question: public static double invSqrtQuick ( final double value ) { if ( USE_JDK_MATH ) { return 1 / Math . sqrt ( value ) ; } return Double . longBitsToDouble ( 0x5FE6EB50C7B537A9L - ( Double . doubleToRawLongBits ( value ) > > 1 ) ) ; }
target:  Quick inverse of square root, with a max relative error of about 3.44e-2 for values in [Double.MIN_NORMAL,Double.MAX_VALUE], and worse accuracy outside this range. This implementation uses zero step of Newton's method. Here are the max relative errors on [Double.MIN_NORMAL,Double.MAX_VALUE] depending on number of steps, if you want to copy-paste this code and use your own number: n=0: about 3.44e-2 n=1: about 1.75e-3 n=2: about 4.6e-6 n=3: about 3.17e-11 n=4: about 3.92e-16 n=5: about 3.03e-16