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selinux: improve symtab string hashing
The number of buckets is calculated by performing a binary AND against the mask of the hash table, which is one less than its size (which is a power of two). This leads to all top bits being discarded, requiring for short or similar inputs a hash function with a good avalanche effect. Use djb2a: # current common prefixes: 7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11 classes: 134 entries and 100/256 buckets used, longest chain length 5, sum of chain length^2 234 roles: 15 entries and 6/16 buckets used, longest chain length 5, sum of chain length^2 57 types: 4448 entries and 3016/8192 buckets used, longest chain length 41, sum of chain length^2 14922 users: 7 entries and 3/8 buckets used, longest chain length 3, sum of chain length^2 17 bools: 306 entries and 221/512 buckets used, longest chain length 4, sum of chain length^2 524 levels: 1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1 categories: 1024 entries and 400/1024 buckets used, longest chain length 4, sum of chain length^2 2740 # patch common prefixes: 7 entries and 5/8 buckets used, longest chain length 2, sum of chain length^2 11 classes: 134 entries and 101/256 buckets used, longest chain length 3, sum of chain length^2 210 roles: 15 entries and 9/16 buckets used, longest chain length 3, sum of chain length^2 31 types: 4448 entries and 3459/8192 buckets used, longest chain length 5, sum of chain length^2 6778 users: 7 entries and 5/8 buckets used, longest chain length 3, sum of chain length^2 13 bools: 306 entries and 236/512 buckets used, longest chain length 5, sum of chain length^2 470 levels: 1 entries and 1/1 buckets used, longest chain length 1, sum of chain length^2 1 categories: 1024 entries and 518/1024 buckets used, longest chain length 7, sum of chain length^2 2992 Signed-off-by: Christian Göttsche <cgzones@googlemail.com> [PM: line length fixes in the commit message] Signed-off-by: Paul Moore <paul@paul-moore.com>
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@ -12,17 +12,17 @@
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static unsigned int symhash(const void *key)
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{
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const char *p, *keyp;
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unsigned int size;
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unsigned int val;
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/*
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* djb2a
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* Public domain from cdb v0.75
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*/
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unsigned int hash = 5381;
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unsigned char c;
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val = 0;
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keyp = key;
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size = strlen(keyp);
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for (p = keyp; (p - keyp) < size; p++)
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val = (val << 4 | (val >> (8 * sizeof(unsigned int) - 4))) ^
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(*p);
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return val;
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while ((c = *(const unsigned char *)key++))
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hash = ((hash << 5) + hash) ^ c;
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return hash;
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}
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static int symcmp(const void *key1, const void *key2)
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