(9)引入哈希桶的概念来实现一个哈希表



前面的讲述了如何用链地址法实现一个哈希表，那么今天来分析一下另一种解决哈希冲突的做法，即为每个Hash值，建立一个Hash桶(Bucket)，桶的容量是固定的，也就是只能处理固定次数的冲突，如1048576个Hash桶，每个桶中有4个表项(Entry)，总计4M个表项。其实这两种的实现思路雷同，就是对Hash表中每个Hash值建立一个冲突表，即将冲突的几个记录以表的形式存储在其中。

大致的思路是这样的：

首先哈希桶的个数是固定的，有用户构建的时候输入，一旦构建，个数就已经固定；查找的时候首先将key值通过哈希函数获取哈希值，根据哈希值获取到对应的哈希桶，然后遍历哈希桶内的pairs数组获取。

主要的数据结构：

struct Pair {
    char *key;
    char *value;
};

struct Bucket {
    unsigned int count;
    Pair *pairs;
};

struct StrMap {
    unsigned int count;
    Bucket *buckets;
};

struct Pair {
    char *key;
    char *value;
};

struct Bucket {
    unsigned int count;
    Pair *pairs;
};

struct StrMap {
    unsigned int count;
    Bucket *buckets;
};

本小节主要是学习一下国外大牛是如何实现哈希表的。完整的代码，请看： 这里 ，一位圣安德鲁斯大学的讲师： KRISTENSSON 博客

strmap.h文件

#ifndef _STRMAP_H_
#define _STRMAP_H_

#ifdef __cplusplus
extern "C"
{
#endif

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

typedef struct StrMap StrMap;

/*
 * This callback function is called once per key-value when iterating over
 * all keys associated to values.
 *
 * Parameters:
 *
 * key: A pointer to a null-terminated C string. The string must not
 * be modified by the client.
 *
 * value: A pointer to a null-terminated C string. The string must
 * not be modified by the client.
 *
 * obj: A pointer to a client-specific object. This parameter may be
 * null.
 *
 * Return value: None.
 */
typedef void(*sm_enum_func)(const char *key, const char *value, const void *obj);

/*
 * Creates a string map.
 *
 * Parameters:
 *
 * capacity: The number of top-level slots this string map
 * should allocate. This parameter must be > 0.
 *
 * Return value: A pointer to a string map object,
 * or null if a new string map could not be allocated.
 */
StrMap * sm_new(unsigned int capacity);

/*
 * Releases all memory held by a string map object.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 * If the supplied string map has been previously released, the
 * behaviour of this function is undefined.
 *
 * Return value: None.
 */
void sm_delete(StrMap *map);

/*
 * Returns the value associated with the supplied key.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter cannot
 * be null.
 *
 * out_buf: A pointer to an output buffer which will contain the value,
 * if it exists and fits into the buffer.
 *
 * n_out_buf: The size of the output buffer in bytes.
 *
 * Return value: If out_buf is set to null and n_out_buf is set to 0 the return
 * value will be the number of bytes required to store the value (if it exists)
 * and its null-terminator. For all other parameter configurations the return value
 * is 1 if an associated value was found and completely copied into the output buffer,
 * 0 otherwise.
 */
int sm_get(const StrMap *map, const char *key, char *out_buf, unsigned int n_out_buf);

/*
 * Queries the existence of a key.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter cannot
 * be null.
 *
 * Return value: 1 if the key exists, 0 otherwise.
 */
int sm_exists(const StrMap *map, const char *key);

/*
 * Associates a value with the supplied key. If the key is already
 * associated with a value, the previous value is replaced.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter
 * cannot be null. The string must have a string length > 0. The
 * string will be copied.
 *
 * value: A pointer to a null-terminated C string. This parameter
 * cannot be null. The string must have a string length > 0. The
 * string will be copied.
 *
 * Return value: 1 if the association succeeded, 0 otherwise.
 */
int sm_put(StrMap *map, const char *key, const char *value);

