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rewrite skiplist.c, a few bug fixes #534

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rewrite skiplist.c, a few bug fixes #534

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6 changes: 2 additions & 4 deletions c-cpp/17_skiplist/SkipList.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -354,10 +354,8 @@ int GetRandom()
int CSkipList::RandomLevel()
{
int level = 1;
for(int i=1; i<MAX_LEVEL; ++i){
if(1 == (GetRandom()%3)){
level++;
}
for(int i=1; i<MAX_LEVEL && GetRandom()%3; ++i){
level++;
}
return level;
}
284 changes: 115 additions & 169 deletions c-cpp/17_skiplist/skiplist.c
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Original file line number Diff line number Diff line change
@@ -1,214 +1,160 @@
// https://www.youtube.com/watch?v=2g9OSRKJuzM&t=17s
// note: 层数从0开始算，非空跳表的最低level是0，空跳表的level是-1。
// note: 因为randomization的关系，跳表允许的最大层数需要事先设定，
// 当n足够大时，跳表的层数（即节点索引的最大层数）将约等于log2(n)。
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <time.h>

// https://www.youtube.com/watch?v=2g9OSRKJuzM&t=17s

#define MAX_LEVEL 15
#define SKIPLIST_P 0.5
#define INVALID_KEY -1

struct node {
int val;
int max_level;
struct node *forward[MAX_LEVEL];
};
typedef int ktype;

struct skip_list {
struct node head;
int max_level;
int max_level_nodes;
};

void node_init(struct node* node)
typedef struct node_
{
memset(node, 0, sizeof(struct node));
}
// dtype data;
ktype key;
int level;
struct node_ **forward;
} node;

void skip_list_init(struct skip_list* sl)
typedef struct
{
node_init(&sl->head);
sl->max_level = 0;
sl->max_level_nodes = 0;
}
node *head;
int level; // 跳表的当前层数，允许的最大层数是head->level
int nodes;
} skip_list;

void random_init()
skip_list *make_skip_list(int max_level)
{
static bool done = false;

if (done)
return;

srandom(time(NULL));
done = true;
skip_list *sl = (skip_list *)malloc(sizeof(skip_list));
sl->level = -1;
sl->head = (node *)malloc(sizeof(node));
sl->head->key = INVALID_KEY;
sl->head->level = max_level;
sl->head->forward = (node **)malloc(sizeof(node *) * max_level); // 注意头节点的指针数量不等于sl的max_level
memset(sl->head->forward, 0, sizeof(node *) * max_level);
return sl;
}

int random_level(void)
int random_level(int max_level)
{
int i, level = 1;

random_init();

for (i = 1; i < MAX_LEVEL; i++)
if (random() % 2 == 1)
level++;

return level;
static const int threshold = SKIPLIST_P * RAND_MAX;
int level = 0;
while (random() < threshold) //在0.5概率下循环平均执行次数趋于2，因此max_level的比较放外面会更快
level++;
return (level < max_level) ? level : max_level;
}

void random_level_test()
node *search(skip_list *sl, ktype key)
{
printf("random level %d\n", random_level());
printf("random level %d\n", random_level());
printf("random level %d\n", random_level());
printf("random level %d\n", random_level());
printf("random level %d\n", random_level());
}

void insert(struct skip_list *sl, int val)
{
int level = random_level();
struct node *update[MAX_LEVEL];
struct node *new, *p;
int i;

new = (struct node*)malloc(sizeof(struct node));
if (!new)
return;

new->max_level = level;
new->val = val;

for (int i = 0; i < MAX_LEVEL; i++)
update[i] = &sl->head;

p = &sl->head;
for (i = level - 1; i >= 0; i--) {
while(p->forward[i] && p->forward[i]->val < val)
p = p->forward[i];

update[i] = p;
node *cur = sl->head;
for (int i = sl->level; i >= 0; i--)
{
while (cur->forward[i] != NULL && cur->forward[i]->key <= key)
cur = cur->forward[i];
if (cur->key == key)
return cur;
}

for (i = 0; i < level; i++) {
new->forward[i] = update[i]->forward[i];
update[i]->forward[i] = new;
}

if (sl->max_level < level) {
sl->max_level = level;
sl->max_level_nodes = 1;
} else if (sl->max_level == level)
sl->max_level_nodes++;
return NULL;
}

struct node *find(struct skip_list* sl, int val)
node *insert(skip_list *sl, ktype key)
{
struct node *node = &sl->head;
int i;

for (i = sl->max_level - 1; i >= 0; i--) {
while (node->forward[i] && node->forward[i]->val < val)
node = node->forward[i];
}

