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85 lines
2.4 KiB
C
85 lines
2.4 KiB
C
#pragma once
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/*
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* https://en.wikipedia.org/wiki/Fowler–Noll–Vo_hash_function
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* http://www.isthe.com/chongo/tech/comp/fnv/index.html
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*/
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/*
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* To use the recommended 32 bit FNV-1a hash, pass FNV1_32A_INIT as the
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* Fnv32_t hashval argument to fnv_32a_buf() or fnv_32a_str().
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*/
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/*
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* 32 bit magic FNV-1a prime
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*/
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#define FNV_32_PRIME ((unsigned int)0x01000193)
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/*
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* 32 bit FNV-1 and FNV-1a non-zero initial basis
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*
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* The FNV-1 initial basis is the FNV-0 hash of the following 32 octets:
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*
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* chongo <Landon Curt Noll> /\../\
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*
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* NOTE: The \'s above are not back-slashing escape characters.
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* They are literal ASCII backslash 0x5c characters.
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*
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* NOTE: The FNV-1a initial basis is the same value as FNV-1 by definition.
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*/
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#define FNV1_32_INIT ((unsigned int)0x811c9dc5)
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#define FNV1_32A_INIT FNV1_32_INIT
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/*
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* If you need an x-bit hash where x is not a power of 2,
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* then we recommend that you compute the FNV hash that is just larger than x-bits
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* and xor-fold the result down to x-bits.By xor-folding we mean shift the excess
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* high order bits down and xor them with the lower x - bits.
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*/
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//For example to produce a 24 bit FNV-1 hash in C we xor-fold fold a 32 bit FNV-1 hash:
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#define MASK_24 (((unsigned int)1<<24)-1) /* i.e., (uint32_t)0xffffff */
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/*
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uint32_t hash;
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void* data;
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size_t data_len;
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hash = fnv_32_buf(data, data_len, FNV1_32_INIT);
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hash = (hash >> 24) ^ (hash & MASK_24);
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*/
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#define FNV_24(hash) ((hash >> 24) ^ (hash & MASK_24))
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//To produce a 16 bit FNV-1 hash in C we xor-fold fold a 32 bit FNV-1 hash:
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#define MASK_16 (((unsigned int)1<<16)-1) /* i.e., (uint32_t)0xffff */
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/*
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uint32_t hash;
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void* data;
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size_t data_len;
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hash = fnv_32_buf(data, data_len, FNV1_32_INIT);
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hash = (hash >> 16) ^ (hash & MASK_16);
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*/
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#define FNV_16(hash) ((hash >> 16) ^ (hash & MASK_16))
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//For tiny x < 16 bit values, we recommend using a 32 bit FNV - 1 hash as follows:
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/* NOTE: for 0 < x < 16 ONLY!!! */
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#define TINY_MASK(x) (((unsigned int)1<<(x))-1)
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/*
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uint32_t hash;
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void* data;
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size_t data_len;
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hash = fnv_32_buf(data, data_len, FNV1_32_INIT);
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hash = (((hash >> x) ^ hash) & TINY_MASK(x));
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*/
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#define TINY_FNV(x, hash) (((hash >> x) ^ hash) & TINY_MASK(x))
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unsigned int fnv_32a_buf(void* buf, size_t len, unsigned int hval);
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unsigned int fnv_32a_str(char* str, unsigned int hval);
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