原创作品，转载请标明：http://blog.csdn.net/Xiejingfa/article/details/51111230
今天为大家带来Redis中zipmap数据结构的分析，该结构定义在zipmap.h和zipmap.c文件里。我把zipmap称作“压缩字典”（不知道这样称呼正不对）是因为zipmap利用字符串实现了一个简单的hash_table结构，又通过固定的字节表示节省空间。
zipmap和前面介绍的ziplist结构十分相似，我们能够对照地进行学习：
Redis中定义了双向链表list，可是这样的链表的每一个节点须要存储两个指针来指向前一个节点和下一个节点，存在着空间利用率不高的缺点（特别是在存储少量数据的情况下）。为了解决问题，Redis引入了压缩列表ziplist，通过存储上一个节点长度和当前节点长度从而在大多数情况下节省了链表指针的存储空间开销。
Redis中定义了字典dict来实现非常多高级语言中的Map结构。这样的结构相对来说比較复杂，并且须要两张散列表来实现rehash操作。在存储少量元素的时候空间利用率也比較低。为此，Redis引入了压缩字典zipmap，就是我们今天要介绍的数据结构。
zipmap实质上是用一个字符串来依次保存key和value。有良好的空间利用率和相对不错的查找速度（以下我们会详细解说）。
假设你还不知道什么是ziplist和dict，请參看我前面的博文：
Redis是基于内存的高效的数据库。从list到ziplist，从dict到zipmap，我们能够看到Redis的作者们为了高效地利用内存空间真是无所不用其极。
以下我们就进入今天的主题，介绍zipmap的实现。
再次强调。zipmap和ziplist有非常多相似的地方，请大家注意比对学习。
1、zipmap的存储结构
zipmap是用连续内存保存key-value对的结构，查询时是依次遍列每一个key-value对，直到查到为止。其存储结构例如以下所看到的：
各部分的含义例如以下：
（1）、zmlen字段
zmlen是1个字节的无符号整型数，表示zipmap当前的键值对数量，最多仅仅能表示253个键值对。当zipmap中的元素数量大于或等于254时，仅仅能通过遍历一遍zipmap来确定其大小。
（2）、key length和value length字段
这两个length字段的编码方式和ziplist有些相似。能够用1个字节或5个字节编码表示，详细例如以下：
- 假设随后的字符串（key或value）长度小于或等于253，直接用1个字节表示其长度。
- 假设随后的字符串（key或value）长度超过254。则用5个字节表示，当中第一个字节值为254，接下来的4个字节才是字符串长度。
（3）、free字段
free表示随后的value后面的空暇字节数。
比方：假设zipmap存在”foo” => “bar”这样一个键值对，随后我们将“bar”设置为“hi”。此时free = 1，表示value字符串后面有1个字节大小的空暇空间。
free字段是一个占1个字节的整型数，它的值一般都比較小。假设空暇区间太大，zipmap会进行调整以使整个map尽可能小，这个过程发生在zipmapSet操作中。
（4）、end字段
end是zipmap的结尾符，占用1个字节，其值为255。
从zipmap的存储结构中我们能够看到，zipmap中的键值对就是顺序存放的key和value字符串。中间加入了相关的字符串长度编码信息以方便我们获取相关的key和value。
而整个zipmap就是在一块内存区中依次存放key-value字符串。
2、zipmap的几个重要操作
与ziplist相比。zipmap的操作要简单很多，Redis甚至未定义额外的结构体。仅仅是通过简单的字符串操作来维护一个zipmap结构。以下我们就来介绍一下zipmap相关的几个操作：
2.1、创建一个空zipmap
一个空的zipmap结构仅仅有两个字节（即zmlen字段和end字段）。结构例如以下：
创建空的zipmap非常easy，这里拿出来讲是为了让大家更好地理解zipmap的存储结构。
unsigned char *zipmapNew(void) {
// 初始化时仅仅有2个字节,第1个字节表示zipmap保存的key-value对的个数。第2个字节为结尾符
unsigned char *zm = zmalloc(2);
// 当前保存的键值对个数为0
zm[0] = 0; /* Length */
zm[1] = ZIPMAP_END;
return zm;
}所以zipmap是不是非常easy？
2.2、查找操作
作为一个hash_table结构。其最基本的操作是依据给定的key值查找对应的value值。
在zipmap中。key-value是顺序存放的，所以仅仅能从前往后遍历以查找目标键值对，但因为每一个键值对中都记录了key和value的长度，所以查找起来也非常方便。
详细例如以下所看到的：
/* 按关键字key查找zipmap，假设totlen不为NULL，函数返回后存放zipmap占用的字节数 */
static unsigned char *zipmapLookupRaw(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned int *totlen) {
// zipmap中第1个字节是zmlen字段。zm+1跳过第1个字节
unsigned char *p = zm+1, *k = NULL;
unsigned int l,llen;
// 从前往后查找
while(*p != ZIPMAP_END) {
unsigned char free;
/* Match or skip the key */
// 确定key字符串的长度
l = zipmapDecodeLength(p);
// 确定保存key字符串长度所须要的字节数，也就是len字段所须要的字节数
llen = zipmapEncodeLength(NULL,l);
// 比較当前key与给定key是否匹配
if (key != NULL && k == NULL && l == klen && !memcmp(p+llen,key,l)) {
/* Only return when the user doesn't care
