问题抛出

用过 Python 的列表吗？就是那种可以存储任意类型数据的，支持随机读取的数据结构。
没有用过的话那就没办法了。

本质上这种列表可以使用数组、链表作为其底层结构，不知道Python中的列表是以什么作为底层结构的。
但是redis的列表既不是用链表，也不是用数组作为其底层实现的，原因也显而易见：数组不方便，弄个二维的？柔性的？怎么写？链表可以实现，通用链表嘛，数据域放 void* 就可以实现列表功能。但是，链表的缺点也很明显，容易造成内存碎片。

在这个大环境下，秉承着“能省就省”的指导思想，请你设计一款数据结构。

结构设计

这个图里要注意，右侧是没有记录“当前元素的大小”的

这个图挺详细哈，都省得我对每一个字段释义了，整挺好。

其他话，文件开头的注释也讲的很清楚了。（ziplist.c）

/* The ziplist is a specially encoded dually linked list that is designed
 * to be very memory efficient. It stores both strings and integer values,
 * where integers are encoded as actual integers instead of a series of
 * characters. It allows push and pop operations on either side of the list
 * in O(1) time. However, because every operation requires a reallocation of
 * the memory used by the ziplist, the actual complexity is related to the
 * amount of memory used by the ziplist.
 *
 * ----------------------------------------------------------------------------
 *
 * ZIPLIST OVERALL LAYOUT
 * ======================
 *
 * The general layout of the ziplist is as follows:
 *
 * <zlbytes> <zltail> <zllen> <entry> <entry> ... <entry> <zlend>
 *
 * NOTE: all fields are stored in little endian, if not specified otherwise.
 *
 * <uint32_t zlbytes> is an unsigned integer to hold the number of bytes that
 * the ziplist occupies, including the four bytes of the zlbytes field itself.
 * This value needs to be stored to be able to resize the entire structure
 * without the need to traverse it first.
 *
 * <uint32_t zltail> is the offset to the last entry in the list. This allows
 * a pop operation on the far side of the list without the need for full
 * traversal.
 *
 * <uint16_t zllen> is the number of entries. When there are more than
 * 2^16-2 entries, this value is set to 2^16-1 and we need to traverse the
 * entire list to know how many items it holds.
 *
 * <uint8_t zlend> is a special entry representing the end of the ziplist.
 * Is encoded as a single byte equal to 255. No other normal entry starts
 * with a byte set to the value of 255.
 *
 * ZIPLIST ENTRIES
 * ===============
 *
 * Every entry in the ziplist is prefixed by metadata that contains two pieces
 * of information. First, the length of the previous entry is stored to be
 * able to traverse the list from back to front. Second, the entry encoding is
 * provided. It represents the entry type, integer or string, and in the case
 * of strings it also represents the length of the string payload.
 * So a complete entry is stored like this:
 *
 * <prevlen> <encoding> <entry-data>
 *
 * Sometimes the encoding represents the entry itself, like for small integers
 * as we'll see later. In such a case the <entry-data> part is missing, and we
 * could have just:
 *
 * <prevlen> <encoding>
 *
 * The length of the previous entry, <prevlen>, is encoded in the following way:
 * If this length is smaller than 254 bytes, it will only consume a single
 * byte representing the length as an unsinged 8 bit integer. When the length
 * is greater than or equal to 254, it will consume 5 bytes. The first byte is
 * set to 254 (FE) to indicate a larger value is following. The remaining 4
 * bytes take the length of the previous entry as value.
 *
 * So practically an entry is encoded in the following way:
 *
 * <prevlen from 0 to 253> <encoding> <entry>
 *
 * Or alternatively if the previous entry length is greater than 253 bytes
 * the following encoding is used:
 *
 * 0xFE <4 bytes unsigned little endian prevlen> <encoding> <entry>
 *
 * The encoding field of the entry depends on the content of the
 * entry. When the entry is a string, the first 2 bits of the encoding first
 * byte will hold the type of encoding used to store the length of the string,
 * followed by the actual length of the string. When the entry is an integer
 * the first 2 bits are both set to 1. The following 2 bits are used to specify
 * what kind of integer will be stored after this header. An overview of the
 * different types and encodings is as follows. The first byte is always enough
