code: private static double interpolateUsingXAndYTables(final double[] xArr, final double[] yArr,
        final int offset, final double x) {
      return cubicInterpolate(xArr[offset], yArr[offset], xArr[offset + 1], yArr[offset + 1],
          xArr[offset + 2], yArr[offset + 2], xArr[offset + 3], yArr[offset + 3], x);
    }
docs: In C: again-two-registers cubic_interpolate_aux L1368