How to implement the “fast inverse square root” in Java?

Question

I've heard of the "fast inverse square root", discussed here, and I wanted to put it in my Java program (just for research purposes, so ignore anything about the native libraries being faster).

I was looking at the code, and the C code directly converts the float into an int with some C pointer magic. If you try to do this in Java with casts, it doesn't work: java truncates the float (as you would expect), and you can't get the pointer of a primitive (as you can in C). So how do you do this?

Answer 1

Remember to benchmark your code before using this.

If it turns out you don't need it, or it's slower on the CPU architecture you are using, then it's better to go without having this obtuse code in your project.

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The Java libraries have a way to get from the float number to the raw bits.

As seen in the Javadoc for java.lang.Float ( http://docs.oracle.com/javase/6/docs/api/java/lang/Float.html ), we have the floatToIntBits function, as well as intBitsToFloat.

This means we can write the "fast inverse square root" in Java as follows:

public static float invSqrt(float x) {
    float xhalf = 0.5f * x;
    int i = Float.floatToIntBits(x);
    i = 0x5f3759df - (i >> 1);
    x = Float.intBitsToFloat(i);
    x *= (1.5f - xhalf * x * x);
    return x;
}

Here is the version for doubles:

public static double invSqrt(double x) {
    double xhalf = 0.5d * x;
    long i = Double.doubleToLongBits(x);
    i = 0x5fe6ec85e7de30daL - (i >> 1);
    x = Double.longBitsToDouble(i);
    x *= (1.5d - xhalf * x * x);
    return x;
}

Source: http://www.actionscript.org/forums/showthread.php3?t=142537

Answer 2

For Riking's answer, even the double one can return stuff like 0.9983227945440889 for the square root of one.

To increase accuracy, you can use this version of it I made:

public static double Q_rsqrt(double number){
    double x = number;
    double xhalf = 0.5d*x;
    long i = Double.doubleToLongBits(x);
    i = 0x5fe6ec85e7de30daL - (i>>1);
    x = Double.longBitsToDouble(i);
    for(int it = 0; it < 4; it++){
        x = x*(1.5d - xhalf*x*x);
    }
    x *= number;
    return x;
}

You can edit how long before the for loop terminates however you want, but 4 times seems to get it down to the maxiumum accuracy for a double. If you want perfect accuracy (or if long strings of decimals where they shouldnt be bother you), use this version.