text: /* The ziplist is a specially encoded dually linked list that is designed
   * to be very memory efficient.
   *
   * Ziplist 是为了尽可能地节约内存而设计的特殊编码双端链表。
   *
   * It stores both strings and integer values,
   * where integers are encoded as actual integers instead of a series of
   * characters.
   *
   * Ziplist 可以储存字符串值和整数值，
   * 其中，整数值被保存为实际的整数，而不是字符数组。
   *
   * 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 允许在列表的两端进行 O(1) 复杂度的 push 和 pop 操作。
   * 但是，因为这些操作都需要对整个 ziplist 进行内存重分配，
   * 所以实际的复杂度和 ziplist 占用的内存大小有关。
   *
   * ----------------------------------------------------------------------------
   *
   * ZIPLIST OVERALL LAYOUT
   * Ziplist 的整体布局：
   *
   * The general layout of the ziplist is as follows:
   * 以下是 ziplist 的一般布局：
   *
   * <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 zlbytes> 是一个无符号整数，保存着 ziplist 使用的内存数量。
   *
   * 通过这个值，程序可以直接对 ziplist 的内存大小进行调整，
   * 而无须为了计算 ziplist 的内存大小而遍历整个列表。
   * <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.
   *
   *
   * <zltail> 保存着到达列表中最后一个节点的偏移量。
   *
   * 这个偏移量使得对表尾的 pop 操作可以在无须遍历整个列表的情况下进行
   * <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.
   * <zllen> 保存着列表中的节点数量。
   *
   * 当 zllen 保存的值大于 2**16-2 时，
   * 程序需要遍历整个列表才能知道列表实际包含了多少个节点。
   *
   * <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.
   *
   * <zlend> 的长度为 1 字节，值为 255 ，标识列表的末尾。
   *
   * ZIPLIST ENTRIES:
   * ZIPLIST 节点：
   *
   * 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:
   *
   * 每个 ziplist 节点的前面都带有一个 header ，这个 header 包含两部分信息：
   *
   * 1)前置节点的长度，在程序从后向前遍历时使用。
   *
   * 2)当前节点所保存的值的类型和长度。
   * <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.
   *
   * 编码前置节点的长度的方法如下：
   *
   * 1) 如果前置节点的长度小于 254 字节，那么程序将使用 1 个字节来保存这个长度值。
   *
   * 2) 如果前置节点的长度大于等于 254 字节，那么程序将使用 5 个字节来保存这个长度值：
   *    a) 第 1 个字节的值将被设为 254 ，用于标识这是一个 5 字节长的长度值。
   *    b) 之后的 4 个字节则用于保存前置节点的实际长度。
   *
   * 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.
   *
   * header 另一部分的内容和节点所保存的值有关。
   *
   * 1) 如果节点保存的是字符串值，
   *    那么这部分 header 的头 2 个位将保存编码字符串长度所使用的类型，
   *    而之后跟着的内容则是字符串的实际长度
   * |00pppppp| - 1 byte
   *      String value with length less than or equal to 63 bytes (6 bits).
   *      "pppppp" represents the unsigned 6 bit length.
  *      字符串的长度小于或等于 63 字节。
   * |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.
   *      字符串的长度小于或等于 16383 字节。
   * |10000000|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
   *      字符串的长度大于或等于 16384 字节。
   *
   * 2) 如果节点保存的是整数值，
   *    那么这部分 header 的头 2 位都将被设置为 1 ，
   *    而之后跟着的 2 位则用于标识节点所保存的整数的类型。
   *      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
   *      节点的值为 int16_t 类型的整数，长度为 2 字节
   *      Integer encoded as int16_t (2 bytes).
   * |11010000| - 5 bytes
   *      节点的值为 int32_t 类型的整数，长度为 4 字节。
   *      Integer encoded as int32_t (4 bytes).
   * |11100000| - 9 bytes
   *      节点的值为 int64_t 类型的整数，长度为 8 字节。
   *      Integer encoded as int64_t (8 bytes).
   * |11110000| - 4 bytes
   *      节点的值为 24 位（3 字节）长的整数。
   *      Integer encoded as 24 bit signed (3 bytes).
   * |11111110| - 2 bytes
   *      节点的值为 8 位（1 字节）长的整数。
   *      Integer encoded as 8 bit signed (1 byte).
   * |1111xxxx| - (with xxxx between 0001 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.
   *      节点的值为介于 0 至 12 之间的无符号整数。
   *      因为 0000 和 1111 都不能使用，所以位的实际值将是 1 至 13 。
   *      程序在取得这 4 个位的值之后，还需要减去 1 ，才能计算出正确的值。
   *      比如说，如果位的值为 0001 = 1 ，那么程序返回的值将是 1 - 1 = 0
   * |11111111| - End of ziplist special entry.
   *      ziplist 的结尾标识
   *
   * Like for the ziplist header,
   * all the integers are represented in little
   * 所有整数都表示为小端字节序。
   * 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>
   * Copyright (c) 2020, Redis Labs, Inc
   * 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.
   */
  
  #include <stdio.h>
  #include <stdlib.h>
  #include <string.h>
  #include <stdint.h>
  #include <limits.h>
  #include "zmalloc.h"
  #include "util.h"
  #include "ziplist.h"
  #include "config.h"
  #include "endianconv.h"
  #include "redisassert.h"
  
  /*
   * ziplist 末端标识符，以及 5 字节长长度标识符
   */
  #define ZIP_END 255         /* Special "end of ziplist" entry. */
  #define ZIP_BIG_PREVLEN 254 /* ZIP_BIG_PREVLEN - 1 is the max number of bytes of
                                 the previous entry, for the "prevlen" field prefixing
                                 each entry, to be represented with just a single byte.
                                 Otherwise it is represented as FE AA BB CC DD, where
                                 AA BB CC DD are a 4 bytes unsigned integer
                                 representing the previous entry len. */
  
  /* Different encoding/length possibilities */
  /*
   * 字符串编码和整数编码的掩码
   */
  #define ZIP_STR_MASK 0xc0
  #define ZIP_INT_MASK 0x30
  /*
   * 字符串编码类型
   */
  #define ZIP_STR_06B (0 << 6)
  #define ZIP_STR_14B (1 << 6)
  #define ZIP_STR_32B (2 << 6)
  /*
   * 整数编码类型
   */
  #define ZIP_INT_16B (0xc0 | 0<<4)
  #define ZIP_INT_32B (0xc0 | 1<<4)
  #define ZIP_INT_64B (0xc0 | 2<<4)
  #define ZIP_INT_24B (0xc0 | 3<<4)
  #define ZIP_INT_8B 0xfe
  
  /* 4 bit integer immediate encoding |1111xxxx| with xxxx between
   *
   * 4 位整数编码的掩码和类型
   * 0001 and 1101. */
  #define ZIP_INT_IMM_MASK 0x0f   /* Mask to extract the 4 bits value. To add
                                     one is needed to reconstruct the value. */
  #define ZIP_INT_IMM_MIN 0xf1    /* 11110001 */
  #define ZIP_INT_IMM_MAX 0xfd    /* 11111101 */
  
  /*
   * 24 位整数的最大值和最小值
   */
  #define INT24_MAX 0x7fffff
  #define INT24_MIN (-INT24_MAX - 1)
  
  /* Macro to determine if the entry is a string. String entries never start
   * with "11" as most significant bits of the first byte. */
  /* Macro to determine type
   *
   * 查看给定编码 enc 是否字符串编码
   */
  #define ZIP_IS_STR(enc) (((enc) & ZIP_STR_MASK) < ZIP_STR_MASK)
  
  /* Utility macros.*/
  
  /* Return total bytes a ziplist is composed of. */
  // 定位到 ziplist 的 bytes 属性，该属性记录了整个 ziplist 所占用的内存字节数
  // 用于取出 bytes 属性的现有值，或者为 bytes 属性赋予新值
  #define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))
  
  /* Return the offset of the last item inside the ziplist. */
  // 定位到 ziplist 的 offset 属性，该属性记录了到达表尾节点的偏移量
  // 用于取出 offset 属性的现有值，或者为 offset 属性赋予新值
  #define ZIPLIST_TAIL_OFFSET(zl) (*((uint32_t*)((zl)+sizeof(uint32_t))))
  
  /* Return the length of a ziplist, or UINT16_MAX if the length cannot be
   * determined without scanning the whole ziplist. */
  // 定位到 ziplist 的 length 属性，该属性记录了 ziplist 包含的节点数量
  // 用于取出 length 属性的现有值，或者为 length 属性赋予新值
  #define ZIPLIST_LENGTH(zl)      (*((uint16_t*)((zl)+sizeof(uint32_t)*2)))
  
  /* The size of a ziplist header: two 32 bit integers for the total
   * bytes count and last item offset. One 16 bit integer for the number
   * of items field. */
  //回 ziplist 表头的大小
  #define ZIPLIST_HEADER_SIZE     (sizeof(uint32_t)*2+sizeof(uint16_t))
  
  /* Size of the "end of ziplist" entry. Just one byte. */
  #define ZIPLIST_END_SIZE        (sizeof(uint8_t))
  
  /* Return the pointer to the first entry of a ziplist. */
  // 返回指向 ziplist 第一个节点（的起始位置）的指针
  #define ZIPLIST_ENTRY_HEAD(zl)  ((zl)+ZIPLIST_HEADER_SIZE)
  
  /* Return the pointer to the last entry of a ziplist, using the
   * last entry offset inside the ziplist header. */
  // 返回指向 ziplist 最后一个节点（的起始位置）的指针
  #define ZIPLIST_ENTRY_TAIL(zl)  ((zl)+intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)))
  
  /* Return the pointer to the last byte of a ziplist, which is, the
   * end of ziplist FF entry. */
  // 返回指向 ziplist 末端 ZIP_END （的起始位置）的指
  #define ZIPLIST_ENTRY_END(zl)   ((zl)+intrev32ifbe(ZIPLIST_BYTES(zl))-1)
  
  /*
  空白 ziplist 示例图
  area        |<---- ziplist header ---->|<-- end -->|
  size          4 bytes   4 bytes 2 bytes  1 byte
              +---------+--------+-------+-----------+
  component   | zlbytes | zltail | zllen | zlend     |
              |         |        |       |           |
  value       |  1011   |  1010  |   0   | 1111 1111 |
              +---------+--------+-------+-----------+
                                         ^
                                         |
                                 ZIPLIST_ENTRY_HEAD
                                         &
  address                        ZIPLIST_ENTRY_TAIL
                                         &
                                 ZIPLIST_ENTRY_END
  非空 ziplist 示例图
  area        |<---- ziplist header ---->|<----------- entries ------------->|<-end->|
  size          4 bytes  4 bytes  2 bytes    ?        ?        ?        ?     1 byte
              +---------+--------+-------+--------+--------+--------+--------+-------+
  component   | zlbytes | zltail | zllen | entry1 | entry2 |  ...   | entryN | zlend |
              +---------+--------+-------+--------+--------+--------+--------+-------+
                                         ^                          ^        ^
  address                                |                          |        |
                                  ZIPLIST_ENTRY_HEAD                |   ZIPLIST_ENTRY_END
                                                                    |
                                                          ZIPLIST_ENTRY_TAIL
  */
  
  /* Increment the number of items field in the ziplist header. Note that this
   * macro should never overflow the unsigned 16 bit integer, since entries are
   * always pushed one at a time. When UINT16_MAX is reached we want the count
   * to stay there to signal that a full scan is needed to get the number of
   * items inside the ziplist. */
  /*
   * 增加 ziplist 的节点数
   *
   * T = O(1)
   */
  #define ZIPLIST_INCR_LENGTH(zl, incr) { \
  if (ZIPLIST_LENGTH(zl) < UINT16_MAX) \
  ZIPLIST_LENGTH(zl) = intrev16ifbe(intrev16ifbe(ZIPLIST_LENGTH(zl))+incr); \
  }
  
  /* We use this function to receive information about a ziplist entry.
   * Note that this is not how the data is actually encoded, is just what we
   * get filled by a function in order to operate more easily. */
  /*
   * 保存 ziplist 节点信息的结构
   */
  typedef struct zlentry {
  
      // prevrawlen ：前置节点的长度
      // prevrawlensize ：编码 prevrawlen 所需的字节大小
      unsigned int prevrawlensize; /* Bytes used to encode the previous entry len*/
  
  
      unsigned int prevrawlen;     /* Previous entry len. */
  
  
      // len ：当前节点值的长度
      // lensize ：编码 len 所需的字节大小
      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. */
  
  
      // 当前节点 header 的大小
      // 等于 prevrawlensize + lensize
      unsigned int headersize;     /* prevrawlensize + lensize. */
  
      // 当前节点值所使用的编码类型
      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. */
  
      // 指向当前节点的指针
      unsigned char *p;            /* Pointer to the very start of the entry, that
                                      is, this points to prev-entry-len field. */
  } zlentry;
  
  #define ZIPLIST_ENTRY_ZERO(zle) { \
  (zle)->prevrawlensize = (zle)->prevrawlen = 0; \
  (zle)->lensize = (zle)->len = (zle)->headersize = 0; \
  (zle)->encoding = 0; \
  (zle)->p = NULL; \
  }
  
  /* Extract the encoding from the byte pointed by 'ptr' and set it into
   * 'encoding' field of the zlentry structure.
   *
   * 从 ptr 中取出节点值的编码类型，并将它保存到 encoding 变量中。
   *
   * T = O(1)
   */
  #define ZIP_ENTRY_ENCODING(ptr, encoding) do {  \
  (encoding) = ((ptr)[0]); \
  if ((encoding) < ZIP_STR_MASK) (encoding) &= ZIP_STR_MASK; \
  } while(0)
  
