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       1                 :            : /* SPDX-License-Identifier: GPL-2.0 */
       2                 :            : #ifndef __LINUX_STRINGHASH_H
       3                 :            : #define __LINUX_STRINGHASH_H
       4                 :            : 
       5                 :            : #include <linux/compiler.h>       /* For __pure */
       6                 :            : #include <linux/types.h>  /* For u32, u64 */
       7                 :            : #include <linux/hash.h>
       8                 :            : 
       9                 :            : /*
      10                 :            :  * Routines for hashing strings of bytes to a 32-bit hash value.
      11                 :            :  *
      12                 :            :  * These hash functions are NOT GUARANTEED STABLE between kernel
      13                 :            :  * versions, architectures, or even repeated boots of the same kernel.
      14                 :            :  * (E.g. they may depend on boot-time hardware detection or be
      15                 :            :  * deliberately randomized.)
      16                 :            :  *
      17                 :            :  * They are also not intended to be secure against collisions caused by
      18                 :            :  * malicious inputs; much slower hash functions are required for that.
      19                 :            :  *
      20                 :            :  * They are optimized for pathname components, meaning short strings.
      21                 :            :  * Even if a majority of files have longer names, the dynamic profile of
      22                 :            :  * pathname components skews short due to short directory names.
      23                 :            :  * (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
      24                 :            :  */
      25                 :            : 
      26                 :            : /*
      27                 :            :  * Version 1: one byte at a time.  Example of use:
      28                 :            :  *
      29                 :            :  * unsigned long hash = init_name_hash;
      30                 :            :  * while (*p)
      31                 :            :  *      hash = partial_name_hash(tolower(*p++), hash);
      32                 :            :  * hash = end_name_hash(hash);
      33                 :            :  *
      34                 :            :  * Although this is designed for bytes, fs/hfsplus/unicode.c
      35                 :            :  * abuses it to hash 16-bit values.
      36                 :            :  */
      37                 :            : 
      38                 :            : /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
      39                 :            : #define init_name_hash(salt)            (unsigned long)(salt)
      40                 :            : 
      41                 :            : /* partial hash update function. Assume roughly 4 bits per character */
      42                 :            : static inline unsigned long
      43                 :            : partial_name_hash(unsigned long c, unsigned long prevhash)
      44                 :            : {
      45                 :          3 :         return (prevhash + (c << 4) + (c >> 4)) * 11;
      46                 :            : }
      47                 :            : 
      48                 :            : /*
      49                 :            :  * Finally: cut down the number of bits to a int value (and try to avoid
      50                 :            :  * losing bits).  This also has the property (wanted by the dcache)
      51                 :            :  * that the msbits make a good hash table index.
      52                 :            :  */
      53                 :            : static inline unsigned int end_name_hash(unsigned long hash)
      54                 :            : {
      55                 :            :         return hash_long(hash, 32);
      56                 :            : }
      57                 :            : 
      58                 :            : /*
      59                 :            :  * Version 2: One word (32 or 64 bits) at a time.
      60                 :            :  * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
      61                 :            :  * exists, which describes major Linux platforms like x86 and ARM), then
      62                 :            :  * this computes a different hash function much faster.
      63                 :            :  *
      64                 :            :  * If not set, this falls back to a wrapper around the preceding.
      65                 :            :  */
      66                 :            : extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int);
      67                 :            : 
      68                 :            : /*
      69                 :            :  * A hash_len is a u64 with the hash of a string in the low
      70                 :            :  * half and the length in the high half.
      71                 :            :  */
      72                 :            : #define hashlen_hash(hashlen) ((u32)(hashlen))
      73                 :            : #define hashlen_len(hashlen)  ((u32)((hashlen) >> 32))
      74                 :            : #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
      75                 :            : 
      76                 :            : /* Return the "hash_len" (hash and length) of a null-terminated string */
      77                 :            : extern u64 __pure hashlen_string(const void *salt, const char *name);
      78                 :            : 
      79                 :            : #endif  /* __LINUX_STRINGHASH_H */