code: static float invsqlook(float a) {
          double d = a * (2.f * (float) INVSQ_LOOKUP_SZ) - (float) INVSQ_LOOKUP_SZ;
          int i = (int) d;
          return INVSQ_LOOKUP[i] + ((float) (d - i)) * (INVSQ_LOOKUP[i + 1] - INVSQ_LOOKUP[i]);
      }
docs: /* interpolated 1./sqrt(p) where .5 <= p < 1.