Redis Ziplist压缩列表的实现

+-----------+-----------+-----------+--------+--------+-----+--------+
|  zlbytes  |  zltail   |   zllen   | entry1 | entry2 | ... |  zlend |
| (4 bytes) | (4 bytes) | (2 bytes) |        |        |     | 1 byte |
+-----------+-----------+-----------+--------+--------+-----+--------+
       ^
       所有字段都是小端序（Little Endian）

[11 00 00 00] [0B 00 00 00] [00 00] [FF]
    |              |            |      |
 zlbytes=11     zltail=11    zllen=0  zlend

<prevlen> <encoding> <entry-data>

// 编码前一条目长度
if (len < 254) {
    p[0] = len;              // 1 byte
} else {
    p[0] = 0xFE;             // 标记
    memcpy(p+1, &len, 4);    // 4 bytes
}

[0f 00 00 00] [0c 00 00 00] [02 00] [00 f3] [02 f6] [ff]
      |              |            |       |       |      |
   zlbytes=15     zltail=12    zllen=2   "2"    "5"    end

[02] [0b] [48 65 6c 6c 6f 20 57 6f 72 6c 64]
 |    |    |
prevlen=2  len=11  "Hello World" (11 bytes)

// 创建空 ziplist
unsigned char *ziplistNew(void);
// 释放 ziplist
void zfree(unsigned char *zl);  // 直接用 zfree

// 在头部或尾部插入
unsigned char *ziplistPush(unsigned char *zl, unsigned char *s, 
                           unsigned int slen, int where);
// where: ZIPLIST_HEAD=0, ZIPLIST_TAIL=1

// 在指定位置插入
unsigned char *ziplistInsert(unsigned char *zl, unsigned char *p,
                             unsigned char *s, unsigned int slen);

// 删除指定位置的 entry
unsigned char *ziplistDelete(unsigned char *zl, unsigned char **p);

// 删除范围内的 entry
unsigned char *ziplistDeleteRange(unsigned char *zl, int index, unsigned int num);

// 替换指定位置的 entry
unsigned char *ziplistReplace(unsigned char *zl, unsigned char *p,
                              unsigned char *s, unsigned int slen);

// 按索引获取 entry（支持负数索引）
unsigned char *ziplistIndex(unsigned char *zl, int index);

// 获取下一个 entry
unsigned char *ziplistNext(unsigned char *zl, unsigned char *p);

// 获取上一个 entry
unsigned char *ziplistPrev(unsigned char *zl, unsigned char *p);

// 获取 entry 的值
unsigned int ziplistGet(unsigned char *p, unsigned char **sval, 
                        unsigned int *slen, long long *lval);

// 获取 ziplist 长度
unsigned int ziplistLen(unsigned char *zl);

// 获取总字节数
size_t ziplistBlobLen(unsigned char *zl);

// 查找匹配的 entry
unsigned char *ziplistFind(unsigned char *zl, unsigned char *p,
                           unsigned char *vstr, unsigned int vlen, 
                           unsigned int skip);

// 比较 entry 和字符串
unsigned int ziplistCompare(unsigned char *p, unsigned char *s, 
                            unsigned int slen);

Before:
[entry A: 2 bytes] [entry B: 253 bytes] [entry C: ...]
       |                   |
    prevlen=1          prevlen=1

After: Insert a new entry before B with 300 bytes
[entry A: 2 bytes] [new entry: 300 bytes] [entry B: 253 bytes] [entry C: ...]
       |                                      |
    prevlen=1                             prevlen=5 (!!!)

unsigned char *__ziplistCascadeUpdate(unsigned char *zl, unsigned char *p) {
    // 1. 计算当前 entry 的长度
    // 2. 检查下一个 entry 的 prevlen 是否需要更新
    // 3. 如果需要，更新并继续检查下一个
    // 4. 直到不需要更新为止
}

// 尝试将字符串编码为整数
int zipTryEncoding(unsigned char *entry, unsigned int entrylen, 
                   long long *v, unsigned char *encoding) {
    if (string2ll((char*)entry, entrylen, &value)) {
        // 可以编码为整数
        if (value >= 0 && value <= 12) {
            *encoding = ZIP_INT_IMM_MIN + value;  // 4-bit immediate
        } else if (value >= INT8_MIN && value <= INT8_MAX) {
            *encoding = ZIP_INT_8B;
        } else if (value >= INT16_MIN && value <= INT16_MAX) {
            *encoding = ZIP_INT_16B;
        } // ... 以此类推
        return 1;
    }
    return 0;  // 无法编码为整数，保持字符串
}

|00pppppp| - 1 byte
     String value with length <= 63 bytes

|01pppppp|qqqqqqqq| - 2 bytes
     String value with length <= 16383 bytes

|10000000|qqqqqqqq|rrrrrrrr|ssssssss|tttttttt| - 5 bytes
     String value with length >= 16384 bytes

|11000000| - 3 bytes total
     Integer encoded as int16_t (2 bytes)

|11010000| - 5 bytes total
     Integer encoded as int32_t (4 bytes)

|11100000| - 9 bytes total
     Integer encoded as int64_t (8 bytes)

|11110000| - 4 bytes total
     Integer encoded as 24 bit signed (3 bytes)

|11111110| - 2 bytes total
     Integer encoded as int8_t (1 byte)

|1111xxxx| - 1 byte (xxxx between 0001 and 1101)
     Immediate 4 bit integer (0-12)

|11111111| - End of ziplist