# 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:

```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

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.