Xiongqi

Rule System for Xiongqi (熊棋, “bear chess”) built on the Game Protocol.

Xiongqi is a two-player abstract strategy game derived from xiàngqí, played on an 8×8 board with on-board pieces only — no drops; captured pieces never re-enter play. It adopts castling exactly as in the Chess specification, performed by the General and a Chariot. The army maps one-to-one onto the Chess army: the Chariot keeps the Rook’s movement, the Bear replaces the Bishop (same diagonal slide — and names the game), the Empress replaces the Queen as a Chariot + Knight compound, the General replaces the King as a royal that steps orthogonally but captures at Chariot range, and the Soldier replaces the Pawn.

1. Board

8×8 grid encoded in CELL.

Valid coordinates: a1–h8

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

River: the midline between ranks 4 and 5. Its only effect is unlocking the Soldier’s sideways step — no piece is restricted by it.

2. Style

Style	SIN	Side
`Chinese`	`C`	first (Red — South, bottom)
`Chinese`	`c`	second (Black — North, top)

3. Pieces

All pieces encoded in EPIN. Following the xiàngqí tradition, the two sides use distinct, homophonic characters for the same piece type; the General, Knight, Chariot and Soldier use the traditional pairs, the Bear (雄/熊, both xióng) and the Empress (妃/騛, both fēi) use Sashité’s own.

EPIN	Name	South (Red)	North (Black)	State / notes
`G^`	General	帥	將	Royal piece, not in check
`-G^`	General			Currently in check (transient)
`E`	Empress	妃	騛	Chariot + Knight compound; no promoted form
`+R`	Chariot	俥	車	Castling immediately performable
`-R`	Chariot			Castling right retained but temporarily blocked
`R`	Chariot			No castling rights (it or its General has moved, or it was born by promotion)
`B`	Bear	雄	熊	Diagonal slide (the Bishop’s movement)
`N`	Knight	傌	馬	All 8 L-jumps, never hobbled
`+S`	Soldier	兵	卒	On its starting rank, eligible for the double step
`-S`	Soldier			Just double-stepped, legally capturable en passant (one turn only)
`S`	Soldier			Has moved; no double step

Black uses the same tokens in lowercase (g^, e, +r, …). There is no Queen in Xiongqi.

Capture mutation: none. A captured piece enters the capturing player’s hand with type and case preserved — no demotion (a promoted Soldier is captured as whatever it became). The case is not flipped, so the hand is permanently inert (droppable-by-nobody), exactly as in Chess — yet it remains part of the canonical position (§8.4).

Hand canonicalization: a piece in hand never carries a +/- marker (+S → S, -R → R) — a recomposition of the position, never written in the PMN of the capturing move (§6).

4. Initial Position

Encoded in FEEN:

-rnbeg^bn-r/+s+s+s+s+s+s+s+s/8/8/8/8/+S+S+S+S+S+S+S+S/-RNBEG^BN-R / C/c

The back rank mirrors the Chess back rank: Chariots a/h, Knights b/g, Bears c/f, the Empress on d (the Queen’s square), the General on e (the King’s square). Every Soldier starts +S (double-step eligible); both Chariots start -R (castling right retained, temporarily blocked).

5. Movements

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

5.1 General

{
  "C:G^": {
    "e4": {
      "e5": [{ "must": { "e5": "empty" } }],
      "d4": [{ "must": { "d4": "empty" } }],
      "e7": [{ "must": { "e5": "empty", "e6": "empty", "e7": "enemy" } }],
      "a4": [{ "must": { "d4": "empty", "c4": "empty", "b4": "empty", "a4": "enemy" } }]
    }
  }
}

Moves one square orthogonally onto an empty square; captures like a Chariot — sliding any distance along a rank or file to the first occupied square, taking it if hostile. No diagonal action whatsoever. Cannot promote.

Facing royals (“flying general”): because the General captures at Chariot range, two Generals on an open shared file or rank give check to each other — an ordinary long-range threat under the standard check rules. Stepping onto such a line is self-check, hence illegal.

5.2 Empress

Chariot + Knight compound: slides any number of squares orthogonally (blockable), and jumps in all 8 L-shape directions (over anything). No diagonal slide — that is the Bear’s domain. The strongest piece. No promoted form.

5.3 Chariot

Any number of squares orthogonally — the Western Rook’s movement. Path must be clear. No promoted form. Participates in castling; state mutation on move: +R/-R → R.

5.4 Bear

Any number of squares diagonally — the Western Bishop’s movement. Path must be clear. No promoted form.

5.5 Knight

L-shape, all 8 directions, jumps over anything — the Western chess Knight, not the hobbled xiàngqí horse. No promoted form.

