forked from mirrors/linux
703b5faf22
Noe that we're mixing in the parent pointer earlier, we don't need to use hash_32() to mix its bits. Instead, we can just take the msbits of the hash value directly. For those applications which use the partial_name_hash(), move the multiply to end_name_hash. Signed-off-by: George Spelvin <linux@sciencehorizons.net> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
78 lines
2.6 KiB
C
78 lines
2.6 KiB
C
#ifndef __LINUX_STRINGHASH_H
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#define __LINUX_STRINGHASH_H
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#include <linux/compiler.h> /* For __pure */
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#include <linux/types.h> /* For u32, u64 */
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#include <linux/hash.h>
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/*
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* Routines for hashing strings of bytes to a 32-bit hash value.
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*
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* These hash functions are NOT GUARANTEED STABLE between kernel
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* versions, architectures, or even repeated boots of the same kernel.
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* (E.g. they may depend on boot-time hardware detection or be
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* deliberately randomized.)
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*
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* They are also not intended to be secure against collisions caused by
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* malicious inputs; much slower hash functions are required for that.
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*
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* They are optimized for pathname components, meaning short strings.
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* Even if a majority of files have longer names, the dynamic profile of
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* pathname components skews short due to short directory names.
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* (E.g. /usr/lib/libsesquipedalianism.so.3.141.)
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*/
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/*
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* Version 1: one byte at a time. Example of use:
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*
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* unsigned long hash = init_name_hash;
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* while (*p)
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* hash = partial_name_hash(tolower(*p++), hash);
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* hash = end_name_hash(hash);
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*
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* Although this is designed for bytes, fs/hfsplus/unicode.c
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* abuses it to hash 16-bit values.
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*/
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/* Hash courtesy of the R5 hash in reiserfs modulo sign bits */
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#define init_name_hash(salt) (unsigned long)(salt)
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/* partial hash update function. Assume roughly 4 bits per character */
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static inline unsigned long
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partial_name_hash(unsigned long c, unsigned long prevhash)
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{
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return (prevhash + (c << 4) + (c >> 4)) * 11;
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}
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/*
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* Finally: cut down the number of bits to a int value (and try to avoid
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* losing bits). This also has the property (wanted by the dcache)
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* that the msbits make a good hash table index.
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*/
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static inline unsigned long end_name_hash(unsigned long hash)
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{
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return __hash_32((unsigned int)hash);
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}
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/*
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* Version 2: One word (32 or 64 bits) at a time.
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* If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h>
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* exists, which describes major Linux platforms like x86 and ARM), then
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* this computes a different hash function much faster.
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*
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* If not set, this falls back to a wrapper around the preceding.
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*/
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extern unsigned int __pure full_name_hash(const void *salt, const char *, unsigned int);
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/*
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* A hash_len is a u64 with the hash of a string in the low
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* half and the length in the high half.
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*/
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#define hashlen_hash(hashlen) ((u32)(hashlen))
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#define hashlen_len(hashlen) ((u32)((hashlen) >> 32))
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#define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash))
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/* Return the "hash_len" (hash and length) of a null-terminated string */
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extern u64 __pure hashlen_string(const void *salt, const char *name);
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#endif /* __LINUX_STRINGHASH_H */
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