colette.go

// Convert between truecolor, xterm(1) 256 color and 16 color.
// Single-header library.
// Arranged from [tmux].
//
// [tmux]: https://github.com/tmux/tmux
package colette

// RgbTo256 converts an RGB triplet to the xterm(1) 256 color palette.
//
// xterm provides a 6x6x6 color cube (16 - 231) and 24 greys (232 - 255). We
// map our RGB color to the closest in the cube, also work out the closest
// grey, and use the nearest of the two.
//
// Note that the xterm has much lower resolution for darker colors (they are
// not evenly spread out), so our 6 levels are not evenly spread: 0x0, 0x5f
// (95), 0x87 (135), 0xaf (175), 0xd7 (215) and 0xff (255). Greys are more
// evenly spread (8, 18, 28 ... 238).
func RgbTo256(r, g, b byte) int {
	q2c := []int{0x00, 0x5f, 0x87, 0xaf, 0xd7, 0xff}
	ir := int(r)
	ig := int(g)
	ib := int(b)
	var qr, qg, qb, cr, cg, cb, idx, greyAvg, greyIdx, grey int

	/* Map RGB to 6x6x6 cube. */
	qr = colorTo6cube(ir)
	cr = q2c[qr]
	qg = colorTo6cube(ig)
	cg = q2c[qg]
	qb = colorTo6cube(ib)
	cb = q2c[qb]

	/* If we have hit the color exactly, return early. */
	if cr == ir && cg == ig && cb == ib {
		return 16 + 36 * qr + 6 * qg + qb
	}

	/* Work out the closest grey (average of RGB). */
	greyAvg = (ir + ig + ib) / 3
	if greyAvg > 238 {
		greyIdx = 23
	} else {
		greyIdx = (greyAvg - 3) / 10
	}
	grey = 8 + (10 * greyIdx)

	/* Is grey or 6x6x6 color closest? */
	if distSq(grey, grey, grey, ir, ig, ib) < distSq(cr, cg, cb, ir, ig, ib) {
		idx = 232 + greyIdx
	} else {
		idx = 16 + 36*qr + 6*qg + qb
	}

	return idx
}

// JoinRgb joins RGB into a color.
func JoinRgb(r, g, b byte) int {
	return int(r)<<16 | int(g)<<8 | int(b)
}

// SplitRgb converts 256 color to RGB color.
func SplitRgb(c int) (r, g, b byte) {
	return byte(c >> 16), byte(c >> 8), byte(c)
}

// Color256ToRgb converts 256 color to RGB color.
func Color256ToRgb(c byte) int {
	return color256ToRgbTable[c]
}

// Color256To16 converts 256 color to 16 color.
func Color256To16(c byte) byte {
	return color256To16Table[c]
}

func distSq(R, G, B, r, g, b int) int {
	return (R-r)*(R-r) + (G-g)*(G-g) + (B-b)*(B-b)
}

func colorTo6cube(v int) int {
	if v < 48 {
		return 0
	}
	if v < 114 {
		return 1
	}
	return (v - 35) / 40
}