* for the total length of the zipmap. */
// 假设totlen为NULL。表示函数调用者不关心zipmap占用的字节数，此时直接返回p，否则先记录下p指针然后继续遍历
if (totlen != NULL) {
k = p;
} else {
return p;
}
}
// p加上llen和l。到了value节点处
p += llen+l;
/* Skip the value as well */
// 确定value字符串的长度
l = zipmapDecodeLength(p);
// 确定保存value字符串长度所须要的字节数，也就是len字段所须要的字节数
p += zipmapEncodeLength(NULL,l);
// 读出free字段的值（前面我们讲过：free仅仅占用一个字节）
free = p[0];
// 跳到下一个key节点的
p += l+1+free; /* +1 to skip the free byte */
}
// 到这里遍历完整个zipmap。得到其占用的字节数
if (totlen != NULL) *totlen = (unsigned int)(p-zm)+1;
return k;
}2.3、插入 & 更新操作
zipmap将插入和更新操作实如今同一个函数zipmapSet中，即存在指定key的键值对则运行更新操作，否则运行插入操作。
函数zipmapSet的运行流程例如以下：
- 调用zipmapLookupRaw函数确定指定key的键值对是否已经在zipmap中。
- 又一次分配空间以确保有足够的空间来容纳新的键值对（插入操作）或新的value值（更新操作）。假设是插入操作，就在zipmap尾部添加空间，假设是更新操作，则通过memmove移动在原键值对处留出足够的空间。
- 在前面两步中已经确定了键值对的位置并有足够的空间。直接写入或更新键值对的信息就可以。
/* 依据key设置value，假设key不存在则创建对应的键值对。參数update用来辨别更新操作和加入操作。 */
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
unsigned int zmlen, offset;
// 计算存储key和value所须要的字节数
unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
unsigned int empty, vempty;
unsigned char *p;
/************************************************************************
* 以下这段代码用于在zipmap留出足够的空间来容纳新插入的键值对或新的value值。尚未写入
************************************************************************/
freelen = reqlen;
if (update) *update = 0;
// 在zipmap中查找key。函数返回后zmlen中保存了zipmap所占用的字节数。
p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p == NULL) {
/* Key not found: enlarge */
// 假设key指定的键值对不存在，则对zipmap扩容，为容纳新的键值对准备内存空间
// zipmapResize运行的是realloc操作
zm = zipmapResize(zm, zmlen+reqlen);
// 此时p指向扩容前zipmap的结尾符，将从这里加入新的键值对
p = zm+zmlen-1;
// 更新zipmap所占用的内存空间大小
zmlen = zmlen+reqlen;
/* Increase zipmap length (this is an insert) */
// 更新zipmap中保存的键值对数量，即zmlen字段
if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
} else {
/* Key found. Is there enough space for the new value? */
/* 找到可对应的键值对。运行更新操作。这里须要考虑value节点的空间大小能否够容纳新值 */
/* Compute the total length: */
if (update) *update = 1;
// 求出旧value节点的空间大小
freelen = zipmapRawEntryLength(p);
if (freelen < reqlen) {
/* Store the offset of this key within the current zipmap, so
* it can be resized. Then, move the tail backwards so this
* pair fits at the current position. */
// 旧节点的空间太小，须要扩容操作，zipmapResize函数会又一次分配空间，所以须要记录p指针的偏移量
offset = p-zm;
zm = zipmapResize(zm, zmlen-freelen+reqlen);
p = zm+offset;
/* The +1 in the number of bytes to be moved is caused by the
* end-of-zipmap byte. Note: the *original* zmlen is used. */
// 移动旧value节点以后的元素以确保有足够的空间容纳新值（ +1是将尾部结尾符一起移动）
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen = zmlen-freelen+reqlen;
freelen = reqlen;
}
}
/* We now have a suitable block where the key/value entry can
