《算法导论》第11章 散列表 (3)开放寻址

前一节介绍是最简单的冲突解决方法-链接法。开放寻址与链接法不同，所有元素都放在散列表内。
在这种方法中，散列表可能会被填满。开放寻址不需要指针，只需要计算出要存取的各个槽。
由于不用存储指针而节省的空间可以提供更多的槽。

有三种技术常用来计算开放寻址法中的探查序列：线性探查、二次探查和双重探查。
下面的实现中，三种方法的差别只在计算探查序列的那一行代码。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define SIZE 20

typedef struct _Entry {
char *key;
char *val;
} Entry;

// 指针数组
Entry *hashmap[SIZE];

// Same as hashcode when String put to HashMap
unsigned hashcode(char *key)
{
// Ensure >> is logical shift
unsigned h = 0;

// String.hashcode()
do h = 31 * h + *key++;
while (*key != '\0');

// HashMap.hash()
h ^= (h >> 20) ^ (h >> 12);
return h ^ (h >> 7) ^ (h >> 4);
}

Entry * hashmap_search(char *key)
{
unsigned h = hashcode(key) % SIZE;
unsigned h2 = h;
Entry *entry = hashmap[h];
while (entry != NULL) {
if (strcmp(entry->key, key) == 0) {
return entry;
}

// 线性探查。不同探查方法差别只在这一行。
h = (h + 1) % SIZE;
entry = hashmap[h];

if (h == h2)
break;
}
return NULL;
}

char * hashmap_insert(char *key, char *val)
{
unsigned h = hashcode(key) % SIZE;
printf("Insert %s - %s to bucket %d\n", key, val, h);

// Find duplicate key, replace it then return old value
unsigned h2 = h;
Entry *entry = hashmap[h];
while (entry != NULL) {
if (strcmp(entry->key, key) == 0) {
char *oldVal = entry->val;
entry->val = val;
return oldVal;
}

// Linear search
h = (h + 1) % SIZE;
entry = hashmap[h];

// Check if loop to initial bucket
if (h == h2)
break;
}

// Not found, create new node to save key&val pair
if (entry == NULL) {
entry = malloc(sizeof(Entry));
entry->key = key;
entry->val = val;
hashmap[h] = entry;
}

return val;
}

void hashmap_print()
{
Entry *entry;
int i;
for (i = 0; i < SIZE; i++) {
entry = hashmap[i];
if (entry == NULL)
printf("%d: null\n", i);
else
printf("%d: %s - %s\n", i, entry->key, entry->val);
}
printf("\n");
}

int main(void)
{
// Compare to String.hashcode() in JDK
printf("%d\n", hashcode("helloworld"));

hashmap_insert("aabb", "value1");
hashmap_insert("ccdd", "value2");
hashmap_insert("i'mcdai", "value3");

int i;
for (i = 0; i < 2 * SIZE + 5; i++) {
char *key = calloc(sizeof(char), 10);
char *val = calloc(sizeof(char), 10);
sprintf(key, "%s%d", "aabbcc", i);
sprintf(val, "%s%d", "val ", i);
hashmap_insert(key, val);
}

// Insert duplicate key
printf("%s\n", hashmap_insert("i'mcdai", "dupdup"));

hashmap_print();

printf("%s\n", hashmap_search("i'mcdai")->val);

hashmap_print();

return 1;
}