VPD1: Vex Position Dataset
VPD1 is an indexed SQLite container for independent chess positions. Its file
extension is .vpd. It is designed for static teacher labeling, where a
consumer needs one complete board immediately without replaying a game.
Position identity
Each row stores a legal six-field FEN. The fullmove number is normalized to
1; it does not affect board state or lc0 evaluation. The halfmove clock is
preserved because it affects draw rules. En-passant state and castling rights
are preserved.
The fen column is unique, so duplicate positions from different games are
removed.
Tables
metadata(key, value) records provenance, format version, sampling seed,
target size, quotas, progress, and completion information.
positions contains:
| Column | Meaning |
|---|---|
id |
Integer row identifier |
random_key |
Indexed deterministic 63-bit key for fast random sampling |
fen |
Normalized six-field FEN |
source_split |
Original train/test split if present |
source_member |
PGN member inside the source archive |
game_number |
Sequential source-game number |
ply |
Ply at which the position was sampled |
result |
Source game result; metadata, not a static WDL label |
side_to_move |
1 for White, 0 for Black |
phase |
0 opening, 1 middlegame, 2 endgame |
piece_count |
Occupied squares |
non_pawn_material |
Combined N/B/R/Q material in pawn units |
material_balance |
White material minus Black material |
legal_moves |
Legal move count |
in_check |
Whether the side to move is in check |
castling_mask |
KQkq availability as four bits |
halfmove_clock |
Fifty-move-rule clock |
Immediate access
By row identifier:
SELECT fen FROM positions WHERE id = 12345;
Near-constant-time random access uses the indexed random_key:
SELECT fen
FROM positions
WHERE random_key >= :random_63_bit_integer
ORDER BY random_key
LIMIT 1;
The supplied vpd.py sample command implements wraparound when the random key
is above the largest stored key.
Parquet derivative
vpd_to_parquet.py converts the same rows into a columnar training dataset.
The output uses Hive directories for source_split and phase; those two
columns are therefore encoded in paths instead of duplicated inside each
file. Each remaining VPD1 column retains its meaning, except phase is exposed
as the strings opening, middlegame, and endgame.
Files use Zstandard compression and 65,536-row groups. _manifest.json
contains the source database checksum plus per-file row counts, sizes, row
groups, ID bounds, and checksums. This representation is optimized for batch
and filtered scans; retain VPD1 SQLite for indexed single-position lookup.
Sampling policy
At most one position per game and phase is selected with reservoir sampling. The default 1.5-million-position build uses fixed phase quotas:
- 15% opening
- 60% middlegame
- 25% endgame
This reduces correlation between adjacent plies and prevents ordinary middlegames from overwhelming openings and endgames.
Phase classification is deterministic:
- opening: ply 20 or earlier with at least 50 combined non-pawn material;
- endgame: at most 20 combined non-pawn material or at most 12 pieces;
- middlegame: everything else.
Variance contract
A database is marked high_variance=true only if every declared threshold in
the JSON report passes. The checks cover corpus size, phase entropy, minimum
phase and result shares, side-to-move balance, legal-move spread, piece-count
spread, material imbalance, castling-state coverage, and positions in check.