src/ziplist.c

/* ziplist是为了提高存储效率而设计的一种特殊编码的双向链表。它可以存储字符串或者整数，
 * 存储整数时是采用整数的二进制而不是字符串形式存储。他能在O(1)的时间复杂度下完成list两端
 * 的push和pop操作。但是因为每次操作都需要重新分配ziplist的内存，所以实际复杂度和ziplist
 * 的内存使用量相关。    
 * ----------------------------------------------------------------------------
 *
 * ZIPLIST整体 
 * ======================
 *
 * ziplist的结构如下 
 *
 * <zlbytes> <zltail> <zllen> <entry> <entry> ... <entry> <zlend>
 *
 * 注意: 除非有特殊声明，所有的域都是按小端存储的。 
 * <zlbytes>: 32位无符号整型，表示整个ziplist所占的空间大小，包含了zlbytes所占的4个字节。
 * 这个字段可以在重置整个ziplist大小时不需要遍历整个list来确定大小，空间换时间。  
 * <zltail>: 32位无符号整型，表示整个list中最后一项所在的偏移量，方便在尾部做pop操作。
 * <zllen>: 16位，表示ziplist中所存储的entry数量，但是注意，这里最多表示2^16-2个entry，
 * 如果是2^16-1有特殊含义，2^16-1表示存储数量超过了2^16-2个，但具体是多少个得遍历一次才
 * 能知道。 
 * <zlend>: 8位，ziplist的末尾表示，值固定是255. 
 *
 * ZIPLIST ENTRIES
 * ===============
 * 
 * ziplist中的每个entry这个entry的元信息，包含两部分，第一部分存储了前面一个entry所占用的字
 * 节数，方便从后往前遍历ziplist。第二部分表示entry的编码形式，它表明了entry的类型，整数还是
 * 字符串，如果是string的话还标识出了string的长度。  
 * 完整的entry是这样的：  
 *
 * <prevlen> <encoding> <entry-data>
 *
 * 部分情况下，encoding字段也可能会是entry本身，比如一些比较小的数字就可以直接存在encoding
 * 字段里了。这种情况下<entry-data>就会缺失，整个entry就会像下面这样。 
 *
 * <prevlen> <encoding>
 *
 * <prevlen> 表示前面一个entry的长度，编码方式如下： 
 * 如果长度小于254，那就只用一个字节来表示长度，如果长度大于等于254就用5个字节，第一个字节
 * 是固定值254(FE)来标识这是个特殊的数据，剩下的4个字节来表示实际的长度。  
 *
 * 所以实际上一个entry可能是下面这样的： 
 * <prevlen from 0 to 253> <encoding> <entry>
 * 当前面一个entry长度超过253时是下面这样的：  
 * 0xFE <4 bytes unsigned little endian prevlen> <encoding> <entry>
 *
 * entry的encoding取决于entry的内容，当entry是个string时，encoding的前两字节存储了字符
 * 串的长度。当entry是一个整数的时候，前两字节默认都是1，后面两字节标识出后面存的是哪种类型的
 * 整数，不同的encoding类型示例如下。第一个字节就足够判断出entry是什么类型了。 
 *
 * |00pppppp| - 1字节
 *      长度小于或者等于63的String类型，'pppppp'无符号6位数标识string长度 
 * |01pppppp|qqqqqqqq| - 2字节
 *      长度小于或者等于16383的String类型(14位)，注意：14位'pppppp'采用大端的方式存储 
 * |10000000|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5字节 
 *      长度大于等于16384的String类型，第二字节开始的qqrrsstt都是用来存储字符串长度的
 *      二进制位，可表示的字符串长度最大2^32-1，第一字节的低6位没有用，所以都是0. 
 *      注意: 32位数采用大端的方式存储  
 * |11000000| - 3字节
 *      存储int16_t (2字节).
 * |11010000| - 5字节
 *      存储int32_t (4字节).
 * |11100000| - 9字节
 *      存储int64_t (8字节).
 * |11110000| - 4字节
 *      24位有符号类型整数 (3字节).
 * |11111110| - 2字节
 *      8位有符号类型整数 (1字节).
 * |1111xxxx| - (xxxx在0001和1101之间) 4位无符号整数.
 *      0到12的无符号整数.编码值实际上是从1到13，因为0000和1111不能使用，要留出一位
 *      表示0，所以应该从编码值中减去1才是准确值 
 * |11111111| - ziplist结尾标识. 
 *
 * 与ziplist头一样，所有的整数都是以小端字节顺序表示的，即使这段代码是在大端系统中编译的。
 *
 * 实际的ZIPLIST样例 
 * ===========================
 *
 * 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:
 * 下面的ziplist包含字符串"2"和"5“，它被压缩后只有15字节，它在内存中的数据16进制表示如下。
 * 
 *  [0f 00 00 00] [0c 00 00 00] [02 00] [00 f3] [02 f6] [ff]
 *        |             |          |       |       |     |
 *     zlbytes        zltail     zllen    "2"     "5"   end
 *
 * 前4字节是数字15，表示整个ziplist总计占用15字节。接下来的4个字节是最后一个ziplist entry
 * 的偏移量，具体偏移量是12，在上面例子中标识最后一个entry 字符串”5“在ziplist中的12位置。
 * 接下来的16位表示ziplist中的元素数量，这里是2，表示只有2个元素。  
 * 接下来的"00 f3"是第一个entry，它里面的prelen是0，因为这是第一个entry，F3对应的
 * 编码是|1111xxxx| - (xxxx在0001和1101之间)，我们需要移除高4位的1111得到"3",然后
 * 减去1，才是真正的值”2“，第一个entry正好被压缩为2字节。下一个entry F5，同样的方式
 * 可以得到"5",最后的FF表示ziplist的结尾。 
 *
 * 在上面的ziplist中添加字符串"Hello World"，假设它插入到”5“的后面，生成的entry16进制表示如下：  
 *
 * [02] [0b] [48 65 6c 6c 6f 20 57 6f 72 6c 64]
 *
 * 第一个字节02，表示前一个entry的长度，下一个字节|00pppppp|，表示是一个短字符串，长度是<pppppp>,
 * 所以这里0B表示这里是一个11个字节长度的字符串。接下的48到最后的64是"Hello World"的ASCII编码。 
 *
 * ----------------------------------------------------------------------------
 *
 * 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.
 */

