text: // @flow
  import parseToRgb from './parseToRgb'
  
  /**
   * Returns a number (float) representing the luminance of a color.
   *
   * @example
   * // Styles as object usage
   * const styles = {
   *   background: getLuminance('#CCCD64') >= getLuminance('#0000ff') ? '#CCCD64' : '#0000ff',
   *   background: getLuminance('rgba(58, 133, 255, 1)') >= getLuminance('rgba(255, 57, 149, 1)') ?
   *                             'rgba(58, 133, 255, 1)' :
   *                             'rgba(255, 57, 149, 1)',
   * }
   *
   * // styled-components usage
   * const div = styled.div`
   *   background: ${getLuminance('#CCCD64') >= getLuminance('#0000ff') ? '#CCCD64' : '#0000ff'};
   *   background: ${getLuminance('rgba(58, 133, 255, 1)') >= getLuminance('rgba(255, 57, 149, 1)') ?
   *                             'rgba(58, 133, 255, 1)' :
   *                             'rgba(255, 57, 149, 1)'};
   *
   * // CSS in JS Output
   *
   * div {
   *   background: "#CCCD64";
   *   background: "rgba(58, 133, 255, 1)";
   * }
   */
  export default function getLuminance(color: string): number {
    if (color === 'transparent') return 0
    const rgbColor: { [string]: number } = parseToRgb(color)
    const [r, g, b] = Object.keys(rgbColor).map(key => {
      const channel = rgbColor[key] / 255
      return channel <= 0.03928
        ? channel / 12.92
        : ((channel + 0.055) / 1.055) ** 2.4
    })
    return parseFloat((0.2126 * r + 0.7152 * g + 0.0722 * b).toFixed(3))
  }
  