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