* be written. If there is too much free space, move the tail
* of the zipmap a few bytes to the front and shrink the zipmap,
* as we want zipmaps to be very space efficient. */
// freelen表示经上步骤后流出来的空余空间大小，reqlen表示插入或更新键值对所须要的空间。两者的差就是free字段的
// 的值，假设该值过大zipmap会自己主动调整。以下这段代码就是完毕调整功能。
empty = freelen-reqlen;
if (empty >= ZIPMAP_VALUE_MAX_FREE) {
/* First, move the tail <empty> bytes to the front, then resize
* the zipmap to be <empty> bytes smaller. */
offset = p-zm;
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen -= empty;
zm = zipmapResize(zm, zmlen);
p = zm+offset;
vempty = 0;
} else {
vempty = empty;
}
/******************************************
* 以下的操作是讲key和value写入zipmap指定位置
*******************************************/
/* Just write the key + value and we are done. */
/* Key: */
// 对key的长度编码并写入zipmap中
p += zipmapEncodeLength(p,klen);
// 写入key字符串
memcpy(p,key,klen);
// 移动指针到value写入位置
p += klen;
/* Value: */
// 对value的长度编码并写入zipmap中
p += zipmapEncodeLength(p,vlen);
// 写入free字段
*p++ = vempty;
// 写入value
memcpy(p,val,vlen);
return zm;
}3、总结
zipmap主要是为了节省内存空间而设计的字符串-字符串映射结构（哈希结构）。它的实现非常easy，代码仅仅有400余行。
zipmap通过在一块连续的内存空间上依次存放key-value对来维持key和value的映射结构，但它跟我们传统意义上的散列结构不同。仅仅能通过依次遍历每一个key-value节点来查找给定key值得value信息。
可是。相比于字典dict，zipmap通过固定的字节编码表示节省了不少空间。另外。因为使用了一块连续的内存空间，zipmap的每一次插入、删除、更新操作都有可能造成空间的又一次分配。
从zipmap的实现上我们能够看到，zipmap不适合用来存放大量的key-value对。
在运用上，Redis中内置的Hash结构在保存的元素数量较少时会採用zipmap来存放键值对，当元素的数量到达给定值后才会转为用哈希表来存储以节省内存。
以下贴出凝视版的代码，供大家參考：
/* String -> String Map data structure optimized for size.
* zipmap是为了节省内存空间而设计的字符串-字符串映射结构（哈希结构）
*
* This file implements a data structure mapping strings to other strings
* implementing an O(n) lookup data structure designed to be very memory
* efficient.
* zipmap实现了一种将字符串与字符串之间映射起来的数据结构，它支持O(n)的查找效率并具有良好的空间利用率。
*
* The Redis Hash type uses this data structure for hashes composed of a small
* number of elements, to switch to a hash table once a given number of
* elements is reached.
* Redis中内置的Hash结构在保存的元素数量较少时会採用zipmap来存放键值对，当元素的数量到达给定值
* 后才会转为用哈希表来存储以节省内存。
*
* Given that many times Redis Hashes are used to represent objects composed
* of few fields, this is a very big win in terms of used memory.
* Redis中的Hash结构常常被用来保存仅仅有少量字段的对象，这样的情况下使用zipmap能非常大程度上节省内存
*
* --------------------------------------------------------------------------
*
* Copyright (c) 2009-2010, Salvatore Sanfilippo <antirez at gmail dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
/* Memory layout of a zipmap, for the map "foo" => "bar", "hello" => "world":
* zipmap的内存空间布局例如以下（假设存放着"foo" => "bar", "hello" => "world"数据）：
*
* <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world"
*
* <zmlen> is 1 byte length that holds the current size of the zipmap.
