Redis源代码分析之ziplist

定义

ziplist.c的头部给出了ziplist的定义

/* ZIPLIST OVERALL LAYOUT:
 * The general layout of the ziplist is as follows:
 * <zlbytes><zltail><zllen><entry><entry><zlend>
 *
 * <zlbytes> is an unsigned integer to hold the number of bytes that the
 * ziplist occupies. This value needs to be stored to be able to resize the
 * entire structure without the need to traverse it first.
 *
 * <zltail> is the offset to the last entry in the list. This allows a pop
 * operation on the far side of the list without the need for full traversal.
 *
 * <zllen> is the number of entries.When this value is larger than 2**16-2,
 * we need to traverse the entire list to know how many items it holds.
 *
 * <zlend> is a single byte special value, equal to 255, which indicates the
 * end of the list.
 *
 * ZIPLIST ENTRIES:
 * Every entry in the ziplist is prefixed by a header that contains two pieces
 * of information. First, the length of the previous entry is stored to be
 * able to traverse the list from back to front. Second, the encoding with an
 * optional string length of the entry itself is stored.
 *
 * The length of the previous entry is encoded in the following way:
 * If this length is smaller than 254 bytes, it will only consume a single
 * byte that takes the length as value. When the length is greater than or
 * equal to 254, it will consume 5 bytes. The first byte is set to 254 to
 * indicate a larger value is following. The remaining 4 bytes take the
 * length of the previous entry as value.
 *
 * The other header field of the entry itself depends on the contents of the
 * entry. When the entry is a string, the first 2 bits of this header will hold
 * the type of encoding used to store the length of the string, followed by the
 * actual length of the string. When the entry is an integer the first 2 bits
 * are both set to 1. The following 2 bits are used to specify what kind of
 * integer will be stored after this header. An overview of the different
 * types and encodings is as follows:
 *
 * |00pppppp| - 1 byte
 *      String value with length less than or equal to 63 bytes (6 bits).
 * |01pppppp|qqqqqqqq| - 2 bytes
 *      String value with length less than or equal to 16383 bytes (14 bits).
 * |10______|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
 *      String value with length greater than or equal to 16384 bytes.
 * |11000000| - 1 byte
 *      Integer encoded as int16_t (2 bytes).
 * |11010000| - 1 byte
 *      Integer encoded as int32_t (4 bytes).
 * |11100000| - 1 byte
 *      Integer encoded as int64_t (8 bytes).
 * |11110000| - 1 byte
 *      Integer encoded as 24 bit signed (3 bytes).
 * |11111110| - 1 byte
 *      Integer encoded as 8 bit signed (1 byte).
 * |1111xxxx| - (with xxxx between 0000 and 1101) immediate 4 bit integer.
 *      Unsigned integer from 0 to 12. The encoded value is actually from
 *      1 to 13 because 0000 and 1111 can not be used, so 1 should be
 *      subtracted from the encoded 4 bit value to obtain the right value.
 * |11111111| - End of ziplist.
 *
 * All the integers are represented in little endian byte order.
 */

由上面的的定义可以看出，ziplist大概看起来就是下面这样子

• 其中zllen为4个字节，保存ziplist字节数；
• zltail保存的是尾节点偏移量，4个字节；
• zllen保存的是ziplist的长度，2个字节，ziplist的长度超出范围，只能通过遍历来知道ziplist的长度；

• 接着是一个个的entry，这是ziplist的节点，entry包含三个字段，第一个是prevlen，记录前一个节点的长度，方便逆向遍历，长度为1个字节或者5个字节，第二个是encoding，里面记录了data段的长度，1个字节到5个字节，最后的一个字段是data段，记录数据；

• 最后是zlend，结束标志，值为255

源代码分析

#define ZIP_END 255				//ziplist结尾zlend
#define ZIP_BIGLEN 254			//zllen如果超过ZIP_BIGLEN的话，将会使用5个字节存储，不能使255，否则就跟zlend重复了

/* Different encoding/length possibilities */
#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 */
#define ZIP_INT_IMM_MASK 0x0f
#define ZIP_INT_IMM_MIN 0xf1    /* 11110001 */
#define ZIP_INT_IMM_MAX 0xfd    /* 11111101 */
#define ZIP_INT_IMM_VAL(v) (v & ZIP_INT_IMM_MASK)

