Hash算法
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2022-07-15 15:39:55
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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);
}
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