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 7 /* 8 * Routines for hashing strings of bytes to a 32-bit hash value. 9 * 10 * These hash functions are NOT GUARANTEED STABLE between kernel 11 * versions, architectures, or even repeated boots of the same kernel. 12 * (E.g. they may depend on boot-time hardware detection or be 13 * deliberately randomized.) 14 * 15 * They are also not intended to be secure against collisions caused by 16 * malicious inputs; much slower hash functions are required for that. 17 * 18 * They are optimized for pathname components, meaning short strings. 19 * Even if a majority of files have longer names, the dynamic profile of 20 * pathname components skews short due to short directory names. 21 * (E.g. /usr/lib/libsesquipedalianism.so.3.141.) 22 */ 23 24 /* 25 * Version 1: one byte at a time. Example of use: 26 * 27 * unsigned long hash = init_name_hash; 28 * while (*p) 29 * hash = partial_name_hash(tolower(*p++), hash); 30 * hash = end_name_hash(hash); 31 * 32 * Although this is designed for bytes, fs/hfsplus/unicode.c 33 * abuses it to hash 16-bit values. 34 */ 35 36 /* Hash courtesy of the R5 hash in reiserfs modulo sign bits */ 37 #define init_name_hash() 0 38 39 /* partial hash update function. Assume roughly 4 bits per character */ 40 static inline unsigned long 41 partial_name_hash(unsigned long c, unsigned long prevhash) 42 { 43 return (prevhash + (c << 4) + (c >> 4)) * 11; 44 } 45 46 /* 47 * Finally: cut down the number of bits to a int value (and try to avoid 48 * losing bits) 49 */ 50 static inline unsigned long end_name_hash(unsigned long hash) 51 { 52 return (unsigned int)hash; 53 } 54 55 /* 56 * Version 2: One word (32 or 64 bits) at a time. 57 * If CONFIG_DCACHE_WORD_ACCESS is defined (meaning <asm/word-at-a-time.h> 58 * exists, which describes major Linux platforms like x86 and ARM), then 59 * this computes a different hash function much faster. 60 * 61 * If not set, this falls back to a wrapper around the preceding. 62 */ 63 extern unsigned int __pure full_name_hash(const char *, unsigned int); 64 65 /* 66 * A hash_len is a u64 with the hash of a string in the low 67 * half and the length in the high half. 68 */ 69 #define hashlen_hash(hashlen) ((u32)(hashlen)) 70 #define hashlen_len(hashlen) ((u32)((hashlen) >> 32)) 71 #define hashlen_create(hash, len) ((u64)(len)<<32 | (u32)(hash)) 72 73 /* Return the "hash_len" (hash and length) of a null-terminated string */ 74 extern u64 __pure hashlen_string(const char *name); 75 76 #endif /* __LINUX_STRINGHASH_H */ 77