#define LRU_BITS 24
typedef struct redisObject {
    unsigned type:4;   // 類型
    unsigned encoding:4; // 編碼
    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;
    void *ptr;
} robj;

/* The actual Redis Object */
#define OBJ_STRING 0    /* String object. */
#define OBJ_LIST 1      /* List object. */
#define OBJ_SET 2       /* Set object. */
#define OBJ_ZSET 3      /* Sorted set object. */
#define OBJ_HASH 4      /* Hash object. */

// code location： src/object.c  
#define OBJ_ENCODING_EMBSTR_SIZE_LIMIT 44
// 創建 strintg 對象
robj *createStringObject(const char *ptr, size_t len) {
	// 如果待保存的字符串的長度小于等于44，使用 embstr 編碼
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT)
        return createEmbeddedStringObject(ptr,len);
    else // 否則使用 raw 編碼
        return createRawStringObject(ptr,len);
}
robj *createEmbeddedStringObject(const char *ptr, size_t len) {
	// 申請 robj + sdshdr + data + 1 的空間
    robj *o = zmalloc(sizeof(robj)+sizeof(struct sdshdr8)+len+1);
    struct sdshdr8 *sh = (void*)(o+1);
    o->type = OBJ_STRING;      // 設置類型
    o->encoding = OBJ_ENCODING_EMBSTR; // 設置編碼
    o->ptr = sh+1;
    o->refcount = 1;
    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
        o->lru = (LFUGetTimeInMinutes()<<8) | LFU_INIT_VAL;
    } else {
        o->lru = LRU_CLOCK();
    }
    sh->len = len;
    sh->alloc = len;
    sh->flags = SDS_TYPE_8;
    if (ptr == SDS_NOINIT)
        sh->buf[len] = '\0';
    else if (ptr) {
        memcpy(sh->buf,ptr,len);
        sh->buf[len] = '\0';
    } else {
        memset(sh->buf,0,len+1);
    }
    return o;
}

