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fix: restrict jump-over to forward movement for ranged pieces #47

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Merged
jwyce merged 1 commit into from
Mar 2, 2026
Merged

fix: restrict jump-over to forward movement for ranged pieces #47

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8 changes: 8 additions & 0 deletions .changeset/rules-update-review.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,8 @@
---
"gungi.js": patch
---

Restrict leap-over behavior for jumping pieces to forward movement only.

- Cannon, archer, and musketeer can no longer jump over blockers when moving sideways or backward.
- Added movement regression tests that verify forward leaps remain legal and non-forward leaps are blocked.
131 changes: 68 additions & 63 deletions src/gungi/move_gen.ts
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Original file line number Diff line number Diff line change
@@ -1,5 +1,5 @@
import type { Board, Color, HandPiece, Move, Piece, PieceType } from './utils';
import { encodeFEN, parseFEN } from './fen';
import { encodeFEN, parseFEN } from "./fen";
import type { Board, Color, HandPiece, Move, Piece, PieceType } from "./utils";
import {
convert,
get,
Expand All @@ -10,7 +10,7 @@ import {
removeTop,
SQUARES,
updateHand,
} from './utils';
} from "./utils";

export const dirs = [
[-1, 1],
Expand Down Expand Up @@ -45,7 +45,7 @@ function getAvailableSquares(
start: [number, number],
origin: [number, number],
length: number,
board: (Piece | null)[][][]
board: (Piece | null)[][][],
) {
const [y, x] = start;
const [py, px] = origin;
Expand Down Expand Up @@ -75,7 +75,7 @@ function getAvailableSquares(
return [];
}

const side = originPiece.color === 'b' ? -1 : 1;
const side = originPiece.color === "b" ? -1 : 1;
const below = getTop(`${reverse.y + side}-${reverse.x}`, board);

if (below && below.square === `${py}-${px}`) break;
Expand All @@ -97,7 +97,12 @@ function getAvailableSquares(
pieceType.musketeer,
pieceType.archer,
];
if (piece && !leapPieces.includes(originPiece.type)) break;
if (piece) {
if (!leapPieces.includes(originPiece.type)) break;

const movingForward = originPiece.color === "w" ? dy < 0 : dy > 0;
if (!movingForward) break;
}

forward.x += dx;
forward.y += dy;
Expand All @@ -112,16 +117,16 @@ export function generateMovesForSquare(square: string, fen: string) {
const piece = getTop(square, board);
if (!piece || turn !== piece.color) return [];

const [py, px] = square.split('-').map(Number);
const [py, px] = square.split("-").map(Number);

const probes = pieceProbes[piece.type];
const squares = probes.flatMap((probe, i) => {
const pval = typeof probe === 'number' ? probe : probe[0];
const pcarry = typeof probe === 'number' ? 1 : probe[1];
const pval = typeof probe === "number" ? probe : probe[0];
const pcarry = typeof probe === "number" ? 1 : probe[1];
if (pval < 1) return [];

let [dy, dx] = dirs[i];
if (piece.color === 'b') {
if (piece.color === "b") {
dy *= -1;
dx *= -1;
}
Expand All @@ -137,16 +142,16 @@ export function generateMovesForSquare(square: string, fen: string) {
const p = getTop(s, board);
const t = get(s, board);

const maxTier = mode === 'advanced' ? 3 : 2;
const marshalCanStack = ['advanced', 'intermediate'].includes(mode);
const maxTier = mode === "advanced" ? 3 : 2;
const marshalCanStack = ["advanced", "intermediate"].includes(mode);

if (!p || !t) {
acc.push(createMove(piece, `${s}-1`, fen, 'route'));
acc.push(createMove(piece, `${s}-1`, fen, "route"));
} else {
// tsuke
if (p.tier < maxTier && p.type !== pieceType.marshal) {
if (piece.type !== pieceType.marshal || marshalCanStack) {
acc.push(createMove(piece, `${s}-${p.tier + 1}`, fen, 'tsuke'));
acc.push(createMove(piece, `${s}-${p.tier + 1}`, fen, "tsuke"));
}