var (
	color256ToRgbTable = []int{
		0x000000, 0x800000, 0x008000, 0x808000,
		0x000080, 0x800080, 0x008080, 0xc0c0c0,
		0x808080, 0xff0000, 0x00ff00, 0xffff00,
		0x0000ff, 0xff00ff, 0x00ffff, 0xffffff,
		0x000000, 0x00005f, 0x000087, 0x0000af,
		0x0000d7, 0x0000ff, 0x005f00, 0x005f5f,
		0x005f87, 0x005faf, 0x005fd7, 0x005fff,
		0x008700, 0x00875f, 0x008787, 0x0087af,
		0x0087d7, 0x0087ff, 0x00af00, 0x00af5f,
		0x00af87, 0x00afaf, 0x00afd7, 0x00afff,
		0x00d700, 0x00d75f, 0x00d787, 0x00d7af,
		0x00d7d7, 0x00d7ff, 0x00ff00, 0x00ff5f,
		0x00ff87, 0x00ffaf, 0x00ffd7, 0x00ffff,
		0x5f0000, 0x5f005f, 0x5f0087, 0x5f00af,
		0x5f00d7, 0x5f00ff, 0x5f5f00, 0x5f5f5f,
		0x5f5f87, 0x5f5faf, 0x5f5fd7, 0x5f5fff,
		0x5f8700, 0x5f875f, 0x5f8787, 0x5f87af,
		0x5f87d7, 0x5f87ff, 0x5faf00, 0x5faf5f,
		0x5faf87, 0x5fafaf, 0x5fafd7, 0x5fafff,
		0x5fd700, 0x5fd75f, 0x5fd787, 0x5fd7af,
		0x5fd7d7, 0x5fd7ff, 0x5fff00, 0x5fff5f,
		0x5fff87, 0x5fffaf, 0x5fffd7, 0x5fffff,
		0x870000, 0x87005f, 0x870087, 0x8700af,
		0x8700d7, 0x8700ff, 0x875f00, 0x875f5f,
		0x875f87, 0x875faf, 0x875fd7, 0x875fff,
		0x878700, 0x87875f, 0x878787, 0x8787af,
		0x8787d7, 0x8787ff, 0x87af00, 0x87af5f,
		0x87af87, 0x87afaf, 0x87afd7, 0x87afff,
		0x87d700, 0x87d75f, 0x87d787, 0x87d7af,
		0x87d7d7, 0x87d7ff, 0x87ff00, 0x87ff5f,
		0x87ff87, 0x87ffaf, 0x87ffd7, 0x87ffff,
		0xaf0000, 0xaf005f, 0xaf0087, 0xaf00af,
		0xaf00d7, 0xaf00ff, 0xaf5f00, 0xaf5f5f,
		0xaf5f87, 0xaf5faf, 0xaf5fd7, 0xaf5fff,
		0xaf8700, 0xaf875f, 0xaf8787, 0xaf87af,
		0xaf87d7, 0xaf87ff, 0xafaf00, 0xafaf5f,
		0xafaf87, 0xafafaf, 0xafafd7, 0xafafff,
		0xafd700, 0xafd75f, 0xafd787, 0xafd7af,
		0xafd7d7, 0xafd7ff, 0xafff00, 0xafff5f,
		0xafff87, 0xafffaf, 0xafffd7, 0xafffff,
		0xd70000, 0xd7005f, 0xd70087, 0xd700af,
		0xd700d7, 0xd700ff, 0xd75f00, 0xd75f5f,
		0xd75f87, 0xd75faf, 0xd75fd7, 0xd75fff,
		0xd78700, 0xd7875f, 0xd78787, 0xd787af,
		0xd787d7, 0xd787ff, 0xd7af00, 0xd7af5f,
		0xd7af87, 0xd7afaf, 0xd7afd7, 0xd7afff,
		0xd7d700, 0xd7d75f, 0xd7d787, 0xd7d7af,
		0xd7d7d7, 0xd7d7ff, 0xd7ff00, 0xd7ff5f,
		0xd7ff87, 0xd7ffaf, 0xd7ffd7, 0xd7ffff,
		0xff0000, 0xff005f, 0xff0087, 0xff00af,
		0xff00d7, 0xff00ff, 0xff5f00, 0xff5f5f,
		0xff5f87, 0xff5faf, 0xff5fd7, 0xff5fff,
		0xff8700, 0xff875f, 0xff8787, 0xff87af,
		0xff87d7, 0xff87ff, 0xffaf00, 0xffaf5f,
		0xffaf87, 0xffafaf, 0xffafd7, 0xffafff,
		0xffd700, 0xffd75f, 0xffd787, 0xffd7af,
		0xffd7d7, 0xffd7ff, 0xffff00, 0xffff5f,
		0xffff87, 0xffffaf, 0xffffd7, 0xffffff,
		0x080808, 0x121212, 0x1c1c1c, 0x262626,
		0x303030, 0x3a3a3a, 0x444444, 0x4e4e4e,
		0x585858, 0x626262, 0x6c6c6c, 0x767676,
		0x808080, 0x8a8a8a, 0x949494, 0x9e9e9e,
		0xa8a8a8, 0xb2b2b2, 0xbcbcbc, 0xc6c6c6,
		0xd0d0d0, 0xdadada, 0xe4e4e4, 0xeeeeee,
	}

	color256To16Table = []byte{
		 0,  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15,
		 0,  4,  4,  4, 12, 12,  2,  6,  4,  4, 12, 12,  2,  2,  6,  4,
		12, 12,  2,  2,  2,  6, 12, 12, 10, 10, 10, 10, 14, 12, 10, 10,
		10, 10, 10, 14,  1,  5,  4,  4, 12, 12,  3,  8,  4,  4, 12, 12,
		 2,  2,  6,  4, 12, 12,  2,  2,  2,  6, 12, 12, 10, 10, 10, 10,
		14, 12, 10, 10, 10, 10, 10, 14,  1,  1,  5,  4, 12, 12,  1,  1,
		 5,  4, 12, 12,  3,  3,  8,  4, 12, 12,  2,  2,  2,  6, 12, 12,
		10, 10, 10, 10, 14, 12, 10, 10, 10, 10, 10, 14,  1,  1,  1,  5,
		12, 12,  1,  1,  1,  5, 12, 12,  1,  1,  1,  5, 12, 12,  3,  3,
		 3,  7, 12, 12, 10, 10, 10, 10, 14, 12, 10, 10, 10, 10, 10, 14,
		 9,  9,  9,  9, 13, 12,  9,  9,  9,  9, 13, 12,  9,  9,  9,  9,
		13, 12,  9,  9,  9,  9, 13, 12, 11, 11, 11, 11,  7, 12, 10, 10,
		10, 10, 10, 14,  9,  9,  9,  9,  9, 13,  9,  9,  9,  9,  9, 13,
		 9,  9,  9,  9,  9, 13,  9,  9,  9,  9,  9, 13,  9,  9,  9,  9,
		 9, 13, 11, 11, 11, 11, 11, 15,  0,  0,  0,  0,  0,  0,  8,  8,
		 8,  8,  8,  8,  7,  7,  7,  7,  7,  7, 15, 15, 15, 15, 15, 15,
	}
)