// toggles the color of a hexagon and all of it's surrounding hexagons.
function toggleHex(target, hex) {
  const firstColIdx = target.row % 2 ? 0 : -1;
  const secondColIdx = target.row % 2 ? 1 : 0;
  const isNeighbor =
    (hex.col === target.col && hex.row === target.row) || // target
    (hex.row === target.row - 1 && (hex.col === target.col + firstColIdx || hex.col === target.col + secondColIdx)) || // above row
    (hex.row === target.row && (hex.col === target.col - 1 || hex.col === target.col + 1)) || // same row
    (hex.row === target.row + 1 && (hex.col === target.col + firstColIdx || hex.col === target.col + secondColIdx)); // below row
  if (isNeighbor) {
    return { ...hex, active: !hex.active };
  }

  return hex;
}

export default class Hexagons extends Component {
  componentDidMount() {
    // add click event handler to hexagons
    this.hexGroup = d3.select(this.svg).append('g');
    this.hexGroup.selectAll('.hex')
      .data(this.state.hexData)
      .enter()
        .append('path')
          ...
          .on('click', d => {
            // I'm not a huge fan of D3's global event object,
            // but it's what we get :)
            d3.event.preventDefault();
            d3.event.stopPropagation();
            this.setState({
              hexData: this.state.hexData.map(hex => toggleHex(d, hex))
            });
          })

  }

  componentDidUpdate() {
    // animate hexagon color every time the component's state changes
    const selection = this.hexGroup.selectAll('.hex')
      .data(this.state.hexData)
      .selectAll('path')
        .transition()
        .duration(500)
        .attr('fill', d => (d.active ? 'rgb(255, 1, 175)' : 'rgb(60, 60, 60)'));
  }

  render() {
    ...
  }
}