// betrayal
Expand All @@ -160,15 +165,15 @@ export function generateMovesForSquare(square: string, fen: string) {

const betrayalOptions = Array.from(enemyCountMap.entries())
.filter(([type, count]) =>
playerHand.some((p) => p.type === type && p.count >= count)
playerHand.some((p) => p.type === type && p.count >= count),
)
.flatMap(([type]) => enemies.filter((e) => e.type === type));

const combos = generateCombinations(betrayalOptions);
acc.push(
...combos.map((combo) =>
createMove(piece, `${s}-${p.tier + 1}`, fen, 'betray', combo)
)
createMove(piece, `${s}-${p.tier + 1}`, fen, "betray", combo),
),
);
}
}
Expand All @@ -178,7 +183,7 @@ export function generateMovesForSquare(square: string, fen: string) {
const newTier = t.filter((p) => p.color === piece.color).length + 1;
const captured = t.filter((p) => p.color !== piece.color);
acc.push(
createMove(piece, `${s}-${newTier}`, fen, 'capture', captured)
createMove(piece, `${s}-${newTier}`, fen, "capture", captured),
);
}
}
Expand All @@ -191,29 +196,29 @@ export function generateArata(piece: HandPiece, fen: string) {
const { board, turn, mode, hand, drafting } = parseFEN(fen);
if (!piece || turn !== piece.color) return [];
const isMarshalPlaced = !hand.some(
(p) => p.type === pieceType.marshal && p.color === piece.color
(p) => p.type === pieceType.marshal && p.color === piece.color,
);
if (!isMarshalPlaced && piece.type !== pieceType.marshal) return [];

const isDraft = drafting.b || drafting.w;
const maxTier = mode === 'advanced' ? 3 : 2;
const maxTier = mode === "advanced" ? 3 : 2;
let ranks: number[] = [];
let maybe: number[] = [];

// if in draft can only place within your first 3 ranks, otherwise can placed as far as your deepest piece
if (isDraft) {
ranks = piece.color === 'w' ? [7, 8, 9] : [1, 2, 3];
ranks = piece.color === "w" ? [7, 8, 9] : [1, 2, 3];
} else {
const start = piece.color === 'b' ? 1 : 9;
const end = piece.color === 'b' ? 9 : 1;
const step = piece.color === 'b' ? 1 : -1;
const start = piece.color === "b" ? 1 : 9;
const end = piece.color === "b" ? 9 : 1;
const step = piece.color === "b" ? 1 : -1;
for (
let rank = start;
piece.color === 'b' ? rank <= end : rank >= end;
piece.color === "b" ? rank <= end : rank >= end;
rank += step
) {
const tops = Array.from({ length: 9 }).map((_, i) =>
getTop(`${rank}-${i + 1}`, board)
getTop(`${rank}-${i + 1}`, board),
);
if (tops.some((p) => p && p.color === piece.color)) {
ranks.push(...maybe, rank);
Expand All @@ -225,7 +230,7 @@ export function generateArata(piece: HandPiece, fen: string) {
}

const squares = ranks.flatMap((rank) =>
Array.from({ length: 9 }, (_, i) => `${rank}-${i + 1}`)
Array.from({ length: 9 }, (_, i) => `${rank}-${i + 1}`),
);

return squares.reduce((acc, s) => {
Expand Down Expand Up @@ -253,11 +258,11 @@ export function generateArata(piece: HandPiece, fen: string) {
if (drafting[piece.color]) {
// If only 1 piece left, must end draft - no option to continue
if (!isLastPiece) {
acc.push(createMove(arata, `${s}-${t}`, fen, 'arata'));
acc.push(createMove(arata, `${s}-${t}`, fen, "arata"));
}
acc.push(createMove(arata, `${s}-${t}`, fen, 'arata', [], true));
acc.push(createMove(arata, `${s}-${t}`, fen, "arata", [], true));
} else {
acc.push(createMove(arata, `${s}-${t}`, fen, 'arata'));
acc.push(createMove(arata, `${s}-${t}`, fen, "arata"));
}
}

Expand All @@ -269,19 +274,19 @@ function createMove(
piece: Piece,
to: string,
fen: string,
type: Move['type'],
type: Move["type"],
captured?: Piece[],
draftFinished?: boolean
draftFinished?: boolean,
) {
const from = piece.tier !== 0 ? `(${piece.square}-${piece.tier})` : '';
const toTier = +(to.split('-').at(-1) ?? 0);
const arata = type === 'arata' ? '新' : '';
const capture = type === 'capture' ? '取' : '';
const from = piece.tier !== 0 ? `(${piece.square}-${piece.tier})` : "";
const toTier = +(to.split("-").at(-1) ?? 0);
const arata = type === "arata" ? "新" : "";
const capture = type === "capture" ? "取" : "";
const tsuke =
type === 'tsuke' || (toTier !== 1 && toTier - piece.tier > 0) ? '付' : '';
type === "tsuke" || (toTier !== 1 && toTier - piece.tier > 0) ? "付" : "";
const betray =
type === 'betray' ? `返${captured?.map((p) => p.type).join('')}` : '';
const draftDone = draftFinished ? '終' : '';
type === "betray" ? `返${captured?.map((p) => p.type).join("")}` : "";
const draftDone = draftFinished ? "終" : "";

