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BirthdayParadoxTest.java
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BirthdayParadoxTest.java
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import java.util.Random;
public class BirthdayParadoxTest {
public static void main(String args[]) {
//Test to see if the birthday paradox holds for Java hashcode() by
//counting the number of rounds to find a collision.
//If we compare the hashes of 1.2*2^16 strings per round, the average
//number of rounds should be 2.
if(args.length != 1) {
System.out.println("Usage: java BirthdayParadoxTest [iterations]");
System.out.println("Exiting.");
return;
}
try {
int iterations = Integer.parseInt(args[0]);
} catch(NumberFormatException e) {
e.printStackTrace();
System.out.println("\nUsage: java BirthdayParadoxTest [iterations]");
System.out.println("iterations must be an integer. Exiting.");
return;
}
int iterations = Integer.parseInt(args[0]);
int totalRounds = 0;
for(int i = 0; i < iterations; i++) {
totalRounds += roundsToFindCollision();
System.out.print(".");
}
System.out.println("\nAverage number of rounds: " + (totalRounds / (float)iterations));
}
public static int roundsToFindCollision() {
//2^16 * 1.2 = 78643.2
String[] msg = new String[78644];
int[] hashes = new int[78644];
Random r = new Random();
int loopCounter = 1;
while(true) { //loop until a collision is found
for(int i = 0; i < 78644; i++) {
msg[i] = "";
for(int j = 0; j < r.nextInt(20) + 10; j++) {
// generate random string, hash the string
msg[i] += (char)(r.nextInt(74) + '0');
hashes[i] = msg[i].hashCode();
}
}
for(int i = 0; i < 78644; i++) {
for(int j = i + 1; j < 78644; j++) {
if(hashes[i] == hashes[j]) {
return loopCounter; //Found a collision, exit method.
}
}
}
loopCounter++;
}
}
}