FreeBSD/Linux Kernel Cross Reference
sys/include/linux/jhash.h

     #ifndef _LINUX_JHASH_H
     #define _LINUX_JHASH_H
     
     /* jhash.h: Jenkins hash support.
      *
      * Copyright (C) 2006. Bob Jenkins (bob_jenkins@burtleburtle.net)
      *
      * http://burtleburtle.net/bob/hash/
      *
     * These are the credits from Bob's sources:
     *
     * lookup3.c, by Bob Jenkins, May 2006, Public Domain.
     *
     * These are functions for producing 32-bit hashes for hash table lookup.
     * hashword(), hashlittle(), hashlittle2(), hashbig(), mix(), and final()
     * are externally useful functions.  Routines to test the hash are included
     * if SELF_TEST is defined.  You can use this free for any purpose.  It's in
     * the public domain.  It has no warranty.
     *
     * Copyright (C) 2009-2010 Jozsef Kadlecsik (kadlec@blackhole.kfki.hu)
     *
     * I've modified Bob's hash to be useful in the Linux kernel, and
     * any bugs present are my fault.
     * Jozsef
     */
    #include <linux/bitops.h>
    #include <linux/unaligned/packed_struct.h>
    
    /* Best hash sizes are of power of two */
    #define jhash_size(n)   ((u32)1<<(n))
    /* Mask the hash value, i.e (value & jhash_mask(n)) instead of (value % n) */
    #define jhash_mask(n)   (jhash_size(n)-1)
    
    /* __jhash_mix -- mix 3 32-bit values reversibly. */
    #define __jhash_mix(a, b, c)                    \
    {                                               \
            a -= c;  a ^= rol32(c, 4);  c += b;     \
            b -= a;  b ^= rol32(a, 6);  a += c;     \
            c -= b;  c ^= rol32(b, 8);  b += a;     \
            a -= c;  a ^= rol32(c, 16); c += b;     \
            b -= a;  b ^= rol32(a, 19); a += c;     \
            c -= b;  c ^= rol32(b, 4);  b += a;     \
    }
    
    /* __jhash_final - final mixing of 3 32-bit values (a,b,c) into c */
    #define __jhash_final(a, b, c)                  \
    {                                               \
            c ^= b; c -= rol32(b, 14);              \
            a ^= c; a -= rol32(c, 11);              \
            b ^= a; b -= rol32(a, 25);              \
            c ^= b; c -= rol32(b, 16);              \
            a ^= c; a -= rol32(c, 4);               \
            b ^= a; b -= rol32(a, 14);              \
            c ^= b; c -= rol32(b, 24);              \
    }
    
    /* An arbitrary initial parameter */
    #define JHASH_INITVAL           0xdeadbeef
    
    /* jhash - hash an arbitrary key
     * @k: sequence of bytes as key
     * @length: the length of the key
     * @initval: the previous hash, or an arbitray value
     *
     * The generic version, hashes an arbitrary sequence of bytes.
     * No alignment or length assumptions are made about the input key.
     *
     * Returns the hash value of the key. The result depends on endianness.
     */
    static inline u32 jhash(const void *key, u32 length, u32 initval)
    {
            u32 a, b, c;
            const u8 *k = key;
    
            /* Set up the internal state */
            a = b = c = JHASH_INITVAL + length + initval;
    
            /* All but the last block: affect some 32 bits of (a,b,c) */
            while (length > 12) {
                    a += __get_unaligned_cpu32(k);
                    b += __get_unaligned_cpu32(k + 4);
                    c += __get_unaligned_cpu32(k + 8);
                    __jhash_mix(a, b, c);
                    length -= 12;
                    k += 12;
            }
            /* Last block: affect all 32 bits of (c) */
            /* All the case statements fall through */
            switch (length) {
            case 12: c += (u32)k[11]<<24;
            case 11: c += (u32)k[10]<<16;
            case 10: c += (u32)k[9]<<8;
            case 9:  c += k[8];
            case 8:  b += (u32)k[7]<<24;
            case 7:  b += (u32)k[6]<<16;
            case 6:  b += (u32)k[5]<<8;
            case 5:  b += k[4];
            case 4:  a += (u32)k[3]<<24;
            case 3:  a += (u32)k[2]<<16;
           case 2:  a += (u32)k[1]<<8;
           case 1:  a += k[0];
                    __jhash_final(a, b, c);
           case 0: /* Nothing left to add */
                   break;
           }
   
           return c;
   }
   
   /* jhash2 - hash an array of u32's
    * @k: the key which must be an array of u32's
    * @length: the number of u32's in the key
    * @initval: the previous hash, or an arbitray value
    *
    * Returns the hash value of the key.
    */
   static inline u32 jhash2(const u32 *k, u32 length, u32 initval)
   {
           u32 a, b, c;
   
           /* Set up the internal state */
           a = b = c = JHASH_INITVAL + (length<<2) + initval;
   
           /* Handle most of the key */
           while (length > 3) {
                   a += k[0];
                   b += k[1];
                   c += k[2];
                   __jhash_mix(a, b, c);
                   length -= 3;
                   k += 3;
           }
   
           /* Handle the last 3 u32's: all the case statements fall through */
           switch (length) {
           case 3: c += k[2];
           case 2: b += k[1];
           case 1: a += k[0];
                   __jhash_final(a, b, c);
           case 0: /* Nothing left to add */
                   break;
           }
   
           return c;
   }
   
   
   /* jhash_3words - hash exactly 3, 2 or 1 word(s) */
   static inline u32 jhash_3words(u32 a, u32 b, u32 c, u32 initval)
   {
           a += JHASH_INITVAL;
           b += JHASH_INITVAL;
           c += initval;
   
           __jhash_final(a, b, c);
   
           return c;
   }
   
   static inline u32 jhash_2words(u32 a, u32 b, u32 initval)
   {
           return jhash_3words(a, b, 0, initval);
   }
   
   static inline u32 jhash_1word(u32 a, u32 initval)
   {
           return jhash_3words(a, 0, 0, initval);
   }
   
   #endif /* _LINUX_JHASH_H */

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