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2014-08-27

2014-08-27 19:06:41 by Henrik Grubbström (Grubba) <grubba@grubba.org>

Cyclic: Change to a power of two hash table.

The hash table used by BEGIN_CYCLIC() et al now has a size that
is a power of 2, so that we can avoid the modulo operation.

The hash function is also modified somewhat to improve its entropy.

7:   #include "global.h"   #include "cyclic.h"    - #define CYCLIC_HASH_SIZE 4711 + #define CYCLIC_HASH_SIZE 0x1000      static CYCLIC *cyclic_hash[CYCLIC_HASH_SIZE];   
22:    h ^= PTR_TO_INT(c->b);    h *= 33;    h ^= PTR_TO_INT(c->th); -  h *= 33; +     -  return h % CYCLIC_HASH_SIZE; + #if SIZEOF_CHAR_P > 4 +  h ^= h>>8; + #endif +  /* Fold h. This is to retain as many bits of h as possible. +  * +  * NB: The "magic" constant below has a 1 bit every 10 bits +  * starting at the least significant, and is == 1 when +  * shifted right 20 bits. Note also that 32 - 20 == 12 +  * and 1<<12 == 0x1000 == CYCLIC_HASH_SIZE. +  * +  * The multiplication has the effect of accumulating +  * the segments of 10 bits of h in the most significant +  * segment, which is then shifted down. +  */ +  h *= 0x100401; +  h >>= 20; +  +  return h & (CYCLIC_HASH_SIZE-1);   }      static void low_unlink_cyclic(CYCLIC *c)