From 2a3093b9f339117c5aac7c3c38762507ee1c096b Mon Sep 17 00:00:00 2001
From: Wei Yang <richard.weiyang@gmail.com>
Date: Sun, 4 Nov 2018 06:34:54 +0800
Subject: [PATCH] skiplist in c

---
 c-cpp/17_skiplist/skiplist.c | 214 +++++++++++++++++++++++++++++++++++
 1 file changed, 214 insertions(+)
 create mode 100644 c-cpp/17_skiplist/skiplist.c

diff --git a/c-cpp/17_skiplist/skiplist.c b/c-cpp/17_skiplist/skiplist.c
new file mode 100644
index 0000000..82621c5
--- /dev/null
+++ b/c-cpp/17_skiplist/skiplist.c
@@ -0,0 +1,214 @@
+#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
+
+struct node {
+	int val;
+	int max_level;
+	struct node *forward[MAX_LEVEL];
+};
+
+struct skip_list {
+	struct node head;
+	int max_level;
+	int max_level_nodes;
+};
+
+void node_init(struct node* node)
+{
+	memset(node, 0, sizeof(struct node));
+}
+
+void skip_list_init(struct skip_list* sl)
+{
+	node_init(&sl->head);
+	sl->max_level = 0;
+	sl->max_level_nodes = 0;
+}
+
+void random_init()
+{
+	static bool done = false;
+
+	if (done)
+		return;
+
+	srandom(time(NULL));
+	done = true;
+}
+
+int random_level(void)
+{
+    int i, level = 1;
+
+    random_init();
+
+    for (i = 1; i < MAX_LEVEL; i++)
+	    if (random() % 2 == 1)
+		    level++;
+
+    return level;
+}
+
+void random_level_test()
+{
+	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;
+	}
+
+	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++;
+}
+
+struct node *find(struct skip_list* sl, int val)
+{
+	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];
+	}
+	else
+		return NULL;
+}
+
+void delete(struct skip_list* sl, int val)
+{
+	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]; 
+	}
+
+	// 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;
+		}
+	}
+}
+
+
+void print_sl(struct 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("\n");
+	}
+}
+
+int main()
+{
+	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);
+	else
+		printf("8 not in sl\n");
+
+	for (i = 0; i < 10; i++) {
+		delete(&sl, i);
+		print_sl(&sl);
+	}
+
+	return 0;
+}