type RGB = { r: number; g: number; b: number }
type HSL = { h: number; s: number; l: number }

function rgbToHsl({ r, g, b }: RGB): HSL {
  r /= 255
  g /= 255
  b /= 255
  const max = Math.max(r, g, b),
    min = Math.min(r, g, b)
  let h: number, s: number
  const l: number = (max + min) / 2

  if (max === min) {
    h = s = 0 // achromatic
  } else {
    const d = max - min
    s = l > 0.5 ? d / (2 - max - min) : d / (max + min)
    switch (max) {
      case r:
        h = (g - b) / d + (g < b ? 6 : 0)
        break
      case g:
        h = (b - r) / d + 2
        break
      case b:
        h = (r - g) / d + 4
        break
    }
    h /= 6
  }

  return { h, s, l }
}

function hslToRgb({ h, s, l }: HSL): RGB {
  let r: number, g: number, b: number

  if (s === 0) {
    r = g = b = l // achromatic
  } else {
    const hue2rgb = (p: number, q: number, t: number) => {
      if (t < 0) t += 1
      if (t > 1) t -= 1
      if (t < 1 / 6) return p + (q - p) * 6 * t
      if (t < 1 / 2) return q
      if (t < 2 / 3) return p + (q - p) * (2 / 3 - t) * 6
      return p
    }

    const q = l < 0.5 ? l * (1 + s) : l + s - l * s
    const p = 2 * l - q
    r = hue2rgb(p, q, h + 1 / 3)
    g = hue2rgb(p, q, h)
    b = hue2rgb(p, q, h - 1 / 3)
  }

  return {
    r: Math.round(r * 255),
    g: Math.round(g * 255),
    b: Math.round(b * 255)
  }
}

function hexToRgb(hex: string): RGB {
  let r = 0,
    g = 0,
    b = 0
  // 3 digits
  if (hex.length == 4) {
    r = parseInt(hex[1] + hex[1], 16)
    g = parseInt(hex[2] + hex[2], 16)
    b = parseInt(hex[3] + hex[3], 16)
  }
  // 6 digits
  else if (hex.length == 7) {
    r = parseInt(hex.slice(1, 3), 16)
    g = parseInt(hex.slice(3, 5), 16)
    b = parseInt(hex.slice(5, 7), 16)
  }
  return { r, g, b }
}

function rgbToHex({ r, g, b }: RGB): string {
  return (
    '#' +
    [r, g, b]
      .map((x) => {
        const hex = x.toString(16)
        return hex.length === 1 ? '0' + hex : hex
      })
      .join('')
  )
}

export function lightenColor(hex: string, amount: number): string {
  let rgb = hexToRgb(hex)
  const hsl = rgbToHsl(rgb)
  hsl.l = Math.min(1, hsl.l + amount)
  rgb = hslToRgb(hsl)
  return rgbToHex(rgb)
}