#define INT24_MAX 0x7fffff
#define INT24_MIN (-INT24_MAX - 1)

/* Macro to determine type */
#define ZIP_IS_STR(enc) (((enc) & ZIP_STR_MASK) < ZIP_STR_MASK)

/* Utility macros */
#define ZIPLIST_BYTES(zl)       (*((uint32_t*)(zl)))  //返回ziplist字节数
#define ZIPLIST_TAIL_OFFSET(zl) (*((uint32_t*)((zl)+sizeof(uint32_t))))	//返回尾节点偏移量
#define ZIPLIST_LENGTH(zl)      (*((uint16_t*)((zl)+sizeof(uint32_t)*2))) //返回ziplist的长度
#define ZIPLIST_HEADER_SIZE     (sizeof(uint32_t)*2+sizeof(uint16_t)) //返回ziplist的头部长度，即zlbytes><zltail><zllen>的长度
#define ZIPLIST_END_SIZE        (sizeof(uint8_t))	//zlend的长度
#define ZIPLIST_ENTRY_HEAD(zl)  ((zl)+ZIPLIST_HEADER_SIZE) //ziplist节点的开始位置
#define ZIPLIST_ENTRY_TAIL(zl)  ((zl)+intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))) //ziplist为尾部节点
#define ZIPLIST_ENTRY_END(zl)   ((zl)+intrev32ifbe(ZIPLIST_BYTES(zl))-1) //ziplist结束符zlend


//增加ziplist的长度
#define ZIPLIST_INCR_LENGTH(zl,incr) { \
	//如果zllen的长度小于2**16 -  1，那么zllen = zllen + 1，否则不加1，这样的话求ziplist长度时候需要遍历整个ziplist才能知道zllen
    if (ZIPLIST_LENGTH(zl) < UINT16_MAX) \
        ZIPLIST_LENGTH(zl) = intrev16ifbe(intrev16ifbe(ZIPLIST_LENGTH(zl))+incr); \
}

//ziplist节点结构体（注：并非用来存储ziplist的原生数据）
typedef struct zlentry {
    unsigned int prevrawlensize, prevrawlen;	//编码前一个节点的长度所需要的字节数，前一个节点的长度
    unsigned int lensize, len; 	//编码本节点长度所需要的字节数，本字节的长度
    unsigned int headersize; 	//ziplist的头长度，包括zlbytes,zltail,zllen
    unsigned char encoding;		//数据编码
    unsigned char *p;			//节点数据
} zlentry;

//初始化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'. */
#define ZIP_ENTRY_ENCODING(ptr, encoding) do {  \
    (encoding) = (ptr[0]); \
    if ((encoding) < ZIP_STR_MASK) (encoding) &= ZIP_STR_MASK; \
} while(0)

void ziplistRepr(unsigned char *zl);

//根据encoding来判断编码整数数据所需要的长度
static unsigned int zipIntSize(unsigned char encoding) {
    switch(encoding) {
    case ZIP_INT_8B:  return 1;		//1个字节的整数
    case ZIP_INT_16B: return 2;		//2
    case ZIP_INT_24B: return 3;		//...
    case ZIP_INT_32B: return 4;
    case ZIP_INT_64B: return 8;
    default: return 0; /* 4 bit immediate */
    }
    assert(NULL);
    return 0;
}

//给p写入encoding数据（字符串和整数分情况），如果p为空，则直接返回存储encoding所需要的长度
static unsigned int zipEncodeLength(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 {
        //如果是整数直接返回1（整数encoding只需要一个字节来保存）
        if (!p) return len;
        buf[0] = encoding;
    }