* When the zipmap length is greater than or equal to 254, this value
* is not used and the zipmap needs to be traversed to find out the length.
* zmlen是1个字节的无符号整型数，表示zipmap当前的键值对数量。最多仅仅能表示253个键值对。当zipmap中的元素数量大于
* 或等于254时。仅仅能通过遍历一遍zipmap来确定其大小。
*
* <len> is the length of the following string (key or value).
* len表示随后的字符串的长度，这些字符串可能是key或者value
*
* <len> lengths are encoded in a single value or in a 5 bytes value.
* If the first byte value (as an unsigned 8 bit value) is between 0 and
* 253, it's a single-byte length. If it is 254 then a four bytes unsigned
* integer follows (in the host byte ordering). A value of 255 is used to
* signal the end of the hash.
* len字段的编码方式和ziplist有些相似，能够用1个字节或5个字节编码表示，详细例如以下：
* （1）假设随后的字符串长度小于或等于253。直接用1个字节表示其长度
* （2）假设随后的字符串长度超过254，则用5个字节表示。当中第一个字节值为254，接下来的4个字节才是字符串长度
*
* <free> is the number of free unused bytes after the string, resulting
* from modification of values associated to a key. For instance if "foo"
* is set to "bar", and later "foo" will be set to "hi", it will have a
* free byte to use if the value will enlarge again later, or even in
* order to add a key/value pair if it fits.
* free表示随后的value的空暇字节数。比方：假设zipmap存在"foo" => "bar"这样一个键值对，随后我们将“bar”设置为
* ”hi"，此时free = 1，表示value字符串后面有1个字节大小的空暇空间。
*
* <free> is always an unsigned 8 bit number, because if after an
* update operation there are more than a few free bytes, the zipmap will be
* reallocated to make sure it is as small as possible.
* free字段是一个占1个字节的整型数，它的值一般都比較小，假设空暇区间太大。zipmap会进行调整以使整个map尽可能小。
*
* zipmap也存在一个结尾符。占用1个字节，其值为255。
*
* 从上面的介绍能够看出：zipmap实质上是用一个字符串数组来依次保存key和value,查询时是依次遍列每一个key-value对，直到查到为止。
*
* The most compact representation of the above two elements hash is actually:
*
* "x02x03foox03x00barx05hellox05x00worldxff"
*
* Note that because keys and values are prefixed length "objects",
* the lookup will take O(N) where N is the number of elements
* in the zipmap and *not* the number of bytes needed to represent the zipmap.
* This lowers the constant times considerably.
*/
#include <stdio.h>
#include <string.h>
#include "zmalloc.h"
#include "endianconv.h"
/* 说明：为了描写叙述方便，这里将key-value键值对的key称为key节点，value称为value节点 */
#define ZIPMAP_BIGLEN 254 // zipmap的元素个数超过253时的标识符
#define ZIPMAP_END 255 // zipmap的结尾符
/* The following defines the max value for the <free> field described in the
* comments above, that is, the max number of trailing bytes in a value. */
/* free字段的最大值，也就是value后面的最大空暇字节数 */
#define ZIPMAP_VALUE_MAX_FREE 4
/* The following macro returns the number of bytes needed to encode the length
* for the integer value _l, that is, 1 byte for lengths < ZIPMAP_BIGLEN and
* 5 bytes for all the other lengths. */
/* 工具宏，用来确定len字段所占用的字节数。简单地測试第一个字节的值与254的大小关系 */
#define ZIPMAP_LEN_BYTES(_l) (((_l) < ZIPMAP_BIGLEN) ? 1 : sizeof(unsigned int)+1)
/* Create a new empty zipmap. */
/* 创建一个空的zipmap结构 */
unsigned char *zipmapNew(void) {
// 初始化时仅仅有2个字节,第1个字节表示zipmap保存的key-value对的个数，第2个字节为结尾符
unsigned char *zm = zmalloc(2);
// 当前保存的键值对个数为0
zm[0] = 0; /* Length */
