| # 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 |
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|
| 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: |
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| | 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 |
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|
| By row identifier: |
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|
| ```sql |
| SELECT fen FROM positions WHERE id = 12345; |
| ``` |
|
|
| Near-constant-time random access uses the indexed `random_key`: |
|
|
| ```sql |
| 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 |
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|
| 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 |
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|
| This reduces correlation between adjacent plies and prevents ordinary |
| middlegames from overwhelming openings and endgames. |
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| Phase classification is deterministic: |
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| - 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 |
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|
| 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. |
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|