/* 压缩列表的实现，具体可以参考博文：https://blog.csdn.net/xindoo/article/details/108923557
 */

#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"

#define ZIP_END 255         /* 标识ziplist尾部的特殊字符 */
#define ZIP_BIG_PREVLEN 254 /* 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
不同的编码长度 */
#define ZIP_STR_MASK 0xc0
#define ZIP_INT_MASK 0x30
#define ZIP_STR_06B (0 << 6)
#define ZIP_STR_14B (1 << 6)
#define ZIP_STR_32B (2 << 6)
#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
 * 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 */

#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. */
#define ZIP_IS_STR(enc) (((enc) & ZIP_STR_MASK) < ZIP_STR_MASK)

/* Utility macros.*/

/* Return total bytes a ziplist is composed of. */
#define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))

/* Return the offset of the last item inside the ziplist. */
#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. */
#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. */
#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. */
#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. */
#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. */
#define ZIPLIST_ENTRY_END(zl)   ((zl)+intrev32ifbe(ZIPLIST_BYTES(zl))-ZIPLIST_END_SIZE)

/* 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. */
#define ZIPLIST_INCR_LENGTH(zl,incr) { \
    if (intrev16ifbe(ZIPLIST_LENGTH(zl)) < UINT16_MAX) \
        ZIPLIST_LENGTH(zl) = intrev16ifbe(intrev16ifbe(ZIPLIST_LENGTH(zl))+incr); \
}

/* Don't let ziplists grow over 1GB in any case, don't wanna risk overflow in
 * zlbytes */
#define ZIPLIST_MAX_SAFETY_SIZE (1<<30)
int ziplistSafeToAdd(unsigned char* zl, size_t add) {
    size_t len = zl? ziplistBlobLen(zl): 0;
    if (len + add > ZIPLIST_MAX_SAFETY_SIZE)
        return 0;
    return 1;
}


