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hllAdd() low level HyperLogLog "add" implemented.

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antirez 2014-03-28 14:42:30 +01:00
parent e3234116ad
commit 5660ff1cc1
1 changed files with 45 additions and 0 deletions
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45
src/hyperloglog.c
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@ -30,6 +30,7 @@
*/
#include "redis.h"
#include "crc64.h"
/* The Redis HyperLogLog implementation is based on the following ideas:
*
@ -53,6 +54,7 @@
#define REDIS_HLL_P 14 /* The greater is P, the smaller the error. */
#define REDIS_HLL_REGISTERS (1<<REDIS_HLL_P) /* With P=14, 16384 registers. */
#define REDIS_HLL_P_MASK (REDIS_HLL_REGISTERS-1) /* Mask to index register. */
#define REDIS_HLL_BITS 6 /* Enough to count up to 63 leading zeroes. */
#define REDIS_HLL_REGISTER_MAX ((1<<REDIS_HLL_BITS)-1)
#define REDIS_HLL_SIZE ((REDIS_HLL_REGISTERS*REDIS_HLL_BITS+7)/8)
@ -180,6 +182,49 @@
/* ========================= HyperLogLog algorithm ========================= */
/* "Add" the element in the hyperloglog data structure.
* Actually nothing is added, but the max 0 pattern counter of the subset
* the element belongs to is incremented if needed.
*
* 'registers' is expected to have room for REDIS_HLL_REGISTERS plus an
* additional byte on the right. This requirement is met by sds strings
* automatically since they are implicitly null terminated.
*
* The function always succeed, however if as a result of the operation
* the approximated cardinality changed, 1 is returned. Otherwise 0
* is returned. */
int hllAdd(uint8_t *registers, uint8_t *ele, size_t elesize) {
uint64_t hash, bit, index;
uint8_t oldcount, count;
/* Count the number of zeroes starting from bit REDIS_HLL_REGISTERS
* (that is a power of two corresponding to the first bit we don't use
* as index). The max run can be 64-P bits.
*
* This may sound like inefficient, but actually in the average case
* there are high probabilities to find a 1 after a few iterations. */
hash = crc64(0,ele,elesize);
bit = REDIS_HLL_REGISTERS;
count = 0;
while((hash & bit) == 0) {
count++;
/* Test the next bit. Note that if we run out of bits in the 64
* bit integer, bit will be set to 0, and the while test will fail,
* so we can save the explicit check and yet the algorithm will
* terminate. */
bit <<= 1;
}
index = hash & REDIS_HLL_P_MASK; /* Index a register inside registers. */
HLL_GET_REGISTER(oldcount,registers,index);
if (count > oldcount) {
HLL_SET_REGISTER(registers,index,count);
return 1;
} else {
return 0;
}
}
/* ========================== HyperLogLog commands ========================== */
/* This command performs a self-test of the HLL registers implementation.