1、hash函數廣泛用於密碼學中,比如銀行客戶的密碼通過函數函數轉換得到一個加密過的新密碼,把這個信息密碼保存在數據庫中,每次客戶登錄時,都會將密碼經過hash算法轉換後,再次與數據庫中的密碼進行比較。由於hash算法是不可逆的,所以可以保證即使得到了數據庫中的密碼,也無法破解得到原先的密碼。
2、hash還可以用來搜索,即用生成的hash值來表示所存儲內容的存儲器中的位置,這樣可以將搜索的時間複雜度降低到O(1)。
下面這些內容均爲轉載:
所有內容均來自於:http://www.partow.net/programming/hashfunctions/
以下總共給出了10種字符串hash函數,在上面的鏈接中可以找到各個Hash函數的描述,以及一些其它的關於hash函數的資料下載。
這些hash函數各自的優缺點不詳。其中,ELFHash函數是在unix系統中被廣泛使用的,也是《算法藝術與信息學競賽》中作者推薦的一個字符串hash函數。DEKHash函數是Knuth在其《The Art of Computer Programming》第三卷中引入的。
- unsigned int RSHash(char* str, unsigned int len)
- {
- unsigned int b = 378551;
- unsigned int a = 63689;
- unsigned int hash = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = hash * a + (*str);
- a = a * b;
- }
- return hash;
- }
- /* End Of RS Hash Function */
- unsigned int JSHash(char* str, unsigned int len)
- {
- unsigned int hash = 1315423911;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash ^= ((hash << 5) + (*str) + (hash >> 2));
- }
- return hash;
- }
- /* End Of JS Hash Function */
- unsigned int PJWHash(char* str, unsigned int len)
- {
- const unsigned int BitsInUnsignedInt = (unsigned int)(sizeof(unsigned int) * 8);
- const unsigned int ThreeQuarters = (unsigned int)((BitsInUnsignedInt * 3) / 4);
- const unsigned int OneEighth = (unsigned int)(BitsInUnsignedInt / 8);
- const unsigned int HighBits = (unsigned int)(0xFFFFFFFF) << (BitsInUnsignedInt - OneEighth);
- unsigned int hash = 0;
- unsigned int test = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = (hash << OneEighth) + (*str);
- if((test = hash & HighBits) != 0)
- {
- hash = (( hash ^ (test >> ThreeQuarters)) & (~HighBits));
- }
- }
- return hash;
- }
- /* End Of P. J. Weinberger Hash Function */
- unsigned int ELFHash(char* str, unsigned int len)
- {
- unsigned int hash = 0;
- unsigned int x = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = (hash << 4) + (*str);
- if((x = hash & 0xF0000000L) != 0)
- {
- hash ^= (x >> 24);
- }
- hash &= ~x;
- }
- return hash;
- }
- /* End Of ELF Hash Function */
- unsigned int BKDRHash(char* str, unsigned int len)
- {
- unsigned int seed = 131; /* 31 131 1313 13131 131313 etc.. */
- unsigned int hash = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = (hash * seed) + (*str);
- }
- return hash;
- }
- /* End Of BKDR Hash Function */
- unsigned int SDBMHash(char* str, unsigned int len)
- {
- unsigned int hash = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = (*str) + (hash << 6) + (hash << 16) - hash;
- }
- return hash;
- }
- /* End Of SDBM Hash Function */
- unsigned int DJBHash(char* str, unsigned int len)
- {
- unsigned int hash = 5381;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = ((hash << 5) + hash) + (*str);
- }
- return hash;
- }
- /* End Of DJB Hash Function */
- unsigned int DEKHash(char* str, unsigned int len)
- {
- unsigned int hash = len;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = ((hash << 5) ^ (hash >> 27)) ^ (*str);
- }
- return hash;
- }
- /* End Of DEK Hash Function */
- unsigned int BPHash(char* str, unsigned int len)
- {
- unsigned int hash = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash = hash << 7 ^ (*str);
- }
- return hash;
- }
- /* End Of BP Hash Function */
- unsigned int FNVHash(char* str, unsigned int len)
- {
- const unsigned int fnv_prime = 0x811C9DC5;
- unsigned int hash = 0;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash *= fnv_prime;
- hash ^= (*str);
- }
- return hash;
- }
- /* End Of FNV Hash Function */
- unsigned int APHash(char* str, unsigned int len)
- {
- unsigned int hash = 0xAAAAAAAA;
- unsigned int i = 0;
- for(i = 0; i < len; str++, i++)
- {
- hash ^= ((i & 1) == 0) ? ( (hash << 7) ^ (*str) * (hash >> 3)) :
- (~((hash << 11) + (*str) ^ (hash >> 5)));
- }
- return hash;
- }
- /* End Of AP Hash Function */