    /* Store this length at p */
    memcpy(p,buf,len);
    return len;
}

//解码ptr位置的encoding，把编码长度赋值给lensize，把data段长度赋值给len
#define ZIP_DECODE_LENGTH(ptr, encoding, lensize, len) do {                    \
    ZIP_ENTRY_ENCODING((ptr), (encoding));                                     \
    if ((encoding) < ZIP_STR_MASK) {                                           \
        if ((encoding) == ZIP_STR_06B) {                                       \
            (lensize) = 1;                                                     \
            (len) = (ptr)[0] & 0x3f;                                           \
        } else if ((encoding) == ZIP_STR_14B) {                                \
            (lensize) = 2;                                                     \
            (len) = (((ptr)[0] & 0x3f) << 8) | (ptr)[1];                       \
        } else if (encoding == ZIP_STR_32B) {                                  \
            (lensize) = 5;                                                     \
            (len) = ((ptr)[1] << 24) |                                         \
                    ((ptr)[2] << 16) |                                         \
                    ((ptr)[3] <<  8) |                                         \
                    ((ptr)[4]);                                                \
        } else {                                                               \
            assert(NULL);                                                      \
        }                                                                      \
    } else {                                                                   \
        (lensize) = 1;                                                         \
        (len) = zipIntSize(encoding);                                          \
    }                                                                          \
} while(0);

//根据前一个节点的长度len在指针p的位置编码prevlen，如果p为NULL，则返回编码该prevlen所需要的长度
static unsigned int zipPrevEncodeLength(unsigned char *p, unsigned int len) {
    if (p == NULL) {
        //如果len小于254，则用一个字节编码即可，如果不是则要5个字节
        return (len < ZIP_BIGLEN) ? 1 : sizeof(len)+1;
    } else {
        if (len < ZIP_BIGLEN) {
            //写数据
            p[0] = len;
            return 1;
        } else {
            p[0] = ZIP_BIGLEN;
            memcpy(p+1,&len,sizeof(len));
            memrev32ifbe(p+1);
            return 1+sizeof(len);
        }
    }
}

//prevlen的编码，用于prevlen如果大于254的情况下
static void zipPrevEncodeLengthForceLarge(unsigned char *p, unsigned int len) {
    if (p == NULL) return;
    //前一个字节赋值为254，后面的四个字节保存len
    p[0] = ZIP_BIGLEN;
    memcpy(p+1,&len,sizeof(len));
    memrev32ifbe(p+1);
}

//返回在ptr位置的prevlen的编码长度
#define ZIP_DECODE_PREVLENSIZE(ptr, prevlensize) do {                          \
    if ((ptr)[0] < ZIP_BIGLEN) {                                               \
        (prevlensize) = 1;                                                     \
    } else {                                                                   \
        (prevlensize) = 5;                                                     \
    }                                                                          \
} while(0);

//解码在ptr位置的prevlen，把它的值保存在prevlen变量中
#define ZIP_DECODE_PREVLEN(ptr, prevlensize, prevlen) do {                     \
    //先获取编码长度
    ZIP_DECODE_PREVLENSIZE(ptr, prevlensize);                                  \
    if ((prevlensize) == 1) {                                                  \
        (prevlen) = (ptr)[0];                                                  \
    } else if ((prevlensize) == 5) {                                           \
        assert(sizeof((prevlensize)) == 4);                                    \
        memcpy(&(prevlen), ((char*)(ptr)) + 1, 4);                             \
        memrev32ifbe(&prevlen);                                                \
    }                                                                          \
} while(0);

//计算新的prevlen与旧的prevlen相差的字节数
static int zipPrevLenByteDiff(unsigned char *p, unsigned int len) {
    unsigned int prevlensize;
    //先获取旧的prevlen的编码长度
    ZIP_DECODE_PREVLENSIZE(p, prevlensize);
    //用现在需要的长度减去旧的prevlen的编码长度，算出diff
    return zipPrevEncodeLength(NULL, len) - prevlensize;
}

//算出整个节点的长度
static unsigned int zipRawEntryLength(unsigned char *p) {
    unsigned int prevlensize, encoding, lensize, len;
    //先获取prevlen的编码长度
    ZIP_DECODE_PREVLENSIZE(p, prevlensize);
    //在获取encoding的编码长度lensize，和解码出data段的长度，len
    ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
    //加起来就是一个节点的总长度，
    return prevlensize + lensize + len;
}

//尝试把字符串entry转换成整形，存在v中
static int zipTryEncoding(unsigned char *entry, unsigned int entrylen, long long *v, unsigned char *encoding) {
    long long value;

    //字符串长度超过32位或者字符串长度为0的时候，直接返回0
    if (entrylen >= 32 || entrylen == 0) return 0;
    //如果可以转换的话
    if (string2ll((char*)entry,entrylen,&value)) {
        //判断转换出来的整数处于什么范围，选择最短的编码长度
        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;
}