zm[1] = ZIPMAP_END;
return zm;
}
/* Decode the encoded length pointed by 'p' */
/* 获取len字段的数值（即随后字符串的长度），其原理非常easy：查看第一个字节的数值。假设该数值小于254，则直接返回，
否则读取接下来的4个字节内容表示的数值。 */
static unsigned int zipmapDecodeLength(unsigned char *p) {
unsigned int len = *p;
if (len < ZIPMAP_BIGLEN) return len;
// 读取随后4个字节的内容
memcpy(&len,p+1,sizeof(unsigned int));
// 统一转化为小端模式表示
memrev32ifbe(&len);
return len;
}
/* Encode the length 'l' writing it in 'p'. If p is NULL it just returns
* the amount of bytes required to encode such a length. */
/* 将长度len编码到p指针指向的内存空间 */
static unsigned int zipmapEncodeLength(unsigned char *p, unsigned int len) {
if (p == NULL) {
return ZIPMAP_LEN_BYTES(len);
} else {
if (len < ZIPMAP_BIGLEN) {
// 长度小于254。仅仅须要1个字节表示
p[0] = len;
return 1;
} else {
// 长度大于等于254。第一个字节赋值为254，接下来的4歌字节才是真正的长度值
p[0] = ZIPMAP_BIGLEN;
memcpy(p+1,&len,sizeof(len));
memrev32ifbe(p+1);
return 1+sizeof(len);
}
}
}
/* Search for a matching key, returning a pointer to the entry inside the
* zipmap. Returns NULL if the key is not found.
*
* If NULL is returned, and totlen is not NULL, it is set to the entire
* size of the zimap, so that the calling function will be able to
* reallocate the original zipmap to make room for more entries. */
/* 按关键字key查找zipmap，假设totlen不为NULL，函数返回后存放zipmap占用的字节数 */
static unsigned char *zipmapLookupRaw(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned int *totlen) {
// zipmap中第1个字节是zmlen字段，zm+1跳过第1个字节
unsigned char *p = zm+1, *k = NULL;
unsigned int l,llen;
// 从前往后查找
while(*p != ZIPMAP_END) {
unsigned char free;
/* Match or skip the key */
// 确定key字符串的长度
l = zipmapDecodeLength(p);
// 确定保存key字符串长度所须要的字节数。也就是len字段所须要的字节数
llen = zipmapEncodeLength(NULL,l);
// 比較当前key与给定key是否匹配
if (key != NULL && k == NULL && l == klen && !memcmp(p+llen,key,l)) {
/* Only return when the user doesn't care
* for the total length of the zipmap. */
// 假设totlen为NULL，表示函数调用者不关心zipmap占用的字节数，此时直接返回p，否则先记录下p指针然后继续遍历
if (totlen != NULL) {
k = p;
} else {
return p;
}
}
// p加上llen和l，到了value节点处
p += llen+l;
/* Skip the value as well */
// 确定value字符串的长度
l = zipmapDecodeLength(p);
// 确定保存value字符串长度所须要的字节数，也就是len字段所须要的字节数
p += zipmapEncodeLength(NULL,l);
// 读出free字段的值（前面我们讲过：free仅仅占用一个字节）
free = p[0];
// 跳到下一个key节点的
p += l+1+free; /* +1 to skip the free byte */
}
// 到这里遍历完整个zipmap，得到其占用的字节数
if (totlen != NULL) *totlen = (unsigned int)(p-zm)+1;
return k;
}
/* 存储由长度为klen的key和长度为vlen的value组成的键值对所须要的字节数*/
static unsigned long zipmapRequiredLength(unsigned int klen, unsigned int vlen) {
unsigned int l;
l = klen+vlen+3;
if (klen >= ZIPMAP_BIGLEN) l += 4;
if (vlen >= ZIPMAP_BIGLEN) l += 4;
return l;
}
/* Return the total amount used by a key (encoded length + payload) */
/* 获取key节点占用的字节数，即len字段 + key字符串长度 */
static unsigned int zipmapRawKeyLength(unsigned char *p) {
// 获取key字符串的长度
unsigned int l = zipmapDecodeLength(p);
// 加上保存key字符串长度所须要的字节数
return zipmapEncodeLength(NULL,l) + l;
}
/* Return the total amount used by a value
* (encoded length + single byte free count + payload) */
