package com.alibaba.json.test;
import junit.framework.TestCase;
import java.text.NumberFormat;
import java.util.Random;
/**
* Created by wenshao on 08/01/2017.
*/
public class JavaHash_CollisionTest extends TestCase {
char[] digLetters = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789_".toCharArray();
//char[] digLetters = "0123456789".toCharArray();
Random r = new Random();
int[] powers = new int[10];
{
for (int i = 0; i < powers.length; ++i) {
powers[i] = (int) Math.pow(digLetters.length, i);
}
}
public void test_fnv_hash() throws Exception {
int COUNT = 1000 * 1000 * 1000;
long id_hash_64 = hash64("name".toCharArray());
int id_hash_32 = (int) id_hash_64;
System.out.println("name : " + id_hash_32 + ", " + id_hash_64);
long v = 0;
long time = System.currentTimeMillis();
NumberFormat format = NumberFormat.getInstance();
for (int len = 1; len <= 7; ++len){
char[] chars = new char[len];
long n = (long) Math.pow(digLetters.length, chars.length);
// h = 31 * h + val[i];
for (; v < n; ++v) {
// long hash = 0x811c9dc5;
long hash = 0;
for (int i = 0; i < chars.length; ++i) {
int power = powers[chars.length - i - 1];
int d = (int) ((v / power) % digLetters.length);
char c = digLetters[d];
hash = 31 * hash + c;
// hash ^= c;
// hash *= 0x1000193;
}
if (hash == id_hash_64) {
int hash_32 = (int) hash;
System.out.println("collision : " + build(v, len) + ", hash64 : " + hash + ", hash 32 " + hash_32);
break;
}
if (v != 0 && v % (1000 * 1000 * 100) == 0) {
long now = System.currentTimeMillis();
long millis = now - time;
time = now;
System.out.println("millis : " + millis + ", " + format.format(v));
}
}
System.out.println("end : " + len);
}
}
String build(long v, int len) {
char[] chars = new char[len];
for (int i = 0; i < chars.length; ++i) {
int power = powers[chars.length - i - 1];
int d = (int) ((v / power) % digLetters.length);
chars[i] = digLetters[d];
}
return new String(chars);
}
static long fnv_hash(char[] chars) {
long hash = 0x811c9dc5;
for (int i = 0; i < chars.length; ++i) {
char c = chars[i];
hash ^= c;
hash *= 0x1000193;
}
return hash;
}
static long hash64(char[] chars) {
long hash = 0x811c9dc5;
for (int i = 0; i < chars.length; ++i) {
char c = chars[i];
hash = 31 * hash + c;
}
return hash;
}
}