完美的网址哈希函数 [英] Perfect Hash Function for URLs

问题描述

>从Base52 url shortener perfect hash function in C > http://lambdajones.com/b52

  const char * b52idx [52] = {
0，1，2，3，4，5，6，7，8，9，
B， C，D，F，G，H，J，K，L，M，
，N，P，Q ，R，S，T，V，W，X，Y，
Z，b，c，d f，g，h，j，k，l，
，m，n，p，q，r，s ，t，v，w，x，
y，z
}; 
 
 #define X 0xff 
 const int b52map [128] = {
 X，X，X，X，X，X，X，X，X，X，X， X，X，X，X，X，X，X，X，X，X，
 X，X，X，X，X，X，X， X，X，X，X，X，X，X，X，X，X，X，X，X，X，X，X，X，
 // 0 1 2 3 4 5 6 7 8 9 
 0,1,2,3,4,5,6,7,8,9，X，X，X，X，X，X，
 // BCDFGHJKLMN 
 X，X，10 ，11,12，X，13,14,15，X，16,17,18,19,20，X，
 // PQRSTVWXYZ 
 21,22,23,24,25，X， 26,27,28,29,30，X，X，X，X，X，
 // bcdfghjklmn 
 X，X，31,32,33，X，34,35,36，X ，37,38,39,40,41，X，
 // pqrstvwxyz 
 42,43,44,45,46，X，47,48,49,50,51，X，X， X，X，X 
}; 
 $ b $＃#ifdef __GNUC__ 
 #define likely（x）__builtin_expect（（x），1）
 #else 
 #define likely（x）（x）
 #endif 
 
 / * 
从00000开始有效 - > zzzzz，适用于380204032网址
返回积分短网址id 
 * / 
 unsigned long long b52（const char * c）{
 unsigned long long x = 0; 
 unsigned long long y = 0; 
 unsigned long long z = 0; 
 
 x | = b52map [c [0]]<< 24 | b52map [c [1]]<< 18 | \ 
 b52map [c [2]]<< 12 | b52map [c [3]]<< 6 | b52map [C [4]]; 
 
 y + =（x / 64）* 12; 
 if（x> 4095）y + = 624 *（x / 4096）; 
 if（x> 262143）y + = 32448 *（x / 262144）; 
 if（x> 16777215）y + = 1687296 *（x / 16777215）; （可能（（z = x  -  y）< 380204033））返回z; 
 
 if 
 else return 380204033; 
 
 
 b52inc（char * id）{
 int x [5] = {
 b52map [id [0]]，b52map [id [1] ]，b52map [id [2]]，b52map [id [3]]，b52map [id [4]] 
}; 
 int y = 5; 
 
 //搜索我们可以增加的第一个字符（51 =='z'）
 //从b52idx表中增加并更新id 
 while（y--）如果（x [y] <51）中断; 
 id [y] = * b52idx [++ x [y]]; 
 
 // if if（x [y] == 51）id [y] ='0（如果我们通过id的上面的z，将它们滚过
，而（y ++< 5） ; 
} 
  

Does anyone know of a perfect hashing function for URLs with 64-bit integers that would perform well for most URLs?

解决方案

Found this marked as a "Base52 url shortener perfect hash function in C" from http://lambdajones.com/b52

  const char *b52idx[52] = {
  "0", "1", "2", "3", "4", "5", "6", "7", "8", "9",
  "B", "C", "D", "F", "G", "H", "J", "K", "L", "M",
  "N", "P", "Q", "R", "S", "T", "V", "W", "X", "Y",
  "Z", "b", "c", "d", "f", "g", "h", "j", "k", "l",
  "m", "n", "p", "q", "r", "s", "t", "v", "w", "x",
  "y", "z"
};

#define X 0xff
const int b52map[128] = {
   X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
   X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
   X, X, X, X, X, X, X, X, X, X, X, X, X, X, X, X,
// 0  1  2  3  4  5  6  7  8  9
   0, 1, 2, 3, 4, 5, 6, 7, 8, 9, X, X, X, X, X, X,
//       B  C  D     F  G  H     J  K  L  M  N
   X, X,10,11,12, X,13,14,15, X,16,17,18,19,20, X,
// P  Q  R  S  T     V  W  X  Y  Z
  21,22,23,24,25, X,26,27,28,29,30, X, X, X, X, X,
//       b  c  d     f  g  h     j  k  l  m  n
   X, X,31,32,33, X,34,35,36, X,37,38,39,40,41, X,
// p  q  r  s  t     v  w  x  y  z
  42,43,44,45,46, X,47,48,49,50,51, X, X, X, X, X
};

#ifdef __GNUC__
#define likely(x) __builtin_expect((x),1)
#else
#define likely(x) (x)
#endif

/*
  valid from 00000 -> zzzzz, good for 380204032 urls
  returns the integral short url id
*/
unsigned long long b52(const char *c) {
  unsigned long long x = 0;
  unsigned long long y = 0;
  unsigned long long z = 0;

  x |= b52map[c[0]] << 24 | b52map[c[1]] << 18 | \
       b52map[c[2]] << 12 | b52map[c[3]] << 6  | b52map[c[4]];

  y += (x/64) * 12;
  if (x > 4095) y += 624 * (x/4096);
  if (x > 262143) y += 32448 * (x/262144);
  if (x > 16777215) y += 1687296 * (x/16777215);

  if (likely((z = x - y) < 380204033)) return z;
  else return 380204033;
}

void b52inc(char *id) {
  int x[5] = {
    b52map[id[0]], b52map[id[1]], b52map[id[2]],b52map[id[3]], b52map[id[4]]
  };
  int y = 5;

  // search for the first character we can increment (51 == 'z')
  // increment from the b52idx table and update id
  while (y--) if (x[y] < 51) break;
  id[y] = *b52idx[++x[y]];

  // if we passed over id's 'z's above, roll them over
  while (y++ < 5) if (x[y] == 51) id[y] = '0';
}

这篇关于完美的网址哈希函数的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！