/*
 * Returns the number of associations between keys and values.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * Return value: The number of associations between keys and values.
 */
int sm_get_count(const StrMap *map);

/*
 * An enumerator over all associations between keys and values.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * enum_func: A pointer to a callback function that will be
 * called by this procedure once for every key associated
 * with a value. This parameter cannot be null.
 *
 * obj: A pointer to a client-specific object. This parameter will be
 * passed back to the client's callback function. This parameter can
 * be null.
 *
 * Return value: 1 if enumeration completed, 0 otherwise.
 */
int sm_enum(const StrMap *map, sm_enum_func enum_func, const void *obj);

#ifdef __cplusplus
}
#endif

#endif

#ifndef _STRMAP_H_
#define _STRMAP_H_

#ifdef __cplusplus
extern "C"
{
#endif

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

typedef struct StrMap StrMap;

/*
 * This callback function is called once per key-value when iterating over
 * all keys associated to values.
 *
 * Parameters:
 *
 * key: A pointer to a null-terminated C string. The string must not
 * be modified by the client.
 *
 * value: A pointer to a null-terminated C string. The string must
 * not be modified by the client.
 *
 * obj: A pointer to a client-specific object. This parameter may be
 * null.
 *
 * Return value: None.
 */
typedef void(*sm_enum_func)(const char *key, const char *value, const void *obj);

/*
 * Creates a string map.
 *
 * Parameters:
 *
 * capacity: The number of top-level slots this string map
 * should allocate. This parameter must be > 0.
 *
 * Return value: A pointer to a string map object, 
 * or null if a new string map could not be allocated.
 */
StrMap * sm_new(unsigned int capacity);

/*
 * Releases all memory held by a string map object.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 * If the supplied string map has been previously released, the
 * behaviour of this function is undefined.
 *
 * Return value: None.
 */
void sm_delete(StrMap *map);

/*
 * Returns the value associated with the supplied key.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter cannot
 * be null.
 *
 * out_buf: A pointer to an output buffer which will contain the value,
 * if it exists and fits into the buffer.
 *
 * n_out_buf: The size of the output buffer in bytes.
 *
 * Return value: If out_buf is set to null and n_out_buf is set to 0 the return
 * value will be the number of bytes required to store the value (if it exists)
 * and its null-terminator. For all other parameter configurations the return value
 * is 1 if an associated value was found and completely copied into the output buffer,
 * 0 otherwise.
 */
int sm_get(const StrMap *map, const char *key, char *out_buf, unsigned int n_out_buf);

/*
 * Queries the existence of a key.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter cannot
 * be null.
 *
 * Return value: 1 if the key exists, 0 otherwise.
 */
int sm_exists(const StrMap *map, const char *key);

/*
 * Associates a value with the supplied key. If the key is already
 * associated with a value, the previous value is replaced.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * key: A pointer to a null-terminated C string. This parameter
 * cannot be null. The string must have a string length > 0. The
 * string will be copied.
 *
 * value: A pointer to a null-terminated C string. This parameter
 * cannot be null. The string must have a string length > 0. The
 * string will be copied.
 *
 * Return value: 1 if the association succeeded, 0 otherwise.
 */
int sm_put(StrMap *map, const char *key, const char *value);

/*
 * Returns the number of associations between keys and values.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * Return value: The number of associations between keys and values.
 */
int sm_get_count(const StrMap *map);

/*
 * An enumerator over all associations between keys and values.
 *
 * Parameters:
 *
 * map: A pointer to a string map. This parameter cannot be null.
 *
 * enum_func: A pointer to a callback function that will be
 * called by this procedure once for every key associated
 * with a value. This parameter cannot be null.
 *
 * obj: A pointer to a client-specific object. This parameter will be
 * passed back to the client's callback function. This parameter can
 * be null.
 *
 * Return value: 1 if enumeration completed, 0 otherwise.
 */
int sm_enum(const StrMap *map, sm_enum_func enum_func, const void *obj);