/* 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. */
typedef struct zlentry {
    unsigned int prevrawlensize; /* 内存中编码后的prevrawlen用了多少字节 */
    unsigned int prevrawlen;     /* 前一个entry占用的长度，主要是为了entry之间跳转 */
    unsigned int lensize;        /* 内存中编码后的len用了多少字节 */
    unsigned int len;            /* 当前entry的长度，如果是string则表示string的长度，如果是整数，则len依赖于具体数值大小。*/
    unsigned int headersize;     /* prevrawlensize + lensize. entry的head部分用了多少字节 */
    unsigned char encoding;      /* 当前entry的编码格式 */
    unsigned char *p;            /* 指向数据域的指针 */
} 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. */
#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 */
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'. */
unsigned int zipStoreEntryEncoding(unsigned char *p, unsigned char encoding, unsigned int rawlen) {
    unsigned char len = 1, buf[5];

    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;
        }
    } else {
        /* Implies integer encoding, so length is always 1. */
        if (!p) return len;
        buf[0] = encoding;
    }

    /* Store this length at p. */
    memcpy(p,buf,len);
    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. */
#define ZIP_DECODE_LENGTH(ptr, encoding, lensize, len) do {                    \
    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) = ((uint32_t)(ptr)[1] << 24) |                               \
                    ((uint32_t)(ptr)[2] << 16) |                               \
                    ((uint32_t)(ptr)[3] <<  8) |                               \
                    ((uint32_t)(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.    */ \
        }                                                                      \
    } 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). */
int zipStorePrevEntryLengthLarge(unsigned char *p, unsigned int len) {
    uint32_t u32;
    if (p != NULL) {
        p[0] = ZIP_BIG_PREVLEN;
        u32 = len;
        memcpy(p+1,&u32,sizeof(u32));
        memrev32ifbe(p+1);
    }
    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. */
unsigned int zipStorePrevEntryLength(unsigned char *p, unsigned int len) {
    if (p == NULL) {
        return (len < ZIP_BIG_PREVLEN) ? 1 : sizeof(uint32_t) + 1;
    } else {
        if (len < ZIP_BIG_PREVLEN) {
            p[0] = len;
            return 1;
        } 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'. */
#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'. */
#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. */
int zipPrevLenByteDiff(unsigned char *p, unsigned int len) {
    unsigned int prevlensize;
    ZIP_DECODE_PREVLENSIZE(p, prevlensize);
    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'. */
int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long *v, unsigned char *encoding) {
    long long value;

    if (entrylen >= 32 || entrylen == 0) return 0;
    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. */
        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;
        }
        *v = value;
        return 1;
    }
    return 0;
}

/* Store integer 'value' at 'p', encoded as 'encoding' */
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 = ((uint64_t)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' */
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 below.
 * 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). */
static inline void zipEntry(unsigned char *p, zlentry *e) {
    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);
    assert(e->lensize != 0); /* check that encoding was valid. */
    e->headersize = e->prevrawlensize + e->lensize;
    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 there'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 reach outside the edge of the ziplist */
        if (OUT_OF_RANGE(p + e->headersize + e->len))
            return 0;
        /* Make sure prevlen doesn't reach 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 reach 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 reach outside the edge of the ziplist */
    if (OUT_OF_RANGE(p + e->headersize + e->len))
        return 0;

    /* Make sure prevlen doesn't reach 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));
}

/* 创建一个新的ziplist 
 * zlbytes|zltail|zllen|entry1|entry2|..|entryN|zlend
 * 占用的空间|尾节点偏移|总结点数|entry|255(标识尾部) */
unsigned char *ziplistNew(void) {
    unsigned int bytes = ZIPLIST_HEADER_SIZE+ZIPLIST_END_SIZE;
    unsigned char *zl = zmalloc(bytes);
    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. */
unsigned char *ziplistResize(unsigned char *zl, size_t len) {
    assert(len < UINT32_MAX);
    zl = zrealloc(zl,len);
    ZIPLIST_BYTES(zl) = intrev32ifbe(len);
    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.
 *
 * 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.
 *
 * The pointer "p" points to the first entry that does NOT need to be
 * updated, i.e. consecutive fields MAY need an update. */
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;

    zipEntry(p, &cur); /* no need for "safe" variant since the input pointer was validated by the function that returned it. */
    firstentrylen = prevlen = cur.headersize + cur.len;
    prevlensize = zipStorePrevEntryLength(NULL, prevlen);
    prevoffset = p - zl;
    p += prevlen;

