connect_four.js

// determines the state of a given Connect Four board
// http://www.codewars.com/kata/529962509ce9545c76000afa/train/javascript

function connectFour(board) {
  horizontal = straight(board, true);
  if (horizontal) return horizontal;

  vertical = straight(board, false);
  if (vertical) return vertical;

  diag = diagonal(board);
  if (diag) return diag;

  if([].concat.apply([], board).indexOf('-') >= 0) return "in progress"
  
  return "draw";
}

function straight(board, horizontal) {
  function row(board, i) { return board[i]; }
  function column(board, i) { return board.map(function(value,index) { return value[i]; }); }

  row_count = horizontal ? 6 : 7;
  col_max = horizontal ? 4 : 3;
  
  for (i = 0; i < row_count; i++) {
    this_row = horizontal ? row(board, i) : column(board, i);
    for (j = 0; j < col_max; j++) {
      var winner = check_slice(this_row.slice(j, j + 4));
      if (winner) return winner;
    }    
  }
}

function diagonal(board) {
  var winner;
  
  for (i = 0; i < 3; i++) {
    for (j = 0; j < 7; j++) {
      if (j < 4) {
        winner = check_slice([board[i][j], board[i+1][j+1], board[i+2][j+2], board[i+3][j+3]]);
        if (winner) return winner;
      }
      if (j > 2) {
        winner = check_slice([board[i][j], board[i+1][j-1], board[i+2][j-2], board[i+3][j-3]]);
        if (winner) return winner;
      }
    }
  }
}

function check_slice(slice) {
  set = new Set(slice);
  if (set.size == 1 && (set.has("R") || set.has("Y"))) {
    return Array.from(set)[0];
  }
}