#ifdef __cplusplus
}
#endif

#endif

strmap.c文件

#include "strmap.h"

typedef struct Pair Pair;

typedef struct Bucket Bucket;

struct Pair {
    char *key;
    char *value;
};

struct Bucket {
    unsigned int count;
    Pair *pairs;
};

struct StrMap {
    unsigned int count;
    Bucket *buckets;
};

static Pair * get_pair(Bucket *bucket, const char *key);
static unsigned long hash(const char *str);

StrMap * sm_new(unsigned int capacity)
{
    StrMap *map;

    map = malloc(sizeof(StrMap));
    if (map == NULL) {
        return NULL;
    }
    map->count = capacity;
    map->buckets = malloc(map->count * sizeof(Bucket));
    if (map->buckets == NULL) {
        free(map);
        return NULL;
    }
    memset(map->buckets, 0, map->count * sizeof(Bucket));
    return map;
}

void sm_delete(StrMap *map)
{
    unsigned int i, j, n, m;
    Bucket *bucket;
    Pair *pair;

    if (map == NULL) {
        return;
    }
    n = map->count;
    bucket = map->buckets;
    i = 0;
    while (i < n) {
        m = bucket->count;
        pair = bucket->pairs;
        j = 0;
        while(j < m) {
            free(pair->key);
            free(pair->value);
            pair++;
            j++;
        }
        free(bucket->pairs);
        bucket++;
        i++;
    }
    free(map->buckets);
    free(map);
}

int sm_get(const StrMap *map, const char *key, char *out_buf, unsigned int n_out_buf)
{
    unsigned int index;
    Bucket *bucket;
    Pair *pair;

    if (map == NULL) {
        return 0;
    }
    if (key == NULL) {
        return 0;
    }
    index = hash(key) % map->count;
    bucket = &(map->buckets[index]);
    pair = get_pair(bucket, key);
    if (pair == NULL) {
        return 0;
    }
    if (out_buf == NULL && n_out_buf == 0) {
        return strlen(pair->value) + 1;
    }
    if (out_buf == NULL) {
        return 0;
    }
    if (strlen(pair->value) >= n_out_buf) {
        return 0;
    }
    strcpy(out_buf, pair->value);
    return 1;
}

int sm_exists(const StrMap *map, const char *key)
{
    unsigned int index;
    Bucket *bucket;
    Pair *pair;

    if (map == NULL) {
        return 0;
    }
    if (key == NULL) {
        return 0;
    }
    index = hash(key) % map->count;
    bucket = &(map->buckets[index]);
    pair = get_pair(bucket, key);
    if (pair == NULL) {
        return 0;
    }
    return 1;
}

int sm_put(StrMap *map, const char *key, const char *value)
{
    unsigned int key_len, value_len, index;
    Bucket *bucket;
    Pair *tmp_pairs, *pair;
    char *tmp_value;
    char *new_key, *new_value;

