服务器安全检查的基本步骤

Generally we compare algorithms by
■ Implementing and debugging them
■ Analyzing their basic properties
■ Formulating a hypothesis about comparative performance
■ Running experiments to validate the hypothesis
 

These steps are nothing more than the time-honored scientific method, applied to the study of algorithms.
 
Suppose you have implemented several sorting algorithms, to validate this hypothesis, we use SortCompare to perform the experiments. We use Stopwatch to compute the running time. The implementation of time() shown here does the job for the basic sorts in this chapter. The “randomly ordered” input model is embedded in the timeRandomInput() method in SortCompare, which generates random Double values, sorts them, and returns the total measured time of the sort for the given number of trials. Using random Double values between 0.0 and 1.0 is much simpler than the alternative of using a library function such as StdRandom.shuffle() and is effective because equal key values are very unlikely. The number of trials is taken as an argument both to take advantage of the law of large numbers (the more trials, the total running time divided by the number of trials is a more accurate estimate of the true average running time) and to help damp out system effects. 

public class Stopwatch {

private final long start;

/**
* Create a stopwatch object.
*/
public Stopwatch() {
start = System.currentTimeMillis();
}

/**
* Return elapsed time (in seconds) since this object was created.
*/
public double elapsedTime() {
long now = System.currentTimeMillis();
return (now - start) / 1000.0;
}

}

 

/**
* Replace this line with class description.
* <p/>
* User: George Sun
* Date: 9/15/13
* Time: 9:45 PM
*/
public class SortCompare {

public static void main(String[] args) {
String alg1 = args[0];
String alg2 = args[1];
int N = Integer.parseInt(args[2]);
int T = Integer.parseInt(args[3]);

double t1 = timeRandomInput(alg1, N, T);
double t2 = timeRandomInput(alg2, N, T);
StdOut.printf("For %d random Doubles\n      %s is: ", N, alg1);
StdOut.printf(" %1f times faster than %s\n", t2 / t1, alg2);
}

public static double timeRandomInput(String alg, int N, int T) {
double total = 0.0;
Double[] a = new Double
;
for (int t = 0; t < T; t++) {
for (int i = 0; i < N; i++) {
a[i] = StdRandom.uniform();
}
total += time(alg, a);
}
return total;
}

public static double time(String alg, Comparable[] a) {
Stopwatch timer = new Stopwatch();
switch (alg) {
case "Insertion":
Insertion.sort(a);
break;
case "Selection":
Selection.sort(a);
break;
case "Shell":
Shell.sort(a);
break;
case "Merge":
Merge.sort(a);
break;
case "Quick":
Quick.sort(a);
break;
case "Heap":
Heap.sort(a);
break;
case "InsertionSentinel":
InsertionWithSentinel.sort(a);
break;
default:
System.err.println("No algorithm specified, end.");
break;
}

return timer.elapsedTime();
}
}