 * to determine the kind of entry.
 *
 * |00pppppp| - 1 byte
 *      String value with length less than or equal to 63 bytes (6 bits).
 *      "pppppp" represents the unsigned 6 bit length.
 * |01pppppp|qqqqqqqq| - 2 bytes
 *      String value with length less than or equal to 16383 bytes (14 bits).
 *      IMPORTANT: The 14 bit number is stored in big endian.
 * |10000000|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
 *      String value with length greater than or equal to 16384 bytes.
 *      Only the 4 bytes following the first byte represents the length
 *      up to 2^32-1. The 6 lower bits of the first byte are not used and
 *      are set to zero.
 *      IMPORTANT: The 32 bit number is stored in big endian.
 * |11000000| - 3 bytes
 *      Integer encoded as int16_t (2 bytes).
 * |11010000| - 5 bytes
 *      Integer encoded as int32_t (4 bytes).
 * |11100000| - 9 bytes
 *      Integer encoded as int64_t (8 bytes).
 * |11110000| - 4 bytes
 *      Integer encoded as 24 bit signed (3 bytes).
 * |11111110| - 2 bytes
 *      Integer encoded as 8 bit signed (1 byte).
 * |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
 *      Unsigned integer from 0 to 12. The encoded value is actually from
 *      1 to 13 because 0000 and 1111 can not be used, so 1 should be
 *      subtracted from the encoded 4 bit value to obtain the right value.
 * |11111111| - End of ziplist special entry.
 *
 * Like for the ziplist header, all the integers are represented in little
 * endian byte order, even when this code is compiled in big endian systems.
 *
 * EXAMPLES OF ACTUAL ZIPLISTS
 * ===========================
 *
 * The following is a ziplist containing the two elements representing
 * the strings "2" and "5". It is composed of 15 bytes, that we visually
 * split into sections:
 *
 *  [0f 00 00 00] [0c 00 00 00] [02 00] [00 f3] [02 f6] [ff]
 *        |             |          |       |       |     |
 *     zlbytes        zltail    entries   "2"     "5"   end
 *
 * The first 4 bytes represent the number 15, that is the number of bytes
 * the whole ziplist is composed of. The second 4 bytes are the offset
 * at which the last ziplist entry is found, that is 12, in fact the
 * last entry, that is "5", is at offset 12 inside the ziplist.
 * The next 16 bit integer represents the number of elements inside the
 * ziplist, its value is 2 since there are just two elements inside.
 * Finally "00 f3" is the first entry representing the number 2. It is
 * composed of the previous entry length, which is zero because this is
 * our first entry, and the byte F3 which corresponds to the encoding
 * |1111xxxx| with xxxx between 0001 and 1101. We need to remove the "F"
 * higher order bits 1111, and subtract 1 from the "3", so the entry value
 * is "2". The next entry has a prevlen of 02, since the first entry is
 * composed of exactly two bytes. The entry itself, F6, is encoded exactly
 * like the first entry, and 6-1 = 5, so the value of the entry is 5.
 * Finally the special entry FF signals the end of the ziplist.
 *
 * Adding another element to the above string with the value "Hello World"
 * allows us to show how the ziplist encodes small strings. We'll just show
 * the hex dump of the entry itself. Imagine the bytes as following the
 * entry that stores "5" in the ziplist above:
 *
 * [02] [0b] [48 65 6c 6c 6f 20 57 6f 72 6c 64]
 *
 * The first byte, 02, is the length of the previous entry. The next
 * byte represents the encoding in the pattern |00pppppp| that means
 * that the entry is a string of length <pppppp>, so 0B means that
 * an 11 bytes string follows. From the third byte (48) to the last (64)
 * there are just the ASCII characters for "Hello World".
 *
 * ----------------------------------------------------------------------------
 *
 * Copyright (c) 2009-2012, Pieter Noordhuis <pcnoordhuis at gmail dot com>
 * Copyright (c) 2009-2017, Salvatore Sanfilippo <antirez at gmail dot com>
 * All rights reserved.
 */

