typedef struct redisObject {
    // 类型
    unsigned type:4;
    // 编码
    unsigned encoding:4;
    unsigned lru:LRU_BITS; /* LRU time (relative to global lru_clock) or
                            * LFU data (least significant 8 bits frequency
                            * and most significant 16 bits access time). */
    int refcount;
    // 指向底层数据结构指针（模拟多态，可以存储任何类型的对象）
    void *ptr;
} robj;

/* The actual Redis Object */
// 字符串对象
#define OBJ_STRING 0    /* String object. */
// 列表对象
#define OBJ_LIST 1      /* List object. */
// 集合对象
#define OBJ_SET 2       /* Set object. */
// 有序集合对象
#define OBJ_ZSET 3      /* Sorted set object. */
// 哈希对象
#define OBJ_HASH 4      /* Hash object. */
/* The "module" object type is a special one that signals that the object
 * is one directly managed by a Redis module. In this case the value points
 * to a moduleValue struct, which contains the object value (which is only
 * handled by the module itself) and the RedisModuleType struct which lists
 * function pointers in order to serialize, deserialize, AOF-rewrite and
 * free the object.
 *
 * Inside the RDB file, module types are encoded as OBJ_MODULE followed
 * by a 64 bit module type ID, which has a 54 bits module-specific signature
 * in order to dispatch the loading to the right module, plus a 10 bits
 * encoding version. */
#define OBJ_MODULE 5    /* Module object. */
#define OBJ_STREAM 6    /* Stream object. */

/* Objects encoding. Some kind of objects like Strings and Hashes can be
 * internally represented in multiple ways. The 'encoding' field of the object
 * is set to one of this fields for this object. */
// 简单动态字符串
#define OBJ_ENCODING_RAW 0     /* Raw representation */
// long 类型的整数
#define OBJ_ENCODING_INT 1     /* Encoded as integer */
// 字典
#define OBJ_ENCODING_HT 2      /* Encoded as hash table */
// 压缩map
#define OBJ_ENCODING_ZIPMAP 3  /* Encoded as zipmap */
// 双端链表
#define OBJ_ENCODING_LINKEDLIST 4 /* No longer used: old list encoding. */
// 压缩链表
#define OBJ_ENCODING_ZIPLIST 5 /* Encoded as ziplist */
// 整数集合
#define OBJ_ENCODING_INTSET 6  /* Encoded as intset */
// 跳跃表
#define OBJ_ENCODING_SKIPLIST 7  /* Encoded as skiplist */
#define OBJ_ENCODING_EMBSTR 8  /* Embedded sds string encoding */
// 快表
#define OBJ_ENCODING_QUICKLIST 9 /* Encoded as linked list of listpacks */
#define OBJ_ENCODING_STREAM 10 /* Encoded as a radix tree of listpacks */
#define OBJ_ENCODING_LISTPACK 11 /* Encoded as a listpack */

char *strEncoding(int encoding) {
    switch(encoding) {
    case OBJ_ENCODING_RAW: return "raw";
    case OBJ_ENCODING_INT: return "int";
    case OBJ_ENCODING_HT: return "hashtable";
    case OBJ_ENCODING_QUICKLIST: return "quicklist";
    case OBJ_ENCODING_ZIPLIST: return "ziplist";
    case OBJ_ENCODING_LISTPACK: return "listpack";
    case OBJ_ENCODING_INTSET: return "intset";
    case OBJ_ENCODING_SKIPLIST: return "skiplist";
    case OBJ_ENCODING_EMBSTR: return "embstr";
    case OBJ_ENCODING_STREAM: return "stream";
    default: return "unknown";
    }
}

robj *createObject(int type, void *ptr) {
    robj *o = zmalloc(sizeof(*o));
    o->type = type;
    o->encoding = OBJ_ENCODING_RAW;
    o->ptr = ptr;
    o->refcount = 1;
    /* Set the LRU to the current lruclock (minutes resolution), or
     * alternatively the LFU counter. */
    // 区分 LRU 和 LFU
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        o->lru = (LFUGetTimeInMinutes()<<8) | LFU_INIT_VAL;
    } else {
        o->lru = LRU_CLOCK();
    }
    return o;
}

/* Return the current time in minutes, just taking the least significant
 * 16 bits. The returned time is suitable to be stored as LDT (last decrement
 * time) for the LFU implementation. */
unsigned long LFUGetTimeInMinutes(void) {
    return (server.unixtime/60) & 65535;
}
/* Given an object last access time, compute the minimum number of minutes
 * that elapsed since the last access. Handle overflow (ldt greater than
 * the current 16 bits minutes time) considering the time as wrapping
 * exactly once. */
unsigned long LFUTimeElapsed(unsigned long ldt) {
    unsigned long now = LFUGetTimeInMinutes();
    if (now >= ldt) return now-ldt;
    return 65535-ldt+now;
}

int serverCron(struct aeEventLoop *eventLoop, long long id, void *clientData) {
    // ...
    /* Update the time cache. */
    updateCachedTime(1);
    // ...
}