/* 获取value节点占用的字节数，即len字段 + 1个字节free字段 + value字符串长度 + 空暇空间大小 */
static unsigned int zipmapRawValueLength(unsigned char *p) {
// 获取value字符串的长度
unsigned int l = zipmapDecodeLength(p);
unsigned int used;
// 获取保存value字符串长度所须要的字节数
used = zipmapEncodeLength(NULL,l);
// p[used]里面存储着空暇空间的大小
used += p[used] + 1 + l;
return used;
}
/* If 'p' points to a key, this function returns the total amount of
* bytes used to store this entry (entry = key + associated value + trailing
* free space if any). */
/* 假设p指向key节点，则该函数返回存储整个键值对所占用的字节数，包含key节点长度 + value节点长度 + 空暇字节数（假设有） */
static unsigned int zipmapRawEntryLength(unsigned char *p) {
unsigned int l = zipmapRawKeyLength(p);
return l + zipmapRawValueLength(p+l);
}
/* 又一次调整zipmap的大小 */
static inline unsigned char *zipmapResize(unsigned char *zm, unsigned int len) {
// 又一次分配空间。注意是realloc，即在原空间又一次分配
zm = zrealloc(zm, len);
// 设置结尾符
zm[len-1] = ZIPMAP_END;
return zm;
}
/* Set key to value, creating the key if it does not already exist.
* If 'update' is not NULL, *update is set to 1 if the key was
* already preset, otherwise to 0. */
/* 依据key设置value。假设key不存在则创建对应的键值对。參数update用来辨别更新操作和加入操作。 */
unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) {
unsigned int zmlen, offset;
// 计算存储key和value所须要的字节数
unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen);
unsigned int empty, vempty;
unsigned char *p;
/************************************************************************
* 以下这段代码用于在zipmap留出足够的空间来容纳新插入的键值对或新的value值。尚未写入
************************************************************************/
freelen = reqlen;
if (update) *update = 0;
// 在zipmap中查找key，函数返回后zmlen中保存了zipmap所占用的字节数。
p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p == NULL) {
/* Key not found: enlarge */
// 假设key指定的键值对不存在，则对zipmap扩容。为容纳新的键值对准备内存空间
// zipmapResize运行的是realloc操作
zm = zipmapResize(zm, zmlen+reqlen);
// 此时p指向扩容前zipmap的结尾符，将从这里加入新的键值对
p = zm+zmlen-1;
// 更新zipmap所占用的内存空间大小
zmlen = zmlen+reqlen;
/* Increase zipmap length (this is an insert) */
// 更新zipmap中保存的键值对数量。即zmlen字段
if (zm[0] < ZIPMAP_BIGLEN) zm[0]++;
} else {
/* Key found. Is there enough space for the new value? */
/* 找到可对应的键值对，运行更新操作。这里须要考虑value节点的空间大小能否够容纳新值 */
/* Compute the total length: */
if (update) *update = 1;
// 求出旧value节点的空间大小
freelen = zipmapRawEntryLength(p);
if (freelen < reqlen) {
/* Store the offset of this key within the current zipmap, so
* it can be resized. Then, move the tail backwards so this
* pair fits at the current position. */
// 旧节点的空间太小，须要扩容操作，zipmapResize函数会又一次分配空间。所以须要记录p指针的偏移量
offset = p-zm;
zm = zipmapResize(zm, zmlen-freelen+reqlen);
p = zm+offset;
/* The +1 in the number of bytes to be moved is caused by the
* end-of-zipmap byte. Note: the *original* zmlen is used. */
// 移动旧value节点以后的元素以确保有足够的空间容纳新值（ +1是将尾部结尾符一起移动）
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen = zmlen-freelen+reqlen;
freelen = reqlen;
}
}
/* We now have a suitable block where the key/value entry can
* be written. If there is too much free space, move the tail
* of the zipmap a few bytes to the front and shrink the zipmap,