    if (map == NULL) {
        return 0;
    }
    if (key == NULL || value == NULL) {
        return 0;
    }
    key_len = strlen(key);
    value_len = strlen(value);
    /* Get a pointer to the bucket the key string hashes to */
    index = hash(key) % map->count;
    bucket = &(map->buckets[index]);
    /* Check if we can handle insertion by simply replacing
     * an existing value in a key-value pair in the bucket.
     */
    if ((pair = get_pair(bucket, key)) != NULL) {
        /* The bucket contains a pair that matches the provided key,
         * change the value for that pair to the new value.
         */
        if (strlen(pair->value) < value_len) {
            /* If the new value is larger than the old value, re-allocate
             * space for the new larger value.
             */
            tmp_value = realloc(pair->value, (value_len + 1) * sizeof(char));
            if (tmp_value == NULL) {
                return 0;
            }
            pair->value = tmp_value;
        }
        /* Copy the new value into the pair that matches the key */
        strcpy(pair->value, value);
        return 1;
    }
    /* Allocate space for a new key and value */
    new_key = malloc((key_len + 1) * sizeof(char));
    if (new_key == NULL) {
        return 0;
    }
    new_value = malloc((value_len + 1) * sizeof(char));
    if (new_value == NULL) {
        free(new_key);
        return 0;
    }
    /* Create a key-value pair */
    if (bucket->count == 0) {
        /* The bucket is empty, lazily allocate space for a single
         * key-value pair.
         */
        bucket->pairs = malloc(sizeof(Pair));
        if (bucket->pairs == NULL) {
            free(new_key);
            free(new_value);
            return 0;
        }
        bucket->count = 1;
    }
    else {
        /* The bucket wasn't empty but no pair existed that matches the provided
         * key, so create a new key-value pair.
         */
        tmp_pairs = realloc(bucket->pairs, (bucket->count + 1) * sizeof(Pair));
        if (tmp_pairs == NULL) {
            free(new_key);
            free(new_value);
            return 0;
        }
        bucket->pairs = tmp_pairs;
        bucket->count++;
    }
    /* Get the last pair in the chain for the bucket */
    pair = &(bucket->pairs[bucket->count - 1]);
    pair->key = new_key;
    pair->value = new_value;
    /* Copy the key and its value into the key-value pair */
    strcpy(pair->key, key);
    strcpy(pair->value, value);
    return 1;
}

int sm_get_count(const StrMap *map)
{
    unsigned int i, j, n, m;
    unsigned int count;
    Bucket *bucket;
    Pair *pair;

    if (map == NULL) {
        return 0;
    }
    bucket = map->buckets;
    n = map->count;
    i = 0;
    count = 0;
    while (i < n) {
        pair = bucket->pairs;
        m = bucket->count;
        j = 0;
        while (j < m) {
            count++;
            pair++;
            j++;
        }
        bucket++;
        i++;
    }
    return count;
}

int sm_enum(const StrMap *map, sm_enum_func enum_func, const void *obj)
{
    unsigned int i, j, n, m;
    Bucket *bucket;
    Pair *pair;

    if (map == NULL) {
        return 0;
    }
    if (enum_func == NULL) {
        return 0;
    }
    bucket = map->buckets;
    n = map->count;
    i = 0;
    while (i < n) {
        pair = bucket->pairs;
        m = bucket->count;
        j = 0;
        while (j < m) {
            enum_func(pair->key, pair->value, obj);
            pair++;
            j++;
        }
        bucket++;
        i++;
    }
    return 1;
}

/*
 * Returns a pair from the bucket that matches the provided key,
 * or null if no such pair exist.
 */
static Pair * get_pair(Bucket *bucket, const char *key)
{
    unsigned int i, n;
    Pair *pair;

    n = bucket->count;
    if (n == 0) {
        return NULL;
    }
    pair = bucket->pairs;
    i = 0;
    while (i < n) {
        if (pair->key != NULL && pair->value != NULL) {
            if (strcmp(pair->key, key) == 0) {
                return pair;
            }
        }
        pair++;
        i++;
    }
    return NULL;
}

/*
 * Returns a hash code for the provided string.
 */
static unsigned long hash(const char *str)
{
    unsigned long hash = 5381;
    int c;

    while (c = *str++) {
        hash = ((hash << 5) + hash) + c;
    }
    return hash;
}

#include "strmap.h"

typedef struct Pair Pair;

typedef struct Bucket Bucket;

struct Pair {
	char *key;
	char *value;
};

struct Bucket {
	unsigned int count;
	Pair *pairs;
};

struct StrMap {
	unsigned int count;
	Bucket *buckets;
};

static Pair * get_pair(Bucket *bucket, const char *key);
static unsigned long hash(const char *str);

