rgb_to_xterm256.h

unsigned char
rgb_to_xterm256(unsigned char r, unsigned char g, unsigned char b) {

    unsigned int color_id = 0;
    unsigned char half_color = (r >= 0x80 && r < 0x87) << 2 |
                               (g >= 0x80 && g < 0x87) << 1 |
                               (b >= 0x80 && b < 0x87);

    static const unsigned char gray_brightness[] = {
        0x00, 0x08, 0x12, 0x1c, 0x26, 0x30, 0x3a, 0x44, 0x4e, 0x58, 0x5f,
        0x62, 0x6c, 0x76, 0x80, 0x87, 0x8a, 0x94, 0x9e, 0xa8, 0xaf, 0xb2,
        0xbc, 0xc0, 0xc6, 0xd0, 0xd7, 0xda, 0xe4, 0xee, 0xff
    };

    static const unsigned char gray_colors[] = {
        0x00, 0xe8, 0xe9, 0xea, 0xeb, 0xec, 0xed, 0xee, 0xef, 0xf0, 0x3b,
        0xf1, 0xf2, 0xf3, 0xf4, 0x66, 0xf5, 0xf6, 0xf7, 0xf8, 0x91, 0xf9,
        0xfa, 0x07, 0xfb, 0xfc, 0xbc, 0xfd, 0xfe, 0xff, 0x0f
    };

    static const unsigned char term_colors[] = { 0x04, 0x02, 0x06,
                                                 0x01, 0x05, 0x03 };

    if (r == g && g == b) {
        unsigned char i = 0;

        while (gray_brightness[i] < r) {
            i++;
        }

        color_id = gray_colors[i];
    } else if (half_color) {
        color_id = term_colors[half_color - 1];
    } else {
        r = ((r << 2) + r) / 255;
        g = ((g << 2) + g) / 255;
        b = ((b << 2) + b) / 255;
        color_id = 16 + ((r << 5) + (r << 2)) + ((g << 2) + (g << 1)) + b;
    }

    return color_id;
}