LXR linux/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 */
  41static inline unsigned long
  42partial_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 */
  52static 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 */
  65extern 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 */
  76extern u64 __pure hashlen_string(const void *salt, const char *name);
  77
  78#endif  /* __LINUX_STRINGHASH_H */
  79