  #define ZIP_ENCODING_SIZE_INVALID 0xff
  
  /* Return the number of bytes required to encode the entry type + length.
   * On error, return ZIP_ENCODING_SIZE_INVALID
   *
   * 返回保存 encoding 编码的值所需的字节数量
   *
   * T = O(1)
   */
  static inline unsigned int zipEncodingLenSize(unsigned char encoding) {
      if (encoding == ZIP_INT_16B || encoding == ZIP_INT_32B ||
          encoding == ZIP_INT_24B || encoding == ZIP_INT_64B ||
          encoding == ZIP_INT_8B)
          return 1;
      if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX)
          return 1;
      if (encoding == ZIP_STR_06B)
          return 1;
      if (encoding == ZIP_STR_14B)
          return 2;
      if (encoding == ZIP_STR_32B)
          return 5;
      return ZIP_ENCODING_SIZE_INVALID;
  }
  
  #define ZIP_ASSERT_ENCODING(encoding) do {                                     \
  assert(zipEncodingLenSize(encoding) != ZIP_ENCODING_SIZE_INVALID);         \
  } while (0)
  
  /* Return bytes needed to store integer encoded by 'encoding' */
  static inline unsigned int zipIntSize(unsigned char encoding) {
      switch (encoding) {
          case ZIP_INT_8B:
              return 1;
          case ZIP_INT_16B:
              return 2;
          case ZIP_INT_24B:
              return 3;
          case ZIP_INT_32B:
              return 4;
          case ZIP_INT_64B:
              return 8;
      }
      if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX)
          return 0; /* 4 bit immediate */
      /* bad encoding, covered by a previous call to ZIP_ASSERT_ENCODING */
      redis_unreachable();
      return 0;
  }
  
  /* Write the encoding header of the entry in 'p'. If p is NULL it just returns
   * the amount of bytes required to encode such a length. Arguments:
   *
   * 'encoding' is the encoding we are using for the entry. It could be
   * ZIP_INT_* or ZIP_STR_* or between ZIP_INT_IMM_MIN and ZIP_INT_IMM_MAX
   * for single-byte small immediate integers.
   *
   * 'rawlen' is only used for ZIP_STR_* encodings and is the length of the
   * string that this entry represents.
   *
   * The function returns the number of bytes used by the encoding/length
   * header stored in 'p'.
   *
   * 编码节点长度值 l ，并将它写入到 p 中，然后返回编码 l 所需的字节数量。
   *
   * 如果 p 为 NULL ，那么仅返回编码 l 所需的字节数量，不进行写入。
   *
   * T = O(1)
   */
  unsigned int zipStoreEntryEncoding(unsigned char *p, unsigned char encoding, unsigned int rawlen) {
      unsigned char len = 1, buf[5];
  
      // 编码字符串
      if (ZIP_IS_STR(encoding)) {
          /* Although encoding is given it may not be set for strings,
           * so we determine it here using the raw length. */
          if (rawlen <= 0x3f) {
              if (!p) return len;
              buf[0] = ZIP_STR_06B | rawlen;
          } else if (rawlen <= 0x3fff) {
              len += 1;
              if (!p) return len;
              buf[0] = ZIP_STR_14B | ((rawlen >> 8) & 0x3f);
              buf[1] = rawlen & 0xff;
          } else {
              len += 4;
              if (!p) return len;
              buf[0] = ZIP_STR_32B;
              buf[1] = (rawlen >> 24) & 0xff;
              buf[2] = (rawlen >> 16) & 0xff;
              buf[3] = (rawlen >> 8) & 0xff;
              buf[4] = rawlen & 0xff;
          }
  
          // 编码整数
      } else {
          /* Implies integer encoding, so length is always 1. */
          if (!p) return len;
          buf[0] = encoding;
      }
  
      /* Store this length at p */
      // 将编码后的长度写入 p
      memcpy(p, buf, len);
  
      // 返回编码所需的字节数
      return len;
  }
  
  /* Decode the entry encoding type and data length (string length for strings,
   * number of bytes used for the integer for integer entries) encoded in 'ptr'.
   * The 'encoding' variable is input, extracted by the caller, the 'lensize'
   * variable will hold the number of bytes required to encode the entry
   * length, and the 'len' variable will hold the entry length.
   * On invalid encoding error, lensize is set to 0.
   *
   * 解码 ptr 指针，取出列表节点的相关信息，并将它们保存在以下变量中：
   *
   * - encoding 保存节点值的编码类型。
   *
   * - lensize 保存编码节点长度所需的字节数。
   *
   * - len 保存节点的长度。
   *
   * T = O(1)
   */
  #define ZIP_DECODE_LENGTH(ptr, encoding, lensize, len) do {                    \
  /* 取出值的编码类型 */                                                     \
  /* 字符串编码 */                                                           \
  if ((encoding) < ZIP_STR_MASK) {                                           \
  if ((encoding) == ZIP_STR_06B) {                                       \
  (lensize) = 1;                                                     \
  (len) = (ptr)[0] & 0x3f;                                           \
  } else if ((encoding) == ZIP_STR_14B) {                                \
  (lensize) = 2;                                                     \
  (len) = (((ptr)[0] & 0x3f) << 8) | (ptr)[1];                       \
  } else if ((encoding) == ZIP_STR_32B) {                                \
  (lensize) = 5;                                                     \
  (len) = ((ptr)[1] << 24) |                                         \
  ((ptr)[2] << 16) |                                         \
  ((ptr)[3] <<  8) |                                         \
  ((ptr)[4]);                                                \
  } else {                                                               \
  (lensize) = 0; /* bad encoding, should be covered by a previous */ \
  (len) = 0;     /* ZIP_ASSERT_ENCODING / zipEncodingLenSize, or  */ \
  /* match the lensize after this macro with 0.    */ \
  }                                                                      \
  /* 整数编码 */                                                             \
  } else {                                                                   \
  (lensize) = 1;                                                         \
  if ((encoding) == ZIP_INT_8B)  (len) = 1;                              \
  else if ((encoding) == ZIP_INT_16B) (len) = 2;                         \
  else if ((encoding) == ZIP_INT_24B) (len) = 3;                         \
  else if ((encoding) == ZIP_INT_32B) (len) = 4;                         \
  else if ((encoding) == ZIP_INT_64B) (len) = 8;                         \
  else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX)   \
  (len) = 0; /* 4 bit immediate */                                   \
  else                                                                   \
  (lensize) = (len) = 0; /* bad encoding */                          \
  }                                                                          \
  } while(0)
  
  /* Encode the length of the previous entry and write it to "p". This only
   * uses the larger encoding (required in __ziplistCascadeUpdate).
   *
   * 将原本只需要 1 个字节来保存的前置节点长度 len 编码至一个 5 字节长的 header 中。
   *
   * T = O(1)
   */
  int zipStorePrevEntryLengthLarge(unsigned char *p, unsigned int len) {
      uint32_t u32;
      if (p != NULL) {
          // 添加 5 字节长度标识
          p[0] = ZIP_BIG_PREVLEN;
          u32 = len;
          // 写入编码
          memcpy(p + 1, &u32, sizeof(u32));
          // 如果有必要的话，进行大小端转换
          memrev32ifbe(p + 1);
      }
      // 返回编码长度
      return 1 + sizeof(uint32_t);
  }
  
  /* Encode the length of the previous entry and write it to "p". Return the
   * number of bytes needed to encode this length if "p" is NULL.
   *
   * 对前置节点的长度 len 进行编码，并将它写入到 p 中，
   * 然后返回编码 len 所需的字节数量。
   *
   * 如果 p 为 NULL ，那么不进行写入，仅返回编码 len 所需的字节数量。
   *
   * T = O(1)
   */
  unsigned int zipStorePrevEntryLength(unsigned char *p, unsigned int len) {
      // 仅返回编码 len 所需的字节数量
      if (p == NULL) {
          return (len < ZIP_BIG_PREVLEN) ? 1 : sizeof(uint32_t) + 1;
          // 写入并返回编码 len 所需的字节数量
      } else {
          if (len < ZIP_BIG_PREVLEN) {
              // 1 字节
              p[0] = len;
              return 1;
              // 5 字节
          } else {
              return zipStorePrevEntryLengthLarge(p, len);
          }
      }
  }
  
  /* Return the number of bytes used to encode the length of the previous
   * entry. The length is returned by setting the var 'prevlensize'.
   *
   * 解码 ptr 指针，
   * 取出编码前置节点长度所需的字节数，并将它保存到 prevlensize 变量中。
   *
   * T = O(1)
   */
  #define ZIP_DECODE_PREVLENSIZE(ptr, prevlensize) do {                          \
  if ((ptr)[0] < ZIP_BIG_PREVLEN) {                                          \
  (prevlensize) = 1;                                                     \
  } else {                                                                   \
  (prevlensize) = 5;                                                     \
  }                                                                          \
  } while(0)
  
  /* Return the length of the previous element, and the number of bytes that
   * are used in order to encode the previous element length.
   * 'ptr' must point to the prevlen prefix of an entry (that encodes the
   * length of the previous entry in order to navigate the elements backward).
   * The length of the previous entry is stored in 'prevlen', the number of
   * bytes needed to encode the previous entry length are stored in
   * 'prevlensize'.
   *
   * 解码 ptr 指针，
   * 取出编码前置节点长度所需的字节数，
   * 并将这个字节数保存到 prevlensize 中。
   *
   * 然后根据 prevlensize ，从 ptr 中取出前置节点的长度值，
   * 并将这个长度值保存到 prevlen 变量中。
   *
   * T = O(1)
   */
  #define ZIP_DECODE_PREVLEN(ptr, prevlensize, prevlen) do {                     \
  ZIP_DECODE_PREVLENSIZE(ptr, prevlensize);                                  \
  if ((prevlensize) == 1) {                                                  \
  (prevlen) = (ptr)[0];                                                  \
  } else { /* prevlensize == 5 */                                            \
  (prevlen) = ((ptr)[4] << 24) |                                         \
  ((ptr)[3] << 16) |                                         \
  ((ptr)[2] <<  8) |                                         \
  ((ptr)[1]);                                                \
  }                                                                          \
  } while(0)
  
  /* Given a pointer 'p' to the prevlen info that prefixes an entry, this
   * function returns the difference in number of bytes needed to encode
   * the prevlen if the previous entry changes of size.
   *
   * So if A is the number of bytes used right now to encode the 'prevlen'
   * field.
   *
   * And B is the number of bytes that are needed in order to encode the
   * 'prevlen' if the previous element will be updated to one of size 'len'.
   *
   * Then the function returns B - A
   *
   * So the function returns a positive number if more space is needed,
   * a negative number if less space is needed, or zero if the same space
   * is needed.
   *
   * 计算编码新的前置节点长度 len 所需的字节数，
   * 减去编码 p 原来的前置节点长度所需的字节数之差。
   *
   * T = O(1)
   */
  int zipPrevLenByteDiff(unsigned char *p, unsigned int len) {
      unsigned int prevlensize;
      // 取出编码原来的前置节点长度所需的字节数
      // T = O(1)
      ZIP_DECODE_PREVLENSIZE(p, prevlensize);
  
      // 计算编码 len 所需的字节数
      // T = O(1)
      return zipStorePrevEntryLength(NULL, len) - prevlensize;
  }
  
  /* Check if string pointed to by 'entry' can be encoded as an integer.
   * Stores the integer value in 'v' and its encoding in 'encoding'.
   *
   * 检查 entry 中指向的字符串能否被编码为整数。
   *
   * 如果可以的话，
   * 将编码后的整数保存在指针 v 的值中，并将编码的方式保存在指针 encoding 的值中。
   *
   * 注意，这里的 entry 和前面代表节点的 entry 不是一个意思。
   *
   * T = O(N)
   */
  int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long *v, unsigned char *encoding) {
      long long value;
  
      // 忽略太长或太短的字符串
      if (entrylen >= 32 || entrylen == 0) return 0;
      // 尝试转换
      // T = O(N)
      if (string2ll((char *) entry, entrylen, &value)) {
          /* Great, the string can be encoded. Check what's the smallest
           * of our encoding types that can hold this value. */
          // 转换成功，以从小到大的顺序检查适合值 value 的编码方式
          if (value >= 0 && value <= 12) {
              *encoding = ZIP_INT_IMM_MIN + value;
          } else if (value >= INT8_MIN && value <= INT8_MAX) {
              *encoding = ZIP_INT_8B;
          } else if (value >= INT16_MIN && value <= INT16_MAX) {
              *encoding = ZIP_INT_16B;
          } else if (value >= INT24_MIN && value <= INT24_MAX) {
              *encoding = ZIP_INT_24B;
          } else if (value >= INT32_MIN && value <= INT32_MAX) {
              *encoding = ZIP_INT_32B;
          } else {
              *encoding = ZIP_INT_64B;
          }
          // 记录值到指针
          *v = value;
  
          // 返回转换成功标识
          return 1;
      }
  
      // 转换失败
      return 0;
  }
  
  /* Store integer 'value' at 'p', encoded as 'encoding'
   *
   * 以 encoding 指定的编码方式，将整数值 value 写入到 p 。
   *
   * T = O(1)
   */
  void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encoding) {
      int16_t i16;
      int32_t i32;
      int64_t i64;
      if (encoding == ZIP_INT_8B) {
          ((int8_t *) p)[0] = (int8_t) value;
      } else if (encoding == ZIP_INT_16B) {
          i16 = value;
          memcpy(p, &i16, sizeof(i16));
          memrev16ifbe(p);
      } else if (encoding == ZIP_INT_24B) {
          i32 = value << 8;
          memrev32ifbe(&i32);
          memcpy(p, ((uint8_t *) &i32) + 1, sizeof(i32) - sizeof(uint8_t));
      } else if (encoding == ZIP_INT_32B) {
          i32 = value;
          memcpy(p, &i32, sizeof(i32));
          memrev32ifbe(p);
      } else if (encoding == ZIP_INT_64B) {
          i64 = value;
          memcpy(p, &i64, sizeof(i64));
          memrev64ifbe(p);
      } else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX) {
          /* Nothing to do, the value is stored in the encoding itself. */
      } else {
          assert(NULL);
      }
  }
  
