source: code/trunk/vendor/github.com/alecthomas/chroma/v2/colour.go@ 67

package chroma

import (
        "fmt"
        "math"
        "strconv"
        "strings"
)

// ANSI2RGB maps ANSI colour names, as supported by Chroma, to hex RGB values.
var ANSI2RGB = map[string]string{
        "#ansiblack":     "000000",
        "#ansidarkred":   "7f0000",
        "#ansidarkgreen": "007f00",
        "#ansibrown":     "7f7fe0",
        "#ansidarkblue":  "00007f",
        "#ansipurple":    "7f007f",
        "#ansiteal":      "007f7f",
        "#ansilightgray": "e5e5e5",
        // Normal
        "#ansidarkgray":  "555555",
        "#ansired":       "ff0000",
        "#ansigreen":     "00ff00",
        "#ansiyellow":    "ffff00",
        "#ansiblue":      "0000ff",
        "#ansifuchsia":   "ff00ff",
        "#ansiturquoise": "00ffff",
        "#ansiwhite":     "ffffff",

        // Aliases without the "ansi" prefix, because...why?
        "#black":     "000000",
        "#darkred":   "7f0000",
        "#darkgreen": "007f00",
        "#brown":     "7f7fe0",
        "#darkblue":  "00007f",
        "#purple":    "7f007f",
        "#teal":      "007f7f",
        "#lightgray": "e5e5e5",
        // Normal
        "#darkgray":  "555555",
        "#red":       "ff0000",
        "#green":     "00ff00",
        "#yellow":    "ffff00",
        "#blue":      "0000ff",
        "#fuchsia":   "ff00ff",
        "#turquoise": "00ffff",
        "#white":     "ffffff",
}

// Colour represents an RGB colour.
type Colour int32

// NewColour creates a Colour directly from RGB values.
func NewColour(r, g, b uint8) Colour {
        return ParseColour(fmt.Sprintf("%02x%02x%02x", r, g, b))
}

// Distance between this colour and another.
//
// This uses the approach described here (https://www.compuphase.com/cmetric.htm).
// This is not as accurate as LAB, et. al. but is *vastly* simpler and sufficient for our needs.
func (c Colour) Distance(e2 Colour) float64 {
        ar, ag, ab := int64(c.Red()), int64(c.Green()), int64(c.Blue())
        br, bg, bb := int64(e2.Red()), int64(e2.Green()), int64(e2.Blue())
        rmean := (ar + br) / 2
        r := ar - br
        g := ag - bg
        b := ab - bb
        return math.Sqrt(float64((((512 + rmean) * r * r) >> 8) + 4*g*g + (((767 - rmean) * b * b) >> 8)))
}

// Brighten returns a copy of this colour with its brightness adjusted.
//
// If factor is negative, the colour is darkened.
//
// Uses approach described here (http://www.pvladov.com/2012/09/make-color-lighter-or-darker.html).
func (c Colour) Brighten(factor float64) Colour {
        r := float64(c.Red())
        g := float64(c.Green())
        b := float64(c.Blue())

        if factor < 0 {
                factor++
                r *= factor
                g *= factor
                b *= factor
        } else {
                r = (255-r)*factor + r
                g = (255-g)*factor + g
                b = (255-b)*factor + b
        }
        return NewColour(uint8(r), uint8(g), uint8(b))
}

// BrightenOrDarken brightens a colour if it is < 0.5 brightness or darkens if > 0.5 brightness.
func (c Colour) BrightenOrDarken(factor float64) Colour {
        if c.Brightness() < 0.5 {
                return c.Brighten(factor)
        }
        return c.Brighten(-factor)
}

// ClampBrightness returns a copy of this colour with its brightness adjusted such that
// it falls within the range [min, max] (or very close to it due to rounding errors).
// The supplied values use the same [0.0, 1.0] range as Brightness.
func (c Colour) ClampBrightness(min, max float64) Colour {
        if !c.IsSet() {
                return c
        }

        min = math.Max(min, 0)
        max = math.Min(max, 1)
        current := c.Brightness()
        target := math.Min(math.Max(current, min), max)
        if current == target {
                return c
        }

        r := float64(c.Red())
        g := float64(c.Green())
        b := float64(c.Blue())
        rgb := r + g + b
        if target > current {
                // Solve for x: target == ((255-r)*x + r + (255-g)*x + g + (255-b)*x + b) / 255 / 3
                return c.Brighten((target*255*3 - rgb) / (255*3 - rgb))
        }
        // Solve for x: target == (r*(x+1) + g*(x+1) + b*(x+1)) / 255 / 3
        return c.Brighten((target*255*3)/rgb - 1)
}

// Brightness of the colour (roughly) in the range 0.0 to 1.0.
func (c Colour) Brightness() float64 {
        return (float64(c.Red()) + float64(c.Green()) + float64(c.Blue())) / 255.0 / 3.0
}

// ParseColour in the forms #rgb, #rrggbb, #ansi<colour>, or #<colour>.
// Will return an "unset" colour if invalid.
func ParseColour(colour string) Colour {
        colour = normaliseColour(colour)
        n, err := strconv.ParseUint(colour, 16, 32)
        if err != nil {
                return 0
        }
        return Colour(n + 1)
}

// MustParseColour is like ParseColour except it panics if the colour is invalid.
//
// Will panic if colour is in an invalid format.
func MustParseColour(colour string) Colour {
        parsed := ParseColour(colour)
        if !parsed.IsSet() {
                panic(fmt.Errorf("invalid colour %q", colour))
        }
        return parsed
}

// IsSet returns true if the colour is set.
func (c Colour) IsSet() bool { return c != 0 }

func (c Colour) String() string   { return fmt.Sprintf("#%06x", int(c-1)) }
func (c Colour) GoString() string { return fmt.Sprintf("Colour(0x%06x)", int(c-1)) }

// Red component of colour.
func (c Colour) Red() uint8 { return uint8(((c - 1) >> 16) & 0xff) }

// Green component of colour.
func (c Colour) Green() uint8 { return uint8(((c - 1) >> 8) & 0xff) }

// Blue component of colour.
func (c Colour) Blue() uint8 { return uint8((c - 1) & 0xff) }

// Colours is an orderable set of colours.
type Colours []Colour

func (c Colours) Len() int           { return len(c) }
func (c Colours) Swap(i, j int)      { c[i], c[j] = c[j], c[i] }
func (c Colours) Less(i, j int) bool { return c[i] < c[j] }

// Convert colours to #rrggbb.
func normaliseColour(colour string) string {
        if ansi, ok := ANSI2RGB[colour]; ok {
                return ansi
        }
        if strings.HasPrefix(colour, "#") {
                colour = colour[1:]
                if len(colour) == 3 {
                        return colour[0:1] + colour[0:1] + colour[1:2] + colour[1:2] + colour[2:3] + colour[2:3]
                }
        }
        return colour
}