tic_tac_toe_magic_squares.js

// theory of tic tac toe win discovery
//
// | 2 | 7 | 6 |
// | 9 | 5 | 1 |
// | 4 | 3 | 8 |
//
// this forms a "magic square" where each
// row, column, or diagonal adds up to 15
//
// as players make moves, the values of
// these spaces are added to an array
//
// to check if a player has won, just
// calculate if any 3 value permutation
// adds up to 15

// this is rather inefficient, but the external API is solid
function combinations (size, array) {
  // return the powerset of a given array
  function powerset (array) {
    var ps = [[]]
    for (var i = 0; i < array.length; i++) {
      for (var j = 0, len = ps.length; j < len; j++) {
        ps.push(ps[j].concat(array[i]))
      }
    }
    return ps
  }

  // filter a nested array for subsets of a given size
  function of_size (size, arr) {
    return arr.filter(function (el) {
      return (el.length === size)
    })
  }

  // filter our powerset for combinations of a given size
  return of_size(size, powerset(array))
}

function has_won (player, board) {
  // given the magic square solution to tic tac toe, just filter
  // player's space arrays for combinations that sum to 15
  function _has_won (spaces) {
    var spaces_combinations = combinations(3, spaces)
    return spaces_combinations.some(function (spcs) {
      return (spcs.reduce(function (x, y) { return x + y }) === 15)
    })
  }

  var player_spaces = board.filter(function (space) {
    // get all spaces owned by the player
    return (space[1] === player)
  }).map(function (space) {
    // build a new list of the scores from those spaces
    return space[0]
  })
  return _has_won(player_spaces)
}

var board = [
  [2, null],
  [7, null],
  [6, null],
  [9, null],
  [5, null],
  [1, null],
  [4, null],
  [3, null],
  [8, null]
]

function cellValue (key, board) {
  return board.filter(function (cell) {
    return cell[0] === key
  }).map(function (cell) {
    return cell[1]
  })[0] // return an Array or just a str?
}

function setCell (key, value, board) {
  board[key][1] = value
}

console.log(' -- before playing -- ')
console.log(board)
setCell(6, 'x', board)
setCell(7, 'x', board)
setCell(8, 'x', board)
console.log(cellValue(8, board))
console.log(' -- after playing -- ')
console.log(board)
console.log(`Did 'X' Win?: ${has_won('x', board)}`)