Checking if a piece in the game of Go is dead.

isDead.js

const spaceAt = (board, x, y) => {
  if (x < 0 || x > board[0].length - 1) {
    return 'outOfBounds'
  }
  if (y < 0 || y > board.length - 1) {
    return 'outOfBounds'
  }
  return {content: board[y][x], x: x, y: y}
}


const DIRECTIONS = [{x: 1, y: 0}, {x: -1, y: 0}, {x: 0, y: 1}, {x: 0, y: -1}];
const neighborsOf = (board, x, y) => {
  const neighbors = [];
  DIRECTIONS.forEach((d) => {
    const neighbor = spaceAt(board, x + d.x, y + d.y);
    if (neighbor !== 'outOfBounds') {
      neighbors.push(neighbor);
    }
  })
  return neighbors;
}

const isEmpty = (space) => {
  return space.content === 'e'
}

const areSameColor = (spaceOne, spaceTwo) => {
  return spaceOne.content === spaceTwo.content;
}

const isUnvisited = (space, visitedSpaces) => {
  return !(visitedSpaces.some(visitedSpace => {
    return visitedSpace.x === space.x && visitedSpace.y === space.y
  }))
}

const findUnvisitedAndSameColor = (space, neighbors, visited) => {
  return neighbors.filter(neighbor => {
    return areSameColor(neighbor, space) && isUnvisited(neighbor, visited)
  })
}

const isDead = (board, x, y) => {
  const startingSpace = spaceAt(board, x, y);
  
  if (startingSpace === 'outOfBounds') {
    return 'outOfBounds'
  }

  if (isEmpty(startingSpace)) {
    return 'emptySpace'
  }

  const visited = [];

  const innerIsDead = (space) => {
    const neighbors = neighborsOf(board, space.x, space.y);

    if (neighbors.find(isEmpty)) {
      return false;
    }

    const unvisitedAndSameColor = findUnvisitedAndSameColor(space, neighbors, visited)

    if (unvisitedAndSameColor.length === 0) {
      return true;
    }

    visited.push(space);

    return unvisitedAndSameColor.every((uv) => {
      return innerIsDead(uv)
    })

  }
  return innerIsDead(startingSpace)
}

const aliveBoard = [
  ['e', 'e', 'e', 'e'],
  ['b', 'b', 'w', 'b'],
  ['b', 'b', 'w', 'b'],
  ['b', 'w', 'w', 'w'],
  ['b', 'b', 'b', 'b'],
];

const deadBoard = [
  ['e', 'e', 'e', 'e'],
  ['b', 'b', 'b', 'b'],
  ['b', 'b', 'w', 'b'],
  ['b', 'w', 'w', 'w'],
  ['b', 'b', 'w', 'b'],
];

console.log(isDead(aliveBoard, 2, 3))
console.log(isDead(deadBoard, 2, 3))