  /* Read integer encoded as 'encoding' from 'p'
   *
   * 以 encoding 指定的编码方式，读取并返回指针 p 中的整数值。
   *
   * T = O(1)
   */
  int64_t zipLoadInteger(unsigned char *p, unsigned char encoding) {
      int16_t i16;
      int32_t i32;
      int64_t i64, ret = 0;
      if (encoding == ZIP_INT_8B) {
          ret = ((int8_t *) p)[0];
      } else if (encoding == ZIP_INT_16B) {
          memcpy(&i16, p, sizeof(i16));
          memrev16ifbe(&i16);
          ret = i16;
      } else if (encoding == ZIP_INT_32B) {
          memcpy(&i32, p, sizeof(i32));
          memrev32ifbe(&i32);
          ret = i32;
      } else if (encoding == ZIP_INT_24B) {
          i32 = 0;
          memcpy(((uint8_t *) &i32) + 1, p, sizeof(i32) - sizeof(uint8_t));
          memrev32ifbe(&i32);
          ret = i32 >> 8;
      } else if (encoding == ZIP_INT_64B) {
          memcpy(&i64, p, sizeof(i64));
          memrev64ifbe(&i64);
          ret = i64;
      } else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX) {
          ret = (encoding & ZIP_INT_IMM_MASK) - 1;
      } else {
          assert(NULL);
      }
      return ret;
  }
  
  /* Fills a struct with all information about an entry.
   * This function is the "unsafe" alternative to the one blow.
   * Generally, all function that return a pointer to an element in the ziplist
   * will assert that this element is valid, so it can be freely used.
   * Generally functions such ziplistGet assume the input pointer is already
   * validated (since it's the return value of another function).
   *
   * 将 p 所指向的列表节点的信息全部保存到 zlentry 中，并返回该 zlentry 。
   *
   * T = O(1)
   */
  static inline void zipEntry(unsigned char *p, zlentry *e) {
  
  
      // e.prevrawlensize 保存着编码前一个节点的长度所需的字节数
      // e.prevrawlen 保存着前一个节点的长度
      // T = O(1)
      ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
      ZIP_ENTRY_ENCODING(p + e->prevrawlensize, e->encoding);
  
      // p + e.prevrawlensize 将指针移动到列表节点本身
      // e.encoding 保存着节点值的编码类型
      // e.lensize 保存着编码节点值长度所需的字节数
      // e.len 保存着节点值的长度
      // T = O(1)
      ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
      assert(e->lensize != 0); /* check that encoding was valid. */
      // 计算头结点的字节数
      e->headersize = e->prevrawlensize + e->lensize;
      // 记录指针
      e->p = p;
  }
  
  /* Fills a struct with all information about an entry.
   * This function is safe to use on untrusted pointers, it'll make sure not to
   * try to access memory outside the ziplist payload.
   * Returns 1 if the entry is valid, and 0 otherwise. */
  static inline int zipEntrySafe(unsigned char *zl, size_t zlbytes, unsigned char *p, zlentry *e, int validate_prevlen) {
      unsigned char *zlfirst = zl + ZIPLIST_HEADER_SIZE;
      unsigned char *zllast = zl + zlbytes - ZIPLIST_END_SIZE;
  #define OUT_OF_RANGE(p) (unlikely((p) < zlfirst || (p) > zllast))
  
      /* If threre's no possibility for the header to reach outside the ziplist,
       * take the fast path. (max lensize and prevrawlensize are both 5 bytes) */
      if (p >= zlfirst && p + 10 < zllast) {
          ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
          ZIP_ENTRY_ENCODING(p + e->prevrawlensize, e->encoding);
          ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
          e->headersize = e->prevrawlensize + e->lensize;
          e->p = p;
          /* We didn't call ZIP_ASSERT_ENCODING, so we check lensize was set to 0. */
          if (unlikely(e->lensize == 0))
              return 0;
          /* Make sure the entry doesn't rech outside the edge of the ziplist */
          if (OUT_OF_RANGE(p + e->headersize + e->len))
              return 0;
          /* Make sure prevlen doesn't rech outside the edge of the ziplist */
          if (validate_prevlen && OUT_OF_RANGE(p - e->prevrawlen))
              return 0;
          return 1;
      }
  
      /* Make sure the pointer doesn't rech outside the edge of the ziplist */
      if (OUT_OF_RANGE(p))
          return 0;
  
      /* Make sure the encoded prevlen header doesn't reach outside the allocation */
      ZIP_DECODE_PREVLENSIZE(p, e->prevrawlensize);
      if (OUT_OF_RANGE(p + e->prevrawlensize))
          return 0;
  
      /* Make sure encoded entry header is valid. */
      ZIP_ENTRY_ENCODING(p + e->prevrawlensize, e->encoding);
      e->lensize = zipEncodingLenSize(e->encoding);
      if (unlikely(e->lensize == ZIP_ENCODING_SIZE_INVALID))
          return 0;
  
      /* Make sure the encoded entry header doesn't reach outside the allocation */
      if (OUT_OF_RANGE(p + e->prevrawlensize + e->lensize))
          return 0;
  
      /* Decode the prevlen and entry len headers. */
      ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
      ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
      e->headersize = e->prevrawlensize + e->lensize;
  
      /* Make sure the entry doesn't rech outside the edge of the ziplist */
      if (OUT_OF_RANGE(p + e->headersize + e->len))
          return 0;
  
      /* Make sure prevlen doesn't rech outside the edge of the ziplist */
      if (validate_prevlen && OUT_OF_RANGE(p - e->prevrawlen))
          return 0;
  
      e->p = p;
      return 1;
  #undef OUT_OF_RANGE
  }
  
  /* Return the total number of bytes used by the entry pointed to by 'p'. */
  static inline unsigned int zipRawEntryLengthSafe(unsigned char *zl, size_t zlbytes, unsigned char *p) {
      zlentry e;
      assert(zipEntrySafe(zl, zlbytes, p, &e, 0));
      return e.headersize + e.len;
  }
  
  /* Return the total number of bytes used by the entry pointed to by 'p'. */
  static inline unsigned int zipRawEntryLength(unsigned char *p) {
      zlentry e;
      zipEntry(p, &e);
      return e.headersize + e.len;
  }
  
  /* Validate that the entry doesn't reach outside the ziplist allocation. */
  static inline void zipAssertValidEntry(unsigned char *zl, size_t zlbytes, unsigned char *p) {
      zlentry e;
      assert(zipEntrySafe(zl, zlbytes, p, &e, 1));
  }
  
  /* Create a new empty ziplist.
   *
   * 创建并返回一个新的 ziplist
   *
   * T = O(1)
   */
  unsigned char *ziplistNew(void) {
  
      // ZIPLIST_HEADER_SIZE 是 ziplist 表头的大小
      // 1 字节是表末端 ZIP_END 的大小
      unsigned int bytes = ZIPLIST_HEADER_SIZE + ZIPLIST_END_SIZE;
      unsigned char *zl = zmalloc(bytes);
  
      // 设置表末端
      ZIPLIST_BYTES(zl) = intrev32ifbe(bytes);
      ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(ZIPLIST_HEADER_SIZE);
      ZIPLIST_LENGTH(zl) = 0;
      zl[bytes - 1] = ZIP_END;
      return zl;
  }
  
  /* Resize the ziplist.
   *
   * 调整 ziplist 的大小为 len 字节。
   *
   * 当 ziplist 原有的大小小于 len 时，扩展 ziplist 不会改变 ziplist 原有的元素。
   *
   * T = O(N)
   */
  unsigned char *ziplistResize(unsigned char *zl, unsigned int len) {
      // 用 zrealloc ，扩展时不改变现有元素
      zl = zrealloc(zl, len);
      // 更新 bytes 属性
      ZIPLIST_BYTES(zl) = intrev32ifbe(len);
      // 重新设置表末端
      zl[len - 1] = ZIP_END;
      return zl;
  }
  
  /* When an entry is inserted, we need to set the prevlen field of the next
   * entry to equal the length of the inserted entry. It can occur that this
   * length cannot be encoded in 1 byte and the next entry needs to be grow
   * a bit larger to hold the 5-byte encoded prevlen. This can be done for free,
   * because this only happens when an entry is already being inserted (which
   * causes a realloc and memmove). However, encoding the prevlen may require
   * that this entry is grown as well. This effect may cascade throughout
   * the ziplist when there are consecutive entries with a size close to
   * ZIP_BIG_PREVLEN, so we need to check that the prevlen can be encoded in
   * every consecutive entry.
   *
   * 当将一个新节点添加到某个节点之前的时候，
   * 如果原节点的 header 空间不足以保存新节点的长度，
   * 那么就需要对原节点的 header 空间进行扩展（从 1 字节扩展到 5 字节）。
   *
   * 但是，当对原节点进行扩展之后，原节点的下一个节点的 prevlen 可能出现空间不足，
   * 这种情况在多个连续节点的长度都接近 ZIP_BIGLEN 时可能发生。
   *
   * 这个函数就用于检查并修复后续节点的空间问题。
   *
   * Note that this effect can also happen in reverse, where the bytes required
   * to encode the prevlen field can shrink. This effect is deliberately ignored,
   * because it can cause a "flapping" effect where a chain prevlen fields is
   * first grown and then shrunk again after consecutive inserts. Rather, the
   * field is allowed to stay larger than necessary, because a large prevlen
   * field implies the ziplist is holding large entries anyway.
   *
   * 反过来说，
   * 因为节点的长度变小而引起的连续缩小也是可能出现的，
   * 不过，为了避免扩展-缩小-扩展-缩小这样的情况反复出现（flapping，抖动），
   * 我们不处理这种情况，而是任由 prevlen 比所需的长度更长。
  
   * The pointer "p" points to the first entry that does NOT need to be
   * updated, i.e. consecutive fields MAY need an update.
   *
   * 注意，程序的检查是针对 p 的后续节点，而不是 p 所指向的节点。
   * 因为节点 p 在传入之前已经完成了所需的空间扩展工作。
   *
   * T = O(N^2)
   */
  unsigned char *__ziplistCascadeUpdate(unsigned char *zl, unsigned char *p) {
      zlentry cur;
      size_t prevlen, prevlensize, prevoffset; /* Informat of the last changed entry. */
      size_t firstentrylen; /* Used to handle insert at head. */
      size_t rawlen, curlen = intrev32ifbe(ZIPLIST_BYTES(zl));
      size_t extra = 0, cnt = 0, offset;
      size_t delta = 4; /* Extra bytes needed to update a entry's prevlen (5-1). */
      unsigned char *tail = zl + intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl));
  
      /* Empty ziplist */
      if (p[0] == ZIP_END) return zl;
  
      // 将 p 所指向的节点的信息保存到 cur 结构中
      zipEntry(p,
               &cur); /* no need for "safe" variant since the input pointer was validated by the function that returned it. */
      // 当前节点的长度
      firstentrylen = prevlen = cur.headersize + cur.len;
      // 计算编码当前节点的长度所需的字节数
      // T = O(1)
      prevlensize = zipStorePrevEntryLength(NULL, prevlen);
      prevoffset = p - zl;
      p += prevlen;
  
      /* Iterate ziplist to find out how many extra bytes do we need to update it. */
      // T = O(N^2)
      while (p[0] != ZIP_END) {
          assert(zipEntrySafe(zl, curlen, p, &cur, 0));
  
          /* Abort when "prevlen" has not changed. */
          // 后续节点编码当前节点的空间已经足够，无须再进行任何处理，跳出
          // 可以证明，只要遇到一个空间足够的节点，
          // 那么这个节点之后的所有节点的空间都是足够的
          if (cur.prevrawlen == prevlen) break;
  
          /* Abort when entry's "prevlensize" is big enough. */
          if (cur.prevrawlensize >= prevlensize) {
              if (cur.prevrawlensize == prevlensize) {
                  zipStorePrevEntryLength(p, prevlen);
              } else {
                  /* This would result in shrinking, which we want to avoid.
                   * So, set "prevlen" in the available bytes. */
                  zipStorePrevEntryLengthLarge(p, prevlen);
              }
              break;
          }
  
          /* cur.prevrawlen means cur is the former head entry. */
          assert(cur.prevrawlen == 0 || cur.prevrawlen + delta == prevlen);
  
          /* Update prev entry's info and advance the cursor. */
          rawlen = cur.headersize + cur.len;
          prevlen = rawlen + delta;
          prevlensize = zipStorePrevEntryLength(NULL, prevlen);
          prevoffset = p - zl;
          p += rawlen;
          extra += delta;
          cnt++;
      }
  
      /* Extra bytes is zero all update has been done(or no need to update). */
      if (extra == 0) return zl;
  