if (node->forward[0] && node->forward[0]->val == val) {
return node->forward[0];
// 构造新节点
int level = random_level(sl->head->level);
if (level > sl->level)
sl->level = level;
node *new_node = (node *)malloc(sizeof(node));
new_node->key = key;
new_node->level = level;
new_node->forward = (node **)malloc(sizeof(node *) * level);
// 插入
node *cur = sl->head;
for (int i = sl->level; i >= 0; i--)
{
while (cur->forward[i] != NULL && cur->forward[i]->key <= key) //同样键值的情况插入到后面
cur = cur->forward[i];
if (level >= i)
{
new_node->forward[i] = cur->forward[i];
cur->forward[i] = new_node;
}
}
else
return NULL;
sl->nodes++;
return new_node;
}

void delete(struct skip_list* sl, int val)
void delete(skip_list *sl, ktype key)
{
struct node *update[MAX_LEVEL];
struct node *p;
int i;

p = &sl->head;

for (i = sl->max_level; i >= 0; i--) {
while (p->forward[i] && p->forward[i]->val < val)
p = p->forward[i];

update[i] = p;
}

if (p->forward[0] == NULL || p->forward[0]->val != val)
return;

if (p->forward[0]->max_level == sl->max_level)
sl->max_level_nodes--;

for (i = sl->max_level-1; i >= 0; i--) {
if (update[i]->forward[i] && update[i]->forward[i]->val == val)
update[i]->forward[i] = update[i]->forward[i]->forward[i];
node *cur = sl->head;
node *target = NULL;
for (int i = sl->level; i >= 0; i--)
{
while (cur->forward[i] != NULL && cur->forward[i]->key < key) //同样键值的情况删除最前面者
cur = cur->forward[i];
if (cur->forward[i] != NULL && cur->forward[i]->key == key) //删除该层的链接
{
target = cur->forward[i];
cur->forward[i] = target->forward[i];
}
}

// fixup max_level and max_level_nodes
if (sl->max_level_nodes == 0) {
//sl->max_level--;
p = &sl->head;
// skip (max_level - 1), direct test (max_level - 2)
// since no nodes on (max_level - 1)
for (i = sl->max_level - 2; i >= 0; i--) {
while (p->forward[i]) {
sl->max_level_nodes++;
p = p->forward[i];
}

if (sl->max_level_nodes) {
sl->max_level = i + 1;
break;
} else
sl->max_level = i;
if (target != NULL)
{
if (target->level == sl->level)
{
while (sl->head->forward[sl->level] == NULL)
sl->level--;
}
free(target->forward);
free(target);
sl->nodes--;
}
}


void print_sl(struct skip_list* sl)
void print_sl(skip_list *sl)
{
struct node *node;
int level;

printf("%d level skip list with %d nodes on top\n",
sl->max_level, sl->max_level_nodes);

for (level = sl->max_level - 1; level >= 0; level--) {
node = &sl->head;
printf("Level[%02d]:", level);
while (node->forward[level]) {
printf("%4d", node->forward[level]->val);
node = node->forward[level];
printf("%d level skip list with %d nodes\n", sl->level, sl->nodes);
for (int i = sl->level; i >= 0; i--)
{
node *cur = sl->head;
printf("Level[%02d]:", i);
while (cur->forward[i])
{
cur = cur->forward[i];
printf("%4d", cur->key);
}
printf("\n");
}
}

int main()
int main(int argc, char* argv[])
{
struct skip_list sl;
struct node *node = NULL;
int i;

skip_list_init(&sl);
print_sl(&sl);

for (i = 0; i < 10; i++)
insert(&sl, i);
print_sl(&sl);

node = find(&sl, 8);
if (node)
printf("find 8 in sl %d\n", node->val);
srandom(1689859321);
skip_list *sl = make_skip_list(15);
print_sl(sl);

ktype a[] = {4, 3, 6, 9, 7, 1, 2, 5, 8};
const int n = sizeof(a) / sizeof(ktype);
for (int i = 0; i < n; i++)
insert(sl, a[i]);
print_sl(sl);

node *cur = search(sl, 8);
if (cur)
printf("find 8 in sl: %d\n", cur->key);
else
printf("8 not in sl\n");

for (i = 0; i < 10; i++) {
delete(&sl, i);
print_sl(&sl);
for (int i = 0; i < n; i++)
{
delete(sl, a[i]);
printf("delete %d\n", a[i]);
print_sl(sl);
}

return 0;
}
}
11 changes: 4 additions & 7 deletions c-cpp/17_skiplist/skiplist_c/skiplist.c
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Original file line number Diff line number Diff line change
Expand Up @@ -78,13 +78,8 @@ int skip_list_level(skiplist * list)
{
int i = 0;
int level = 1;
for (i = 1; i < list->head->max_level; i++)
{
if ((rand()%2) == 1)
{
level++;
}
}
for (i = 1; i < list->head->max_level && rand() % 2; i++)
level++;

return level;
}
Expand Down Expand Up @@ -152,6 +147,7 @@ int skip_list_insert(skiplist *list,int key,int value)
}

list->count++;
free(update);
return 0;
}

Expand Down Expand Up @@ -221,6 +217,7 @@ int skip_list_delete(skiplist * list, int key ,int *value)
return 3;/*未找到节点*/
}

free(update);
return 0 ;
}

Expand Down