Ōgi

Rule System for Ōgi (王棋) built on the Game Protocol.

Ōgi is a two-player abstract strategy game derived from shōgi, played on an 8×8 board with directional pieces, captures kept in hand, and drops. It also adopts castling — kingside and queenside — exactly as in the Chess specification.

1. Board

8×8 grid encoded in CELL.

Valid coordinates: a1–h8

FEEN serialization: rank 8 → rank 1, file a → file h, / between ranks.

2. Style

Style	SIN	Side
`Japanese`	`J`	first (Sente 先手)
`Japanese`	`j`	second (Gote 後手)

3. Pieces

All pieces encoded in EPIN. Both sides use the same character for a given piece (shōgi convention — armies are told apart by orientation); the abbreviations are Sashité’s.

EPIN	Name	Character	State / notes
`K^`	King	王 / 玉	Royal piece, not in check
`-K^`	King		Currently in check (transient)
`I`	Princess	姫	Bishop + Knight compound; no promoted form
`+R`	Rook	飛	Castling immediately performable
`-R`	Rook		Castling right retained but temporarily blocked
`R`	Rook		No castling rights (moved, or dropped from hand)
`B`	Bishop	角
`N`	Knight	騎
`+F`	Fu	歩	On its starting rank, eligible for the double step
`-F`	Fu		Reserved in the vocabulary; never set in Ōgi — Fu capture straight forward, so en passant cannot arise
`F`	Fu		Has moved or was dropped; no double step
`T`	Tokin	と	Promoted Fu — Gold-General movement. A dedicated letter, not a `+F` prefix, to avoid ambiguity with double-step eligibility

Gote uses the same tokens in lowercase.

Jeweled King: in Ōgi vs Ōgi, the 玉 glyph is rendered for the lower-rated player (Sente if ratings tie). Purely cosmetic display metadata — both Kings share the token K^ and the canonical position is unaffected.

Capture mutation — two rules, in order:

Demotion: a captured Tokin demotes to a Fu (t → f).
Side flip: the case flips unconditionally — the piece adopts the capturing player’s camp (f → F, r → R).

Both change the piece’s identity, so the PMN of the capturing move carries the captured piece as it enters the hand (/F, /R — §6).

Hand canonicalization: a piece in hand never carries a +/- marker (+f → f, -r → r before the flip). This is a recomposition of the position, not a mutation of the capture, and is never a reason for a PMN suffix by itself.

Every captured piece therefore sits in the capturer’s own camp, unpromoted, and is droppable in principle. There is no way to hold a Tokin in hand.

4. Initial Position

Encoded in FEEN:

-rnbik^bn-r/+f+f+f+f+f+f+f+f/8/8/8/8/+F+F+F+F+F+F+F+F/-RNBIK^BN-R / J/j

Rooks on the a/h files, Knights b/g, Bishops c/f, the Princess on d, the King on e, Fu across rank 2 — the same file layout for both armies. Every Fu starts +F (double-step eligible); both Rooks start -R (castling right retained, temporarily blocked by the occupied home rank).

5. Movements

All pseudo-legal moves encoded in GGN. Patterns shown are representative.

5.1 King

One square in any of the 8 directions. Cannot promote. When the King moves, every Rook that could castle with it (+R/-R) becomes R. See §5.8 for castling.

5.2 Princess

{
  "J:I": {
    "d4": {
      "g7": [
        { "must": { "e5": "empty", "f6": "empty", "g7": "empty" } },
        { "must": { "e5": "empty", "f6": "empty", "g7": "enemy" } }
      ],
      "e6": [{ "must": { "e6": "empty" } }, { "must": { "e6": "enemy" } }],
      "c6": [{ "must": { "c6": "empty" } }, { "must": { "c6": "enemy" } }]
    }
  }
}

Bishop + Knight compound: slides any number of squares diagonally (blockable), and jumps in all 8 L-shape directions (over anything). No orthogonal slide — that is the Rook’s domain. No promoted form.

5.3 Rook

Any number of squares orthogonally. Path must be clear. No promoted form. Participates in castling; state mutation on move: +R/-R → R.

5.4 Bishop

Any number of squares diagonally. Path must be clear. No promoted form.

5.5 Knight

L-shape, all 8 directions, jumps — the Western chess Knight, not the forward-only shōgi knight. No promoted form.

5.6 Fu

{
  "J:+F": {
    "e2": {
      "e3": [{ "must": { "e3": "empty" } }, { "must": { "e3": "enemy" } }],
      "e4": [{ "must": { "e3": "empty", "e4": "empty" } }]
    }
  },
  "J:F": {
    "e3": {
      "e4": [{ "must": { "e4": "empty" } }, { "must": { "e4": "enemy" } }]
    }
  }
}

Moves and captures one square straight forward — there is no diagonal capture. The double step (two squares forward, both empty, never a capture) requires the +F token: stripped on the Fu’s first move, never regained — a captured-then-dropped Fu does not recover it.