    /* Iterate ziplist to find out how many extra bytes do we need to update it. */
    while (p[0] != ZIP_END) {
        assert(zipEntrySafe(zl, curlen, p, &cur, 0));

        /* Abort when "prevlen" has not changed. */
        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. */
    if (tail == zl + prevoffset) {
        /* When 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;
    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) {
            /* "cur" is the previous head entry, update its prevlen with firstentrylen. */
            zipStorePrevEntryLength(p, firstentrylen);
        } else {
            /* An entry's prevlen can only increment 4 bytes. */
            zipStorePrevEntryLength(p, cur.prevrawlen+delta);
        }
        /* Forward to previous entry. */
        prevoffset -= cur.prevrawlen;
        cnt--;
    }
    return zl;
}

/* Delete "num" entries, starting at "p". Returns pointer to the ziplist. */
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));

    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 = p-first.p; /* 删除元素后减少的内存空间(字节) */
    if (totlen > 0) {
        uint32_t set_tail;
        if (p[0] != ZIP_END) {
            /* 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. */
            nextdiff = zipPrevLenByteDiff(p,first.prevrawlen);

            /* 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;
            assert(p >= first.p && p<zl+zlbytes-1);
            zipStorePrevEntryLength(p,first.prevrawlen);

            /* Update offset for tail */
            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));
            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. */
            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. */
            set_tail = (first.p-zl)-first.prevrawlen;
        }

        /* 更新ziplist大小 */
        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 */
        if (nextdiff != 0)
            zl = __ziplistCascadeUpdate(zl,p);
    }
    return zl;
}

/* 在p位置插入数据 *s. */
unsigned char *__ziplistInsert(unsigned char *zl, unsigned char *p, unsigned char *s, unsigned int slen) {
    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;

    /* 找到前一个节点计算出prevlensize和prevlen */
    if (p[0] != ZIP_END) {
        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
    } else {
        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);
        if (ptail[0] != ZIP_END) {
            prevlen = zipRawEntryLengthSafe(zl, curlen, ptail);
        }
    }

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

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

    /* Store offset because a realloc may change the address of zl. */
    offset = p-zl;
    // 计算出需要的内存容量，然后重新生成一个新大小的zl替换掉原来的zl
    newlen = curlen+reqlen+nextdiff;
    zl = ziplistResize(zl,newlen);
    p = zl+offset;

    /* 迁移数据，然后更新tail的offset */
    if (p[0] != ZIP_END) {
        /* Subtract one because of the ZIP_END bytes */
        memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);

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

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

        /* When the tail contains more than one entry, we need to take
         * "nextdiff" in account as well. Otherwise, a change in the
         * size of prevlen doesn't have an effect on the *tail* offset. */
        assert(zipEntrySafe(zl, newlen, p+reqlen, &tail, 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. */
        ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
    }

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

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

/* 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;

    /* larger values can't be stored into ZIPLIST_BYTES */
    assert(zlbytes < UINT32_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;
}

unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, unsigned int slen, int where) {
    unsigned char *p;
    p = (where == ZIPLIST_HEAD) ? ZIPLIST_ENTRY_HEAD(zl) : ZIPLIST_ENTRY_END(zl);
    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. */
unsigned char *ziplistIndex(unsigned char *zl, int index) {
    unsigned char *p;
    unsigned int prevlensize, prevlen = 0;
    size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
    if (index < 0) {
        index = (-index)-1;
        p = ZIPLIST_ENTRY_TAIL(zl);
        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. */
            ZIP_DECODE_PREVLENSIZE(p, prevlensize);
            assert(p + prevlensize < zl + zlbytes - ZIPLIST_END_SIZE);
            ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            while (prevlen > 0 && index--) {
                p -= prevlen;
                assert(p >= zl + ZIPLIST_HEADER_SIZE && p < zl + zlbytes - ZIPLIST_END_SIZE);
                ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            }
        }
    } else {
        p = ZIPLIST_ENTRY_HEAD(zl);
        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. */
            p += zipRawEntryLengthSafe(zl, zlbytes, p);
            if (p[0] == ZIP_END)
                break;
        }
    }
    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. */
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). */
    if (p[0] == ZIP_END) {
        return NULL;
    }

    p += zipRawEntryLength(p);
    if (p[0] == ZIP_END) {
        return NULL;
    }

    zipAssertValidEntry(zl, zlbytes, p);
    return p;
}

/* Return pointer to previous entry in ziplist. */
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. */
    if (p[0] == ZIP_END) {
        p = ZIPLIST_ENTRY_TAIL(zl);
        return (p[0] == ZIP_END) ? NULL : p;
    } else if (p == ZIPLIST_ENTRY_HEAD(zl)) {
        return NULL;
    } else {
        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
        assert(prevlen > 0);
        p-=prevlen;
        size_t zlbytes = intrev32ifbe(ZIPLIST_BYTES(zl));
        zipAssertValidEntry(zl, zlbytes, p);
        return p;
    }
}

/*  获取“p”指向的entry，并根据entry的编码，把里面的字符串或者数值解析后写入到sstr或者sval中。
 * sstr总是设置为空，以便能够确定是否确定字符串指针或数值指针被设置了值。如果“p”指向ziplist的
 * 结尾，则返回0，否则返回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;

    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)) {
        if (sstr) {
            *slen = entry.len;
            *sstr = p+entry.headersize;
        }
    } else {
        if (sval) {
            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);
        }
    }
    return 1;
}

/* Insert an entry at "p". */
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. */
unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p) {
    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. */
unsigned char *ziplistDeleteRange(unsigned char *zl, int index, unsigned int num) {
    unsigned char *p = ziplistIndex(zl,index);
    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. */
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;

    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)) {
        /* Raw compare */
        if (entry.len == slen) {
            return memcmp(p+entry.headersize,sstr,slen) == 0;
        } else {
            return 0;
        }
    } else {
        /* Try to compare encoded values. Don't compare encoding because
         * different implementations may encoded integers differently. */
        if (zipTryEncoding(sstr,slen,&sval,&sencoding)) {
          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.
 * 在list中查找特定的节点 */
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);
    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 */
            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. */
                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. */
                if (vencoding != UCHAR_MAX) {
                    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 */
        p = q + e.len;
    }

    return NULL;
}

/* 返回ziplist的长度. */
unsigned int ziplistLen(unsigned char *zl) {
    unsigned int len = 0;
    // 如果zllen字段是小于65535，说明zllen是有效的，直接返回zllen，否则需要遍历获取长度
    if (intrev16ifbe(ZIPLIST_LENGTH(zl)) < UINT16_MAX) {
        len = intrev16ifbe(ZIPLIST_LENGTH(zl));
    } 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. */
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 int header_count = intrev16ifbe(ZIPLIST_LENGTH(zl));
    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, header_count, 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 'p' really does point to the end of the ziplist. */
    if (p != zl + bytes - ZIPLIST_END_SIZE)
        return 0;

    /* Make sure the <zltail> entry really do point to the start of the last entry. */
    if (prev != NULL && prev != ZIPLIST_ENTRY_TAIL(zl))
        return 0;

    /* Check that the count in the header is correct */
    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 = picks[0].index, pickindex = 0;
    p = ziplistIndex(zl, zipindex);
    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"
#include "testhelp.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];

    snprintf(buf, sizeof(buf), "100");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    snprintf(buf, sizeof(buf), "128000");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    snprintf(buf, sizeof(buf), "-100");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
    snprintf(buf, sizeof(buf), "4294967296");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_HEAD);
    snprintf(buf, sizeof(buf), "non integer");
    zl = ziplistPush(zl, (unsigned char*)buf, strlen(buf), ZIPLIST_TAIL);
    snprintf(buf,sizeof(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 = 0;