StrMap * sm_new(unsigned int capacity)
{
	StrMap *map;
	
	map = malloc(sizeof(StrMap));
	if (map == NULL) {
		return NULL;
	}
	map->count = capacity;
	map->buckets = malloc(map->count * sizeof(Bucket));
	if (map->buckets == NULL) {
		free(map);
		return NULL;
	}
	memset(map->buckets, 0, map->count * sizeof(Bucket));
	return map;
}

void sm_delete(StrMap *map)
{
	unsigned int i, j, n, m;
	Bucket *bucket;
	Pair *pair;

	if (map == NULL) {
		return;
	}
	n = map->count;
	bucket = map->buckets;
	i = 0;
	while (i < n) {
		m = bucket->count;
		pair = bucket->pairs;
		j = 0;
		while(j < m) {
			free(pair->key);
			free(pair->value);
			pair++;
			j++;
		}
		free(bucket->pairs);
		bucket++;
		i++;
	}
	free(map->buckets);
	free(map);
}

int sm_get(const StrMap *map, const char *key, char *out_buf, unsigned int n_out_buf)
{
	unsigned int index;
	Bucket *bucket;
	Pair *pair;

	if (map == NULL) {
		return 0;
	}
	if (key == NULL) {
		return 0;
	}
	index = hash(key) % map->count;
	bucket = &(map->buckets[index]);
	pair = get_pair(bucket, key);
	if (pair == NULL) {
		return 0;
	}
	if (out_buf == NULL && n_out_buf == 0) {
		return strlen(pair->value) + 1;
	}
	if (out_buf == NULL) {
		return 0;
	}
	if (strlen(pair->value) >= n_out_buf) {
		return 0;
	}
	strcpy(out_buf, pair->value);
	return 1;
}

int sm_exists(const StrMap *map, const char *key)
{
	unsigned int index;
	Bucket *bucket;
	Pair *pair;

	if (map == NULL) {
		return 0;
	}
	if (key == NULL) {
		return 0;
	}
	index = hash(key) % map->count;
	bucket = &(map->buckets[index]);
	pair = get_pair(bucket, key);
	if (pair == NULL) {
		return 0;
	}
	return 1;
}

int sm_put(StrMap *map, const char *key, const char *value)
{
	unsigned int key_len, value_len, index;
	Bucket *bucket;
	Pair *tmp_pairs, *pair;
	char *tmp_value;
	char *new_key, *new_value;

	if (map == NULL) {
		return 0;
	}
	if (key == NULL || value == NULL) {
		return 0;
	}
	key_len = strlen(key);
	value_len = strlen(value);
	/* Get a pointer to the bucket the key string hashes to */
	index = hash(key) % map->count;
	bucket = &(map->buckets[index]);
	/* Check if we can handle insertion by simply replacing
	 * an existing value in a key-value pair in the bucket.
	 */
	if ((pair = get_pair(bucket, key)) != NULL) {
		/* The bucket contains a pair that matches the provided key,
		 * change the value for that pair to the new value.
		 */
		if (strlen(pair->value) < value_len) {
			/* If the new value is larger than the old value, re-allocate
			 * space for the new larger value.
			 */
			tmp_value = realloc(pair->value, (value_len + 1) * sizeof(char));
			if (tmp_value == NULL) {
				return 0;
			}
			pair->value = tmp_value;
		}
		/* Copy the new value into the pair that matches the key */
		strcpy(pair->value, value);
		return 1;
	}
	/* Allocate space for a new key and value */
	new_key = malloc((key_len + 1) * sizeof(char));
	if (new_key == NULL) {
		return 0;
	}
	new_value = malloc((value_len + 1) * sizeof(char));
	if (new_value == NULL) {
		free(new_key);
		return 0;
	}
	/* Create a key-value pair */
	if (bucket->count == 0) {
		/* The bucket is empty, lazily allocate space for a single
		 * key-value pair.
		 */
		bucket->pairs = malloc(sizeof(Pair));
		if (bucket->pairs == NULL) {
			free(new_key);
			free(new_value);
			return 0;
		}
		bucket->count = 1;
	}
	else {
		/* The bucket wasn't empty but no pair existed that matches the provided
		 * key, so create a new key-value pair.
		 */
		tmp_pairs = realloc(bucket->pairs, (bucket->count + 1) * sizeof(Pair));
		if (tmp_pairs == NULL) {
			free(new_key);
			free(new_value);
			return 0;
		}
		bucket->pairs = tmp_pairs;
		bucket->count++;
	}
	/* Get the last pair in the chain for the bucket */
	pair = &(bucket->pairs[bucket->count - 1]);
	pair->key = new_key;
	pair->value = new_value;
	/* Copy the key and its value into the key-value pair */
	strcpy(pair->key, key);
	strcpy(pair->value, value);
	return 1;
}

