include/linux/stringhash.h

   1 #ifndef __LINUX_STRINGHASH_H
   2 #define __LINUX_STRINGHASH_H
   3
   4 #include <linux/compiler.h>     /* For __pure */
   5 #include <linux/types.h>        /* For u32, u64 */
   6 #include <linux/hash.h>
   7
   8 /*
   9  * Routines for hashing strings of bytes to a 32-bit hash value.
  10  *
  11  * These hash functions are NOT GUARANTEED STABLE between kernel
  12  * versions, architectures, or even repeated boots of the same kernel.
  13  * (E.g. they may depend on boot-time hardware detection or be
  14  * deliberately randomized.)
  15  *
  16  * They are also not intended to be secure against collisions caused by
  17  * malicious inputs; much slower hash functions are required for that.
  18  *
  19  * They are optimized for pathname components, meaning short strings.
  20  * Even if a majority of files have longer names, the dynamic profile of
  21  * pathname components skews short due to short directory names.
  22  * (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
  23  */
  24
  25 /*
  26  * Version 1: one byte at a time.  Example of use:
  27  *
  28  * unsigned long hash = init_name_hash;
  29  * while (*p)
  30  *      hash = partial_name_hash(tolower(*p++), hash);
  31  * hash = end_name_hash(hash);
  32  *
  33  * Although this is designed for bytes, fs/hfsplus/unicode.c
  34  * abuses it to hash 16-bit values.
  35  */
  36
  37 /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
  38 #define init_name_hash(salt)            (unsigned long)(salt)
  39
  40 /* partial hash update function. Assume roughly 4 bits per character */
  41 static inline unsigned long
  42 partial_name_hash(unsigned long c, unsigned long prevhash)
  43 {
  44         return (prevhash + (c << 4) + (c >> 4)) * 11;
  45 }
  46
  47 /*
  48  * Finally: cut down the number of bits to a int value (and try to avoid
  49  * losing bits).  This also has the property (wanted by the dcache)
  50  * that the msbits make a good hash table index.
  51  */
  52 static inline unsigned long end_name_hash(unsigned long hash)
  53 {
  54         return __hash_32((unsigned int)hash);
  55 }
  56
  57 /*
  58  * Version 2: One word (32 or 64 bits) at a time.
  59  * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
  60  * exists, which describes major Linux platforms like x86 and ARM), then
  61  * this computes a different hash function much faster.
  62  *
  63  * If not set, this falls back to a wrapper around the preceding.
  64  */
  65 extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int);
  66
  67 /*
  68  * A hash_len is a u64 with the hash of a string in the low
  69  * half and the length in the high half.
  70  */
  71 #define hashlen_hash(hashlen) ((u32)(hashlen))
  72 #define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
  73 #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
  74
  75 /* Return the "hash_len" (hash and length) of a null-terminated string */
  76 extern u64 __pure hashlen_string(const void *salt, const char *name);
  77
  78 #endif  /* __LINUX_STRINGHASH_H */