const move: Move = {
piece: piece.type,
Expand All @@ -291,7 +296,7 @@ function createMove(
type,
san: `${arata}${piece.type}${from}${capture}(${to})${betray || tsuke}${draftDone}`,
before: fen,
after: '',
after: "",
draftFinished,
captured,
};
Expand All @@ -305,52 +310,52 @@ function createMove(

function makeMove(move: Move, fen: string) {
let { board, hand, mode, turn, drafting, moveNumber } = parseFEN(fen);
const [rank, file, tier] = move.to.split('-');
const [rank, file, tier] = move.to.split("-");
const to = {
type: move.piece,
color: move.color,
square: `${rank}-${file}`,
tier: +tier,
};

if (move.type === 'route' || move.type === 'tsuke') {
if (move.type === "route" || move.type === "tsuke") {
removeTop(move.from!, board);
} else if (move.type === 'capture') {
} else if (move.type === "capture") {
removeTop(move.from!, board);
remove(`${rank}-${file}`, move.captured!, board);
} else if (move.type === 'betray') {
} else if (move.type === "betray") {
removeTop(move.from!, board);
convert(`${rank}-${file}`, move.captured!, board);
updateHand(move.captured!, hand, true);
} else if (move.type === 'arata') {
} else if (move.type === "arata") {
updateHand([to], hand);
if (move.draftFinished) {
drafting[move.color] = false;
if (move.color === 'b') drafting.w = false;
if (move.color === "b") drafting.w = false;
}
}

put(to, board);

if (turn === 'b' && (drafting.w || !drafting.b)) moveNumber++;
if (turn === 'w' && !drafting.b && !drafting.w && move.draftFinished)
if (turn === "b" && (drafting.w || !drafting.b)) moveNumber++;
if (turn === "w" && !drafting.b && !drafting.w && move.draftFinished)
moveNumber++;

if (drafting.w === drafting.b) {
if (!move.draftFinished || turn !== 'w') {
turn = turn === 'b' ? 'w' : 'b';
if (!move.draftFinished || turn !== "w") {
turn = turn === "b" ? "w" : "b";
}
} else {
if (!drafting.b && turn === 'b') {
turn = 'w';
} else if (!drafting.w && turn === 'w') {
turn = 'b';
if (!drafting.b && turn === "b") {
turn = "w";
} else if (!drafting.w && turn === "w") {
turn = "b";
}
}

let gameOverSAN = '';
let gameOverSAN = "";
if (move.captured?.some((p) => p.type === pieceType.marshal))
gameOverSAN = '#';
gameOverSAN = "#";

return {
after: encodeFEN({ board, hand, mode, turn, drafting, moveNumber }),
Expand Down Expand Up @@ -382,16 +387,16 @@ function getAttackedSquares(square: string, board: Board): string[] {
const piece = getTop(square, board);
if (!piece) return [];

const [py, px] = square.split('-').map(Number);
const [py, px] = square.split("-").map(Number);
const probes = pieceProbes[piece.type];

return probes.flatMap((probe, i) => {
const pval = typeof probe === 'number' ? probe : probe[0];
const pcarry = typeof probe === 'number' ? 1 : probe[1];
const pval = typeof probe === "number" ? probe : probe[0];
const pcarry = typeof probe === "number" ? 1 : probe[1];
if (pval < 1) return [];

let [dy, dx] = dirs[i];
if (piece.color === 'b') {
if (piece.color === "b") {
dy *= -1;
dx *= -1;
}
Expand All @@ -407,9 +412,9 @@ function getAttackedSquares(square: string, board: Board): string[] {
export function isSquareAttacked(
square: string,
byColor: Color,
board: Board
board: Board,
): boolean {
const [targetRank, targetFile] = square.split('-').map(Number);
const [targetRank, targetFile] = square.split("-").map(Number);
const targetSquare = `${targetRank}-${targetFile}`;

for (const sq of SQUARES) {
Expand All @@ -427,7 +432,7 @@ export function isSquareAttacked(

export function inCheck(color: Color, fen: string): boolean {
const { board } = parseFEN(fen);
const oppositeColor = color === 'w' ? 'b' : 'w';
const oppositeColor = color === "w" ? "b" : "w";

// Find marshal position
for (const sq of SQUARES) {
Expand Down
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