/** Drops a piece in the given column and updates the game state accordingly. */
function dropPiece(grid, status, game, column) {
  // Check if move is valid.
  if (game.state != "playing") {
    return;  // or throw new Error("can't play in this game")
  }
  // Compute the lowest empty row in the selected column.
  let row = computeEmptyRow(game, column);
  let cell = row * 7 + column;
  // Update game state.
  game.board[cell] = game.next;
  // Check for winner / tie, will set game.state if game ended.
  checkWinner(game, cell);
  // Swap active player
  if (game.state == "playing") {
    togglePlayer(game);
  }
  // Update the HTML view.
  updateHtml(grid, game);
  // Update game status area to reflect active player / winner / tie
  updateStatus(status, game);
}