int sm_get_count(const StrMap *map)
{
	unsigned int i, j, n, m;
	unsigned int count;
	Bucket *bucket;
	Pair *pair;

	if (map == NULL) {
		return 0;
	}
	bucket = map->buckets;
	n = map->count;
	i = 0;
	count = 0;
	while (i < n) {
		pair = bucket->pairs;
		m = bucket->count;
		j = 0;
		while (j < m) {
			count++;
			pair++;
			j++;
		}
		bucket++;
		i++;
	}
	return count;
}

int sm_enum(const StrMap *map, sm_enum_func enum_func, const void *obj)
{
	unsigned int i, j, n, m;
	Bucket *bucket;
	Pair *pair;

	if (map == NULL) {
		return 0;
	}
	if (enum_func == NULL) {
		return 0;
	}
	bucket = map->buckets;
	n = map->count;
	i = 0;
	while (i < n) {
		pair = bucket->pairs;
		m = bucket->count;
		j = 0;
		while (j < m) {
			enum_func(pair->key, pair->value, obj);
			pair++;
			j++;
		}
		bucket++;
		i++;
	}
	return 1;
}

/*
 * Returns a pair from the bucket that matches the provided key,
 * or null if no such pair exist.
 */
static Pair * get_pair(Bucket *bucket, const char *key)
{
	unsigned int i, n;
	Pair *pair;

	n = bucket->count;
	if (n == 0) {
		return NULL;
	}
	pair = bucket->pairs;
	i = 0;
	while (i < n) {
		if (pair->key != NULL && pair->value != NULL) {
			if (strcmp(pair->key, key) == 0) {
				return pair;
			}
		}
		pair++;
		i++;
	}
	return NULL;
}

/*
 * Returns a hash code for the provided string.
 */
static unsigned long hash(const char *str)
{
	unsigned long hash = 5381;
	int c;

	while (c = *str++) {
		hash = ((hash << 5) + hash) + c;
	}
	return hash;
}

前一节与这节这两种实现方法看似比较类似，但也有差异：

基于哈希桶的情况下，由于Hash桶容量的限制，所以，有可能发生Hash表填不满的情况，也就是，虽然Hash表里面还有空位，但是新建的表项由于冲突过多，而不能装入Hash表中。不过，这样的实现也有其好处，就是查表的最大开销是可以确定的，因为最多处理的冲突数是确定的，所以算法的时间复杂度为O(1)+O(m)，其中m为Hash桶容量。

而另一种通过链表的实现，由于Hash桶的容量是无限的，因此，只要没有超出Hash表的最大容量，就能够容纳新建的表项。但是，一旦发生了Hash冲突严重的情况，就会造成Hash桶的链表过长，大大降低查找效率。在最坏的情况下，时间复杂度退化为O(n)，其中n为Hash表的总容量。当然，这种情况的概率小之又小，几乎是可以忽略的。