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/**************************************************************************/
/* */
/* OCaml */
/* */
/* Xavier Leroy, projet Cambium, INRIA Paris */
/* */
/* Copyright 2020 Institut National de Recherche en Informatique et */
/* en Automatique. */
/* */
/* All rights reserved. This file is distributed under the terms of */
/* the GNU Lesser General Public License version 2.1, with the */
/* special exception on linking described in the file LICENSE. */
/* */
/**************************************************************************/
#define CAML_INTERNALS
/* A dictionary data structure implemented as skip lists
(see William Pugh, "Skip lists: a probabilistic alternative to
balanced binary trees", Comm. ACM 33(6), 1990). */
#include <stddef.h>
#include "caml/config.h"
#include "caml/memory.h"
#include "caml/misc.h"
#include "caml/skiplist.h"
/* Size of struct skipcell, in bytes, without the forward array */
#define SIZEOF_SKIPCELL sizeof(struct skipcell)
/* Generate a random level for a new node: 0 with probability 3/4,
1 with probability 3/16, 2 with probability 3/64, etc.
We use a simple linear congruential PRNG (see Knuth vol 2) instead
of random(), because we need exactly 32 bits of pseudo-random data
(i.e. 2 * (NUM_LEVELS - 1)). Moreover, the congruential PRNG
is faster and guaranteed to be deterministic (to reproduce bugs). */
static uint32_t random_seed = 0;
static int random_level(void)
{
uint32_t r;
int level = 0;
/* Linear congruence with modulus = 2^32, multiplier = 69069
(Knuth vol 2 p. 106, line 15 of table 1), additive = 25173. */
r = random_seed = random_seed * 69069 + 25173;
/* Knuth (vol 2 p. 13) shows that the least significant bits are
"less random" than the most significant bits with a modulus of 2^m,
so consume most significant bits first */
while ((r & 0xC0000000U) == 0xC0000000U) { level++; r = r << 2; }
CAMLassert(level < NUM_LEVELS);
return level;
}
/* Initialize a skip list */
void caml_skiplist_init(struct skiplist * sk)
{
for (int i = 0; i < NUM_LEVELS; i++) sk->forward[i] = NULL;
sk->level = 0;
}
/* Search a skip list */
int caml_skiplist_find(struct skiplist * sk, uintnat key, uintnat * data)
{
struct skipcell ** e, * f;
e = sk->forward;
for (int i = sk->level; i >= 0; i--) {
while (1) {
f = e[i];
if (f == NULL || f->key > key) break;
if (f->key == key) {
*data = f->data;
return 1;
}
e = f->forward;
}
}
return 0;
}
int caml_skiplist_find_below(struct skiplist * sk, uintnat k,
uintnat * key, uintnat * data)
{
struct skipcell ** e, * f, * last = NULL;
e = sk->forward;
for (int i = sk->level; i >= 0; i--) {
while (1) {
f = e[i];
if (f == NULL || f->key > k) break;
last = f;
e = f->forward;
}
}
if (!last) {
return 0;
} else {
*key = last-> key; *data = last->data; return 1;
}
}
/* Insertion in a skip list */
int caml_skiplist_insert(struct skiplist * sk,
uintnat key, uintnat data)
{
struct skipcell ** update[NUM_LEVELS];
struct skipcell ** e, * f;
int new_level;
/* Init "cursor" to list head */
e = sk->forward;
/* Find place to insert new node */
for (int i = sk->level; i >= 0; i--) {
while (1) {
f = e[i];
if (f == NULL || f->key >= key) break;
e = f->forward;
}
update[i] = &e[i];
}
f = e[0];
/* If already present, update data */
if (f != NULL && f->key == key) {
f->data = data;
return 1;
}
/* Insert additional element, updating list level if necessary */
new_level = random_level();
if (new_level > sk->level) {
for (int i = sk->level + 1; i <= new_level; i++)
update[i] = &sk->forward[i];
sk->level = new_level;
}
f = caml_stat_alloc(SIZEOF_SKIPCELL +
(new_level + 1) * sizeof(struct skipcell *));
f->key = key;
f->data = data;
for (int i = 0; i <= new_level; i++) {
f->forward[i] = *update[i];
*update[i] = f;
}
return 0;
}
/* Deletion in a skip list */
int caml_skiplist_remove(struct skiplist * sk, uintnat key)
{
struct skipcell ** update[NUM_LEVELS];
struct skipcell ** e, * f;
/* Init "cursor" to list head */
e = sk->forward;
/* Find element in list */
for (int i = sk->level; i >= 0; i--) {
while (1) {
f = e[i];
if (f == NULL || f->key >= key) break;
e = f->forward;
}
update[i] = &e[i];
}
f = e[0];
/* If not found, nothing to do */
if (f == NULL || f->key != key) return 0;
/* Rebuild list without node */
for (int i = 0; i <= sk->level; i++) {
if (*update[i] == f)
*update[i] = f->forward[i];
}
/* Reclaim list element */
caml_stat_free(f);
/* Down-correct list level */
while (sk->level > 0 &&
sk->forward[sk->level] == NULL)
sk->level--;
return 1;
}
/* Empty a skip list */
void caml_skiplist_empty(struct skiplist * sk)
{
for (struct skipcell *e = sk->forward[0], *next; e != NULL; e = next) {
next = e->forward[0];
caml_stat_free(e);
}
for (int i = 0; i <= sk->level; i++) sk->forward[i] = NULL;
sk->level = 0;
}
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