qmk-firmware/quantum/rgb_matrix_animations/solid_reactive_cross.h

#ifdef RGB_MATRIX_KEYREACTIVE_ENABLED
#if !defined(DISABLE_RGB_MATRIX_SOLID_REACTIVE_CROSS) || !defined(DISABLE_RGB_MATRIX_SOLID_REACTIVE_MULTICROSS)

#ifndef DISABLE_RGB_MATRIX_SOLID_REACTIVE_CROSS
RGB_MATRIX_EFFECT(SOLID_REACTIVE_CROSS)
#endif

#ifndef DISABLE_RGB_MATRIX_SOLID_REACTIVE_MULTICROSS
RGB_MATRIX_EFFECT(SOLID_REACTIVE_MULTICROSS)
#endif

#ifdef RGB_MATRIX_CUSTOM_EFFECT_IMPLS

static HSV SOLID_REACTIVE_CROSS_math(HSV hsv,  int16_t dx, int16_t dy, uint8_t dist, uint16_t tick) {
    uint16_t effect = tick + dist;
    dx = dx < 0 ? dx * -1 : dx;
    dy = dy < 0 ? dy * -1 : dy;
    dx = dx * 16 > 255 ? 255 : dx * 16;
    dy = dy * 16 > 255 ? 255 : dy * 16;
    effect += dx > dy ? dy : dx;
    if (effect > 255)
        effect = 255;
    hsv.v = qadd8(hsv.v, 255 - effect);
    return hsv;
}

#ifndef DISABLE_RGB_MATRIX_SOLID_REACTIVE_CROSS
bool SOLID_REACTIVE_CROSS(effect_params_t* params) {
    return effect_runner_reactive_splash(qsub8(g_last_hit_tracker.count, 1), params, &SOLID_REACTIVE_CROSS_math);
}
#endif

#ifndef DISABLE_RGB_MATRIX_SOLID_REACTIVE_MULTICROSS
bool SOLID_REACTIVE_MULTICROSS(effect_params_t* params) {
    return effect_runner_reactive_splash(0, params, &SOLID_REACTIVE_CROSS_math);
}
#endif

#endif // RGB_MATRIX_CUSTOM_EFFECT_IMPLS
#endif // !defined(DISABLE_RGB_MATRIX_SOLID_REACTIVE_CROSS) || !defined(DISABLE_RGB_MATRIX_SOLID_REACTIVE_MULTICROSS)
#endif // RGB_MATRIX_KEYREACTIVE_ENABLED