//保存值在p位置，以encoding的格式保存
static void zipSaveInteger(unsigned char *p, int64_t value, unsigned char encoding) {
    int16_t i16;
    int32_t i32;
    int64_t i64;
    //判断encoding的类型，根据类型来写入value值在data段
    if (encoding == ZIP_INT_8B) {
        ((int8_t*)p)[0] = (int8_t)value;
    } else if (encoding == ZIP_INT_16B) {
        i16 = value;
        memcpy(p,&i16,sizeof(i16));
        memrev16ifbe(p);
    } else if (encoding == ZIP_INT_24B) {
        i32 = value<<8;
        memrev32ifbe(&i32);
        memcpy(p,((uint8_t*)&i32)+1,sizeof(i32)-sizeof(uint8_t));
    } else if (encoding == ZIP_INT_32B) {
        i32 = value;
        memcpy(p,&i32,sizeof(i32));
        memrev32ifbe(p);
    } else if (encoding == ZIP_INT_64B) {
        i64 = value;
        memcpy(p,&i64,sizeof(i64));
        memrev64ifbe(p);
    } else if (encoding >= ZIP_INT_IMM_MIN && encoding <= ZIP_INT_IMM_MAX) {
        /* Nothing to do, the value is stored in the encoding itself. */
    } else {
        assert(NULL);
    }
}

//根据encoding来读取Integer，原理同上，注释略
static 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;
}

//解码节点信息，保存在结构体e中
static void zipEntry(unsigned char *p, zlentry *e) {
    //获取prevlen，以及其编码长度
    ZIP_DECODE_PREVLEN(p, e->prevrawlensize, e->prevrawlen);
    //获取encoding和其编码长度，和data段的长度len
    ZIP_DECODE_LENGTH(p + e->prevrawlensize, e->encoding, e->lensize, e->len);
    //计算节点“头”的长度，即prevlen的编码长度加上encoding的编码长度
    e->headersize = e->prevrawlensize + e->lensize;
    e->p = p;
}

//创建一个新的ziplist
unsigned char *ziplistNew(void) {
    unsigned int bytes = ZIPLIST_HEADER_SIZE+1;
    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;
}

//从新分配zl的大小
static unsigned char *ziplistResize(unsigned char *zl, unsigned int len) {
    zl = zrealloc(zl,len);
    ZIPLIST_BYTES(zl) = intrev32ifbe(len);
    zl[len-1] = ZIP_END;
    return zl;
}

//连锁更新函数，因为当插入一个新的节点时候，新节点的后一个元素的prevlen要重新编码，由此
//新节点的后后个元素就又要更新prevlen了...由此导致一系列的连锁更新
static unsigned char *__ziplistCascadeUpdate(unsigned char *zl, unsigned char *p) {
    size_t curlen = intrev32ifbe(ZIPLIST_BYTES(zl)), rawlen, rawlensize;
    size_t offset, noffset, extra;
    unsigned char *np;
    zlentry cur, next;

    //如果p还没到达尾部的话
    while (p[0] != ZIP_END) {
        //解码p位置上的节点，把相关信息保存在cur结构体中
        zipEntry(p, &cur);
        //整个节点的大小
        rawlen = cur.headersize + cur.len;
        //编码这个节点的大小所需要的字节数
        rawlensize = zipPrevEncodeLength(NULL,rawlen);

        //如果p位置所在的节点后面是zlend，也就是后面没有节点了，跳出
        if (p[rawlen] == ZIP_END) break;
        //把下一个节点的信息读取到next结构体中
        zipEntry(p+rawlen, &next);

        //如果下一个节点的prevlen编码长度与编码rawlen所需要的长度一样，跳出
        if (next.prevrawlen == rawlen) break;

        //如果下一个节点的prevlen编码长度不够
        if (next.prevrawlensize < rawlensize) {
           
            offset = p-zl;
            //计算需要的额外空间extra
            extra = rawlensize-next.prevrawlensize;
            //重新分配
            zl = ziplistResize(zl,curlen+extra);
            p = zl+offset;

            //np为下一个节点的位置
            np = p+rawlen;
            //noffset为下一个节点的偏移量
            noffset = np-zl;

