/*
* Compute entropy value for an array of bytes.
*
* The returned number represents entropy per bit!
* For good long number seed (8bytes seed) it should be in range <2.75,3> (higher is better)
*
* For large set of bytes (>100) it should be almost 8 (means almost 8 random bits per byte).
*/
public static float entropy(byte[] f) {
int counts[] = new int[256];
float entropy = 0;
float total = f.length;
for (byte b : f) counts[b+128]++;
for (int c : counts) {
if (c == 0) continue;
float p = c / total;
/* Compute entropy per bit in byte.
*
* To compute entropy per byte compute log with base 256 = log(p)/log(256).
*/
entropy -= p * Math.log(p)/Math.log(2);
}
return entropy;
}
public static double lnF(double what) {
return (what < 1e-06) ? 0 : what * Math.log(what);
}