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A faster and more robust code of zslRandomLevel using RAND_MAX (#5539)

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1. since ZSKIPLIST_P is float, using it directly inside the condition used to causes floating point code to be used (gcc/x86)
2. In some operating system(eg.Windows), the largest value returned from random() is 0x7FFF(15bit), so after bitwise AND with 0xFFFF, the probability of the less operation returning true in the while loop's condition is no more equal to ZSKIPLIST_P.
3. In case some library has random() returning int in range [0~ZSKIPLIST_P*65535], the while loop will be an infinite loop.
4. on Linux where RAND_MAX is higher than 0xFFFF, this change actually improves precision (despite not matching the result against a float value)
This commit is contained in:
Henry
2022-03-02 00:40:39 -08:00
committed by GitHub
parent 9b15dd288e
commit feb032fd42
1 changed files with 2 additions and 1 deletions
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src/t_zset.c
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@@ -120,8 +120,9 @@ void zslFree(zskiplist *zsl) {
* (both inclusive), with a powerlaw-alike distribution where higher
* levels are less likely to be returned. */
int zslRandomLevel(void) {
static const int threshold = ZSKIPLIST_P*RAND_MAX;
int level = 1;
while ((random()&0xFFFF) < (ZSKIPLIST_P * 0xFFFF))
while (random() < threshold)
level += 1;
return (level<ZSKIPLIST_MAXLEVEL) ? level : ZSKIPLIST_MAXLEVEL;
}