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47 lines
1.3 KiB
C
47 lines
1.3 KiB
C
#ifndef _SCOUTFS_NAME_HASH_H_
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#define _SCOUTFS_NAME_HASH_H_
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#include "hash.h"
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/*
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* Test a bit number as though an array of bytes is a large len-bit
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* big-endian value. nr 0 is the LSB of the final byte, nr (len - 1) is
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* the MSB of the first byte.
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*/
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static int test_be_bytes_bit(int nr, const char *bytes, int len)
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{
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return bytes[(len - 1 - nr) >> 3] & (1 << (nr & 7));
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}
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/*
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* Generate a 32bit "fingerprint" of the name by extracting 32 evenly
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* distributed bits from the name. The intent is to have the sort order
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* of the fingerprints reflect the memcmp() sort order of the names
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* while mapping large names down to small fs keys.
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*
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* Names that are smaller than 32bits are biased towards the high bits
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* of the fingerprint so that most significant bits of the fingerprints
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* consistently reflect the initial characters of the names.
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*/
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static inline u32 dirent_name_fingerprint(const char *name, unsigned int name_len)
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{
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int name_bits = name_len * 8;
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int skip = max(name_bits / 32, 1);
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u32 fp = 0;
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int f;
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int n;
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for (f = 31, n = name_bits - 1; f >= 0 && n >= 0; f--, n -= skip)
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fp |= !!test_be_bytes_bit(n, name, name_bits) << f;
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return fp;
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}
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static inline u64 dirent_name_hash(const char *name, unsigned int name_len)
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{
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return scoutfs_hash32(name, name_len) |
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((u64)dirent_name_fingerprint(name, name_len) << 32);
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}
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#endif
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