            //如果np不是尾节点，则更新zltail，如果np是尾节点，则不用更新，因为np的位置相对之前没有变
            if ((zl+intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))) != np) {
                ZIPLIST_TAIL_OFFSET(zl) =
                    intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+extra);
            }

            //内存移动
            memmove(np+rawlensize,
                np+next.prevrawlensize,
                curlen-noffset-next.prevrawlensize-1);
            //把rawlen编码进np的prevlen字段中
            zipPrevEncodeLength(np,rawlen);

            //跟新p和curlen
            p += rawlen;
            curlen += extra;
        } else {
            //如果下一个节点的prevlen的编码长度比rawlensize要大
            if (next.prevrawlensize > rawlensize) {
                //这说明要缩小np的prevlen编码长度，但是我们采用懒惰策略，暂时不缩小prelen的编码长度，而是把rawlen先写入np的prevlen中
                zipPrevEncodeLengthForceLarge(p+rawlen,rawlen);
            } else {
                //如果长度刚刚好，则编码rawlen到np的prevlen字段中
                zipPrevEncodeLength(p+rawlen,rawlen);
            }

            /* Stop here, as the raw length of "next" has not changed. */
            break;
        }
    }
    return zl;
}

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

    zipEntry(p, &first);
    for (i = 0; p[0] != ZIP_END && i < num; i++) {
        p += zipRawEntryLength(p);
        deleted++;
    }

    totlen = p-first.p;
    if (totlen > 0) {
        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);
            p -= nextdiff;
            zipPrevEncodeLength(p,first.prevrawlen);

            /* Update offset for tail */
            ZIPLIST_TAIL_OFFSET(zl) =
                intrev32ifbe(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. */
            zipEntry(p, &tail);
            if (p[tail.headersize+tail.len] != ZIP_END) {
                ZIPLIST_TAIL_OFFSET(zl) =
                   intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
            }

            /* Move tail to the front of the ziplist */
            memmove(first.p,p,
                intrev32ifbe(ZIPLIST_BYTES(zl))-(p-zl)-1);
        } else {
            /* The entire tail was deleted. No need to move memory. */
            ZIPLIST_TAIL_OFFSET(zl) =
                intrev32ifbe((first.p-zl)-first.prevrawlen);
        }

        /* Resize and update length */
        offset = first.p-zl;
        zl = ziplistResize(zl, intrev32ifbe(ZIPLIST_BYTES(zl))-totlen+nextdiff);
        ZIPLIST_INCR_LENGTH(zl,-deleted);
        p = zl+offset;

        /* 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;
}

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

    //判断位置p是不是zlend
    if (p[0] != ZIP_END) {
    	//把位置p中所在的节点的[编码前一个节点长度所需的字节数]和[前一个节点的长度]计算出来
        ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
    } else {
    	//如果位置p是zlend，
        unsigned char *ptail = ZIPLIST_ENTRY_TAIL(zl);	//返回尾节点
        //如果尾节点不为zlend的话，也就是ziplist不为空时
        if (ptail[0] != ZIP_END) {
        	//把尾部节点的长度计算出来
            prevlen = zipRawEntryLength(ptail);
        }
    }

    //判断字符串数据是否可以用整数来表示，如果可以，则用整数类型表示，以节省空间
    if (zipTryEncoding(s,slen,&value,&encoding)) {
        //获取存储该整数数据所需要的字节数
        reqlen = zipIntSize(encoding);
    } else {
        //如果不能转化为整数数据，直接把字符串长度付给reqlen
        reqlen = slen;
    }
    
    //reqlen加上编码prevlen需要的字节数
    reqlen += zipPrevEncodeLength(NULL,prevlen);
    //reqlen加上编码len需要的字节数
    reqlen += zipEncodeLength(NULL,encoding,slen);

    //因为加入新节点之后，新节点的后一个节点的prevlen字段先前保存的时候新节点的前一个节点的长度
    //现在保存的是新节点的长度，所以长度可能有所改变，长度改变用nextdiff来计算
    nextdiff = (p[0] != ZIP_END) ? zipPrevLenByteDiff(p,reqlen) : 0;