看完了么？接下来就是基操阶段了，对于任何一种数据结构，基操无非增删查改。

实际节点

typedef struct zlentry {
    unsigned int prevrawlensize; /* Bytes used to encode the previous entry len*/
    unsigned int prevrawlen;     /* Previous entry len. */
    unsigned int lensize;        /* Bytes used to encode this entry type/len.
                                    For example strings have a 1, 2 or 5 bytes
                                    header. Integers always use a single byte.*/
    unsigned int len;            /* Bytes used to represent the actual entry.
                                    For strings this is just the string length
                                    while for integers it is 1, 2, 3, 4, 8 or
                                    0 (for 4 bit immediate) depending on the
                                    number range. */
    unsigned int headersize;     /* prevrawlensize + lensize. */
    unsigned char encoding;      /* Set to ZIP_STR_* or ZIP_INT_* depending on
                                    the entry encoding. However for 4 bits
                                    immediate integers this can assume a range
                                    of values and must be range-checked. */
    unsigned char *p;            /* Pointer to the very start of the entry, that
                                    is, this points to prev-entry-len field. */
} zlentry;

基本操作

我觉得这张图还是要再摆一下：

这个图里要注意，右侧是没有记录“当前元素的大小”的

增

真实插入的是这个函数：

讲真，头皮有点发麻。那么我们等下还是用老套路，按步骤拆开来看。

/* Insert item at "p". */
unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
    size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen;
    unsigned int prevlensize, prevlen = 0;
    size_t offset;
    int nextdiff = 0;
    unsigned char encoding = 0;
    long long value = 123456789; /* initialized to avoid warning. Using a value
                                    that is easy to see if for some reason
                                    we use it uninitialized. */
    zlentry tail;

    /* Find out prevlen for the entry that is inserted. */
    if (p[0] != ZIP_END) {
        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
    } else {
        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
        if (ptail[0] != ZIP_END) {
            prevlen = zipRawEntryLength(ptail);
        }
    }

    /* See if the entry can be encoded */
    if (zipTryEncoding(s,slen,&value,&encoding)) {
        /* 'encoding' is set to the appropriate integer encoding */
        reqlen = zipIntSize(encoding);
    } else {
        /* 'encoding' is untouched, however zipStoreEntryEncoding will use the
         * string length to figure out how to encode it. */
        reqlen = slen;
    }
    /* We need space for both the length of the previous entry and
     * the length of the payload. */
    reqlen += zipStorePrevEntryLength(NULL,prevlen);
    reqlen += zipStoreEntryEncoding(NULL,encoding,slen);

    /* When the insert position is not equal to the tail, we need to
     * make sure that the next entry can hold this entry's length in
     * its prevlen field. */
    int forcelarge = 0;
    nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;
    if (nextdiff == -4 && reqlen < 4) {
        nextdiff = 0;
        forcelarge = 1;
    }

    /* Store offset because a realloc may change the address of zl. */
    offset = p-zl;
    zl = ziplistResize(zl,curlen+reqlen+nextdiff);
    p = zl+offset;

    /* Apply memory move when necessary and update tail offset. */
    if (p[0] != ZIP_END) {
        /* Subtract one because of the ZIP_END bytes */
        memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);

        /* Encode this entry's raw length in the next entry. */
        if (forcelarge)
            zipStorePrevEntryLengthLarge(p+reqlen,reqlen);
        else
            zipStorePrevEntryLength(p+reqlen,reqlen);

        /* Update offset for tail */
        ZIPLIST_TAIL_OFFSET(zl) =
            intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+reqlen);

        /* When the tail contains more than one entry, we need to take
         * "nextdiff" in account as well. Otherwise, a change in the
         * size of prevlen doesn't have an effect on the *tail* offset. */
        zipEntry(p+reqlen, &tail);
        if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
            ZIPLIST_TAIL_OFFSET(zl) =
                intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
        }
    } else {
        /* This element will be the new tail. */
        ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
    }

