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The DSI utilities are a mishmash of classes accumulated during the last twenty years in projects developed at the DSI (Dipartimento di Scienze dell'Informazione, i.e., Information Sciences Department), now DI (Dipartimento di Informatica, i.e., Informatics Department), of the Universita` degli Studi di Milano.
package it.unimi.dsi.test;
import it.unimi.dsi.util.XoRoShiRo128PlusRandomGenerator;
public class LeastSignificantBitSpeedTest {
/**
* The set of least-significant bits for a given byte. -1
* is used if no bits are set (so as to not be confused with "index of zero"
* meaning that the least significant bit is the 0th (1st) bit).
*
* @see #leastSignificantBit(long)
*/
private static final int[] LEAST_SIGNIFICANT_BIT = {
-1, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
7, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
6, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
5, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0,
4, 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0
};
/**
* Computes the least-significant bit of the specified long
* that is set to 1. Zero-indexed.
*
* @param value the long whose least-significant bit is desired.
* @return the least-significant bit of the specified long.
* -1 is returned if there are no bits set.
*/
// REF: http://stackoverflow.com/questions/757059/position-of-least-significant-bit-that-is-set
// REF: http://www-graphics.stanford.edu/~seander/bithacks.html
public static int leastSignificantBit(final long value) {
if(value == 0L) return -1/*by contract*/;
if((value & 0xFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 0) & 0xFF)] + 0;
if((value & 0xFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 8) & 0xFF)] + 8;
if((value & 0xFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 16) & 0xFF)] + 16;
if((value & 0xFFFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 24) & 0xFF)] + 24;
if((value & 0xFFFFFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 32) & 0xFF)] + 32;
if((value & 0xFFFFFFFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 40) & 0xFF)] + 40;
if((value & 0xFFFFFFFFFFFFFFL) != 0) return LEAST_SIGNIFICANT_BIT[(int)((value >>> 48) & 0xFF)] + 48;
return LEAST_SIGNIFICANT_BIT[(int)((value >>> 56) & 0xFFL)] + 56;
}
public static int javaLsb(final long value) {
return value == 0 ? -1 : Long.numberOfTrailingZeros(value);
}
private final static byte[] LSB_TABLE = {
0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4, 62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5, 63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11, 54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6
};
private static int deBrujin(long x) {
return LSB_TABLE[(int)(((x & -x) * 0x03f79d71b4ca8b09L) >>> 58)];
}
public static void main(final String a[]) {
final int n = Integer.parseInt(a[0]);
final XoRoShiRo128PlusRandomGenerator r = new XoRoShiRo128PlusRandomGenerator(1);
long start, elapsed;
int x = 42;
final long value[] = new long[n];
for(int i = n; i-- != 0;) value[i] = r.nextLong();
for(int k = 10; k-- !=0;) {
System.out.print("Java: ");
start = System.nanoTime();
for(int i = n; i-- != 0;) x ^= javaLsb(value[i]);
elapsed = System.nanoTime() - start;
System.out.println("elapsed " + elapsed + ", " + (double)elapsed / n + " ns/call");
System.out.print("Test-based: ");
start = System.nanoTime();
for(int i = n; i-- != 0;) x ^= leastSignificantBit(value[i]);
elapsed = System.nanoTime() - start;
System.out.println("elapsed " + elapsed + ", " + (double)elapsed / n + " ns/call");
System.out.print("De Brujin: ");
start = System.nanoTime();
for(int i = n; i-- != 0;) x ^= deBrujin(value[i]);
elapsed = System.nanoTime() - start;
System.out.println("elapsed " + elapsed + ", " + (double)elapsed / n + " ns/call");
}
if (x == 0) System.out.println(0);
}
}