    //对存储ziplist的内存进行realloc，因为zl的地址可能会改变，所以要先保存好p到zl的offset
    offset = p-zl;
    zl = ziplistResize(zl,curlen+reqlen+nextdiff);
    p = zl+offset;		//恢复p

    //如果插入位置不在ziplist的尾部
    if (p[0] != ZIP_END) {
        //数据后移，为新节点腾出空间
        memmove(p+reqlen,p-nextdiff,curlen-offset-1+nextdiff);

        //更新新节点的后一个节点的prevlen
        zipPrevEncodeLength(p+reqlen,reqlen);

        //更新tail位置，看到这里大家可能有点懵逼，这里为什么不用加上nextdiff的长度的呢，别急，继续往下看
        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. */
        //把新节点后一个节点的信息提取到*tail结构体中
        zipEntry(p+reqlen, &tail);
        //如果新节点后一个节点不是尾节点，那么就要加上nextdiff（解答了上面的疑惑）
        if (p[reqlen+tail.headersize+tail.len] != ZIP_END) {
            ZIPLIST_TAIL_OFFSET(zl) =
                intrev32ifbe(intrev32ifbe(ZIPLIST_TAIL_OFFSET(zl))+nextdiff);
        }
    } else {
    	//如果新节点就是ziplist的为节点，那么就更新zltail
        ZIPLIST_TAIL_OFFSET(zl) = intrev32ifbe(p-zl);
    }

    //如果nextdiff不为0，那么就执行连锁更新
    if (nextdiff != 0) {
        offset = p-zl;
        zl = __ziplistCascadeUpdate(zl,p+reqlen);
        p = zl+offset;
    }

    //把prelen写入新节点中
    p += zipPrevEncodeLength(p,prevlen);
    //把len(encoding)写入新节点中
    p += zipEncodeLength(p,encoding,slen);
    //如果数据是字符串，那么宝贝数据到data段
    if (ZIP_IS_STR(encoding)) {
        memcpy(p,s,slen);
    } else {
    	//如果是整数就整数
        zipSaveInteger(p,value,encoding);
    }
    //给ziplist的长度加1
    ZIPLIST_INCR_LENGTH(zl,1);
    return zl;
}


 //合并两个压缩表first和second，两个中比较大的一个压缩表会被realloc以足以存储合并后的结果，另一个会被释放掉
unsigned char *ziplistMerge(unsigned char **first, unsigned char **second) {
    //任何一个参数为NULL的话，返回NULL
    if (first == NULL || *first == NULL || second == NULL || *second == NULL)
        return NULL;

    //如果这两个ziplist是同一个ziplist的话，返回NULL
    if (*first == *second)
        return NULL;

    //第一个ziplist的字节数
    size_t first_bytes = intrev32ifbe(ZIPLIST_BYTES(*first));
    //第一个ziplist的节点数
    size_t first_len = intrev16ifbe(ZIPLIST_LENGTH(*first));

    //第二个ziplist的字节数
    size_t second_bytes = intrev32ifbe(ZIPLIST_BYTES(*second));
    //第二个ziplist的节点数
    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. */
    //如果第一个ziplist的节点数大于第二个ziplist的话
    if (first_len >= second_len) {
        //那么就把second合并到first
        target = *first;
        target_bytes = first_bytes;
        source = *second;
        source_bytes = second_bytes;
        append = 1;
    } else {
        //否则，把first合并到second
        target = *second;
        target_bytes = second_bytes;
        source = *first;
        source_bytes = first_bytes;
        append = 0;
    }

    //计算合并后的字节数
    size_t zlbytes = first_bytes + second_bytes -
                     ZIPLIST_HEADER_SIZE - ZIPLIST_END_SIZE;
    //计算合并后的节点数
    size_t zllength = first_len + second_len;

    //zllength应该小于2**16 - 1，否则把2**16 - 1赋给zllength
    zllength = zllength < UINT16_MAX ? zllength : UINT16_MAX;

    //分别求出firs和second的尾节点偏移
    size_t first_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*first));
    size_t second_offset = intrev32ifbe(ZIPLIST_TAIL_OFFSET(*second));

