]
coordinates = delete 'I' ['A'..'Z']

showPoint :: Point -> String
showPoint (x, y) = coordinates !! x : show (19 - y)

We'll pad moves out to four spaces, and show them in pairs.

pad :: String -> String
pad s = s ++ replicate (4 - length s) ' '

showMove :: MoveGo -> String
showMove Pass = "pass"
showMove (Play p) = pad (showPoint p)

showMoves :: [MoveGo] -> String
showMoves = unlines . showMoves' 1 where
    showMoves' n [ ]       = []
    showMoves' n [m]       = unwords [show n ++ ".", showMove m]              : []
    showMoves' n (m:m':ms) = unwords [show n ++ ".", showMove m, showMove m'] : showMoves' (n+2) ms

Finally, we just need some plumbing:

main :: IO ()
main = putStr . showMoves . parse =<< getContents

And there we have it: a complete program to parse an SGF file, extract the moves, and print them!