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Add userspace version of our dirent name hash

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Signed-off-by: Zach Brown <zab@versity.com>
This commit is contained in:
Zach Brown
2023-03-28 13:09:12 -07:00
committed by Auke Kok
parent 4697424c7c
commit 660f46a3b4
1 changed files with 46 additions and 0 deletions
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46
utils/src/name_hash.h Normal file
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#ifndef _SCOUTFS_NAME_HASH_H_
#define _SCOUTFS_NAME_HASH_H_
#include "hash.h"
/*
* Test a bit number as though an array of bytes is a large len-bit
* big-endian value. nr 0 is the LSB of the final byte, nr (len - 1) is
* the MSB of the first byte.
*/
static int test_be_bytes_bit(int nr, const char *bytes, int len)
{
return bytes[(len - 1 - nr) >> 3] & (1 << (nr & 7));
}
/*
* Generate a 32bit "fingerprint" of the name by extracting 32 evenly
* distributed bits from the name. The intent is to have the sort order
* of the fingerprints reflect the memcmp() sort order of the names
* while mapping large names down to small fs keys.
*
* Names that are smaller than 32bits are biased towards the high bits
* of the fingerprint so that most significant bits of the fingerprints
* consistently reflect the initial characters of the names.
*/
static inline u32 dirent_name_fingerprint(const char *name, unsigned int name_len)
{
int name_bits = name_len * 8;
int skip = max(name_bits / 32, 1);
u32 fp = 0;
int f;
int n;
for (f = 31, n = name_bits - 1; f >= 0 && n >= 0; f--, n -= skip)
fp |= !!test_be_bytes_bit(n, name, name_bits) << f;
return fp;
}
static inline u64 dirent_name_hash(const char *name, unsigned int name_len)
{
return scoutfs_hash32(name, name_len) |
((u64)dirent_name_fingerprint(name, name_len) << 32);
}
#endif