5.6 Soldier

{
  "C:+S": {
    "e2": {
      "e3": [{ "must": { "e3": "empty" } }, { "must": { "e3": "enemy" } }],
      "e4": [{ "must": { "e3": "empty", "e4": "empty" } }]
    }
  },
  "C:S": {
    "e5": {
      "e6": [{ "must": { "e6": "empty" } }, { "must": { "e6": "enemy" } }],
      "d5": [{ "must": { "d5": "empty" } }, { "must": { "d5": "enemy" } }],
      "f5": [{ "must": { "f5": "empty" } }, { "must": { "f5": "enemy" } }]
    }
  }
}

Moves and captures straight forward — never diagonally, never backward. After crossing the river (Red on ranks 5–8, Black on ranks 1–4) it gains a sideways step — move or capture, same rank — shown in the second sample. The double step (two squares forward, both empty, never a capture) requires the +S token, stripped on the Soldier’s first move and never regained. A sideways step can never reach the last rank, so promotion always results from a forward move.

5.7 Castling

{
  "C:G^": {
    "e1": {
      "g1": [{ "must": { "f1": "empty", "g1": "empty", "h1": "C:+R" } }],
      "c1": [{ "must": { "d1": "empty", "c1": "empty", "b1": "empty", "a1": "C:+R" } }]
    }
  }
}

Identical mechanics to Chess §5.7, with the General in the King’s role and a Chariot in the Rook’s. Black mirrors on rank 8. Never ambiguous with the Chariot-style capture: castling is the General’s only non-capturing displacement of more than one square and lands on an empty square; a two-file General displacement onto an occupied square is an ordinary capture — no Chariot relocates, and the castling rights are gone (the General has moved).

5.8 En passant

{
  "C:S": {
    "e6": {
      "f6": [{ "must": { "f5": "c:-s", "f6": "empty" } }]
    }
  }
}

When an enemy Soldier double-steps past yours, capture it on the immediately following turn by a sideways step onto the skipped square — the only capture whose victim is not on the destination square (it sits directly behind it). Worked geometry: Black plays f7→f5 (skipped square f6); a Red Soldier on e6 or g6 — necessarily across the river — steps sideways onto f6, removing the Soldier on f5. A Soldier already on the landing rank (e5/g5) instead captures f5 by an ordinary sideways capture, no en passant involved.

Canonicality condition: the -S flag is set only if the capture is actually legal (a Soldier is positioned for the sideways take and the capture would not self-check). Otherwise the token stays plain S.

6. Special Move Notation

Encoded in PMN.

Move	PMN	Description
Kingside castling (Red)	`e1~g1`	General e1→g1, Chariot h1→f1
Queenside castling (Red)	`e1~c1`	General e1→c1, Chariot a1→d1
Kingside castling (Black)	`e8~g8`	General e8→g8, Chariot h8→f8
Queenside castling (Black)	`e8~c8`	General e8→c8, Chariot a8→d8
En passant (Red)	`e6~f6`	Soldier e6→f6, captures Soldier on f5
En passant (Black)	`e3~f3`	Soldier e3→f3, captures Soldier on f4
Promotion	`e7-e8=E`	Soldier e7→e8, promotes to Empress (player’s choice among R, N, B, E; `=` names the piece the actor becomes)

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. A capture carries a / suffix only when the captured piece’s letter or case changes on entering the hand — never in Xiongqi, whose capture mutation is empty (§3): the en passant capture of a -s is written e6~f6.

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. The General is never actually captured
`resignation`	Resigning player loses

7.2 Draw

CGSN Status	Condition
`nomove`	The player to move has no pseudo-legal move at all — the pseudo-legal set (GGN) is empty. Tested before `checkmate` and `stalemate`, and takes precedence even if the General is in check (CGSN §9.1)
`stalemate`	No legal move, not in check. A deliberate departure from xiàngqí (where no-move loses), aligned on FIDE
`insufficient`	Dead position, material-only detection: General versus General is the only case — almost any extra material mates a General cornered by Chariot-range checks, and a lone Soldier promotes to an Empress
`repetition`	The same canonical position occurs a third time (see §8.4). Follows FIDE — perpetual check remains a defender’s draw, unlike xiàngqí where it loses
`movelimit`	50 full moves by each player with no capture and no unpromoted Soldier move
`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 is illegal if it leaves the active player’s General in check — including stepping the General onto an open line facing the enemy General. A General in check carries the transient -G^ marker.

8.2 Castling constraints

As in Chess: illegal if the General is in check, passes through, or lands on an attacked square — attack detection includes the enemy General’s Chariot-range capture (the flying-general line). +R/-R encode executability; a Chariot born by Soldier promotion is plain R and never castles.

8.3 Promotion

Mandatory when a Soldier ends a forward move on the last rank, and a genuine player choice among Chariot, Knight, Bear, Empress — never a Soldier, never a General, never a Queen (none exists). The move record must carry the chosen piece; the Soldier token is replaced outright, so a promoted Soldier is thereafter indistinguishable from an original piece of that type.

8.4 Position identity (threefold repetition)

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

8.5 En passant timing

The -S/-s flag expires at the start of the next move of the player who double-stepped, whether or not the capture was taken.

9. Reference

Xiongqi rules page

10. License

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