      /* Update tail offset after loop. */
  
      // 当 next 节点不是表尾节点时，更新列表到表尾节点的偏移量
      //
      // 不用更新的情况（next 为表尾节点）：
      //
      // |     | next |      ==>    |     | new next          |
      //       ^                          ^
      //       |                          |
      //     tail                        tail
      //
      // 需要更新的情况（next 不是表尾节点）：
      //
      // | next |     |   ==>     | new next          |     |
      //        ^                        ^
      //        |                        |
      //    old tail                 old tail
      //
      // 更新之后：
      //
      // | new next          |     |
      //                     ^
      //                     |
      //                  new tail
      // T = O(1)
      if (tail == zl + prevoffset) {
          /* When the the last entry we need to update is also the tail, update tail offset
           * unless this is the only entry that was updated (so the tail offset didn't change). */
          if (extra - delta != 0) {
              ZIPLIST_TAIL_OFFSET(zl) =
                      intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)) + extra - delta);
          }
      } else {
          /* Update the tail offset in cases where the last entry we updated is not the tail. */
          ZIPLIST_TAIL_OFFSET(zl) =
                  intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)) + extra);
      }
  
      /* Now "p" points at the first unchanged byte in original ziplist,
       * move data after that to new ziplist. */
      offset = p - zl;
      zl = ziplistResize(zl, curlen + extra);
      p = zl + offset;
  
      // 向后移动 cur 节点之后的数据，为 cur 的新 header 腾出空间
      //
      // 示例：
      //
      // | header | value |  ==>  | header |    | value |  ==>  | header      | value |
      //                                   |<-->|
      //                            为新 header 腾出的空间
      // T = O(N)
      memmove(p + extra, p, curlen - offset - 1);
      p += extra;
  
      /* Iterate all entries that need to be updated tail to head. */
      while (cnt) {
          zipEntry(zl + prevoffset,
                   &cur); /* no need for "safe" variant since we already iterated on all these entries above. */
          rawlen = cur.headersize + cur.len;
          /* Move entry to tail and reset prevlen. */
          memmove(p - (rawlen - cur.prevrawlensize),
                  zl + prevoffset + cur.prevrawlensize,
                  rawlen - cur.prevrawlensize);
          p -= (rawlen + delta);
          if (cur.prevrawlen == 0) {
              // 运行到这里，
              // 说明 cur 节点的长度正好可以编码到 next 节点的 header 中
              // T = O(1)
              /* "cur" is the previous head entry, update its prevlen with firstentrylen. */
              zipStorePrevEntryLength(p, firstentrylen);
          } else {
              /* An entry's prevlen can only increment 4 bytes. */
              // 执行到这里，说明 next 节点编码前置节点的 header 空间有 5 字节
              // 而编码 rawlen 只需要 1 字节
              // 但是程序不会对 next 进行缩小，
              // 所以这里只将 rawlen 写入 5 字节的 header 中就算了。
              // T = O(1)
              zipStorePrevEntryLength(p, cur.prevrawlen + delta);
          }
          /* Foward to previous entry. */
          prevoffset -= cur.prevrawlen;
          cnt--;
      }
      return zl;
  }
  
  /* Delete "num" entries, starting at "p". Returns pointer to the ziplist.
   *
   * 从位置 p 开始，连续删除 num 个节点。
   *
   * 函数的返回值为处理删除操作之后的 ziplist 。
   *
   * T = O(N^2)
   */
  unsigned char *__ziplistDelete(unsigned char *zl, unsigned char *p, unsigned int num) {
      unsigned int i, totlen, deleted = 0;
      size_t offset;
      int nextdiff = 0;
      zlentry first, tail;
      size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
  
      // 计算被删除节点总共占用的内存字节数
      // 以及被删除节点的总个数
      // T = O(N)
      zipEntry(p,
               &first); /* no need for "safe" variant since the input pointer was validated by the function that returned it. */
      for (i = 0; p[0] != ZIP_END && i < num; i++) {
          p += zipRawEntryLengthSafe(zl, zlbytes, p);
          deleted++;
      }
  
      assert(p >= first.p);
      // totlen 是所有被删除节点总共占用的内存字节数
      totlen = p - first.p; /* Bytes taken by the element(s) to delete. */
      if (totlen > 0) {
          uint32_t set_tail;
          if (p[0] != ZIP_END) {
              // 执行这里，表示被删除节点之后仍然有节点存在
              /* Storing `prevrawlen` in this entry may increase or decrease the
               * number of bytes required compare to the current `prevrawlen`.
               * There always is room to store this, because it was previously
               * stored by an entry that is now being deleted. */
              // 因为位于被删除范围之后的第一个节点的 header 部分的大小
              // 可能容纳不了新的前置节点，所以需要计算新旧前置节点之间的字节数差
              // T = O(1)
              nextdiff = zipPrevLenByteDiff(p, first.prevrawlen);
              // 如果有需要的话，将指针 p 后退 nextdiff 字节，为新 header 空出空间
              /* Note that there is always space when p jumps backward: if
               * the new previous entry is large, one of the deleted elements
               * had a 5 bytes prevlen header, so there is for sure at least
               * 5 bytes free and we need just 4. */
              p -= nextdiff;
              // 将 first 的前置节点的长度编码至 p 中
              // T = O(1)
              assert(p >= first.p && p < zl + zlbytes - 1);
              zipStorePrevEntryLength(p, first.prevrawlen);
  
              /* Update offset for tail */
              // 更新到达表尾的偏移量
              // T = O(1)
              set_tail = intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)) - totlen;
  
              /* 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. */
              assert(zipEntrySafe(zl, zlbytes, p, &tail, 1));
              // 如果被删除节点之后，有多于一个节点
              // 那么程序需要将 nextdiff 记录的字节数也计算到表尾偏移量中
              // 这样才能让表尾偏移量正确对齐表尾节点
              // T = O(1)
              if (p[tail.headersize + tail.len] != ZIP_END) {
                  set_tail = set_tail + nextdiff;
              }
  
              /* Move tail to the front of the ziplist */
              /* since we asserted that p >= first.p. we know totlen >= 0,
               * so we know that p > first.p and this is guaranteed not to reach
               * beyond the allocation, even if the entries lens are corrupted. */
              // 从表尾向表头移动数据，覆盖被删除节点的数据
              // T = O(N)
              size_t bytes_to_move = zlbytes - (p - zl) - 1;
              memmove(first.p, p, bytes_to_move);
          } else {
              /* The entire tail was deleted. No need to move memory. */
              // 执行这里，表示被删除节点之后已经没有其他节点了
  
              set_tail = (first.p - zl) - first.prevrawlen;
          }
  
          /* Resize the ziplist */
          // 缩小并更新 ziplist 的长度
          offset = first.p - zl;
          zlbytes -= totlen - nextdiff;
          zl = ziplistResize(zl, zlbytes);
          p = zl + offset;
  
          /* Update record count */
          ZIPLIST_INCR_LENGTH(zl, -deleted);
  
          /* Set the tail offset computed above */
          assert(set_tail <= zlbytes - ZIPLIST_END_SIZE);
          ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(set_tail);
  
          /* When nextdiff != 0, the raw length of the next entry has changed, so
           * we need to cascade the update throughout the ziplist */
          // 如果 p 所指向的节点的大小已经变更，那么进行级联更新
          // 检查 p 之后的所有节点是否符合 ziplist 的编码要求
          // T = O(N^2)
          if (nextdiff != 0)
              zl = __ziplistCascadeUpdate(zl, p);
      }
      return zl;
  }
  
  /* Insert item at "p".
   * 根据指针 p 所指定的位置，将长度为 slen 的字符串 s 插入到 zl 中。
   *
   * 函数的返回值为完成插入操作之后的 ziplist
   *
   * T = O(N^2)
   */
  unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
      // 记录当前 ziplist 的长度
      size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), reqlen, newlen;
      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) {
  
          // 如果 p[0] 不指向列表末端，说明列表非空，并且 p 正指向列表的其中一个节点
          // 那么取出 p 所指向节点的信息，并将它保存到 entry 结构中
          // 然后用 prevlen 变量记录前置节点的长度
          // （当插入新节点之后 p 所指向的节点就成了新节点的前置节点）
          // T = O(1)
          ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
      } else {
          // 如果 p 指向表尾末端，那么程序需要检查列表是否为：
          // 1)如果 ptail 也指向 ZIP_END ，那么列表为空；
          // 2)如果列表不为空，那么 ptail 将指向列表的最后一个节点。
          unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
          if (ptail[0] != ZIP_END) {
  
              // 表尾节点为新节点的前置节点
  
              // 取出表尾节点的长度
              // T = O(1)
              prevlen = zipRawEntryLengthSafe(zl, curlen, ptail);
          }
      }
  
      /* See if the entry can be encoded */
      // 尝试看能否将输入字符串转换为整数，如果成功的话：
      // 1)value 将保存转换后的整数值
      // 2)encoding 则保存适用于 value 的编码方式
      // 无论使用什么编码， reqlen 都保存节点值的长度
      // T = O(N)
      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. */
  
      // 计算编码前置节点的长度所需的大小
      // T = O(1)
      reqlen += zipStorePrevEntryLength(NULL, prevlen);
  
      // 计算编码当前节点值所需的大小
      // T = O(1)
      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. */
  
      // 只要新节点不是被添加到列表末端，
      // 那么程序就需要检查看 p 所指向的节点（的 header）能否编码新节点的长度。
      // nextdiff 保存了新旧编码之间的字节大小差，如果这个值大于 0
      // 那么说明需要对 p 所指向的节点（的 header ）进行扩展
      // T = O(1)
      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. */
      // 因为重分配空间可能会改变 zl 的地址
      // 所以在分配之前，需要记录 zl 到 p 的偏移量，然后在分配之后依靠偏移量还原 p
      offset = p - zl;
  
      // curlen 是 ziplist 原来的长度
      // reqlen 是整个新节点的长度
      // nextdiff 是新节点的后继节点扩展 header 的长度（要么 0 字节，要么 4 个字节）
      // T = O(N)
      newlen = curlen + reqlen + nextdiff;
      zl = ziplistResize(zl, newlen);
      p = zl + offset;
  
      /* Apply memory move when necessary and update tail offset. */
      if (p[0] != ZIP_END) {
          // 新元素之后还有节点，因为新元素的加入，需要对这些原有节点进行调整
  
          /* Subtract one because of the ZIP_END bytes */
          // 移动现有元素，为新元素的插入空间腾出位置
          // T = O(N)
          memmove(p + reqlen, p - nextdiff, curlen - offset - 1 + nextdiff);
  
          /* Encode this entry's raw length in the next entry. */
          // 将新节点的长度编码至后置节点
          // p+reqlen 定位到后置节点
          // reqlen 是新节点的长度
          // T = O(1)
          if (forcelarge)
              zipStorePrevEntryLengthLarge(p + reqlen, reqlen);
          else
              zipStorePrevEntryLength(p + reqlen, reqlen);
  
          /* Update offset for tail */
          // 更新到达表尾的偏移量，将新节点的长度也算上
          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. */
          assert(zipEntrySafe(zl, newlen, p + reqlen, &tail, 1));
          // 如果新节点的后面有多于一个节点
          // 那么程序需要将 nextdiff 记录的字节数也计算到表尾偏移量中
          // 这样才能让表尾偏移量正确对齐表尾节点
          // T = O(1)
          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. */
          // 新元素是新的表尾节点
          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 */
      // 当 nextdiff != 0 时，新节点的后继节点的（header 部分）长度已经被改变，
      // 所以需要级联地更新后续的节点
      if (nextdiff != 0) {
          offset = p - zl;
          // T  = O(N^2)
          zl = __ziplistCascadeUpdate(zl, p + reqlen);
          p = zl + offset;
      }
  
      /* Write the entry */
      // 一切搞定，将前置节点的长度写入新节点的 header
      p += zipStorePrevEntryLength(p, prevlen);
      // 将节点值的长度写入新节点的 header
      p += zipStoreEntryEncoding(p, encoding, slen);
      // 写入节点值
      if (ZIP_IS_STR(encoding)) {
          // T = O(N)
          memcpy(p, s, slen);
      } else {
          // T = O(1)
          zipSaveInteger(p, value, encoding);
      }
      // 更新列表的节点数量计数器
      // T = O(1)
      ZIPLIST_INCR_LENGTH(zl, 1);
      return zl;
  }
  
  /* Merge ziplists 'first' and 'second' by appending 'second' to 'first'.
   *
   * NOTE: The larger ziplist is reallocated to contain the new merged ziplist.
   * Either 'first' or 'second' can be used for the result.  The parameter not
   * used will be free'd and set to NULL.
   *
   * After calling this function, the input parameters are no longer valid since
   * they are changed and free'd in-place.
   *
   * The result ziplist is the contents of 'first' followed by 'second'.
   *
   * On failure: returns NULL if the merge is impossible.
   * On success: returns the merged ziplist (which is expanded version of either
   * 'first' or 'second', also frees the other unused input ziplist, and sets the
   * input ziplist argument equal to newly reallocated ziplist return value. */
  unsigned char *ziplistMerge(unsigned char **first, unsigned char **second) {
      /* If any params are null, we can't merge, so NULL. */
      if (first == NULL || *first == NULL || second == NULL || *second == NULL)
          return NULL;
  
      /* Can't merge same list into itself. */
      if (*first == *second)
          return NULL;
  
      size_t first_bytes = intrev32ifbe(ZIPLIST_BYTES(*first));
      size_t first_len = intrev16ifbe(ZIPLIST_LENGTH(*first));
  
      size_t second_bytes = intrev32ifbe(ZIPLIST_BYTES(*second));
      size_t second_len = intrev16ifbe(ZIPLIST_LENGTH(*second));
  
      int append;
      unsigned char *source, *target;
      size_t target_bytes, source_bytes;
      /* Pick the largest ziplist so we can resize easily in-place.
       * We must also track if we are now appending or prepending to
       * the target ziplist. */
      if (first_len >= second_len) {
          /* retain first, append second to first. */
          target = *first;
          target_bytes = first_bytes;
          source = *second;
          source_bytes = second_bytes;
          append = 1;
      } else {
          /* else, retain second, prepend first to second. */
          target = *second;
          target_bytes = second_bytes;
          source = *first;
          source_bytes = first_bytes;
          append = 0;
      }
  