    //给target分配空间（两个ziplist合起来的空间）
    target = zrealloc(target, zlbytes);
    if (append) {
        //如果把second(source)加到first(target)
        //把second拷贝到first后
        memcpy(target + target_bytes - ZIPLIST_END_SIZE,
               source + ZIPLIST_HEADER_SIZE,
               source_bytes - ZIPLIST_HEADER_SIZE);
    } else {
        //如果把first(source)加到second(target)
        //先把second后移，腾出空间给first
        memmove(target + source_bytes - ZIPLIST_END_SIZE,
                target + ZIPLIST_HEADER_SIZE,
                target_bytes - ZIPLIST_HEADER_SIZE);
        //把first拷贝到腾出的空间里面
        memcpy(target, source, source_bytes - ZIPLIST_END_SIZE);
    }

    //更新ziplist的zlbytes
    ZIPLIST_BYTES(target) = intrev32ifbe(zlbytes);
    //更新ziplist的zllen
    ZIPLIST_LENGTH(target) = intrev16ifbe(zllength);

    //更新尾节点偏移
    ZIPLIST_TAIL_OFFSET(target) = intrev32ifbe(
                                   (first_bytes - ZIPLIST_END_SIZE) +
                                   (second_offset - ZIPLIST_HEADER_SIZE));

    //连锁更新
    target = __ziplistCascadeUpdate(target, target+first_offset);

    //释放资源
    if (append) {
        zfree(*second);
        *second = NULL;
        *first = target;
    } else {
        zfree(*first);
        *first = NULL;
        *second = target;
    }
    return target;
}

//把s加到ziplist开始或者末尾
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);
}

//返回index位置上的节点，如果不存在返回NULL, 如果index未负数，从尾节点开始向前遍历
unsigned char *ziplistIndex(unsigned char *zl, int index) {
    unsigned char *p;
    unsigned int prevlensize, prevlen = 0;
    //如果为负数，从后往前遍历
    if (index < 0) {
        index = (-index)-1;
        p = ZIPLIST_ENTRY_TAIL(zl);
        if (p[0] != ZIP_END) {
            ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            while (prevlen > 0 && index--) {
                p -= prevlen;
                ZIP_DECODE_PREVLEN(p, prevlensize, prevlen);
            }
        }
    } else {
        p = ZIPLIST_ENTRY_HEAD(zl);
        while (p[0] != ZIP_END && index--) {
            p += zipRawEntryLength(p);
        }
    }
    return (p[0] == ZIP_END || index > 0) ? NULL : p;
}

//返回在p位置的节点的下一个节点
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p) {
    ((void) 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;
    }

    return p;
}

//返回上一个节点
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);
        return p-prevlen;
    }
}

//获取p位置上的值，如果是字符串，则存于sstr中，如果是整数，则存于sval中
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;

    //提取节点信息
    zipEntry(p, &entry);
    //如果是字符串
    if (ZIP_IS_STR(entry.encoding)) {
        if (sstr) {
            *slen = entry.len;
            *sstr = p+entry.headersize;
        }
    } else {
        if (sval) {
            //如果是整数直接根据enconding提取p+entry.headersize上的数据存在sval中
            *sval = zipLoadInteger(p+entry.headersize,entry.encoding);
        }
    }
    return 1;
}

//包装函数，在p位置插入字符窜s，slen为s的长度
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;
}

//删除指定位置上的节点
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);
}

//比较p位置的节点与sstr的数据是否相等
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;

    //提取当前节点的信息到entry结构体
    zipEntry(p, &entry);
    if (ZIP_IS_STR(entry.encoding)) {
        //如果是当前节点的数据段是字符串的话，进行字符串比较
        if (entry.len == slen) {
            return memcmp(p+entry.headersize,sstr,slen) == 0;
        } else {
            return 0;
        }
    } else {
       	//如果不是字符串，先尝试把sstr转换成整数，然后再对其进行比较
        if (zipTryEncoding(sstr,slen,&sval,&sencoding)) {
          zval = zipLoadInteger(p+entry.headersize,entry.encoding);
          return zval == sval;
        }
    }
    return 0;
}

//给出vstr，查找与之相等的节点，skip指定每隔skip个节点比较一次，如果没有找到，返回NULL
unsigned char *ziplistFind(unsigned char *p, unsigned char *vstr, unsigned int vlen, unsigned int skip) {
    int skipcnt = 0;
    unsigned char vencoding = 0;
    long long vll = 0;

