/**
 * 纯规则模块：坐标换算、落子合法性、胜负判定。
 * 不依赖任何 I/O，便于单独推演和测试。
 */
import { BOARD_SIZE, CELL_COUNT, WIN_COUNT, type Cell, type Player } from '../shared/protocol.js'

export function idx(x: number, y: number): number {
  return y * BOARD_SIZE + x
}

export function inBounds(x: number, y: number): boolean {
  return x >= 0 && x < BOARD_SIZE && y >= 0 && y < BOARD_SIZE
}

export function createBoard(): Cell[] {
  return new Array<Cell>(CELL_COUNT).fill(0)
}

/**
 * 以最后落子点为中心，向横、竖、两条斜线四个方向统计同色连子。
 * 返回该方向上的完整连子下标数组（长度 >= 5 即为获胜），否则返回 null。
 */
export function findWinLine(board: Cell[], x: number, y: number, player: Player): number[] | null {
  const dirs: Array<[number, number]> = [
    [1, 0],
    [0, 1],
    [1, 1],
    [1, -1],
  ]

  for (const [dx, dy] of dirs) {
    const line: number[] = [idx(x, y)]

    // 正方向延伸
    for (let cx = x + dx, cy = y + dy; inBounds(cx, cy) && board[idx(cx, cy)] === player; cx += dx, cy += dy) {
      line.push(idx(cx, cy))
    }
    // 反方向延伸，插到数组头部以保持连线有序
    for (let cx = x - dx, cy = y - dy; inBounds(cx, cy) && board[idx(cx, cy)] === player; cx -= dx, cy -= dy) {
      line.unshift(idx(cx, cy))
    }

    if (line.length >= WIN_COUNT) return line
  }

  return null
}

export function isEmptyCell(board: Cell[], x: number, y: number): boolean {
  return inBounds(x, y) && board[idx(x, y)] === 0
}

export function opponent(p: Player): Player {
  return p === 1 ? 2 : 1
}