      /* Calculate final bytes (subtract one pair of metadata) */
      size_t zlbytes = first_bytes + second_bytes -
                       ZIPLIST_HEADER_SIZE - ZIPLIST_END_SIZE;
      size_t zllength = first_len + second_len;
  
      /* Combined zl length should be limited within UINT16_MAX */
      zllength = zllength < UINT16_MAX ? zllength : UINT16_MAX;
  
      /* Save offset positions before we start ripping memory apart. */
      size_t first_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*first));
      size_t second_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*second));
  
      /* Extend target to new zlbytes then append or prepend source. */
      target = zrealloc(target, zlbytes);
      if (append) {
          /* append == appending to target */
          /* Copy source after target (copying over original [END]):
           *   [TARGET - END, SOURCE - HEADER] */
          memcpy(target + target_bytes - ZIPLIST_END_SIZE,
                 source + ZIPLIST_HEADER_SIZE,
                 source_bytes - ZIPLIST_HEADER_SIZE);
      } else {
          /* !append == prepending to target */
          /* Move target *contents* exactly size of (source - [END]),
           * then copy source into vacated space (source - [END]):
           *   [SOURCE - END, TARGET - HEADER] */
          memmove(target + source_bytes - ZIPLIST_END_SIZE,
                  target + ZIPLIST_HEADER_SIZE,
                  target_bytes - ZIPLIST_HEADER_SIZE);
          memcpy(target, source, source_bytes - ZIPLIST_END_SIZE);
      }
  
      /* Update header metadata. */
      ZIPLIST_BYTES(target) = intrev32ifbe(zlbytes);
      ZIPLIST_LENGTH(target) = intrev16ifbe(zllength);
      /* New tail offset is:
       *   + N bytes of first ziplist
       *   - 1 byte for [END] of first ziplist
       *   + M bytes for the offset of the original tail of the second ziplist
       *   - J bytes for HEADER because second_offset keeps no header. */
      ZIPLIST_TAIL_OFFSET(target) = intrev32ifbe(
              (first_bytes - ZIPLIST_END_SIZE) +
              (second_offset - ZIPLIST_HEADER_SIZE));
  
      /* __ziplistCascadeUpdate just fixes the prev length values until it finds a
       * correct prev length value (then it assumes the rest of the list is okay).
       * We tell CascadeUpdate to start at the first ziplist's tail element to fix
       * the merge seam. */
      target = __ziplistCascadeUpdate(target, target + first_offset);
  
      /* Now free and NULL out what we didn't realloc */
      if (append) {
          zfree(*second);
          *second = NULL;
          *first = target;
      } else {
          zfree(*first);
          *first = NULL;
          *second = target;
      }
      return target;
  }
  
  /*
   * 将长度为 slen 的字符串 s 推入到 zl 中。
   *
   * where 参数的值决定了推入的方向：
   * - 值为 ZIPLIST_HEAD 时，将新值推入到表头。
   * - 否则，将新值推入到表末端。
   *
   * 函数的返回值为添加新值后的 ziplist 。
   *
   * T = O(N^2)
   */
  unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {
      // 根据 where 参数的值，决定将值推入到表头还是表尾
      unsigned char *p;
      p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
      // 返回添加新值后的 ziplist
      // T = O(N^2)
      return __ziplistInsert(zl, p, s, slen);
  }
  
  /* Returns an offset to use for iterating with ziplistNext. When the given
   * index is negative, the list is traversed back to front. When the list
   * doesn't contain an element at the provided index, NULL is returned.
   * 根据给定索引，遍历列表，并返回索引指定节点的指针。
   *
   * 如果索引为正，那么从表头向表尾遍历。
   * 如果索引为负，那么从表尾向表头遍历。
   * 正数索引从 0 开始，负数索引从 -1 开始。
   *
   * 如果索引超过列表的节点数量，或者列表为空，那么返回 NULL 。
   *
   * T = O(N)
   */
  unsigned char *ziplistIndex(unsigned char *zl, int index) {
      unsigned char *p;
      unsigned int prevlensize, prevlen = 0;
      size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
      // 处理负数索引
      if (index < 0) {
  
          // 将索引转换为正数
          index = (-index) - 1;
  
          // 定位到表尾节点
          p = ZIPLIST_ENTRY_TAIL(zl);
  
          // 如果列表不为空，那么。。。
          if (p[0] != ZIP_END) {
              /* No need for "safe" check: when going backwards, we know the header
               * we're parsing is in the range, we just need to assert (below) that
               * the size we take doesn't cause p to go outside the allocation. */
              // 从表尾向表头遍历
              ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
              while (prevlen > 0 && index--) {
                  // 前移指针
                  p -= prevlen;
                  assert(p >= zl + ZIPLIST_HEADER_SIZE && p < zl + zlbytes - ZIPLIST_END_SIZE);
                  // T = O(1)
                  ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
              }
          }
          // 处理正数索引
      } else {
          // 定位到表头节点
          p = ZIPLIST_ENTRY_HEAD(zl);
  
          // T = O(N)
          while (index--) {
              /* Use the "safe" length: When we go forward, we need to be careful
               * not to decode an entry header if it's past the ziplist allocation. */
              // 后移指针
              // T = O(1)
              p += zipRawEntryLengthSafe(zl, zlbytes, p);
              if (p[0] == ZIP_END)
                  break;
          }
      }
  
      // 返回结果
      if (p[0] == ZIP_END || index > 0)
          return NULL;
      zipAssertValidEntry(zl, zlbytes, p);
      return p;
  }
  
  /* Return pointer to next entry in ziplist.
   *
   * zl is the pointer to the ziplist
   * p is the pointer to the current element
   *
   * The element after 'p' is returned, otherwise NULL if we are at the end.
   * 返回 p 所指向节点的后置节点。
   *
   * 如果 p 为表末端，或者 p 已经是表尾节点，那么返回 NULL 。
   *
   * T = O(1)
   */
  unsigned char *ziplistNext(unsigned char *zl, unsigned char *p) {
      ((void) zl);
      size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
  
      /* "p" could be equal to ZIP_END, caused by ziplistDelete,
       * and we should return NULL. Otherwise, we should return NULL
       * when the *next* element is ZIP_END (there is no next entry). */
      // p 已经指向列表末端
      if (p[0] == ZIP_END) {
          return NULL;
      }
  
      // 指向后一节点
      p += zipRawEntryLength(p);
      if (p[0] == ZIP_END) {
          // p 已经是表尾节点，没有后置节点
          return NULL;
      }
  
      zipAssertValidEntry(zl, zlbytes, p);
      return p;
  }
  
  /* Return pointer to previous entry in ziplist.
   * 返回 p 所指向节点的前置节点。
   *
   * 如果 p 所指向为空列表，或者 p 已经指向表头节点，那么返回 NULL 。
   *
   * T = O(1)
   */
  unsigned char *ziplistPrev(unsigned char *zl, unsigned char *p) {
      unsigned int prevlensize, prevlen = 0;
  
      /* Iterating backwards from ZIP_END should return the tail. When "p" is
       * equal to the first element of the list, we're already at the head,
       * and should return NULL. */
      // 如果 p 指向列表末端（列表为空，或者刚开始从表尾向表头迭代）
      // 那么尝试取出列表尾端节点
      if (p[0] == ZIP_END) {
          p = ZIPLIST_ENTRY_TAIL(zl);
          // 尾端节点也指向列表末端，那么列表为空
          return (p[0] == ZIP_END) ? NULL : p;
  
          // 如果 p 指向列表头，那么说明迭代已经完成
      } else if (p == ZIPLIST_ENTRY_HEAD(zl)) {
          return NULL;
  
          // 既不是表头也不是表尾，从表尾向表头移动指针
      } else {
          // 计算前一个节点的节点数
          ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
          assert(prevlen > 0);
          // 移动指针，指向前一个节点
          p -= prevlen;
          size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
          zipAssertValidEntry(zl, zlbytes, p);
          return p;
      }
  }
  
  /* Get entry pointed to by 'p' and store in either '*sstr' or 'sval' depending
   * on the encoding of the entry. '*sstr' is always set to NULL to be able
   * to find out whether the string pointer or the integer value was set.
   * Return 0 if 'p' points to the end of the ziplist, 1 otherwise.
   * 取出 p 所指向节点的值：
   *
   * - 如果节点保存的是字符串，那么将字符串值指针保存到 *sstr 中，字符串长度保存到 *slen
   *
   * - 如果节点保存的是整数，那么将整数保存到 *sval
   *
   * 程序可以通过检查 *sstr 是否为 NULL 来检查值是字符串还是整数。
   *
   * 提取值成功返回 1 ，
   * 如果 p 为空，或者 p 指向的是列表末端，那么返回 0 ，提取值失败。
   *
   * T = O(1)
   */
  unsigned int ziplistGet(unsigned char *p, unsigned char **sstr, unsigned int *slen, long long *sval) {
      zlentry entry;
      if (p == NULL || p[0] == ZIP_END) return 0;
      if (sstr) *sstr = NULL;
  
      // 取出 p 所指向的节点的各项信息，并保存到结构 entry 中
      // T = O(1)
      zipEntry(p,
               &entry); /* no need for "safe" variant since the input pointer was validated by the function that returned it. */
      // 节点的值为字符串，将字符串长度保存到 *slen ，字符串保存到 *sstr
      // T = O(1)
      if (ZIP_IS_STR(entry.encoding)) {
          if (sstr) {
              *slen = entry.len;
              *sstr = p + entry.headersize;
          }
          // 节点的值为整数，解码值，并将值保存到 *sval
          // T = O(1)
      } else {
          if (sval) {
              *sval = zipLoadInteger(p + entry.headersize, entry.encoding);
          }
      }
      return 1;
  }
  
  /* Insert an entry at "p".
   *
   * 将包含给定值 s 的新节点插入到给定的位置 p 中。
   *
   * 如果 p 指向一个节点，那么新节点将放在原有节点的前面。
   *
   * T = O(N^2)
  */unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
      return __ziplistInsert(zl, p, s, slen);
  }
  
  /* Delete a single entry from the ziplist, pointed to by *p.
   * Also update *p in place, to be able to iterate over the
   * ziplist, while deleting entries.
   *
   * 从 zl 中删除 *p 所指向的节点，
   * 并且原地更新 *p 所指向的位置，使得可以在迭代列表的过程中对节点进行删除。
   *
   * T = O(N^2)
  */unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p) {
      // 因为 __ziplistDelete 时会对 zl 进行内存重分配
      // 而内存充分配可能会改变 zl 的内存地址
      // 所以这里需要记录到达 *p 的偏移量
      // 这样在删除节点之后就可以通过偏移量来将 *p 还原到正确的位置
      size_t offset = *p - zl;
      zl = __ziplistDelete(zl, *p, 1);
  
      /* Store pointer to current element in p, because ziplistDelete will
       * do a realloc which might result in a different "zl"-pointer.
       * When the delete direction is back to front, we might delete the last
       * entry and end up with "p" pointing to ZIP_END, so check this. */
      *p = zl + offset;
      return zl;
  }
  
  /* Delete a range of entries from the ziplist.
   *
   * 从 index 索引指定的节点开始，连续地从 zl 中删除 num 个节点。
   *
   * T = O(N^2)
   */
  unsigned char *ziplistDeleteRange(unsigned char *zl, int index, unsigned int num) {
      // 根据索引定位到节点
      // T = O(N)
       unsigned char *p = ziplistIndex(zl,index);
      // 连续删除 num 个节点
      // T = O(N^2)
       return (p == NULL) ? zl : __ziplistDelete(zl,p,num);
  }
  
  /* Replaces the entry at p. This is equivalent to a delete and an insert,
   * but avoids some overhead when replacing a value of the same size. */
  unsigned char *ziplistReplace(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
  
      /* get metadata of the current entry */
      zlentry entry;
      zipEntry(p, &entry);
  
      /* compute length of entry to store, excluding prevlen */
      unsigned int reqlen;
      unsigned char encoding = 0;
      long long value = 123456789; /* initialized to avoid warning. */
      if (zipTryEncoding(s, slen, &value, &encoding)) {
          reqlen = zipIntSize(encoding); /* encoding is set */
      } else {
          reqlen = slen; /* encoding == 0 */
      }
      reqlen += zipStoreEntryEncoding(NULL, encoding, slen);
  
      if (reqlen == entry.lensize + entry.len) {
          /* Simply overwrite the element. */
          p += entry.prevrawlensize;
          p += zipStoreEntryEncoding(p, encoding, slen);
          if (ZIP_IS_STR(encoding)) {
              memcpy(p, s, slen);
          } else {
              zipSaveInteger(p, value, encoding);
          }
      } else {
          /* Fallback. */
          zl = ziplistDelete(zl, &p);
          zl = ziplistInsert(zl, p, s, slen);
      }
      return zl;
  }
  
  /* Compare entry pointer to by 'p' with 'sstr' of length 'slen'. */
  /* Return 1 if equal.
   *
   * 将 p 所指向的节点的值和 sstr 进行对比。
   *
   * 如果节点值和 sstr 的值相等，返回 1 ，不相等则返回 0 。
   *
   * T = O(N)
   */
  unsigned int ziplistCompare(unsigned char *p, unsigned char *sstr, unsigned int slen) {
      zlentry entry;
      unsigned char sencoding;
      long long zval, sval;
      if (p[0] == ZIP_END) return 0;
  
