Log2vsApproxLog2.java

package test;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Random;
import java.util.stream.IntStream;

public class Log2vsApproxLog2 {

	public static void main(String[] args) {
		final int TOTAL_REPETITIONS = 1000;
		
		final int RUNS = 10;
		final int REPEATS_PER_RUN = 100000;
		
		final int WARMUPS = 5;
		final int ARRAY_SIZE = 100000000;
		
		Map<String, List<Long>> totalStats = new HashMap<>();
		for(int mode=0; mode<2; mode++) {
			String key = mode%2 == 0 ? "log2" : "approxLog2";
			totalStats.put(key, new ArrayList<Long>());
		}
		
		IntStream.range(0, TOTAL_REPETITIONS + WARMUPS).sequential().forEach(repetition -> {
			Random r = new Random(System.currentTimeMillis());
			
			Timer timer = new Timer();
			double[] randoms = new double[ARRAY_SIZE];
			for(int i=0; i<randoms.length; i++)
				randoms[i] = r.nextDouble();
			System.out.println(String.format("\ngenerating %d random double values took %d ms", randoms.length, timer.durationAsMilliS()));
			
			Map<String, List<Long>> runStats = new HashMap<>();
			Timer nt;
			
			for(int mode=0; mode<2; mode++) {
				String key = (mode + repetition) %2 == 0 ? "log2" : "approxLog2";
				if(!runStats.containsKey(key))
					runStats.put(key, new ArrayList<Long>());
				
				for(int run = 0; run < RUNS + WARMUPS; run++) {
					if(key.equals("log2")) {
						nt = new Timer();
						for(int i=0; i<REPEATS_PER_RUN; i++)
							for(double d: randoms)
								log2(d);
						long time = nt.durationAsNanoS();
						if(run >= WARMUPS)
							runStats.get(key).add(time);
					} else {
						nt = new Timer();
						for(int i=0; i<REPEATS_PER_RUN; i++)
							for(double d: randoms)
								approxLog2(d);
						long time = nt.durationAsNanoS();
						if(run >= WARMUPS)
							runStats.get(key).add(time);
					}
				}
			}
			
			if(repetition >= WARMUPS) {
				System.out.println("Stats: " + runStats);
				for(String key: runStats.keySet()) {
					long average = (long)runStats.get(key).stream().mapToLong((x) -> x).summaryStatistics().getAverage();
					totalStats.get(key).add(average);
					System.out.println("Average time for " + key + " -> "	+ average + " ns");
				}
			}  else {
				System.out.println("warmup round");
			}
			
		});
		
		System.out.println("\n\nFINAL RESULTS FOR " + TOTAL_REPETITIONS + " REPETITIONS");
		for(String key: totalStats.keySet()) 
			System.out.println(String.format("Total Stats time for %s -> %s", key, totalStats.get(key).stream().mapToLong((x) -> x).summaryStatistics()));
		
	}
	

	static final double lg2 = Math.log(2);
	
	public static double log2(double n) {
		return (Math.log(n) / lg2);
	}
	
    public static double approxLog2(double value) {
        final long valueBits = Double.doubleToRawLongBits(value);
        final long exponent = ((valueBits & 0x7ff0_0000_0000_0000L) >>> 52) - 1024;
        final double m = Double.longBitsToDouble((valueBits & 0x800fffffffffffffL) | 0x3ff0000000000000L);
        return (m * (2 - (1.0 / 3) * m) + exponent - (2.0 / 3.0));
    } 

    
	static class Timer {
		final static long KILO = 1000L;
		
		private long start;
		
		public Timer() {
			start = System.nanoTime();
		}
		
		public long durationAsNanoS() {
			return System.nanoTime() - start;
		}
		
		public long durationAsMicroS() {
			return durationAsNanoS()/KILO;
		}
		
		public long durationAsMilliS() {
			return durationAsMicroS()/KILO;
		}
	}
	
}