palette example

template<typename PixelType>
struct waveshare5in65f_palette {
private:
    constexpr static gfx::gfx_result index_to_mapped(int idx,PixelType* result) {
        switch(idx) {
            case 0:
                return gfx::convert(gfx::rgb_pixel<16>(0,0,0),result);
            case 1:
                return gfx::convert(gfx::rgb_pixel<16>(31,63,31),result);
            case 2:
                return gfx::convert(gfx::rgb_pixel<16>(0,63,0),result);
            case 3:
                return gfx::convert(gfx::rgb_pixel<16>(0,0,31),result);
            case 4:
                return gfx::convert(gfx::rgb_pixel<16>(31,0,0),result);
            case 5:
                return gfx::convert(gfx::rgb_pixel<16>(31,63,0),result);
            default: //case 6:
                return gfx::convert(gfx::rgb_pixel<16>(31,31,0),result);
        }
    }
public:
    using type = waveshare5in65f_palette;
    using pixel_type = gfx::pixel<gfx::channel_traits<gfx::channel_name::index,4,0,6>>;
    using mapped_pixel_type = PixelType;
    constexpr static const bool writable = false;
    constexpr static const size_t size = 7;
    gfx::gfx_result map(pixel_type pixel,mapped_pixel_type* mapped_pixel) const {
        return index_to_mapped(pixel.channel<gfx::channel_name::index>(),mapped_pixel);
    }
    gfx::gfx_result nearest(mapped_pixel_type mapped_pixel,pixel_type* pixel) const {
        if(nullptr==pixel) {
            return gfx::gfx_result::invalid_argument;
        }
        mapped_pixel_type mpx;
        gfx::gfx_result r = index_to_mapped(0,&mpx);
        if(gfx::gfx_result::success!=r) {
            return r;
        }
        double least = mpx.difference(mapped_pixel);
        if(0.0==least) {
            pixel->native_value = 0;
            return gfx::gfx_result::success;
        }
        int ii=0;
        for(int i = 1;i<size;++i) {
            r=index_to_mapped(i,&mpx);
            if(gfx::gfx_result::success!=r) {
                return r;
            }
            double cmp = mpx.difference(mapped_pixel);
            if(0.0==cmp) {
                ii=i;
                least = 0.0;
                break;
            }
            if(cmp<least) {
                least = cmp;
                ii=i;
            }
        }

        pixel->channel<gfx::channel_name::index>(ii);
        return gfx::gfx_result::success;
    }
};