      // 取出节点
      zipEntry(p,&entry);
      /* no need for "safe" variant since the input pointer was validated by the function that returned it. */
      if (ZIP_IS_STR(entry.encoding)) {
          // 节点值为字符串，进行字符串对比
          /* Raw compare */
          if (entry.len == slen) {
              // T = O(N)
               return memcmp(p+entry.headersize,sstr,slen) == 0;
          } else {
              return 0;
          }
      } else {
          // 节点值为整数，进行整数对比
          /* Try to compare encoded values. Don't compare encoding because
         * different implementations may encoded integers differently. */
          if (zipTryEncoding(sstr, slen, &sval, &sencoding)) {
              // T = O(1)
               zval = zipLoadInteger(p+entry.headersize,entry.encoding);
              return zval == sval;
          }
      }
      return 0;
  }
  
  /* Find pointer to the entry equal to the specified entry. Skip 'skip' entries
   * between every comparison. Returns NULL when the field could not be found.
   *
   * 寻找节点值和 vstr 相等的列表节点，并返回该节点的指针。
   *
   * Skip 'skip' entries between every comparison.
   *
   * 每次比对之前都跳过 skip 个节点。
   *
   * Returns NULL when the field could not be found.
   *
   * 如果找不到相应的节点，则返回 NULL 。
   *
   * T = O(N^2)
   */
  unsigned char *
  ziplistFind(unsigned char *zl, unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip) {
      int skipcnt = 0;
      unsigned char vencoding = 0;
      long long vll = 0;
      size_t zlbytes = ziplistBlobLen(zl);
  
      // 只要未到达列表末端，就一直迭代
      // T = O(N^2)
      while (p[0] != ZIP_END) {
          struct zlentry e;
          unsigned char *q;
  
          assert(zipEntrySafe(zl, zlbytes, p, &e, 1));
          q = p + e.prevrawlensize + e.lensize;
  
          if (skipcnt == 0) {
              /* Compare current entry with specified entry */
              // 对比字符串值
              // T = O(N)
              if (ZIP_IS_STR(e.encoding)) {
                  if (e.len == vlen && memcmp(q, vstr, vlen) == 0) {
                      return p;
                  }
              } else {
                  /* Find out if the searched field can be encoded. Note that
                   * we do it only the first time, once done vencoding is set
                   * to non-zero and vll is set to the integer value. */
                  // 因为传入值有可能被编码了，
                  // 所以当第一次进行值对比时，程序会对传入值进行解码
                  // 这个解码操作只会进行一次
                  if (vencoding == 0) {
                      if (!zipTryEncoding(vstr, vlen, &vll, &vencoding)) {
                          /* If the entry can't be encoded we set it to
                           * UCHAR_MAX so that we don't retry again the next
                           * time. */
                          vencoding = UCHAR_MAX;
                      }
                      /* Must be non-zero by now */
                      assert(vencoding);
                  }
  
                  /* Compare current entry with specified entry, do it only
                   * if vencoding != UCHAR_MAX because if there is no encoding
                   * possible for the field it can't be a valid integer. */
                  // 对比整数值
                  if (vencoding != UCHAR_MAX) {
                      // T = O(1)
                      long long ll = zipLoadInteger(q, e.encoding);
                      if (ll == vll) {
                          return p;
                      }
                  }
              }
  
  /* Reset skip count */
              skipcnt = skip;
          } else {
              /* Skip entry */
              skipcnt--;
          }
  
  /* Move to next entry */
  // 后移指针，指向后置节点
          p = q + e.len;
      }
  
  // 没有找到指定的节点
   return NULL;
  }
  
  /* Return length of ziplist.
   *
   * 返回 ziplist 中的节点个数
   *
   * T = O(N)
   */
  unsigned int ziplistLen(unsigned char *zl) {
      unsigned int len = 0;
      // 节点数小于 UINT16_MAX
      // T = O(1)
      if (intrev16ifbe(ZIPLIST_LENGTH(zl)) < UINT16_MAX) {
          len = intrev16ifbe(ZIPLIST_LENGTH(zl));
          // 节点数大于 UINT16_MAX 时，需要遍历整个列表才能计算出节点数
          // T = O(N)
      } else {
          unsigned char *p = zl + ZIPLIST_HEADER_SIZE;
          size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
          while (*p != ZIP_END) {
              p += zipRawEntryLengthSafe(zl, zlbytes, p);
              len++;
          }
  
          /* Re-store length if small enough */
          if (len < UINT16_MAX) ZIPLIST_LENGTH(zl) = intrev16ifbe(len);
      }
      return len;
  }
  
  /* Return ziplist blob size in bytes.
   *
   * 返回整个 ziplist 占用的内存字节数
   *
   * T = O(1)
  */size_t ziplistBlobLen(unsigned char *zl) {
      return intrev32ifbe(ZIPLIST_BYTES(zl));
  }
  
  void ziplistRepr(unsigned char *zl) {
      unsigned char *p;
      int index = 0;
      zlentry entry;
      size_t zlbytes = ziplistBlobLen(zl);
  
      printf(
              "{total bytes %u} "
              "{num entries %u}\n"
              "{tail offset %u}\n",
              intrev32ifbe(ZIPLIST_BYTES(zl)),
              intrev16ifbe(ZIPLIST_LENGTH(zl)),
              intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)));
      p = ZIPLIST_ENTRY_HEAD(zl);
      while (*p != ZIP_END) {
          assert(zipEntrySafe(zl, zlbytes, p, &entry, 1));
          printf(
                  "{\n"
                  "\taddr 0x%08lx,\n"
                  "\tindex %2d,\n"
                  "\toffset %5lu,\n"
                  "\thdr+entry len: %5u,\n"
                  "\thdr len%2u,\n"
                  "\tprevrawlen: %5u,\n"
                  "\tprevrawlensize: %2u,\n"
                  "\tpayload %5u\n",
                  (long unsigned) p,
                  index,
                  (unsigned long) (p - zl),
                  entry.headersize + entry.len,
                  entry.headersize,
                  entry.prevrawlen,
                  entry.prevrawlensize,
                  entry.len);
          printf("\tbytes: ");
          for (unsigned int i = 0; i < entry.headersize + entry.len; i++) {
              printf("%02x|", p[i]);
          }
          printf("\n");
          p += entry.headersize;
          if (ZIP_IS_STR(entry.encoding)) {
              printf("\t[str]");
              if (entry.len > 40) {
                  if (fwrite(p, 40, 1, stdout) == 0) perror("fwrite");
                  printf("...");
              } else {
                  if (entry.len &&
                      fwrite(p, entry.len, 1, stdout) == 0)
                      perror("fwrite");
              }
          } else {
              printf("\t[int]%lld", (long long) zipLoadInteger(p, entry.encoding));
          }
          printf("\n}\n");
          p += entry.len;
          index++;
      }
      printf("{end}\n\n");
  }
  
  /* Validate the integrity of the data structure.
   * when `deep` is 0, only the integrity of the header is validated.
   * when `deep` is 1, we scan all the entries one by one. */
  int ziplistValidateIntegrity(unsigned char *zl, size_t size, int deep,
                               ziplistValidateEntryCB entry_cb, void *cb_userdata) {
      /* check that we can actually read the header. (and ZIP_END) */
      if (size < ZIPLIST_HEADER_SIZE + ZIPLIST_END_SIZE)
          return 0;
  
      /* check that the encoded size in the header must match the allocated size. */
      size_t bytes = intrev32ifbe(ZIPLIST_BYTES(zl));
      if (bytes != size)
          return 0;
  
      /* the last byte must be the terminator. */
      if (zl[size - ZIPLIST_END_SIZE] != ZIP_END)
          return 0;
  
      /* make sure the tail offset isn't reaching outside the allocation. */
      if (intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl)) > size - ZIPLIST_END_SIZE)
          return 0;
  
      if (!deep)
          return 1;
  
      unsigned int count = 0;
      unsigned char *p = ZIPLIST_ENTRY_HEAD(zl);
      unsigned char *prev = NULL;
      size_t prev_raw_size = 0;
      while (*p != ZIP_END) {
          struct zlentry e;
          /* Decode the entry headers and fail if invalid or reaches outside the allocation */
          if (!zipEntrySafe(zl, size, p, &e, 1))
              return 0;
  
          /* Make sure the record stating the prev entry size is correct. */
          if (e.prevrawlen != prev_raw_size)
              return 0;
  
          /* Optionally let the caller validate the entry too. */
          if (entry_cb && !entry_cb(p, cb_userdata))
              return 0;
  
          /* Move to the next entry */
          prev_raw_size = e.headersize + e.len;
          prev = p;
          p += e.headersize + e.len;
          count++;
      }
  
      /* Make sure the <zltail> entry really do point to the start of the last entry. */
      if (prev != ZIPLIST_ENTRY_TAIL(zl))
          return 0;
  
      /* Check that the count in the header is correct */
      unsigned int header_count = intrev16ifbe(ZIPLIST_LENGTH(zl));
      if (header_count != UINT16_MAX && count != header_count)
          return 0;
  
      return 1;
  }
  
  /* Randomly select a pair of key and value.
   * total_count is a pre-computed length/2 of the ziplist (to avoid calls to ziplistLen)
   * 'key' and 'val' are used to store the result key value pair.
   * 'val' can be NULL if the value is not needed. */
  void ziplistRandomPair(unsigned char *zl, unsigned long total_count, ziplistEntry *key, ziplistEntry *val) {
      int ret;
      unsigned char *p;
  
      /* Avoid div by zero on corrupt ziplist */
      assert(total_count);
  
      /* Generate even numbers, because ziplist saved K-V pair */
      int r = (rand() % total_count) * 2;
      p = ziplistIndex(zl, r);
      ret = ziplistGet(p, &key->sval, &key->slen, &key->lval);
      assert(ret != 0);
  
      if (!val)
          return;
      p = ziplistNext(zl, p);
      ret = ziplistGet(p, &val->sval, &val->slen, &val->lval);
      assert(ret != 0);
  }
  
  /* int compare for qsort */
  int uintCompare(const void *a, const void *b) {
      return (*(unsigned int *) a - *(unsigned int *) b);
  }
  
  /* Helper method to store a string into from val or lval into dest */
  static inline void ziplistSaveValue(unsigned char *val, unsigned int len, long long lval, ziplistEntry *dest) {
      dest->sval = val;
      dest->slen = len;
      dest->lval = lval;
  }
  
  /* Randomly select count of key value pairs and store into 'keys' and
   * 'vals' args. The order of the picked entries is random, and the selections
   * are non-unique (repetitions are possible).
   * The 'vals' arg can be NULL in which case we skip these. */
  void ziplistRandomPairs(unsigned char *zl, unsigned int count, ziplistEntry *keys, ziplistEntry *vals) {
      unsigned char *p, *key, *value;
      unsigned int klen = 0, vlen = 0;
      long long klval = 0, vlval = 0;
  
      /* Notice: the index member must be first due to the use in uintCompare */
      typedef struct {
          unsigned int index;
          unsigned int order;
      } rand_pick;
      rand_pick *picks = zmalloc(sizeof(rand_pick) * count);
      unsigned int total_size = ziplistLen(zl) / 2;
  
      /* Avoid div by zero on corrupt ziplist */
      assert(total_size);
  
      /* create a pool of random indexes (some may be duplicate). */
      for (unsigned int i = 0; i < count; i++) {
          picks[i].index = (rand() % total_size) * 2; /* Generate even indexes */
          /* keep track of the order we picked them */
          picks[i].order = i;
      }
  
      /* sort by indexes. */
      qsort(picks, count, sizeof(rand_pick), uintCompare);
  
      /* fetch the elements form the ziplist into a output array respecting the original order. */
      unsigned int zipindex = 0, pickindex = 0;
      p = ziplistIndex(zl, 0);
      while (ziplistGet(p, &key, &klen, &klval) && pickindex < count) {
          p = ziplistNext(zl, p);
          assert(ziplistGet(p, &value, &vlen, &vlval));
          while (pickindex < count && zipindex == picks[pickindex].index) {
              int storeorder = picks[pickindex].order;
              ziplistSaveValue(key, klen, klval, &keys[storeorder]);
              if (vals)
                  ziplistSaveValue(value, vlen, vlval, &vals[storeorder]);
              pickindex++;
          }
          zipindex += 2;
          p = ziplistNext(zl, p);
      }
  
      zfree(picks);
  }
  
  /* Randomly select count of key value pairs and store into 'keys' and
   * 'vals' args. The selections are unique (no repetitions), and the order of
   * the picked entries is NOT-random.
   * The 'vals' arg can be NULL in which case we skip these.
   * The return value is the number of items picked which can be lower than the
   * requested count if the ziplist doesn't hold enough pairs. */
  unsigned int ziplistRandomPairsUnique(unsigned char *zl, unsigned int count, ziplistEntry *keys, ziplistEntry *vals) {
      unsigned char *p, *key;
      unsigned int klen = 0;
      long long klval = 0;
      unsigned int total_size = ziplistLen(zl) / 2;
      unsigned int index = 0;
      if (count > total_size)
          count = total_size;
  
      /* To only iterate once, every time we try to pick a member, the probability
       * we pick it is the quotient of the count left we want to pick and the
       * count still we haven't visited in the dict, this way, we could make every
       * member be equally picked.*/
      p = ziplistIndex(zl, 0);
      unsigned int picked = 0, remaining = count;
      while (picked < count && p) {
          double randomDouble = ((double) rand()) / RAND_MAX;
          double threshold = ((double) remaining) / (total_size - index);
          if (randomDouble <= threshold) {
              assert(ziplistGet(p, &key, &klen, &klval));
              ziplistSaveValue(key, klen, klval, &keys[picked]);
              p = ziplistNext(zl, p);
              assert(p);
              if (vals) {
                  assert(ziplistGet(p, &key, &klen, &klval));
                  ziplistSaveValue(key, klen, klval, &vals[picked]);
              }
              remaining--;
              picked++;
          } else {
              p = ziplistNext(zl, p);
              assert(p);
          }
          p = ziplistNext(zl, p);
          index++;
      }
      return picked;
  }
  