    //如果没达到zlend
    while (p[0] != ZIP_END) {
        unsigned int prevlensize, encoding, lensize, len;
        unsigned char *q;

        //解码出当前节点的prevlensize
        ZIP_DECODE_PREVLENSIZE(p, prevlensize);
        //解码出len和lensize
        ZIP_DECODE_LENGTH(p + prevlensize, encoding, lensize, len);
        //求出数据段的开始地址q
        q = p + prevlensize + lensize;

        if (skipcnt == 0) {
            //比较当前节点的数据是否跟给出的vstr相同，如果相同则返回当前节点的开始地址
            //如果是字符串数据
            if (ZIP_IS_STR(encoding)) {
                if (len == vlen && memcmp(q, vstr, vlen) == 0) {
                    return p;
                }
            } else {
                //接下来是编码输入数据vstr，当vencoding为0的时候，说明其还没有被编码，下面的代码对它进行编码
                if (vencoding == 0) {
                    if (!zipTryEncoding(vstr, vlen, &vll, &vencoding)) {
                        //如果不能被转化成整形数据，那么把vencoding设成255，这样的话下次就不会再对其进行编码了
                        vencoding = UCHAR_MAX;
                    }
                    /* Must be non-zero by now */
                    assert(vencoding);
                }

                //如果vencoding不是UCHAR_MAX，那么说明vstr已经被转换成整形数据
                if (vencoding != UCHAR_MAX) {
                    long long ll = zipLoadInteger(q, encoding);
        			//ll等于vll的话，那就返回当前节点的开始指针
                    if (ll == vll) {
                        return p;
                    }
                }
            }

            /* Reset skip count */
            skipcnt = skip;
        } else {
            /* Skip entry */
            skipcnt--;
        }

        //更新p，移动到下一个节点
        p = q + len;

    }

    return NULL;
}

//返回ziplist的长度
unsigned int ziplistLen(unsigned char *zl) {
    unsigned int len = 0;
    //如果zllen字段的长度小于UINT16_MAX(2**16 - 1)，那么直接返回
    if (intrev16ifbe(ZIPLIST_LENGTH(zl)) < UINT16_MAX) {
        len = intrev16ifbe(ZIPLIST_LENGTH(zl));
    } else {
    	//否则，需要遍历整个ziplist，然后返回其长度，整个过程很低效
        unsigned char *p = zl+ZIPLIST_HEADER_SIZE;
        while (*p != ZIP_END) {
            p += zipRawEntryLength(p);
            len++;
        }

        //如果遍历之后发现长度小于UINT16_MAX的话，重新更改zllen字段，改成实际长度，避免下次再次遍历
        if (len < UINT16_MAX) ZIPLIST_LENGTH(zl) = intrev16ifbe(len);
    }
    return len;
}

//返回ziplist的字节数
size_t ziplistBlobLen(unsigned char *zl) {
    return intrev32ifbe(ZIPLIST_BYTES(zl));
}

//列出ziplist的基本信息，注释略
void ziplistRepr(unsigned char *zl) {
    unsigned char *p;
    int index = 0;
    zlentry entry;

    printf(
        "{total bytes %d} "
        "{length %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) {
        zipEntry(p, &entry);
        printf(
            "{"
                "addr 0x%08lx, "
                "index %2d, "
                "offset %5ld, "
                "rl: %5u, "
                "hs %2u, "
                "pl: %5u, "
                "pls: %2u, "
                "payload %5u"
            "} ",
            (long unsigned)p,
            index,
            (unsigned long) (p-zl),
            entry.headersize+entry.len,
            entry.headersize,
            entry.prevrawlen,
            entry.prevrawlensize,
            entry.len);
        p += entry.headersize;
        if (ZIP_IS_STR(entry.encoding)) {
            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("%lld", (long long) zipLoadInteger(p,entry.encoding));
        }
        printf("\n");
        p += entry.len;
        index++;
    }
    printf("{end}\n\n");
}

总结

时间问题只写了几个核心函数的注释和一些小函数的注释，不过读懂了那几个函数其它函数其实也就大同小异，先出门喝杯咖啡散散步，什么时候有空补上没有写完的注释。

## 2016/11/22 已经更新完所有注释 ##

能力有限，可能会有错误，欢迎指出，大家一起交流学习。