5.7 Tokin

One square orthogonally (all 4) or diagonally forward (2) — the shōgi Gold-General pattern, 6 destinations. Cannot promote further.

5.8 Castling

Identical mechanics to Chess §5.7: King two squares toward a +R Rook (e1~g1 / e1~c1 for Sente, mirrored on rank 8 for Gote), Rook lands on the crossed square, conditions 1–6 as in FIDE. A castling is a move, not a drop. Drops interact only through the conditions: an opponent may drop a piece between King and Rook (blocking the path) or one that attacks a transit square — the Rook shows -R until the condition clears.

5.9 Drops

Instead of moving, a player may drop a piece from hand onto an empty square.

The unified drop rule: a piece is droppable iff its EPIN side (case) matches the dropping player’s side. With the capture-time side flip, every piece in hand qualifies in principle.

Fu restrictions — a Fu drop is illegal when:

Last rank — the Fu would sit where it has no legal move (rank 8 for Sente, rank 1 for Gote).
Nifu (二歩) — the dropping player already has an unpromoted Fu on that file (a Tokin does not count).
Uchifuzume (打ち歩詰め) — the drop would deliver checkmate (check by Fu drop is allowed; only mate is forbidden).

Drop mechanics: a drop never captures, never promotes, and never confers state — a dropped Fu is plain F (no double step), a dropped Rook is plain R (a drop never creates a castling right, even onto a home-rank corner). No other piece is constrained by the no-future-move rule on an 8×8 board.

6. Special Move Notation

Encoded in PMN.

Move	PMN	Description
Kingside castling (Sente)	`e1~g1`	King e1→g1, Rook h1→f1
Queenside castling (Sente)	`e1~c1`	King e1→c1, Rook a1→d1
Kingside castling (Gote)	`e8~g8`	King e8→g8, Rook h8→f8
Queenside castling (Gote)	`e8~c8`	King e8→c8, Rook a8→d8
Promotion	`e7-e8=T`	Fu e7→e8, promotes to Tokin (automatic — sole form)
Drop	`F*e4`	Fu from hand placed on e4 (`*e4` when the piece is implied by context)
Capture	`d4+e5/R`	Rook captures a Gote rook, which enters Sente’s hand as `R` (side flip, §3)
Capture of a Tokin	`d4+e5/F`	The captured Tokin demotes and flips: it enters the hand as a Fu

PMN writes the tokens as they stand at the moment of the move; the transient +/- markers are recomposed afterwards and never appear in a move. The / suffix carries the captured piece as it enters the hand (§3); Ōgi’s side flip means every capture of an Ōgi piece carries one.

7. Termination

Status values from CGSN with game-specific interpretation. A game has exactly one of three results — 1–0, ½–½, or 0–1; there are no partial results.

7.1 Decisive

CGSN Status	Result
`checkmate`	The player to move is in check with no legal escape — they lose
`resignation`	Resigning player loses

7.2 Draw

CGSN Status	Condition
`nomove`	The player to move has no pseudo-legal move or drop at all — the pseudo-legal set (GGN) is empty. Tested before `checkmate` and `stalemate`, and takes precedence even if the King is in check (CGSN §9.1)
`stalemate`	No legal move or drop available, not in check. A deliberate departure from shōgi (where no-move loses), aligned on FIDE
`insufficient`	Never triggers in pure Ōgi — captures never remove material from play (board + hands hold 2 Kings and 30 pieces, invariantly), so no materially dead configuration is reachable and the engine performs no detection
`repetition`	The same canonical position occurs a third time (see §8.4). Follows FIDE, not shōgi’s fourfold sennichite; perpetual check remains a defender’s draw
`movelimit`	50 full moves by each player with no capture and no unpromoted Fu move. A drop — even a Fu drop — is not a move and does not reset the counter
`agreement`	Mutual agreement

Illegal moves are not a termination cause: a rejected move is simply refused — the player keeps the turn. Time controls are out of scope of this rule system.

8. Constraints

8.1 Check constraint

A move or drop is illegal if it leaves the active player’s King in check. A King in check carries the transient -K^ marker.

8.2 Castling constraints

As in Chess: illegal if the King is in check, passes through, or lands on an attacked square. +R/-R encode executability; only an original Rook on its original corner ever castles — never a dropped one.

8.3 Promotion

Mandatory and automatic when a Fu ends a move on the last rank: it becomes a Tokin (T), the sole promoted form — no prompt, no choice. No other piece promotes; drops never promote.

8.4 Position identity (threefold repetition)

Two positions are the same iff their canonical FEEN strings are equal: same squares and piece state (+F, T, +R/-R, -K^ included), same hand contents for each player, same player to move.

8.5 Uchifuzume look-ahead

The one drop restriction needing look-ahead: simulate the candidate Fu drop, then test whether the opponent has any legal reply; if none and their King is in check, the drop is illegal. Nifu and last-rank are local board checks.

9. Reference

Ōgi rules page

10. License

Open Web Foundation Agreement 1.0 (OWFa 1.0) — OWFa 1.0