  #ifdef REDIS_TEST
  #include <sys/time.h>
  #include "adlist.h"
  #include "sds.h"
  
  #define debug(f, ...) { if (DEBUG) printf(f, __VA_ARGS__); }
  
  static unsigned char *createList() {
      unsigned char *zl = ziplistNew();
      zl = ziplistPush(zl, (unsigned char*)"foo", 3, ZIPLIST_TAIL);
      zl = ziplistPush(zl, (unsigned char*)"quux", 4, ZIPLIST_TAIL);
      zl = ziplistPush(zl, (unsigned char*)"hello", 5, ZIPLIST_HEAD);
      zl = ziplistPush(zl, (unsigned char*)"1024", 4, ZIPLIST_TAIL);
      return zl;
  }
  
  static unsigned char *createIntList() {
      unsigned char *zl = ziplistNew();
      char buf[32];
  
      sprintf(buf, "100");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
      sprintf(buf, "128000");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
      sprintf(buf, "-100");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
      sprintf(buf, "4294967296");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
      sprintf(buf, "non integer");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
      sprintf(buf, "much much longer non integer");
      zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
      return zl;
  }
  
  static long long usec(void) {
      struct timeval tv;
      gettimeofday(&tv,NULL);
      return (((long long)tv.tv_sec)*1000000)+tv.tv_usec;
  }
  
  static void stress(int pos, int num, int maxsize, int dnum) {
      int i,j,k;
      unsigned char *zl;
      char posstr[2][5] = { "HEAD", "TAIL" };
      long long start;
      for (i = 0; i < maxsize; i+=dnum) {
          zl = ziplistNew();
          for (j = 0; j < i; j++) {
              zl = ziplistPush(zl,(unsigned char*)"quux",4,ZIPLIST_TAIL);
          }
  
          /* Do num times a push+pop from pos */
          start = usec();
          for (k = 0; k < num; k++) {
              zl = ziplistPush(zl,(unsigned char*)"quux",4,pos);
              zl = ziplistDeleteRange(zl,0,1);
          }
          printf("List size: %8d, bytes: %8d, %dx push+pop (%s): %6lld usec\n",
                 i,intrev32ifbe(ZIPLIST_BYTES(zl)),num,posstr[pos],usec()-start);
          zfree(zl);
      }
  }
  
  static unsigned char *pop(unsigned char *zl, int where) {
      unsigned char *p, *vstr;
      unsigned int vlen;
      long long vlong;
  
      p = ziplistIndex(zl,where == ZIPLIST_HEAD ? 0 : -1);
      if (ziplistGet(p,&vstr,&vlen,&vlong)) {
          if (where == ZIPLIST_HEAD)
              printf("Pop head: ");
          else
              printf("Pop tail: ");
  
          if (vstr) {
              if (vlen && fwrite(vstr,vlen,1,stdout) == 0) perror("fwrite");
          }
          else {
              printf("%lld", vlong);
          }
  
          printf("\n");
          return ziplistDelete(zl,&p);
      } else {
          printf("ERROR: Could not pop\n");
          exit(1);
      }
  }
  
  static int randstring(char *target, unsigned int min, unsigned int max) {
      int p = 0;
      int len = min+rand()%(max-min+1);
      int minval, maxval;
      switch(rand() % 3) {
          case 0:
              minval = 0;
              maxval = 255;
              break;
              case 1:
                  minval = 48;
                  maxval = 122;
                  break;
                  case 2:
                      minval = 48;
                      maxval = 52;
                      break;
                      default:
                          assert(NULL);
      }
  
      while(p < len)
          target[p++] = minval+rand()%(maxval-minval+1);
      return len;
  }
  
  static void verify(unsigned char *zl, zlentry *e) {
      int len = ziplistLen(zl);
      zlentry _e;
  
      ZIPLIST_ENTRY_ZERO(&_e);
  
      for (int i = 0; i < len; i++) {
          memset(&e[i], 0, sizeof(zlentry));
          zipEntry(ziplistIndex(zl, i), &e[i]);
  
          memset(&_e, 0, sizeof(zlentry));
          zipEntry(ziplistIndex(zl, -len+i), &_e);
  
          assert(memcmp(&e[i], &_e, sizeof(zlentry)) == 0);
      }
  }
  
  static unsigned char *insertHelper(unsigned char *zl, char ch, size_t len, unsigned char *pos) {
      assert(len <= ZIP_BIG_PREVLEN);
      unsigned char data[ZIP_BIG_PREVLEN] = {0};
      memset(data, ch, len);
      return ziplistInsert(zl, pos, data, len);
  }
  
  static int compareHelper(unsigned char *zl, char ch, size_t len, int index) {
      assert(len <= ZIP_BIG_PREVLEN);
      unsigned char data[ZIP_BIG_PREVLEN] = {0};
      memset(data, ch, len);
      unsigned char *p = ziplistIndex(zl, index);
      assert(p != NULL);
      return ziplistCompare(p, data, len);
  }
  
  static size_t strEntryBytesSmall(size_t slen) {
      return slen + zipStorePrevEntryLength(NULL, 0) + zipStoreEntryEncoding(NULL, 0, slen);
  }
  
  static size_t strEntryBytesLarge(size_t slen) {
      return slen + zipStorePrevEntryLength(NULL, ZIP_BIG_PREVLEN) + zipStoreEntryEncoding(NULL, 0, slen);
  }
  
  /* ./redis-server test ziplist <randomseed> --accurate */
  int ziplistTest(int argc, char **argv, int accurate) {
      unsigned char *zl, *p;
      unsigned char *entry;
      unsigned int elen;
      long long value;
      int iteration;
  
      /* If an argument is given, use it as the random seed. */
      if (argc >= 4)
          srand(atoi(argv[3]));
  
      zl = createIntList();
      ziplistRepr(zl);
  
      zfree(zl);
  
      zl = createList();
      ziplistRepr(zl);
  
      zl = pop(zl,ZIPLIST_TAIL);
      ziplistRepr(zl);
  
      zl = pop(zl,ZIPLIST_HEAD);
      ziplistRepr(zl);
  
      zl = pop(zl,ZIPLIST_TAIL);
      ziplistRepr(zl);
  
      zl = pop(zl,ZIPLIST_TAIL);
      ziplistRepr(zl);
  
      zfree(zl);
  
      printf("Get element at index 3:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 3);
          if (!ziplistGet(p, &entry, &elen, &value)) {
              printf("ERROR: Could not access index 3\n");
              return 1;
          }
          if (entry) {
              if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              printf("\n");
          } else {
              printf("%lld\n", value);
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Get element at index 4 (out of range):\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 4);
          if (p == NULL) {
              printf("No entry\n");
          } else {
              printf("ERROR: Out of range index should return NULL, returned offset: %ld\n", (long)(p-zl));
              return 1;
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Get element at index -1 (last element):\n");
      {
          zl = createList();
          p = ziplistIndex(zl, -1);
          if (!ziplistGet(p, &entry, &elen, &value)) {
              printf("ERROR: Could not access index -1\n");
              return 1;
          }
          if (entry) {
              if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              printf("\n");
          } else {
              printf("%lld\n", value);
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Get element at index -4 (first element):\n");
      {
          zl = createList();
          p = ziplistIndex(zl, -4);
          if (!ziplistGet(p, &entry, &elen, &value)) {
              printf("ERROR: Could not access index -4\n");
              return 1;
          }
          if (entry) {
              if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              printf("\n");
          } else {
              printf("%lld\n", value);
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Get element at index -5 (reverse out of range):\n");
      {
          zl = createList();
          p = ziplistIndex(zl, -5);
          if (p == NULL) {
              printf("No entry\n");
          } else {
              printf("ERROR: Out of range index should return NULL, returned offset: %ld\n", (long)(p-zl));
              return 1;
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate list from 0 to end:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 0);
          while (ziplistGet(p, &entry, &elen, &value)) {
              printf("Entry: ");
              if (entry) {
                  if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              } else {
                  printf("%lld", value);
              }
              p = ziplistNext(zl,p);
              printf("\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate list from 1 to end:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 1);
          while (ziplistGet(p, &entry, &elen, &value)) {
              printf("Entry: ");
              if (entry) {
                  if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              } else {
                  printf("%lld", value);
              }
              p = ziplistNext(zl,p);
              printf("\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate list from 2 to end:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 2);
          while (ziplistGet(p, &entry, &elen, &value)) {
              printf("Entry: ");
              if (entry) {
                  if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              } else {
                  printf("%lld", value);
              }
              p = ziplistNext(zl,p);
              printf("\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate starting out of range:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, 4);
          if (!ziplistGet(p, &entry, &elen, &value)) {
              printf("No entry\n");
          } else {
              printf("ERROR\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate from back to front:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, -1);
          while (ziplistGet(p, &entry, &elen, &value)) {
              printf("Entry: ");
              if (entry) {
                  if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              } else {
                  printf("%lld", value);
              }
              p = ziplistPrev(zl,p);
              printf("\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Iterate from back to front, deleting all items:\n");
      {
          zl = createList();
          p = ziplistIndex(zl, -1);
          while (ziplistGet(p, &entry, &elen, &value)) {
              printf("Entry: ");
              if (entry) {
                  if (elen && fwrite(entry,elen,1,stdout) == 0) perror("fwrite");
              } else {
                  printf("%lld", value);
              }
              zl = ziplistDelete(zl,&p);
              p = ziplistPrev(zl,p);
              printf("\n");
          }
          printf("\n");
          zfree(zl);
      }
  
      printf("Delete inclusive range 0,0:\n");
      {
          zl = createList();
          zl = ziplistDeleteRange(zl, 0, 1);
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Delete inclusive range 0,1:\n");
      {
          zl = createList();
          zl = ziplistDeleteRange(zl, 0, 2);
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Delete inclusive range 1,2:\n");
      {
          zl = createList();
          zl = ziplistDeleteRange(zl, 1, 2);
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Delete with start index out of range:\n");
      {
          zl = createList();
          zl = ziplistDeleteRange(zl, 5, 1);
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Delete with num overflow:\n");
      {
          zl = createList();
          zl = ziplistDeleteRange(zl, 1, 5);
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Delete foo while iterating:\n");
      {
          zl = createList();
          p = ziplistIndex(zl,0);
          while (ziplistGet(p,&entry,&elen,&value)) {
              if (entry && strncmp("foo",(char*)entry,elen) == 0) {
                  printf("Delete foo\n");
                  zl = ziplistDelete(zl,&p);
              } else {
                  printf("Entry: ");
                  if (entry) {
                      if (elen && fwrite(entry,elen,1,stdout) == 0)
                          perror("fwrite");
                  } else {
                      printf("%lld",value);
                  }
                  p = ziplistNext(zl,p);
                  printf("\n");
              }
          }
          printf("\n");
          ziplistRepr(zl);
          zfree(zl);
      }
  
      printf("Replace with same size:\n");
      {
          zl = createList(); /* "hello", "foo", "quux", "1024" */
          unsigned char *orig_zl = zl;
          p = ziplistIndex(zl, 0);
          zl = ziplistReplace(zl, p, (unsigned char*)"zoink", 5);
          p = ziplistIndex(zl, 3);
          zl = ziplistReplace(zl, p, (unsigned char*)"yy", 2);
          p = ziplistIndex(zl, 1);
          zl = ziplistReplace(zl, p, (unsigned char*)"65536", 5);
          p = ziplistIndex(zl, 0);
          assert(!memcmp((char*)p,
                         "\x00\x05zoink"
                         "\x07\xf0\x00\x00\x01" /* 65536 as int24 */
                         "\x05\x04quux" "\x06\x02yy" "\xff",
                         23));
          assert(zl == orig_zl); /* no reallocations have happened */
          zfree(zl);
          printf("SUCCESS\n\n");
      }
  
      printf("Replace with different size:\n");
      {
          zl = createList(); /* "hello", "foo", "quux", "1024" */
          p = ziplistIndex(zl, 1);
          zl = ziplistReplace(zl, p, (unsigned char*)"squirrel", 8);
          p = ziplistIndex(zl, 0);
          assert(!strncmp((char*)p,
                          "\x00\x05hello" "\x07\x08squirrel" "\x0a\x04quux"
                                                             "\x06\xc0\x00\x04" "\xff",
                                                             28));
          zfree(zl);
          printf("SUCCESS\n\n");
      }
  
      printf("Regression test for >255 byte strings:\n");
      {
          char v1[257] = {0}, v2[257] = {0};
          memset(v1,'x',256);
          memset(v2,'y',256);
          zl = ziplistNew();
          zl = ziplistPush(zl,(unsigned char*)v1,strlen(v1),ZIPLIST_TAIL);
          zl = ziplistPush(zl,(unsigned char*)v2,strlen(v2),ZIPLIST_TAIL);
  
