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Gregory Burd 2024-04-08 11:24:01 -04:00
parent 0275f505fc
commit 32d06df14e
1 changed files with 25 additions and 11 deletions
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36
include/sl.h
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@ -61,10 +61,10 @@
*
* A skiplist is a sorted list with O(log(n)) on average for most operations.
* It is a probabilistic datastructure, meaning that it does not guarantee
* O(log(n)) it approximates it over time. This implementation includes
* improves the probability by integrating the splay list algorithm for
* rebalancing trading off a bit of computational overhead and code complexity
* for a nearly always optimal, or "perfect" skiplist.
* O(log(n)) it approximates it over time. This implementation improves the
* probability by integrating the splay list algorithm for rebalancing trading
* off a bit of computational overhead and code complexity for a nearly always
* optimal, or "perfect" skiplist.
*
* Conceptually, a skiplist is arranged as follows:
*
@ -235,8 +235,8 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
#define __SKIP_IS_LAST_ENTRY_T2B() if (lvl == 0)
#define __SKIP_ALL_ENTRIES_B2T(field, elm) for (size_t lvl = 0; lvl < slist->slh_max_height; lvl++)
#define __SKIP_ENTRIES_B2T(field, elm) for (size_t lvl = 0; lvl < elm->field.sle_height; lvl++)
#define __SKIP_ENTRIES_B2T_FROM(field, elm, off) for (size_t lvl = off; lvl < elm->field.sle_height; lvl++)
#define __SKIP_ENTRIES_B2T(field, elm) for (size_t lvl = 0; lvl <= elm->field.sle_height; lvl++)
#define __SKIP_ENTRIES_B2T_FROM(field, elm, off) for (size_t lvl = off; lvl <= elm->field.sle_height; lvl++)
#define __SKIP_IS_LAST_ENTRY_B2T() if (lvl + 1 == elm->field.sle_height)
/* Iterate over the subtree to the left (v, or 'lt') and right (u) or "CHu" and "CHv". */
@ -263,6 +263,7 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
struct decl { \
size_t slh_length, slh_max_height; \
void *slh_aux; \
int slh_prng_state; \
decl##_node_t *slh_head; \
decl##_node_t *slh_tail; \
struct { \
@ -289,6 +290,19 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
size_t pu; /* sum of hits from intersection to level[1] */ \
} __skiplist_path_##decl##_t; \
\
/* Xorshift algorithm for PRNG */ \
static uint32_t __##decl##_xorshift32(int *state) \
{ \
uint32_t x = *state; \
if (x == 0) \
x = 123456789; \
x ^= x << 13; \
x ^= x >> 17; \
x ^= x << 5; \
*state = x; \
return x; \
} \
\
/** \
* -- __skip_compare_entries_fn_ \
* \
@ -379,12 +393,12 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
* Skiplist. For example, when `max = 6` this function returns 0 with \
* probability 0.5, 1 with 0.25, 2 with 0.125, etc. until 6 with 0.5^7. \
*/ \
static int __skip_toss_##decl(size_t max) \
static int __skip_toss_##decl(decl##_t *slist, size_t max) \
{ \
size_t level = 0; \
double probability = 0.5; \
\
double random_value = (double)rand() / RAND_MAX; /* NOLINT(*-msc50-cpp) */ \
double random_value = (double)__##decl##_xorshift32(&slist->slh_prng_state) / RAND_MAX; \
while (random_value < probability && level < max) { \
level++; \
probability *= 0.5; \
@ -456,9 +470,9 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
* seed the PRNG in a predictable way and have reproducible random numbers. \
*/ \
if (max < 0) \
srand(-max); \
slist->slh_prng_state = -max; \
else \
srand(((unsigned int)time(NULL) ^ getpid())); \
slist->slh_prng_state = ((unsigned int)time(NULL) ^ getpid()); \
fail:; \
return rc; \
} \
@ -787,7 +801,7 @@ void __attribute__((format(printf, 4, 5))) __skip_diag_(const char *file, int li
} \
/* Coin toss to determine level of this new node [0, max) */ \
cur_height = slist->slh_head->field.sle_height; \
new_height = __skip_toss_##decl(slist->slh_max_height - 1); \
new_height = __skip_toss_##decl(slist, slist->slh_max_height - 1); \
new->field.sle_height = new_height; \
/* Trim the path to at most the new height for the new node. */ \
for (i = cur_height + 1; i <= new_height; i++) { \