    /* When nextdiff != 0, the raw length of the next entry has changed, so
     * we need to cascade the update throughout the ziplist */
    if (nextdiff != 0) {
        offset = p-zl;
        zl = __ziplistCascadeUpdate(zl,p+reqlen);
        p = zl+offset;
    }

    /* Write the entry */
    p += zipStorePrevEntryLength(p,prevlen);
    p += zipStoreEntryEncoding(p,encoding,slen);
    if (ZIP_IS_STR(encoding)) {
        memcpy(p,s,slen);
    } else {
        zipSaveInteger(p,value,encoding);
    }
    ZIPLIST_INCR_LENGTH(zl,1);
    return zl;
}

对“链表”插入数据有几个步骤？
1、偏移
2、插进去
3、缝合

那这个“列表”，比较特殊一点，特殊在哪里？特殊在它比较紧凑，而且数据类型，其实也就两种，要么integer，要么string。所以它的步骤是？
1、数据重新编码
2、解析数据并分配空间
3、接入数据

重新编码

什么是重新编码？插入一个元素，是不是需要对：“前一个元素的大小、本身大小、当前元素编码” 这些数据进行一个统计，然后一并插入。就编这个。

插入位置无非三个，头中尾。
头：前一个元素大小为0，因为前面没有元素。
中：待插入位置后一个元素记录的“前一个元素大小”，当然，之后本身大小就成为了后一个元素眼中的“前一个元素大小”。
尾：那就要把三个字段加起来了。

具体怎么重新编码就不看了吧，这篇本来就已经很长了。

解析数据

再往下就是解析数据了。
首先尝试将数据解析为整数，如果可以解析，就按照压缩列表整数类型编码存储；如果解析失败，就按照压缩列表字节数组类型编码存储。

解析之后，数值存储在 value 中，编码格式存储在 encoding中。如果解析成功，还要计算整数所占字节数。变量 reqlen 存储当前元素所需空间大小，再累加其他两个字段的空间大小，就是本节点所需空间大小了。

重新分配空间

看注释这架势，咋滴，还存在没地方给它塞？

来我们看看。

这里的分配空间不是简单的就新插进来的数据多少空间就分配多少，如果没有仔细阅读上面那段英文的话，嗯，可以选择绕回去仔细阅读一下那个节点组成。特别是那个：

/*
* The length of the previous entry, <prevlen>, is encoded in the following way:
* If this length is smaller than 254 bytes, it will only consume a single
* byte representing the length as an unsinged 8 bit integer. When the length
* is greater than or equal to 254, it will consume 5 bytes. The first byte is
* set to 254 (FE) to indicate a larger value is following. The remaining 4
* bytes take the length of the previous entry as value.
*/

所以这个 previous 就是个不确定因素。有可能人家本来是 1 1 排列的，中间插进来一个之后变成 1 1 5 排列了；也有可能人家是1 5 排列的、5 1 排列的，总之就是不确定。

所以，在 entryX 的位置插入一个数据之后，entryX+1 的 previous 可能不变，可能加四，也可能减四，谁也说不准。说不准那不就得测一下嘛。所以就测一下，仅此而已。

接入数据

数据怎么接入？鉴于这里真心不是链表，是列表。
所以，按数组那一套来。对。

很麻烦吧。其实不麻烦，你在redis里见过它给你中间插入的机会了吗？更不要说头插了，你见过它给你头插的机会了吗？

插个题外话：大数据插入时，数组不一定输给链表。在尾插的时候，数组的优势是远超链表的（当然，仅限于尾插）。在我两个月前的博客里有做过这一系列的实验。

删就不写了吧，增的逆操作，从系列开始就没写过删。不过这里删就不可避免的大量数据进行复制了（如果不真删，只是做个删除标志呢？这样会省时间，但是时候会造成内存碎片化。不过可以设计一个定期调整内存的函数，比方说重用三分之一的块之后紧凑一下？内存不够用的时候紧凑一下？STL就是这么干的）。

查也没啥好讲的了吧，这个数据结构的应用场景一般就是对键进行检索，这里就是个值，不一样的是这个值是一串的。
所以除了提供原有的前后向遍历之外，还提供了 range 查询，不难的。