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如果有看过之前我分析的ziplist压缩列表的分析的话，理解这个我觉得不是什么特别的难题。ziplist压缩列表和zipmap都采用了动态分配字节的做法表示长度，比如通过固定的字节表示节省了不少的空间。同样带来的问题就是复杂的指针移动，和字符位置移动。但总的来说，一定是利大于弊了，要不然设计者也不会这么做。ziplist保存的使用一个列表，zipmap就保存的则是一个个键值对，通过key:value key:value的形式连着。下面我给出zipmap的结构构成，zipmap其实也就是一个超级长的字符串。
<zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world" /* String -> String Map data structure optimized for size. * This file implements a data structure mapping strings to other strings * implementing an O(n) lookup data structure designed to be very memory * efficient. * * 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. * * 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. * * zipmap压缩表和ziplist十分类似，都做到了内存操作效率比较高的 * -------------------------------------------------------------------------- * * 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": * * <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. * <zmeln>占有着1个字节，所以他的最多可代表的数量是254,当zipmap中的元素记录超过这个数时， * 那只能从前往后后遍历算大小了，和ziplist是不一样的。 * * <len> is the length of the following string (key or 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 * 252, it's a single-byte length. If it is 253 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. The special value 254 is used to mark * empty space that can be used to add new key/value pairs. * <len>代表了后面字符串key 或 value的值的长度，长度一般被编码1个字节或5个字节表示，这个和ziplist类似 * 如果后面的字符串长度小于等于252个，可与用单字节表示，其他253,254等长度被用来表示其他作用了，当超过这个数时候 * 则直接按5字节的方式存储长度。 * * <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>一般来表示后面的value长度的空闲值，当key：value=“foo”:"bar",后来被改为“foo”:"hi"，空闲长度就为1了 * * <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>的数字一般比较小，如果空闲太大，zipmap会进行调整大小使map整体变得尽可能小 * * The most compact representation of the above two elements hash is actually: * 这是一个例子： * <zmlen><len>"foo"<len><free>"bar"<len>"hello"<len><free>"world" * <总键值对数><第一个key的长度>key字符<第一个value的长度><空闲长度开始都为0>后面同前 * "\x02\x03foo\x03\x00bar\x05hello\x05\x00world\xff" * * 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. *//* 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. */unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update) { unsigned int zmlen, offset; unsigned int freelen, reqlen = zipmapRequiredLength(klen,vlen); unsigned int empty, vempty; unsigned char *p; freelen = reqlen; if (update) *update = 0; //寻找key的位置 p = zipmapLookupRaw(zm,key,klen,&zmlen); if (p == NULL) { /* Key not found: enlarge */ //key的位置没有找到，调整zipmap的大小，准备添加操作 zm = zipmapResize(zm, zmlen+reqlen); p = zm+zmlen-1; zmlen = zmlen+reqlen; /* Increase zipmap length (this is an insert) */ //如果头字节还没有达到最大值，则递增 if (zm[0] < ZIPMAP_BIGLEN) zm[0]++; } else { /* Key found. Is there enough space for the new value? */ /* Compute the total length: */ if (update) *update = 1; //key的位置以及找到，判断是否有空间插入新的值 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. */ //如果没有空间插入新的值，则调整大小 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. */ //移动空间以便增加新的值 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. */ 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; } /* Just write the key + value and we are done. */ /* Key: */ //定位到插入的位置，首先写入key值 p += zipmapEncodeLength(p,klen); memcpy(p,key,klen); p += klen; /* Value: */ //key值后面是value值，再次写入 p += zipmapEncodeLength(p,vlen); *p++ = vempty; memcpy(p,val,vlen); return zm;}/* Return the number of entries inside a zipmap *//* 返回map的长度 */unsigned int zipmapLen(unsigned char *zm) { unsigned int len = 0; //如果第一个长度小于最大值，则直接返回 if (zm[0] < ZIPMAP_BIGLEN) { len = zm[0]; } else { //否则变量计算长度 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;} 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);/* String -> String Map data structure optimized for size. * * See zipmap.c for more info. * * -------------------------------------------------------------------------- * * 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. */#ifndef _ZIPMAP_H#define _ZIPMAP_Hunsigned char *zipmapNew(void); //创建一个新的压缩图unsigned char *zipmapSet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char *val, unsigned int vlen, int *update); //设置压缩图中的某个键值对unsigned char *zipmapDel(unsigned char *zm, unsigned char *key, unsigned int klen, int *deleted); //删除压缩图上的某个键值对unsigned char *zipmapRewind(unsigned char *zm); //将在zipmapNext中被调用到unsigned char *zipmapNext(unsigned char *zm, unsigned char **key, unsigned int *klen, unsigned char **value, unsigned int *vlen); //取得此键值对的下一个键值对int zipmapGet(unsigned char *zm, unsigned char *key, unsigned int klen, unsigned char **value, unsigned int *vlen); //获取某个键值对int zipmapExists(unsigned char *zm, unsigned char *key, unsigned int klen); //某个key值在zipmap中是否存在unsigned int zipmapLen(unsigned char *zm); //zipmap压缩图的总键值对数size_t zipmapBlobLen(unsigned char *zm); //压缩图的序列化到文件中所需大小void zipmapRepr(unsigned char *p); //输出的压缩图的具体信息，用于测试#endifgithub：https://github.com/linyiqun/Redis-Code
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