Hash算法

 

据说，Hash的优劣顺序为：BKDRHash, APHash, DJBHash, JSHash, RSHash, SDBMHash, PJWHash, ELFHash。应该是从速度区分吧。因为BKDRHash最快（据说也最好背），ELFHash最准确（冲突率最低）。

 

	public static int BKDRHash(String str) { // BKDR Hash Function

		int seed = 31; // 31 131 1313 13131 131313 etc..

		int hash = 0;

		int i = 0;

		while (i < str.length()) {

			hash = hash * seed + str.charAt(i);

			i++;

		}
		//return (hash & 0x7FFFFFFF);
		return hash;

	}

 

一个HashMap的简单实现：

 

package com.hash.test;

public class TestHash {

	public static int BKDRHash(String str) { // BKDR Hash Function

		int seed = 31; // 31 131 1313 13131 131313 etc..
		int hash = 0;
		int i = 0;
		while (i < str.length()) {
			hash = hash * seed + str.charAt(i);
			i++;
		}
		// return (hash & 0x7FFFFFFF);
		return hash;
	}

	public static String[] data = new String[20];

	public static void put(String str) {
		int key = BKDRHash(str) % 20;
		System.out.println(key);
		data[key] = str;
	}

	/**
	 * Test
	 */
	public static void main(String[] args) {

		String str = "key";
		for (int i = 0; i < 20; i++) {
			put(str + i);
		}
		System.out.println(data[BKDRHash(str + 1) % 20]);

	}

}

 

 

 

附：各种哈希函数的C语言程序代码

 

unsigned int SDBMHash(char *str)


{
	unsigned int hash = 0;
 
	while (*str)
	{
		// equivalent to: hash = 65599*hash + (*str++);
		hash = (*str++) + (hash << 6) + (hash << 16) - hash;
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// RS Hash 
unsigned int RSHash(char *str)
{
	unsigned int b = 378551;
	unsigned int a = 63689;
	unsigned int hash = 0;
 
	while (*str)
	{
		hash = hash * a + (*str++);
		a *= b;
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// JS Hash 
unsigned int JSHash(char *str)
{
	unsigned int hash = 1315423911;
 
	while (*str)
	{
		hash ^= ((hash << 5) + (*str++) + (hash >> 2));
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// P. J. Weinberger Hash 
unsigned int PJWHash(char *str)
{
	unsigned int BitsInUnignedInt = (unsigned int)(sizeof(unsigned int) * 8);
	unsigned int ThreeQuarters	= (unsigned int)((BitsInUnignedInt  * 3) / 4);
	unsigned int OneEighth = (unsigned int)(BitsInUnignedInt / 8);
	unsigned int HighBits = (unsigned int)(0xFFFFFFFF) << (BitsInUnignedInt 

                                               - OneEighth);
	unsigned int hash	= 0;
	unsigned int test	= 0;
 
	while (*str)
	{
		hash = (hash << OneEighth) + (*str++);
		if ((test = hash & HighBits) != 0)
		{
			hash = ((hash ^ (test >> ThreeQuarters)) & (~HighBits));
		}
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// ELF Hash 
unsigned int ELFHash(char *str)
{
	unsigned int hash = 0;
	unsigned int x	= 0;
 
	while (*str)
	{
		hash = (hash << 4) + (*str++);
		if ((x = hash & 0xF0000000L) != 0)
		{
			hash ^= (x >> 24);
			hash &= ~x;
		}
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// BKDR Hash 
unsigned int BKDRHash(char *str)
{
	unsigned int seed = 131; // 31 131 1313 13131 131313 etc..
	unsigned int hash = 0;
 
	while (*str)
	{
		hash = hash * seed + (*str++);
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// DJB Hash 
unsigned int DJBHash(char *str)
{
	unsigned int hash = 5381;
 
	while (*str)
	{
		hash += (hash << 5) + (*str++);
	}
 
	return (hash & 0x7FFFFFFF);
}
 
// AP Hash 
unsigned int APHash(char *str)
{
	unsigned int hash = 0;
	int i;
 
	for (i=0; *str; i++)
	{
		if ((i & 1) == 0)
		{
			hash ^= ((hash << 7) ^ (*str++) ^ (hash >> 3));
		}
		else
		{
			hash ^= (~((hash << 11) ^ (*str++) ^ (hash >> 5)));
		}
	}
 
	return (hash & 0x7FFFFFFF);
}