shared/bot/WinMasks.ts

import { isBoardCoordInBounds } from '../board.ts'

export type WinDirection = readonly [number, number]

export interface WinMask {
  readonly cellIndices: readonly number[]
  readonly beforeIndex: number | null
  readonly afterIndex: number | null
  readonly direction: WinDirection
  readonly words: Uint32Array
}

export interface WinMaskSet {
  readonly boardSize: number
  readonly cellCount: number
  readonly wordCount: number
  readonly masks: readonly WinMask[]
  readonly byCellIndex: readonly (readonly WinMask[])[]
}

const DIRECTIONS: readonly WinDirection[] = [[1, 0], [0, 1], [1, 1], [1, -1]]
const cache = new Map<number, WinMaskSet>()

export function getWinMasks(boardSize: number): WinMaskSet {
  const cached = cache.get(boardSize)
  if (cached) return cached

  const cellCount = boardSize * boardSize
  const wordCount = Math.ceil(cellCount / 32)
  const masks: WinMask[] = []
  const byCellIndex: WinMask[][] = Array.from({ length: cellCount }, () => [])

  for (const direction of DIRECTIONS) {
    const [dx, dy] = direction
    for (let y = 0; y < boardSize; y++) {
      for (let x = 0; x < boardSize; x++) {
        const endX = x + dx * 4
        const endY = y + dy * 4
        if (!isBoardCoordInBounds(endX, endY, boardSize)) continue

        const cellIndices: number[] = []
        const words = new Uint32Array(wordCount)
        for (let i = 0; i < 5; i++) {
          const index = toIndex(x + dx * i, y + dy * i, boardSize)
          cellIndices.push(index)
          words[index >>> 5] |= 1 << (index & 31)
        }

        const beforeX = x - dx
        const beforeY = y - dy
        const afterX = x + dx * 5
        const afterY = y + dy * 5
        const mask: WinMask = {
          cellIndices,
          beforeIndex: isBoardCoordInBounds(beforeX, beforeY, boardSize)
            ? toIndex(beforeX, beforeY, boardSize)
            : null,
          afterIndex: isBoardCoordInBounds(afterX, afterY, boardSize)
            ? toIndex(afterX, afterY, boardSize)
            : null,
          direction,
          words,
        }
        masks.push(mask)
        for (const index of cellIndices) byCellIndex[index].push(mask)
      }
    }
  }

  const result: WinMaskSet = { boardSize, cellCount, wordCount, masks, byCellIndex }
  cache.set(boardSize, result)
  return result
}

function toIndex(x: number, y: number, boardSize: number): number {
  return y * boardSize + x
}