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。
"foo""bar""hello""world"
里面涉及了几个变量zmlen，len,free，下面给出完整的解释：
/* 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
* 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":
*
* "foo""bar""hello""world"
*
* 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.
* 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。
*
* is the length of the following string (key or value).
* 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.
* 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