    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> */
int ziplistTest(int argc, char **argv, int flags) {
    int accurate = (flags & REDIS_TEST_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 = snprintf(buf,sizeof(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 = snprintf(buf,sizeof(buf),"%lld",(0LL + rand()) >> 20);
                        break;
                    case 1:
                        buflen = snprintf(buf,sizeof(buf),"%lld",(0LL + rand()));
                        break;
                    case 2:
                        buflen = snprintf(buf,sizeof(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 = snprintf(buf,sizeof(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);
        }

        printf("Benchmark ziplistCompare with string\n");
        {
            unsigned long long start = usec();
            for (int i = 0; i < 2000; i++) {
                unsigned char *eptr = ziplistIndex(zl,0);
                while (eptr != NULL) {
                    ziplistCompare(eptr,(unsigned char*)"nothing",7);
                    eptr = ziplistNext(zl,eptr);
                }
            }
            printf("Done. usec=%lld\n", usec()-start);
        }

        printf("Benchmark ziplistCompare with number\n");
        {
            unsigned long long start = usec();
            for (int i = 0; i < 2000; i++) {
                unsigned char *eptr = ziplistIndex(zl,0);
                while (eptr != NULL) {
                    ziplistCompare(eptr,(unsigned char*)"99999",5);
                    eptr = ziplistNext(zl,eptr);
                }
            }
            printf("Done. usec=%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);
    }

    printf("__ziplistInsert nextdiff == -4 && reqlen < 4 (issue #7170):\n");
    {
        zl = ziplistNew();

        /* We set some values to almost reach the critical point - 254 */
        char A_252[253] = {0}, A_250[251] = {0};
        memset(A_252, 'A', 252);
        memset(A_250, 'A', 250);

        /* After the rpush, the list look like: [one two A_252 A_250 three 10] */
        zl = ziplistPush(zl, (unsigned char*)"one", 3, ZIPLIST_TAIL);
        zl = ziplistPush(zl, (unsigned char*)"two", 3, ZIPLIST_TAIL);
        zl = ziplistPush(zl, (unsigned char*)A_252, strlen(A_252), ZIPLIST_TAIL);
        zl = ziplistPush(zl, (unsigned char*)A_250, strlen(A_250), ZIPLIST_TAIL);
        zl = ziplistPush(zl, (unsigned char*)"three", 5, ZIPLIST_TAIL);
        zl = ziplistPush(zl, (unsigned char*)"10", 2, ZIPLIST_TAIL);
        ziplistRepr(zl);

        p = ziplistIndex(zl, 2);
        if (!ziplistCompare(p, (unsigned char*)A_252, strlen(A_252))) {
            printf("ERROR: not \"A_252\"\n");
            return 1;
        }

        /* When we remove A_252, the list became: [one two A_250 three 10]
         * A_250's prev node became node two, because node two quite small
         * So A_250's prevlenSize shrink to 1, A_250's total size became 253(1+2+250)
         * The prev node of node three is still node A_250.
         * We will not shrink the node three's prevlenSize, keep it at 5 bytes */
        zl = ziplistDelete(zl, &p);
        ziplistRepr(zl);

        p = ziplistIndex(zl, 3);
        if (!ziplistCompare(p, (unsigned char*)"three", 5)) {
            printf("ERROR: not \"three\"\n");
            return 1;
        }

        /* We want to insert a node after A_250, the list became: [one two A_250 10 three 10]
         * Because the new node is quite small, node three prevlenSize will shrink to 1 */
        zl = ziplistInsert(zl, p, (unsigned char*)"10", 2);
        ziplistRepr(zl);

        /* Last element should equal 10 */
        p = ziplistIndex(zl, -1);
        if (!ziplistCompare(p, (unsigned char*)"10", 2)) {
            printf("ERROR: not \"10\"\n");
            return 1;
        }

        zfree(zl);
    }

    printf("ALL TESTS PASSED!\n");
    return 0;
}
#endif