* as we want zipmaps to be very space efficient. */
// freelen表示经上步骤后流出来的空余空间大小，reqlen表示插入或更新键值对所须要的空间，两者的差就是free字段的
// 的值。假设该值过大zipmap会自己主动调整。以下这段代码就是完毕调整功能。
empty = freelen-reqlen;
if (empty >= ZIPMAP_VALUE_MAX_FREE) {
/* First, move the tail <empty> bytes to the front, then resize
* the zipmap to be <empty> bytes smaller. */
offset = p-zm;
memmove(p+reqlen, p+freelen, zmlen-(offset+freelen+1));
zmlen -= empty;
zm = zipmapResize(zm, zmlen);
p = zm+offset;
vempty = 0;
} else {
vempty = empty;
}
/******************************************
* 以下的操作是讲key和value写入zipmap指定位置
*******************************************/
/* Just write the key + value and we are done. */
/* Key: */
// 对key的长度编码并写入zipmap中
p += zipmapEncodeLength(p,klen);
// 写入key字符串
memcpy(p,key,klen);
// 移动指针到value写入位置
p += klen;
/* Value: */
// 对value的长度编码并写入zipmap中
p += zipmapEncodeLength(p,vlen);
// 写入free字段
*p++ = vempty;
// 写入value
memcpy(p,val,vlen);
return zm;
}
/* Remove the specified key. If 'deleted' is not NULL the pointed integer is
* set to 0 if the key was not found, to 1 if it was found and deleted. */
/* 依据key删除指定的键值对 */
unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted) {
unsigned int zmlen, freelen;
// 看推断该键值对是否在zipmap中，假设不存在则直接返回
unsigned char *p = zipmapLookupRaw(zm,key,klen,&zmlen);
if (p) {
// 以下三句代码运行删除操作，事实上就是内存块的移动操作
freelen = zipmapRawEntryLength(p);
memmove(p, p+freelen, zmlen-((p-zm)+freelen+1));
zm = zipmapResize(zm, zmlen-freelen);
/* Decrease zipmap length */
if (zm[0] < ZIPMAP_BIGLEN) zm[0]--;
if (deleted) *deleted = 1;
} else {
if (deleted) *deleted = 0;
}
return zm;
}
/* Call before iterating through elements via zipmapNext() */
/* zipmapNext迭代器函数，还记得前面我们分析过zipmap第一个字节是zmlen字段吗？以下这个函数就是跳过第一个字节返回
指向第一个键值对的首地址 */
unsigned char *zipmapRewind(unsigned char *zm) {
return zm+1;
}
/* This function is used to iterate through all the zipmap elements.
* In the first call the first argument is the pointer to the zipmap + 1.
* In the next calls what zipmapNext returns is used as first argument.
* Example:
*
* unsigned char *i = zipmapRewind(my_zipmap);
* while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
* printf("%d bytes key at $p
", klen, key);
* printf("%d bytes value at $p
", vlen, value);
* }
*/
/* zipmap的迭代器式遍历函数，典型使用方法例如以下：
*
* unsigned char *i = zipmapRewind(my_zipmap);
* while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
* printf("%d bytes key at $p
", klen, key);
* printf("%d bytes value at $p
", vlen, value);
* }
*/
unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen) {
// 假设达到尾部。直接返回NULL
if (zm[0] == ZIPMAP_END) return NULL;
// 获取key
if (key) {
*key = zm;
*klen = zipmapDecodeLength(zm);
*key += ZIPMAP_LEN_BYTES(*klen);
}
zm += zipmapRawKeyLength(zm);
// 获取value
if (value) {
// +1是为了跳过free字段，该字段占用一个字节
*value = zm+1;
*vlen = zipmapDecodeLength(zm);
*value += ZIPMAP_LEN_BYTES(*vlen);
}
// 此时zm指向下一个键值对的首地址
zm += zipmapRawValueLength(zm);
return zm;
}
/* Search a key and retrieve the pointer and len of the associated value.