          /* Pop values again and compare their value. */
          p = ziplistIndex(zl,0);
          assert(ziplistGet(p,&entry,&elen,&value));
          assert(strncmp(v1,(char*)entry,elen) == 0);
          p = ziplistIndex(zl,1);
          assert(ziplistGet(p,&entry,&elen,&value));
          assert(strncmp(v2,(char*)entry,elen) == 0);
          printf("SUCCESS\n\n");
          zfree(zl);
      }
  
      printf("Regression test deleting next to last entries:\n");
      {
          char v[3][257] = {{0}};
          zlentry e[3] = {{.prevrawlensize = 0, .prevrawlen = 0, .lensize = 0,
                           .len = 0, .headersize = 0, .encoding = 0, .p = NULL}};
          size_t i;
  
          for (i = 0; i < (sizeof(v)/sizeof(v[0])); i++) {
              memset(v[i], 'a' + i, sizeof(v[0]));
          }
  
          v[0][256] = '\0';
          v[1][  1] = '\0';
          v[2][256] = '\0';
  
          zl = ziplistNew();
          for (i = 0; i < (sizeof(v)/sizeof(v[0])); i++) {
              zl = ziplistPush(zl, (unsigned char *) v[i], strlen(v[i]), ZIPLIST_TAIL);
          }
  
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1);
          assert(e[1].prevrawlensize == 5);
          assert(e[2].prevrawlensize == 1);
  
          /* Deleting entry 1 will increase `prevrawlensize` for entry 2 */
          unsigned char *p = e[1].p;
          zl = ziplistDelete(zl, &p);
  
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1);
          assert(e[1].prevrawlensize == 5);
  
          printf("SUCCESS\n\n");
          zfree(zl);
      }
  
      printf("Create long list and check indices:\n");
      {
          unsigned long long start = usec();
          zl = ziplistNew();
          char buf[32];
          int i,len;
          for (i = 0; i < 1000; i++) {
              len = sprintf(buf,"%d",i);
              zl = ziplistPush(zl,(unsigned char*)buf,len,ZIPLIST_TAIL);
          }
          for (i = 0; i < 1000; i++) {
              p = ziplistIndex(zl,i);
              assert(ziplistGet(p,NULL,NULL,&value));
              assert(i == value);
  
              p = ziplistIndex(zl,-i-1);
              assert(ziplistGet(p,NULL,NULL,&value));
              assert(999-i == value);
          }
          printf("SUCCESS. usec=%lld\n\n", usec()-start);
          zfree(zl);
      }
  
      printf("Compare strings with ziplist entries:\n");
      {
          zl = createList();
          p = ziplistIndex(zl,0);
          if (!ziplistCompare(p,(unsigned char*)"hello",5)) {
              printf("ERROR: not \"hello\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"hella",5)) {
              printf("ERROR: \"hella\"\n");
              return 1;
          }
  
          p = ziplistIndex(zl,3);
          if (!ziplistCompare(p,(unsigned char*)"1024",4)) {
              printf("ERROR: not \"1024\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"1025",4)) {
              printf("ERROR: \"1025\"\n");
              return 1;
          }
          printf("SUCCESS\n\n");
          zfree(zl);
      }
  
      printf("Merge test:\n");
      {
          /* create list gives us: [hello, foo, quux, 1024] */
          zl = createList();
          unsigned char *zl2 = createList();
  
          unsigned char *zl3 = ziplistNew();
          unsigned char *zl4 = ziplistNew();
  
          if (ziplistMerge(&zl4, &zl4)) {
              printf("ERROR: Allowed merging of one ziplist into itself.\n");
              return 1;
          }
  
          /* Merge two empty ziplists, get empty result back. */
          zl4 = ziplistMerge(&zl3, &zl4);
          ziplistRepr(zl4);
          if (ziplistLen(zl4)) {
              printf("ERROR: Merging two empty ziplists created entries.\n");
              return 1;
          }
          zfree(zl4);
  
          zl2 = ziplistMerge(&zl, &zl2);
          /* merge gives us: [hello, foo, quux, 1024, hello, foo, quux, 1024] */
          ziplistRepr(zl2);
  
          if (ziplistLen(zl2) != 8) {
              printf("ERROR: Merged length not 8, but: %u\n", ziplistLen(zl2));
              return 1;
          }
  
          p = ziplistIndex(zl2,0);
          if (!ziplistCompare(p,(unsigned char*)"hello",5)) {
              printf("ERROR: not \"hello\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"hella",5)) {
              printf("ERROR: \"hella\"\n");
              return 1;
          }
  
          p = ziplistIndex(zl2,3);
          if (!ziplistCompare(p,(unsigned char*)"1024",4)) {
              printf("ERROR: not \"1024\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"1025",4)) {
              printf("ERROR: \"1025\"\n");
              return 1;
          }
  
          p = ziplistIndex(zl2,4);
          if (!ziplistCompare(p,(unsigned char*)"hello",5)) {
              printf("ERROR: not \"hello\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"hella",5)) {
              printf("ERROR: \"hella\"\n");
              return 1;
          }
  
          p = ziplistIndex(zl2,7);
          if (!ziplistCompare(p,(unsigned char*)"1024",4)) {
              printf("ERROR: not \"1024\"\n");
              return 1;
          }
          if (ziplistCompare(p,(unsigned char*)"1025",4)) {
              printf("ERROR: \"1025\"\n");
              return 1;
          }
          printf("SUCCESS\n\n");
          zfree(zl);
      }
  
      printf("Stress with random payloads of different encoding:\n");
      {
          unsigned long long start = usec();
          int i,j,len,where;
          unsigned char *p;
          char buf[1024];
          int buflen;
          list *ref;
          listNode *refnode;
  
          /* Hold temp vars from ziplist */
          unsigned char *sstr;
          unsigned int slen;
          long long sval;
  
          iteration = accurate ? 20000 : 20;
          for (i = 0; i < iteration; i++) {
              zl = ziplistNew();
              ref = listCreate();
              listSetFreeMethod(ref,(void (*)(void*))sdsfree);
              len = rand() % 256;
  
              /* Create lists */
              for (j = 0; j < len; j++) {
                  where = (rand() & 1) ? ZIPLIST_HEAD : ZIPLIST_TAIL;
                  if (rand() % 2) {
                      buflen = randstring(buf,1,sizeof(buf)-1);
                  } else {
                      switch(rand() % 3) {
                          case 0:
                              buflen = sprintf(buf,"%lld",(0LL + rand()) >> 20);
                              break;
                              case 1:
                                  buflen = sprintf(buf,"%lld",(0LL + rand()));
                                  break;
                                  case 2:
                                      buflen = sprintf(buf,"%lld",(0LL + rand()) << 20);
                                      break;
                                      default:
                                          assert(NULL);
                      }
                  }
  
                  /* Add to ziplist */
                  zl = ziplistPush(zl, (unsigned char*)buf, buflen, where);
  
                  /* Add to reference list */
                  if (where == ZIPLIST_HEAD) {
                      listAddNodeHead(ref,sdsnewlen(buf, buflen));
                  } else if (where == ZIPLIST_TAIL) {
                      listAddNodeTail(ref,sdsnewlen(buf, buflen));
                  } else {
                      assert(NULL);
                  }
              }
  
              assert(listLength(ref) == ziplistLen(zl));
              for (j = 0; j < len; j++) {
                  /* Naive way to get elements, but similar to the stresser
                   * executed from the Tcl test suite. */
                  p = ziplistIndex(zl,j);
                  refnode = listIndex(ref,j);
  
                  assert(ziplistGet(p,&sstr,&slen,&sval));
                  if (sstr == NULL) {
                      buflen = sprintf(buf,"%lld",sval);
                  } else {
                      buflen = slen;
                      memcpy(buf,sstr,buflen);
                      buf[buflen] = '\0';
                  }
                  assert(memcmp(buf,listNodeValue(refnode),buflen) == 0);
              }
              zfree(zl);
              listRelease(ref);
          }
          printf("Done. usec=%lld\n\n", usec()-start);
      }
  
      printf("Stress with variable ziplist size:\n");
      {
          unsigned long long start = usec();
          int maxsize = accurate ? 16384 : 16;
          stress(ZIPLIST_HEAD,100000,maxsize,256);
          stress(ZIPLIST_TAIL,100000,maxsize,256);
          printf("Done. usec=%lld\n\n", usec()-start);
      }
  
      /* Benchmarks */
      {
          zl = ziplistNew();
          iteration = accurate ? 100000 : 100;
          for (int i=0; i<iteration; i++) {
              char buf[4096] = "asdf";
              zl = ziplistPush(zl, (unsigned char*)buf, 4, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)buf, 40, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)buf, 400, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)buf, 4000, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"1", 1, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"10", 2, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"100", 3, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"1000", 4, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"10000", 5, ZIPLIST_TAIL);
              zl = ziplistPush(zl, (unsigned char*)"100000", 6, ZIPLIST_TAIL);
          }
  
          printf("Benchmark ziplistFind:\n");
          {
              unsigned long long start = usec();
              for (int i = 0; i < 2000; i++) {
                  unsigned char *fptr = ziplistIndex(zl, ZIPLIST_HEAD);
                  fptr = ziplistFind(zl, fptr, (unsigned char*)"nothing", 7, 1);
              }
              printf("%lld\n", usec()-start);
          }
  
          printf("Benchmark ziplistIndex:\n");
          {
              unsigned long long start = usec();
              for (int i = 0; i < 2000; i++) {
                  ziplistIndex(zl, 99999);
              }
              printf("%lld\n", usec()-start);
          }
  
          printf("Benchmark ziplistValidateIntegrity:\n");
          {
              unsigned long long start = usec();
              for (int i = 0; i < 2000; i++) {
                  ziplistValidateIntegrity(zl, ziplistBlobLen(zl), 1, NULL, NULL);
              }
              printf("%lld\n", usec()-start);
          }
  
          zfree(zl);
      }
  
      printf("Stress __ziplistCascadeUpdate:\n");
      {
          char data[ZIP_BIG_PREVLEN];
          zl = ziplistNew();
          iteration = accurate ? 100000 : 100;
          for (int i = 0; i < iteration; i++) {
              zl = ziplistPush(zl, (unsigned char*)data, ZIP_BIG_PREVLEN-4, ZIPLIST_TAIL);
          }
          unsigned long long start = usec();
          zl = ziplistPush(zl, (unsigned char*)data, ZIP_BIG_PREVLEN-3, ZIPLIST_HEAD);
          printf("Done. usec=%lld\n\n", usec()-start);
          zfree(zl);
      }
  
      printf("Edge cases of __ziplistCascadeUpdate:\n");
      {
          /* Inserting a entry with data length greater than ZIP_BIG_PREVLEN-4
           * will leads to cascade update. */
          size_t s1 = ZIP_BIG_PREVLEN-4, s2 = ZIP_BIG_PREVLEN-3;
          zl = ziplistNew();
  
          zlentry e[4] = {{.prevrawlensize = 0, .prevrawlen = 0, .lensize = 0,
                           .len = 0, .headersize = 0, .encoding = 0, .p = NULL}};
  
          zl = insertHelper(zl, 'a', s1, ZIPLIST_ENTRY_HEAD(zl));
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1 && e[0].prevrawlen == 0);
          assert(compareHelper(zl, 'a', s1, 0));
          ziplistRepr(zl);
  
          /* No expand. */
          zl = insertHelper(zl, 'b', s1, ZIPLIST_ENTRY_HEAD(zl));
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1 && e[0].prevrawlen == 0);
          assert(compareHelper(zl, 'b', s1, 0));
  
          assert(e[1].prevrawlensize == 1 && e[1].prevrawlen == strEntryBytesSmall(s1));
          assert(compareHelper(zl, 'a', s1, 1));
  
          ziplistRepr(zl);
  
          /* Expand(tail included). */
          zl = insertHelper(zl, 'c', s2, ZIPLIST_ENTRY_HEAD(zl));
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1 && e[0].prevrawlen == 0);
          assert(compareHelper(zl, 'c', s2, 0));
  
          assert(e[1].prevrawlensize == 5 && e[1].prevrawlen == strEntryBytesSmall(s2));
          assert(compareHelper(zl, 'b', s1, 1));
  
          assert(e[2].prevrawlensize == 5 && e[2].prevrawlen == strEntryBytesLarge(s1));
          assert(compareHelper(zl, 'a', s1, 2));
  
          ziplistRepr(zl);
  
          /* Expand(only previous head entry). */
          zl = insertHelper(zl, 'd', s2, ZIPLIST_ENTRY_HEAD(zl));
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1 && e[0].prevrawlen == 0);
          assert(compareHelper(zl, 'd', s2, 0));
  
          assert(e[1].prevrawlensize == 5 && e[1].prevrawlen == strEntryBytesSmall(s2));
          assert(compareHelper(zl, 'c', s2, 1));
  
          assert(e[2].prevrawlensize == 5 && e[2].prevrawlen == strEntryBytesLarge(s2));
          assert(compareHelper(zl, 'b', s1, 2));
  
          assert(e[3].prevrawlensize == 5 && e[3].prevrawlen == strEntryBytesLarge(s1));
          assert(compareHelper(zl, 'a', s1, 3));
  
          ziplistRepr(zl);
  
          /* Delete from mid. */
          unsigned char *p = ziplistIndex(zl, 2);
          zl = ziplistDelete(zl, &p);
          verify(zl, e);
  
          assert(e[0].prevrawlensize == 1 && e[0].prevrawlen == 0);
          assert(compareHelper(zl, 'd', s2, 0));
  
          assert(e[1].prevrawlensize == 5 && e[1].prevrawlen == strEntryBytesSmall(s2));
          assert(compareHelper(zl, 'c', s2, 1));
  
          assert(e[2].prevrawlensize == 5 && e[2].prevrawlen == strEntryBytesLarge(s2));
          assert(compareHelper(zl, 'a', s1, 2));
  
          ziplistRepr(zl);
  
          zfree(zl);
      }
  
      return 0;
  }
  #endif
  