/* Try to encode a string object in order to save space */
robj *tryObjectEncoding(robj *o) {
    long value;
    sds s = o->ptr;
    size_t len;
    /* Make sure this is a string object, the only type we encode
     * in this function. Other types use encoded memory efficient
     * representations but are handled by the commands implementing
     * the type. */
    // 這里只對string對象進行編碼，其他類型的編碼都有對應的具體實現處理
    serverAssertWithInfo(NULL,o,o->type == OBJ_STRING);
    /* We try some specialized encoding only for objects that are
     * RAW or EMBSTR encoded, in other words objects that are still
     * in represented by an actually array of chars. */
    // 非sds string對象，直接返回原對象
    if (!sdsEncodedObject(o)) return o;
    /* It's not safe to encode shared objects: shared objects can be shared
     * everywhere in the "object space" of Redis and may end in places where
     * they are not handled. We handle them only as values in the keyspace. */
    // 如果該對象由其他對象共享，不能編碼，如果編碼可能影響到其他對象的使用
     if (o->refcount > 1) return o;
    /* Check if we can represent this string as a long integer.
     * Note that we are sure that a string larger than 20 chars is not
     * representable as a 32 nor 64 bit integer. */
    // 檢查能否把一個字符串表示為長整型數，長度要小于等于20
    len = sdslen(s);
    if (len <= 20 && string2l(s,len,&value)) {
        /* This object is encodable as a long. Try to use a shared object.
         * Note that we avoid using shared integers when maxmemory is used
         * because every object needs to have a private LRU field for the LRU
         * algorithm to work well. */
         // 如果可以被編碼為long型，且編碼后的值小于OBJ_SHARED_INTEGERS(10000)，且未配
         // 置LRU替換淘汰策略, 就使用這個數的共享對象，相當于所有小于10000的數都是用的同一個robj
        if ((server.maxmemory == 0 ||
            !(server.maxmemory_policy & MAXMEMORY_FLAG_NO_SHARED_INTEGERS)) &&
            value >= 0 &&
            value < OBJ_SHARED_INTEGERS)
        {
            decrRefCount(o);
            incrRefCount(shared.integers[value]);
            return shared.integers[value];
        } else {
            if (o->encoding == OBJ_ENCODING_RAW) {
                sdsfree(o->ptr);
                o->encoding = OBJ_ENCODING_INT;
                o->ptr = (void*) value;
                return o;
            } else if (o->encoding == OBJ_ENCODING_EMBSTR) {
                decrRefCount(o);
                return createStringObjectFromLongLongForValue(value);
            }
        }
    }
    // 不能轉為long的字符串
    /* If the string is small and is still RAW encoded,
     * try the EMBSTR encoding which is more efficient.
     * In this representation the object and the SDS string are allocated
     * in the same chunk of memory to save space and cache misses. */
    // 如果字符串的長度太小，小于等于44
    if (len <= OBJ_ENCODING_EMBSTR_SIZE_LIMIT) {
        robj *emb;
        // 如果當前編碼是embstr，直接返回原對象，否則轉為embstr編碼，返回
        if (o->encoding == OBJ_ENCODING_EMBSTR) return o;
        emb = createEmbeddedStringObject(s,sdslen(s));
        decrRefCount(o);
        return emb;
    }
    /* We can't encode the object...
     *
     * Do the last try, and at least optimize the SDS string inside
     * the string object to require little space, in case there
     * is more than 10% of free space at the end of the SDS string.
     *
     * We do that only for relatively large strings as this branch
     * is only entered if the length of the string is greater than
     * OBJ_ENCODING_EMBSTR_SIZE_LIMIT. */
    // 如果前面編碼沒有成功，這里做最后一步，當編碼類型為raw，且它的sds可用空間超過10%，
    // 嘗試釋放調這部分內存
    trimStringObjectIfNeeded(o);
    /* Return the original object. */
    // 返回原對象
    return o;
}

robj *getDecodedObject(robj *o) {
    robj *dec;
	// 如果編碼是 embstr 和 raw ，只是把引用計數加一，然后返回原對象
    if (sdsEncodedObject(o)) {
        incrRefCount(o);
        return o;
    }
    // 如果編碼是 int 進行解碼，返回新的對象
    if (o->type == OBJ_STRING && o->encoding == OBJ_ENCODING_INT) {
        char buf[32];
        ll2string(buf,32,(long)o->ptr);
        dec = createStringObject(buf,strlen(buf));
        return dec;
    } else {
        serverPanic("Unknown encoding type");
    }
}

void incrRefCount(robj *o) {
    if (o->refcount < OBJ_FIRST_SPECIAL_REFCOUNT) {
        o->refcount++; // 引用計數加一
    } else {
        if (o->refcount == OBJ_SHARED_REFCOUNT) {
            /* Nothing to do: this refcount is immutable. */
        } else if (o->refcount == OBJ_STATIC_REFCOUNT) {
            serverPanic("You tried to retain an object allocated in the stack");
        }
    }
}
// 減少引用計數
void decrRefCount(robj *o) {
	// 釋放空間
    if (o->refcount == 1) {
        switch(o->type) {
        case OBJ_STRING: freeStringObject(o); break;
        case OBJ_LIST: freeListObject(o); break;
        case OBJ_SET: freeSetObject(o); break;
        case OBJ_ZSET: freeZsetObject(o); break;
        case OBJ_HASH: freeHashObject(o); break;
        case OBJ_MODULE: freeModuleObject(o); break;
        case OBJ_STREAM: freeStreamObject(o); break;
        default: serverPanic("Unknown object type"); break;
        }
        zfree(o);
    } else {
        if (o->refcount <= 0) serverPanic("decrRefCount against refcount <= 0");
        if (o->refcount != OBJ_SHARED_REFCOUNT) o->refcount--; // 計數減一
    }
}