// @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))
}