* If the key is found the function returns 1, otherwise 0. */
/* 依据key值查找对应的value值，事实上是对zipmapLookupRaw的包装 */
int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen) {
unsigned char *p;
if ((p = zipmapLookupRaw(zm,key,klen,NULL)) == NULL) return 0;
p += zipmapRawKeyLength(p);
*vlen = zipmapDecodeLength(p);
*value = p + ZIPMAP_LEN_BYTES(*vlen) + 1;
return 1;
}
/* Return 1 if the key exists, otherwise 0 is returned. */
/* 推断某个key是否存在 */
int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen) {
return zipmapLookupRaw(zm,key,klen,NULL) != NULL;
}
/* Return the number of entries inside a zipmap */
/* 返回zipmap中键值对的个数，假设zmlen字段的值小于254。值zmlen的值就是所要求得返回值，否则须要遍历整个zipmap */
unsigned int zipmapLen(unsigned char *zm) {
unsigned int len = 0;
if (zm[0] < ZIPMAP_BIGLEN) {
len = zm[0];
} else {
unsigned char *p = zipmapRewind(zm);
while((p = zipmapNext(p,NULL,NULL,NULL,NULL)) != NULL) len++;
/* Re-store length if small enough */
if (len < ZIPMAP_BIGLEN) zm[0] = len;
}
return len;
}
/* Return the raw size in bytes of a zipmap, so that we can serialize
* the zipmap on disk (or everywhere is needed) just writing the returned
* amount of bytes of the C array starting at the zipmap pointer. */
/* 获取整个zipmap占用的字节数。事实上是对zipmapLookupRaw的包装 */
size_t zipmapBlobLen(unsigned char *zm) {
unsigned int totlen;
zipmapLookupRaw(zm,NULL,0,&totlen);
return totlen;
}
#ifdef ZIPMAP_TEST_MAIN
/* 格式化输出函数 */
void zipmapRepr(unsigned char *p) {
unsigned int l;
printf("{status %u}",*p++);
while(1) {
if (p[0] == ZIPMAP_END) {
printf("{end}");
break;
} else {
unsigned char e;
l = zipmapDecodeLength(p);
printf("{key %u}",l);
p += zipmapEncodeLength(NULL,l);
if (l != 0 && fwrite(p,l,1,stdout) == 0) perror("fwrite");
p += l;
l = zipmapDecodeLength(p);
printf("{value %u}",l);
p += zipmapEncodeLength(NULL,l);
e = *p++;
if (l != 0 && fwrite(p,l,1,stdout) == 0) perror("fwrite");
p += l+e;
if (e) {
printf("[");
while(e--) printf(".");
printf("]");
}
}
}
printf("
");
}
/* 以下是一些測试代码 */
int main(void) {
unsigned char *zm;
zm = zipmapNew();
zm = zipmapSet(zm,(unsigned char*) "name",4, (unsigned char*) "foo",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "surname",7, (unsigned char*) "foo",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "age",3, (unsigned char*) "foo",3,NULL);
zipmapRepr(zm);
zm = zipmapSet(zm,(unsigned char*) "hello",5, (unsigned char*) "world!",6,NULL);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "bar",3,NULL);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "!",1,NULL);
zipmapRepr(zm);
zm = zipmapSet(zm,(unsigned char*) "foo",3, (unsigned char*) "12345",5,NULL);
zipmapRepr(zm);
zm = zipmapSet(zm,(unsigned char*) "new",3, (unsigned char*) "xx",2,NULL);
zm = zipmapSet(zm,(unsigned char*) "noval",5, (unsigned char*) "",0,NULL);
zipmapRepr(zm);
zm = zipmapDel(zm,(unsigned char*) "new",3,NULL);
zipmapRepr(zm);
printf("
Look up large key:
");
{
unsigned char buf[512];
unsigned char *value;
unsigned int vlen, i;
for (i = 0; i < 512; i++) buf[i] = 'a';
zm = zipmapSet(zm,buf,512,(unsigned char*) "long",4,NULL);
if (zipmapGet(zm,buf,512,&value,&vlen)) {
printf(" <long key> is associated to the %d bytes value: %.*s
",
vlen, vlen, value);
}
}
printf("
Perform a direct lookup:
");
{
unsigned char *value;
unsigned int vlen;
if (zipmapGet(zm,(unsigned char*) "foo",3,&value,&vlen)) {
printf(" foo is associated to the %d bytes value: %.*s
",
vlen, vlen, value);
}
}
printf("
Iterate through elements:
");
{
unsigned char *i = zipmapRewind(zm);
unsigned char *key, *value;
unsigned int klen, vlen;
while((i = zipmapNext(i,&key,&klen,&value,&vlen)) != NULL) {
printf(" %d:%.*s => %d:%.*s
", klen, klen, key, vlen, vlen, value);
}
}
return 0;
}
#endif